B. E, ST
J
JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
Volume XXXVII July, 1941
CONTENTS
Page
Twenty-Fifth Anniversary of the Society of Motion Picture En-
gineers 3
Salute to the SMPE WILL H. HAYS 5
Another Milestone. . „ EMERY HUSE 7
Twenty-Five Years of Service F. H. RICHARDSON 9
Recent Advances in the Theory of the Photographic Process . .
C. E. K. MEES 10
Recommended Procedure and Equipment Specifications for
Educational 16-Mm Projection — A Report of the Commit-
tee on Non-Theatrical Equipment
Part I. General ^Recommendations 22
Part II. The Optical Properties of Commercially Available
Screens for 16-Mm Projection 47
Part III. Performance Specifications for 16-Mm Projection
Equipment for Educational Service 57
Supplement. Resolution Tests on 16-Mm Projection Lenses 70
Report of the Standards Committee 76
Report of the Theater Engineering Committee 78
Television Report, Order, Rules, and Regulations of the Federal
Communications Commission 87
Characteristics of Intermittent Carbon Arcs
F. T. BOWDITCH, R. B. DULL, AND H. G. MACPHERSON 98
Development and Current Uses of the Acoustic Envelope
H. BURRIS-MEYER 109
Current Literature 115
1941 Fall Convention at New York, October 20th-23rd 117
JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
SYLVAN HARRIS, EDITOR
BOARD OF EDITORS
A C. DOWNES, Chairman
I I CRABTUB A. N. GOLDSMITH E. W. KELLOGG
H K A. M. GUNDELFINGER C. R. SAWYER
A. C. HARDY
Sub*rriptkm to non-members, $8.00 per annum ; to members, $5.00 per annum,
! E tfc
thru annual membership dues; single copies, $1.00. A discount
oa MibKriptton or single copies of 15 per cent is allowed to accredited agencies.
Order from the Society of Motion Picture Engineers, Inc., 20th and Northampton
4Jton. Pm.. or Hotel Pennsylvania, New York, N. Y.
t*ubit*bed monthly at Easton, Pa., by the Society of Motion Picture Engineers.
Publication Office, 20th & Northampton Sts., Easton, Pa.
General and Editorial Office, Hotel Pennsylvania, New York, N. Y.
West Coast Office. Suite 226. Equitable Bldg., Hollywood, Calif.
Entered a* second class matter January 15, 1930, at the Post Office at Easton,
Pa . under the Act of March 3, 1879. Copyrighted, 1941, by the Society of
Motion Picture Engineers, Inc.
OFFICERS OF THE SOCIETY
EMERY HUSB, 6706 Santa Monica Blvd., Hollywood, Calif.
fut: E. ALLAN WILLIFORD, 30 East 42nd St., New York, N. Y.
Yut-Presidcnt: HERBERT GRIFFIN, 90 Gold St., New York, N. Y.
•/•.*Ci««rs»c Vic*-President: D. E. HYNDMAN, 350 Madison Ave., New York
Vic*-Prtsidenl: ARTHUR C. DOWNES, Box 6087, Cleveland, Ohio.
esident: A. S. DICKINSON, 28 W. 44th St., New York, N. Y.
resident: W. C. KUNZMANN, Box 6087, Cleveland, Ohio
r: PAUL J. LARSBN. 44 Beverly Rd., Summit, N. J.
Trmimrtr: GBORGB FRIEDL, JR.. 90 Gold St., New York, N. Y.
GOVERNORS
BATSBL. 501 N. LaSalle St.. Indianapolis, Ind.
DUWUY. 1801 Larchmont Ave., Chicago, 111.
•Jon* C. FRAYNB. 6601 Romaine St., Hollywood, Calif.
GOLDSMITH. 680 Fifth Ave., New York, N. Y.
••L HARDY. Massachusetts Institute of Technology, Cambridge. Mass.
RTMR. 5451 Marathon St., Hollywood. Calif
.
195 Broadway. New York. N Y
Snoot. 35-11 35th St., Astoria, L. L, N. Y.
Term expire* December 31. 1941.
•*Ttm eipire* December 31. 1942.
SOCIETY OF MOTION PICTURE ENGINEERS
Incorporated at Washington, D. C.
July 24. 1916
Incorporators
C. FRANCIS JENKINS
DONALD J. BELL
PAUL H. CROMELIN
C. A. WILLATT
FRANCIS B. CANNOCK
W. BURTON WESTCOTT
PAUL BROCKETT
E. KENDALL GILLETT
HERBERT MILES
J. P. LYONS
Washington, D. C.
Chicago, 111.
New York, N. Y.
Boston, Mass.
New York, N. Y.
Boston, Mass
Washington, D. C.
New York, N. Y.
New York, N. Y.
Cleveland, Ohio
The object of the Society shall be ... Advancement in the theory
and practice of motion picture engineering and the allied arts and sciences,
the standardization of the mechanisms and practices employed therein,
and the maintenance of a high professional standing among its members.
Membership of the Society
The membership of the Society at the first meeting, held at the
Hotel Astor, New York, N. Y., October 2-3, 1916, numbered twenty-
six persons, as follows :
C. FRANCIS JENKINS CARL E. AKELEY M. D. COPPLE
DONALD J. BELL H. T. WILKINS F. H. RICHARDSON
PAUL H. CROMELIN R. G. HASTINGS H. T. EDWARDS
C. A. WILLAT H. B. COLES MAX MAYER
FRANCIS B. CANNOCK HARVEY M. WIBLE WM. C. KUNZMANN
W. BURTON WESCOTT H. A. CAMPE A. S. VICTOR
PAUL BROCKETT R. E. VOM SAAL E. M. PORTER
E. KENDALL GILLETT BARTON A. PROCTOR N. I. BROWN
HERBERT MILES HERMANN KELLNER
From this modest beginning the Society has grown to nearly 1300
members distributed all over the world, a tribute to its success in
fulfilling the object stated on the previous page. Every branch of
the motion picture industry, including both the artistic and the
scientific aspects of photography, processing, distribution and pro-
jection is well represented in the Society, not only by those directly
engaged in these professions but by chemists, engineers, and research
workers interested in perfecting the apparatus and materials involved.
Presidents of the Society
C. FRANCIS JENKINS 1916-1918
H. A. CAMPE 1919-1921
LAWRENCE C. PORTER 1922-1923
LOYD A. JONES 1924-1925
WILLARD B. COOK 1926-1928
LAWRENCE C. PORTER 1929-1930
JOHN I. CRABTREE 1930-1931
ALFRED N. GOLDSMITH 1932-1934
HOMER G. TASKER 1935-1936
S. K. WOLF 1937-1938
E. ALLAN WILLIFORD 1939-1940
EMERY HUSE 1941-
SALUTE TO THE SMPE
WILL H. HAYS
When your Society was founded twenty-five years ago, the mo-
tion picture, with slow and faltering steps, was just beginning to
grope its way into the hearts and affections of the public. The
pioneers of that seemingly far away period had enthusiastic confi-
dence in this youngster among the arts, but the world at large too often
looked down its nose at the "movies." The child grew and devel-
oped, soon was taking prodigious strides, until today the motion pic-
ture is the most democratic of the arts of our century, and the uni-
versal entertainment of all the people everywhere.
Of the past, with its heartaches and its exhilarations, with its de-
feats and its unparalleled triumphs, we can be justly proud, but it
is the present and the future that now concern us most.
5
6 SALUTE TO THE SMPE
If we are going to develop this art-industry to its fullest poten-
tialities, as I know we are, then the work in no small measure will have
to be done by the technicians and engineers of your group. In a basic
sense, the motion picture is a mechanical art, the product of technical
wizardry. During my long years of association with the industry, I
have never ceased to have a feeling of awe on learning of each fabulous
scientific advance that has come from the laboratories and workshops,
from the minds of men seeking constantly to improve the art. How
many times have the unknowing said, "Well, this is it .... nothing more
is possible!" And then some Aladdin among you has rubbed a magic
lamp and brought forth a new wonder to dazzle and stun the imagina-
tion.
You have given the screen voice, color, undreamed-of realism.
Knowing what you have done, what you are capable of, I won't
even hazard a guess what the motion pictures will be twenty-five
years from now when the Society of Motion Picture Engineers cele-
brates its Golden Jubilee. But I do know this: whatever the future
holds you will contribute greatly to its course.
The motion picture is a collaborative art, requiring many minds
and many hands. Some 275 arts and crafts and professions partici-
pate in the making of a single film in our studios. It is this coopera-
tion of talents, harmoniously integrated, which has made the screen
one of the greatest constructive forces of modern times. Our industry
must always combine depth of human interest and human under-
standing with foresight, tenacity, sound judgment, and unswerving
devotion to the public welfare.
The entire industry rejoices to extend greetings and best wishes on
this occasion of your Society's 25th Anniversary.
WILL H. HAYS,
President, Motion Picture Producers
and Distributors of America,
ANOTHER MILESTONE
EMERY HUSE
President
July 24, 1941, marks the Twenty- Fifth Anniversary of the Society
of Motion Picture Engineers. In 1916 when a group of twenty-six
technical men, headed by Mr. C. Francis Jenkins of Washington,
D. C., met with the idea of formulating a motion picture engineering
society, little did they realize what might come of their idea. The
Society as a mere infant passed through the First World War with
only a few scars. As the years passed the Society grew in member-
ship and in strength until it eventually became a nationwide organi-
zation. Some years after its inception it began to reach out into the
world for membershfp and as a result of its far-reaching activities it
has become without question the outstanding motion picture engi-
neering society in the world today.
Some idea of the growth of the Society, particularly during the past
ten or twelve years, can be had if one knows that in 1928, at the time
the Pacific Coast Section of the Society was organized, the total mem-
bership of the national organization was less than the current mem-
bership of the Pacific Coast Section alone. The Society is now made
up of two Sections in addition to that on the Pacific Coast — the Mid-
West Section, located in Chicago, and the East Coast Section, with
headquarters in New York. The East Coast Section is fundamen-
tally the parent body of the Society. From the standpoint of mem-
bership from all over the world, the Society now boasts of approxi-
mately thirteen hundred motion picture engineers.
It is most unfortunate for the affairs of men that the world today is
in such a state of turmoil, but it is the purpose of the Society of Mo-
tion Picture Engineers during these trying times to maintain its nor-
mal activities as far as it is possible to do so. It is firmly believed
that in times of war, peacetime activities and efforts must go on and
the Society must remain a worthy outlet for the accumulated knowl-
edge in the minds of men doing peacetime work, or even war work,
provided the latter is connected with motion picture engineering. If
7
8 ANOTHER MILESTONE
we are able to live up to these worthy desires it seems certain that
when this period of emergency is over the importance and prestige of
the Society will be maintained, and we believe we will have laid a firm
foundation upon which a better peacetime program in the field of
motion picture engineering may be built.
This issue of the JOURNAL of the Society of Motion Picture Engi-
neers, which is dedicated to the Twenty-Fifth Anniversary of the
Society, marks a definite milestone in the accomplishments of the
Society. It must be proved that these accomplishments have not
been made in vain, and it is up to each and every member of the
Society to dedicate himself to the perpetuation of the ideals of this
Society. This can be done best by looking ahead to the Fiftieth
Semi- Annual Convention of the Society which will take place in New
York City, October 20 to 23, 1941. We must all put our shoulders to
the wheel and see to it that this Convention is the most outstanding
ever held by our Society.
President
TWENTY-FIVE YEARS OF SERVICE
F. H. RICHARDSON
Historically speaking, twenty-five years is an infinitesimal portion
of time, but in relation to the motion picture industry, twenty-five
years covers almost the entire period of birth, growth, and adolescence
of the industry. Twenty-five years ago I sat at a meeting with
twenty-five other men who had somehow chosen "moving pictures"
as their interest and livelihood, and we put this Society to work for us.
We had no idea the Society was going to last for twenty-five years
or that it would grow to the technical importance it now holds for the
entire industry. We had a job to do, and we set about to do it, and
the formation of the Society was one means of helping us to do it.
The Society grew slowly at first, because the industry was flounder-
ing about, trying to find itself ; but soon it grew more rapidly as the
movies began to expand into the enormous industry we have today;
and when sound came into the picture .... It is needless to go into
details. Today the Society's influence encompasses the entire world;
it has members in all important countries of the world; and consti-
tutes the most important source of information on the up-to-date prog-
ress and technical developments of motion picture engineering.
I am proud to have been one of the founders of the Society, and all
through the years I have tried to contribute whatever I could to the
betterment of the industry and of the Society. Projection has been
my principal interest, because I started as a projectionist — or "opera-
tor," in those days — and I am indeed happy in the fact that, with the
aid of a few others, I have been helpful in arousing the interest of the
Society in the humble art of "operating moving picture machines."
I trust and feel confident that the Society will continue successfully
this work begun so many years ago, and I can but repeat that I am
proud to have had a part in all this work. May the Society prosper
and find success in all its endeavors.
J
RECENT ADVANCES IN THE THEORY OF THE
PHOTOGRAPHIC PROCESS*
C. E. KENNETH MEES**
Summary. — A photographic film consists of a layer of gelatin coated on cellulose
base in which are dispersed a great number of very small siher bromide crystals.
When exposed to light, electrons are liberated in the crystals and these collect at certain
Points, where they are neutralized by silver ions which deposit atoms of metallic siher.
This metallic silver deposited in definite specks forms what is known as the latent
image, which makes possible the development of the crystal. The surface of each
silver bromide crystal in the gelatin layer of an emulsion immersed in the developer is
protected by charged layers of bromide and potassium ions. The development of the
grain is initiated by the break in this charged layer caused by the presence of the silver
latent image. When the developer acts on the silver bromide crystal, metallic silver
is produced in a ribbon-like form, a tangled mass of which forms the developed silver
grain.
Behind all our technology there lies the basic theory of the photo-
graphic process — the chemistry and physics of the formation and
structure of the photographic material, its reaction to light, its be-
havior in the developer when the image is produced, and the prop-
erties of that image.
The science of photography is founded on the two great sister
sciences, chemistry and physics, and it was only as our knowledge of
these grew that progress could be made on the problems of photo-
graphic science. Until recently, photographic science tended to con-
sist of a chaos of observations, some of them of real value and others
of very doubtful value, with little in the way of theories to connect
them properly. It is only within the last few years that fact after
fact has been falling into place in an ordered network. At the present
time we can say that while much remains to be done, we have a very
clear and definite science of photography — something which can be
written out and generalized upon and to which the missing parts can
be added as more work is done.
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 6,
1941.
** Kodak Research Laboratories, Rochester, N. Y.
10
<> The Society is not responsible for statements by authors <>
ADVANCES IN PHOTOGRAPHIC PROCESSES 1 1
Strictly speaking, many light-sensitive substances could be used
for making photographic images, and the science of photography
should be co-extensive with photochemistry itself; that is, with the
chemistry and physics of light-sensitive substances. But in practice,
this is not the case, and the art of photography is almost entirely con-
fined to the use of silver salts, so that the science of photography is
necessarily preoccupied with the very complex system of silver halide
crystals dispersed in gelatin. Information as to the reactions which
go on in the simpler systems used in photochemical investigations
throws little light on the photographic process.
If we enlarge a photographic film under a microscope to about the
limit of resolution of the microscope ; that is, to some 2500 diameters,
we shall find that it consists of a very complex system. On the base,
which is cellulose nitrate or acetate, there is coated a layer of gelatin
containing silver halide crystals. These silver halide crystals are
composed of silver bromide containing a small amount of silver io-
dide, and they may be dyed to sensitize them to the longer wave-
lengths of light. The crystals vary considerably in size but are of
the same general shape. They are triangles and hexagons, which are
the natural forms of silver bromide, and they are held in photographic
gelatin (Fig. 1). Analysis would show that the film also contains a
number of materials — glycerine, hardeners, and other things adapted
to control its properties. When this film is exposed to light, the
silver bromide crystals are affected in some way by an extraordi-
narily small amount of light, and they suffer some change. That
change must take place in two steps and not quite instantaneously,
although it occurs in a very short fraction of a second. The reason
for this conclusion is that the amount of change produced depends
somewhat upon the rate at which the light is supplied. This is
what is known as the ''reciprocity effect." If the light is supplied
rapidly, somewhat more effect is produced than if the light is ap-
plied very slowly — as if, for instance, a faucet were running into a
bucket and the bucket had a small hole in it. But the analogy is not
good because when the exposure is over, the change that has occurred
is permanent; the image will keep for long periods. When Andre's
photographs were found at the Pole thirty years after his balloon
fell on the ice and were developed, they were quite satisfactory, the
latent image having been preserved by the cold in spite of immersion
in sea water.
The silver bromide crystals in the emulsion depend for their sensi-
12 C. E. K. MEES [J. S. M. P. E
tivity upon the gelatin in which they are suspended. Emulsion
makers have known for many years that some gelatins were active
and would give sensitive emulsions and that others were inactive.
In an arduous research, this was traced by Sheppard to the presence
in the gelatin of traces of free sulfur compounds, which are presum-
ably derived from the plants which the calves and their mothers ate.
When gelatin is made from little animals, like rabbits, which avoid
the hot-tasting plants, such as mustard, which contain sulfur, the
gelatin does not contain these sulfur compounds, so that it was not
» .v^» P-
*.$> 1
FIG. 1. Silver bromide grains in a photographic emulsion.
improper to state that "if cows didn't like mustard we wouldn't
have any movies!" The sulfur compounds in the gelatin react with
the silver bromide and produce specks of silver sulfide. These specks
of silver sulfide in some way increase the sensitivity of the silver
bromide crystal to light.
Recently, a thoroughly consistent theory of the effect of light upon
the silver bromide grains has come out of the work of our laboratories
and from Professors Gurney and Mott of Bristol, England. In the
first place, if we consider the energy diagram (Fig. 2) of a silver bro-
mide crystal, we shall find that we have two energy levels, the 5 and
July, 1941] ADVANCES IN PHOTOGRAPHIC PROCESSES 13
P levels, in which the electrons may be situated. The S band is
normally empty and is referred to as a "conduction band." The P
band is normally completely filled with the electrons. Upon ex-
posure of a silver bromide crystal to light which is absorbed in the
long-wavelength end of the characteristic absorption band, the elec-
trons are transferred from the lower P band to the 5 band, and the
crystal becomes conducting. This property is well known in other
materials, as well as in silver bromide, as "photo-conductance," and
the silver bromide crystal exposed to light may be imagined to be
filled with a sort of gas of conducting electrons. Also, when light is
absorbed by the silver bromide, electrons are released. This is the
primary photographic process — the thing that happens instantly
when light falls on the crystal. The electrons move about with
great speed inside the crystal and will very frequently reach the
ZERO POTENTIAL ENERGV
) FILLED
MOMOUCnON
' LEVELS
BC AqS, AqBP ^ ^ ^ ^ ^
Aq SPECK
FIG. 2. Energy diagram of the silver bromide crystal.
boundaries of the crystal, but when they reach a sensitivity speck,
they will be trapped by it and the sensitivity speck will become nega-
tively charged by the electrons that it has absorbed. Naturally,
the sensitivity specks will themselves be giving out electrons slowly
if they are at normal temperatures, just as does any other solid body.
During an ordinary exposure, the amount of electrons given out by
the sensitivity specks will be very small; while those which will be
absorbed from the electrons freed by light will be very great. After
the formation of the free electrons by light, therefore, the sensitivity
specks acquire a negative charge by the absorption of these free elec-
trons.
In a crystal, there is always available, of course, a certain amount
of silver ions which are formed inside the lattice. As soon as the
sensitivity specks acquire negative charges, these silver ions are at-
tracted to the specks, each negative charge neutralizes one silver ion
and produces a deposit of a silver atom at the sensitivity speck, so
14 C. E. K. MEES [J. S. M. P. E.
that every electron freed by the original light exposure is eventually
transformed into a silver atom deposited on a sensitivity speck.
This theory of the effect of exposure was suggested by Sheppard
and Trivelli of our laboratory over ten years ago under the title of
"the concentration speck theory," but they were unable to give a
satisfactory mechanism for the formation of the concentration speck
although they saw that in some way the effect of light must be to
produce a silver deposit at the sensitivity specks. The new theory
of Webb and Gurney and Mott shows that the effect occurs in two
stages: first, the release of free electrons, which occurs instantane-
ously ; and then the transformation of the free electrons by neutraliza-
tion through the silver ions into silver atoms at the sensitivity specks.
This accounts for the reciprocity effect. When the light acts, free
electrons are formed and go to the sensitivity specks, but the sensi-
tivity specks are continually losing electrons and, consequently, if
the light is weak, there will not be as many silver atoms deposited at
the sensitivity specks as there should be. A certain minimum con-
centration of electrons must be built up in the crystal before the elec-
trons begin to be trapped by the sensitivity specks. This explana-
tion is shown to fit the facts because, when the loss of electrons from
the sensitivity specks is reduced by greatly lowering the temperature,
the rate at which the light is supplied no longer affects the resulting
image.
The action of light, then, on the silver halide crystals is, first, to
produce instantaneously a charge of free electrons. Then these elec-
trons are attracted to the sensitivity specks, and their charge is
neutralized by silver ions, with the result that metallic silver is de-
posited around each sensitivity speck and forms the permanent
nucleus which we call the "latent image."
The great efficiency of the photographic process is due to the very
small amount of work which is done by light in forming an image and
the very large amount of work which is done by the chemical de-
veloper.
A photographic developer is a reducing agent; that is, it is a sub-
stance which is itself oxidized by silver bromide and, in being oxi-
dized, reduces the silver bromide to metallic silver. The matter is,
however, very complicated, and we are only beginning to understand
the behavior of photographic development. In the first place, not
all reducing agents, by any means, are photographic developers.
If the reducing agents are too strong, they reduce the unexposed
July, 1941] ADVANCES IN PHOTOGRAPHIC PROCESSES 1.-,
silver bromide and the whole of the film turns black, no image being
formed. If the reducing agents are too weak, they will not reduce
the silver bromide after exposure. In order that the substance may
be a developer, it must have a certain power of reduction or, as we
should say in electrochemical terms, a certain "reduction potential."
But there are substances which fall in the correct range of reduction
potentials, so far as we can measure it, which still are not photo-
graphic developers. There are others which are photographic de-
FIG. 3. Filamentary structure of a silver grain
(X40,000).
velopers in the sense that they show an image on an exposed film but
are not useful photographic developers because they do not develop
satisfactory images in a reasonable time.
Our knowledge of the mechanism of development has been greatly
assisted by the information as to the structure of the developed silver
obtained by the use of the electron microscope. The grains of de-
veloped silver show little structure under the highest magnification
of the ordinary microscope. It was obvious that they could not be
compact masses of silver since their volume is much too great for
their mass if the structure was compact, and it was generally thought
that the grains had a spongy structure, somewhat similar to that of
16
C. E. K. MEES
[J. S. M. P. E.
I
§-
I
—
July, 1941] ADVANCES IN PHOTOGRAPHIC PROCESSES
17
«RO POTENTIAL
coke. The electron microscope enables photographs to be taken
with equally good definition at magnifications about twenty times
higher than those which are possible with the ordinary microscope,
and when this instrument was applied to the photomicrography of
developed silver, it was found that the silver had a most unexpected
ribbon-like structure, so that the grains appear like masses of sea-
weed (Fig. 3). This filamentary structure of developed silver is very
surprising, and the fact that it is so unusual makes possible some de-
ductions as to the formation of silver. It might be thought that the
ribbons were produced by the
formation of silver in interstices
in the silver halide grains, but
this is seen to be impossible when
we examine the development of
extremely small silver bromide
grains, such as those which are
used in emulsions of the Lipp-
mann type. Each single crystal
turns into a filament of silver,
which is much longer than the
diameter of the crystal, so that
it is evident that filamentary
silver must be ejected from the
crystal when development occurs.
A series of pictures-showing the
stages of development of grains
are very instructive (Fig. 4).
The grains were deliberately
selected to be very small and
the photographs show clearly the ejection of the ribbons of silver
and their growth from the grains until the whole grain has been
converted into a spongy mass of silver filaments. It seems to be
clear, therefore, that the old idea that the grains dissolved in the
developer and then silver was precipitated and coagulated around
the exposed crystalline grains is quite incorrect. Instead, we
have to imagine that the developer reacts with the exposed
silver bromide grain and from it forces filaments of silver arising pre-
sumably from the silver silver-bromide interface. As more silver is
produced, new spots in the grain become the sources of development
until the whole grain is converted into silver.
FIG. 5.
Diagram of grain with pro-
tective double layer.
18 C. E. K. MEES [J. S. M. p. E.
In a study of the initiation of development, it must be remem-
bered that the problem is not why an exposed grain develops, so much
as why an unexposed grain does not develop. If silver bromide is
precipitated in the presence of an excess of silver nitrate, it is spon-
taneously developable without exposure to light. Moreover, silver
bromide even in the absence of free silver and without exposure to
light is readily reduced in a developing solution if it is precipitated in
the absence of gelatin, and there is no doubt that the adsorption of
gelatin to the silver bromide protects it from the action of the de-
veloper. This protection may be considered to be due to a nega-
tively charged electric layer which surrounds the silver bromide grain
formed with an excess of bromide, the function of the gelatin being
to protect the charged layer. Dr. J. H. Webb depicts the exposed
silver halide grain as a plate, as shown in Fig. 5 in which the charged
condition around the grain is represented schematically. The sur-
face of the silver bromide grain itself has an excess of bromide ions
which give rise to a negatively charged surface. However, just out-
side this negative charge, a positive layer of potassium ions must be
present to neutralize the negative charge. Without such a neutraliz-
ing layer of positive ions, it would be impossible for the surface of the
silver bromide grain to be covered with negative bromine ions, since
the amount of such a charge in so small a region would give rise to ex-
plosive forces. A double charge layer, consisting of negative bro-
mine ions on the grain and positive potassium ions in the gelatin
just outside, may be considered to exist around the surface of each
silver bromide grain. Grains with such a double layer (in solution)
would move under an electric field as negatively charged bodies,
since the negatively charged grain would be forced in one direction
by the field, and the surrounding movable positive ion layer in the
opposite direction, but as at any point in the liquid there would be
positive ions to form the surrounding positive shell, the double
charge layer would be maintained. That the surface charge on the
particles and surrounding charge layers do neutralize each other in
the manner outlined is proved by the fact that the colloidal suspen-
sion does not possess a net charge of either sign, but is neutral as a
whole.
It may be assumed that a grain, with its double charge layer, be-
haves toward outside charges and also toward charges located inside
the grain as a neutral body. An electron placed inside such a double
charge layer would experience no force nor, in the same way, would
July, 1941] ADVANCES IN PHOTOGRAPHIC PROCESSES
19
an electron placed outside such a double layer. However, there is a
marked difference in potential between the inside and outside of the
grain, and the total jump in this potential occurs in the region between
the two charge layers. The potential gradient between these charge
layers accordingly gives rise to a strong electrical force between the
layers, and an electron placed between them would experience a force
toward the outside. It is considered that the double charge layer
UOUBUE CHARGE-'
LAVE.R
FIG. 6. Diagram of grain with latent image.
acts in this way as an effective potential barrier to the entrance of an
electron into the silver bromide grain of the emulsion and prevents
the charged ions of the developer from attacking the grain.
The condition existing in the exposed grain containing a latent
image silver speck may be seen in Fig. 6. This shows a greatly en-
larged scale model of a charged grain surface with a clump of silver
atoms on the surface, which is supposed to represent the latent image
produced by exposure to light. The clump shown includes 220
atoms, with approximately the correct spacing. This size was
20 C. E. K. MEES [J. S. M. P. E.
chosen as representing a fair mean of the values given by various
workers.
It is assumed that development of a grain is initiated by the break
in the double charge layer caused by the silver speck, permitting the
negative developer ions to reach this silver speck. The latent image
speck is viewed as an electrode penetrating into the grain. The
tendency on the part of the developer ions to release electrons to the
silver causes electrons to pass to the electrode and charge it nega-
tively. This occurs if the electrons of the developer ions are situated
in levels above the highest occupied energy levels of the silver metal.
The penetration of this negative electrode into the silver bromide
grain upsets the neutral electrical condition previously existing in the
grain, and there arises an attractive force for the positively charged
silver ions in the neighborhood of the latent image speck. Some
loose positive silver ions always exist in the crystal lattice owing to
temperature motion, and these diffuse to the speck under the attrac-
tion of the negative charge there and enlarge the silver speck. Thus,
it is supposed that the original silver speck of the latent image com-
mences to grow by this mechanism. As this proceeds, the protective
double layer is more and more ruptured, and a rapidly increasing
area of the silver halide grain is exposed to the attack of the developer.
The reduction of the grain therefore proceeds at an ever-increasing
rate, and the grain is very soon reduced throughout to metallic silver.
In the initial stages of development only, is a silver bromide grain
protected from a developer; after the barrier is once penetrated, it
rapidly approaches the status of an unprotected grain, which, as
pointed out, is developable very rapidly.
This is only a very preliminary sketch of the action of develop-
ment. Undoubtedly, the adsorption of the developer to the de-
veloping grain plays some part in the reaction. It concentrates the
developer ions at the point where they are required and undoubtedly
also the actual reaction of the developer with the grain, and its be-
havior as a reducing substance is catalyzed by the silver of the latent
image.
A great change has taken place in the technic of the motion pic-
ture studio in the last twelve years as a result of the application of
panchromatic films. Negative films in motion picture work are
now invariably panchromatic, and their greatly improved quality
compared with the earlier materials is due to the advances that have
been made in the preparation of sensitizing dyes. These sensitizing
July, 1941] ADVANCES IN PHOTOGRAPHIC PROCESSES 21
dyes are what are known as "polymethine" dyes, most of them being
the class of dyes which are known as "cyanines." There are basic
dyes in which the two nuclei are linked by a chain of CH groups.
Since many different nuclei can be used, the chain can be of different
lengths and various substituents can be inserted in a molecule, so
that very many dyes are available, and since they all have properties
peculiar to their structure, a wide range of sensitizing can be obtained.
The cyanine dyes show very beautiful crystals. They have bright
colors, and many of them are pleochroic, so that they show iridescent
effects.
In addition to the advances in practical photography which have
followed the use of the sensitizing dyes we have achieved a consider-
able knowledge of the way in which they work. It seems clear that
the optimum concentration for the sensitizing of a silver halide grain
is a single layer of dye molecules attached to the whole surface of the
grain, as if the flat plate of silver halide were covered with a little
velvet pile of dye molecules, all of them firmly attached to the silver
halide lattice, but free to resonate to the light which they absorb.
The dye molecules appear to be arranged edge-on, so that for the best
sensitizing the surface is covered with leaf-like molecules piled edge-
wise in as close packing as is compatible with their own structure
and the structure of the crystal, forming a parallel pile or edge-on
layer one molecule thick. The new surface of the crystal with the
dye on it has no affinity for water, but there is an attraction between
the molecules oriented in this way, so that colliding particles may
tend to aggregate and precipitate. Dyes which otherwise might be
sensitizers may fail to sensitize because their molecules are so shaped
that they can not form this flat leaf -like structure, and the best sen-
sitizers are presumably those which form the structure easily. When
the light is absorbed by the dye molecule, it must liberate an electron,
but it is not yet possible to decide whether this electron itself acts to
form the latent image in the silver bromide or whether the energy is
transferred to the silver bromide which liberates the electron into
the conduction band.
There are many obscure points which still require elucidation in
the theory of the photographic process, but very rapid progress has
been made recently and we are beginning to understand the funda-
mentals of the process by which pictures are made.
RECOMMENDED PROCEDURE AND EQUIPMENT
SPECIFICATIONS FOR EDUCATIONAL
16-MM PROJECTION*
A REPORT OF THE COMMITTEE ON NON-THEATRICAL EQUIPMENT
MAY, 1941
This report has been prepared in response to a request from the
Committee on Scientific Aids to Learning, of the National Research
Council.
The report is in three parts. Part I is a general discussion of the
problems that enter into the selection and use of 16-mm motion
picture equipment for educational institutions. It includes recom-
mendations for such comparative tests of equipment as can properly
be made without testing laboratory facilities.
Part II is a report on the optical characteristics of the screens
available at the present time for non-theatrical projection.
Part III consists of a set of detailed technical specifications de-
fining acceptable performance of 16-mm projection equipment for
educational uses. The character of these specifications is neces-
sarily such that they can be interpreted and applied only by a fully
equipped testing laboratory.
Committee on Non-Theatrical Equipment
J. A. MAURER, Chairman
J. G. BLACK R. C. HOLSLAG W. H. OFFENHAUSER
F. E. CARLSON R. KINGSLAKE L. T. SACHTLEBEN
F. M. HALL D. F. LYMAN A. SHAPIRO
J. A. HAMMOND L. R. MARTIN M. G. TOWNSLEY
M. HOB ART R. F. MITCHELL A. G. ZIMMERMAN
PART I
GENERAL RECOMMENDATIONS
Objectives. — In the selection of motion picture equipment for
classroom use, the object should be to provide a picture that can be
viewed to good advantage by everyone in the classroom. Likewise,
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 27,
1941.
22
NON-THEATRICAL EQUIPMENT REPORT 23
equipment should be selected to provide good reproduced sound in
every part of the classroom.
Standards of quality in educational projection ought, if anything,
to be higher than those in the theatrical motion picture field. The
pupil does not come to the classroom to be entertained, but to learn.
In order to learn from the screen, he must watch it diligently, even
though he may happen to be seated in a position that affords him
only an oblique and distorted view of the picture. In order to learn
from the sound, he must be able to understand reproduced speech
without effort, and he must be able to obtain a true impression of the
character of natural sounds and of the tone qualities of musical
instruments when these are used in the films.
In a motion picture theater, if one has to sit in an unfavorable
location, as a rule he is subjected to this annoyance for only a single
performance. In the schoolroom, however, he may be required to
keep the same seat day after day. If this seat does not give him a good
view of the picture and a good opportunity to hear the sound, he is
under a permanent handicap.
It is because of these considerations that in several instances this
report recommends narrower limits than are commonly accepted in
theatrical projection practice. The Committee believes that in the
educational field there should be no compromise with respect to the
conditions that are necessary to secure substantially equal perform-
ance for all persons in the classroom.
Basic Steps in Equipment Selection. — Intelligent selection of equip-
ment means a great deal more than the mere selection of high-quality
equipment. It means the coordinated selection of equipment items
in relation to the conditions under which they are to be used. The
projector, the unit on which most attention is generally focused,
should, as a rule, be the last to be selected.
The first consideration, and one of the most important, is the size
of the screen to be provided. This is determined primarily by the
maximum distance from which the picture will be viewed by the
students.
After the picture size has been determined, the right type of screen
surface must be selected, as determined by the shape of the room, or,
rather, by the seating arrangement of the spectators in the room.
The picture size and the type of screen surface, together with the
degree to which light can be excluded from the room, determine the
light output required from the projector. The selection of the pro-
24 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
jector should be made from those types which provide as nearly
as possible the correct light output.
A similar requirement exists with respect to the power-handling
capacity of the sound-reproducing system, in relation to the acoustic
properties of the classroom. Fortunately, the power output of the
sound-reproducing system can be controlled more conveniently than
the light-projecting system of the projector; therefore it is sufficient
to ascertain by practical test that the maximum sound power output
of the projector selected is sufficient for the room in which it is to be
used.
Complete equipment for the projection of films in a classroom in-
cludes a suitable stand for supporting the projector firmly in the
proper location. Facilities for darkening the room during projection
are also needed.
Recommendations with respect to each of these problems will be
given, hi the general order in which they have been mentioned.
Picture Size. — In the past the Society of Motion Picture Engineers
has conducted an extensive survey of theaters1 to determine, among
other things, the most desirable picture size in relation to the dis-
tance from the screen to the farthest spectators. The result of this
survey agrees well with the conclusion that follows theoretically
from a study of the ability of the average human eye to see fine
details under the conditions of watching a motion picture. It is
found that a distance equal to 6 times the width of the screen is the
greatest at which all of the details in the picture can be seen easily.
// is recommended, therefore, that a picture width equal to 1/6 of the
distance from the farthest row of seats to the screen position be adopted
for classroom projection.
Other considerations dictate a minimum viewing distance. If the
observer is sitting too close to the screen, even though the screen
image is focused as sharply as possible, it will appear to be out of
focus, because it does not contain enough fine details to appear sharp
at that distance. Under this condition the spectator experiences a
type of eye-strain caused by the instinctive attempt of the brain and
the eye muscles to focus a sharper picture than is present on the
screen. Since this is impossible, the eye muscles are kept in con-
tinuous activity, and there is nervous as well as physical fatigue. It
is also a matter of common observation that when one sits too close
to a motion picture, the eye movements needed to follow the action
on the acreen are excessively rapid, and may result in eye-strain.
July, 1941] NON -THEATRICAL EQUIPMENT REPORT 25
It is recommended that no pupils be seated closer to the screen than
twice the picture width. In most classrooms this requires the placing
of the screen on or near the front wall of the classroom, in order to
have it far enough from the front row of seats.
In order to adjust the size of the projected image exactly to fill
the screen, the projector rather than the screen should be moved.
The ideal arrangement is to provide a fixed stand for the projector
at the correct distance from the screen. If this is not done, a stand
on wheels which can be locked in position when the right location
has been found should be provided with each projector. A descrip-
tion of one suitable type of stand will be found in the booklet en-
titled "Projecting Motion Pictures in the Classroom," Vol. IV, No.
5, of the series entitled "Motion Pictures in Education," published
by The American Council on Education, Washington, D. C. A stand
not differing greatly from this design is available from at least one
equipment manufacturer.
If the projector is equipped with the usual lens of 2-inch focal
length, the screen will be filled when the distance to the projection
lens is 5*/4 times the screen width. Thus the correct location for the
projector is a little more than 5/6 as far from the front wall of the
classroom as the farthest row of seats. While in some cases it is
more convenient to have the projector at the rear of all of the rows of
seats, it should be placed in this location only if the resulting picture
width is not greater than l/2 the distance from the front row of seats
to the screen.
Projection lenses of several focal lengths greater than 2 inches are
available for use with 16-mm projectors. These are useful in cases
where the construction of the room or auditorium makes it necessary
to place the projector in a location that would give too large a picture
with the 2-inch lens. The lens of longer focus gives a smaller picture
in the ratio of 2 inches to the focal length of the lens used.
In general, however, it is best to use the 2-inch lens and obtain
the correct picture size by locating the projector correctly. Struc-
tural limitations in most of the projectors now available make it
necessary to reduce the angular aperture, or "speed" of the longer
focus projection lenses in order that they may be mounted on the
machine. This reduction of lens aperture reduces the light trans-
mitted to the screen. If the amount of light obtained is still within
the limits recommended later in this report for the size and type of
screen used, performance will be satisfactory. The need for a lens
26 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
of long focal length is most likely to arise, however, in an auditorium
or lecture room where the amount of light required is already equal
to the maximum that can be obtained from the projector. In such
a case the sacrifice of optical efficiency that is involved in the use of
a lens of long focal length and reduced relative aperture should be
made only when there would be real difficulty in locating the pro-
jector correctly for the use of the 2-inch lens.
It may reasonably be inquired whether or not there is a best loca-
tion from which to view a motion picture. This question receives
a definite answer from a consideration of the perspective relation-
ships in the viewing of the picture. It can readily be demonstrated2
that the perspective in a projected picture will be entirely correct
from the standpoint of the observer only when the observer's distance
from the screen bears the same relation to the distance from the
projector to the screen as the focal length of the lens used to take the
picture bears to the focal length of the lens used to project it. Since
most 16-mm motion pictures are photographed with 1-inch lenses,
and the result obtained by reducing films from the 35-mm size is
almost exactly the same as if the pictures had been taken in the 16-
mm size with a 1-inch lens, it follows that whenever 16-mm films are
projected with the usual 2-inch lens, the best position from which to
view the picture is half way between the projector and the screen.
In this location the spectator's judgment of the relative sizes of
objects near and far from the camera will be correct. All those
sitting closer to the screen receive the impression that the more dis-
tant objects in the picture are too large. Those sitting farther from
the screen receive the impression that the near objects in the picture
are too large.
The above considerations relating to perspective are not of vital
importance, as may be seen from the fact that they are frequently
neglected in motion picture theater construction. Nevertheless,
they indicate that whenever a picture slightly wider than 1/6 the
distance from the screen to the farthest row of seats can be used with-
out making the width greater than l/2 the distance to the nearest
row of seats, it is just as well to use this larger picture, since this
practice results in placing the point of best perspective rendition more
nearly in the center of the audience. It should be remembered,
however, that a larger picture requires more light from the projector.
Limitation of Viewing Angle. — All those spectators who view the
picture from positions near a line drawn perpendicular to the center
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 27
of the screen see a nearly undistorted picture. As the observer moves
away from this line toward the side of the room, his view of the pic-
ture becomes distorted. When the viewing angle approaches 40
degrees the screen appears square instead of oblong. Careful tests
made by Tuttle3 demonstrated that this amount of distortion is
SCRFFN
. BOUNDARY OF
SEATING AREA..
FIG. 1. Recommended seating area for wide room, with matte type screen.
definitely objectionable, even to spectators who see the picture under
conditions such that they are unaware of the cause of the distortion.
This Committee recommends that for school projection the viewing
angle be limited to 30 degrees. This condition is approximately ful-
filled when no row of seats is longer than its distance from the screen.
In a nearly square classroom or auditorium this recommendation
calls for the seating arrangement shown in Fig. 1. Any seats out-
28 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. p. E.
side the limits indicated should not be occupied during the projection
of motion pictures.
Selection of Screen Surface. — Screens fall into three general classes
as to the manner in which they reflect light. First is the matte-
surface type, coated with flat white paint or consisting of fabric or
rubber so treated as to reflect light in the same manner as white
paint. These screens reflect the light that falls upon them in such
a way that their brightnesses are approximately the same at all
angles of view. A picture projected on such a screen is ^almost
as bright when viewed from an angle of 30 degrees, or even from an
angle of 60 degrees, as it is when viewed perpendicularly.
The reflecting power of screens in different directions is customarily
expressed in terms of a theoretical screen which reflects all the light
falling upon it in such a way as to be equally bright at all viewing
angles. Taking the coefficient of reflection of such a screen as 100
per cent in every direction, a good matte-surface screen will have a
coefficient of reflection of 85 per cent along the perpendicular to the
screen surface, and a coefficient of reflection of between 75 and 80
per cent at an angle of 30 degrees.
The second type of screen has a surface covered with small glass
beads. These beads have the property of reflecting the light in-
ternally, and at the same time directing it by refraction in such a
way that the largest proportion of the light is sent back in the direc-
tion from which it came. This is true even when the light strikes
the screen at an angle. Because the beaded screen sends most of
the light back toward the projector, those sitting along the centerline
of the classroom see a much brighter picture than would be provided
even by the perfectly reflecting theoretical screen described above.
The brightness along the axis of projection corresponds to a coef-
ficient of reflection (as defined above) of about 350 per cent. At
greater viewing angles, however, the picture is much less bright.
At an angle of 22 degrees the picture brightness is the same as is
obtained with an average matte surface screen, and at all greater
viewing angles it is considerably less.
The third type of screen is coated with fine particles of metal,
usually aluminum, which reflect the light like so many little mirrors.
Screens of this type show a pronounced "hot-spot," which is near the
center of the screen for those sitting near the centerline of the room,
and moves over to one edge as the spectator moves away from the
centerline. The condition is particularly bad for those sitting near
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 29
the ends of the front row of seats. In this location one side of the
picture may appear as much as ten times as bright as the other side.
Metal-surfaced screens are necessary for certain types of projection
in which polarized light is used, but they should not be used in classroom
projection of motion pictures. This recommendation is made regard-
less of whether the screen surface is of smooth or rough texture. The
rough textured screens diffuse the light more than the smooth sur-
faced types, but none of the surfaces examined by the Committee
showed sufficient diffusion to avoid the difficulties mentioned above.
The choice between the matte-surface type of screen and the
beaded screen depends upon the maximum viewing angle. When the
classroom is nearly square, requiring a maximum viewing angle of 30
degrees (that is to say, a seating arrangement similar to that shown
in Fig. 1), only the matte-surf ace type should be used. If the classroom
is oblong, with the picture at one end, so that the maximum mewing angle
can be limited to 20 degrees, the beaded type is the best, because of its
ability to concentrate most of the reflected light within this narrow
angle and thus furnish a bright picture with less light from the
projector.
An easy rule to follow is that with the beaded screen no row of seats
should be longer than 2/3 of its distance from the screen. In many
oblong classrooms this condition can be met by not using the seats
in the extreme front corners of the seating space during the projec-
tion of motion pictures. A beaded screen should not be used if the
viewing angle for any- large number of spectators will exceed 20 degrees.
The proper arrangement of the spectators is shown in Fig. 2.
It has been mentioned that the beaded screen sends most of the
reflected light back in the direction from which it came, even when
that direction is not at right angles to the screen. For this reason,
whenever a beaded screen is used, the projector should be located only
just high enough for the projected beam of light to clear the heads of the
spectators. If, as is sometimes done, the projector is located in the
balcony of an auditorium or gymnasium, far above the heads of the
spectators, it can readily be seen that the beaded type of screen is
entirely unsuitable, since it sends most of the light back to the
neighborhood of the projector where there are relatively few specta-
tors. Meanwhile, all those seated on the main floor of the room see
a relatively dim picture.
It should be emphasized that the above paragraphs apply to the
selection of screens from among the types commercially available at
30
NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
the time of this report. It is entirely possible that new types will
be developed that will provide brighter pictures than the present
matte screens over the range of viewing angles up to 30 degrees
without exhibiting the "hot-spot" difficulty now experienced with the
metal-surfaced screens.
SCREEN
I BOUNDARY I
I OF SEMI NQ AREA.
FIG. 2. Recommended seating area for narrow
room, with beaded screen. Note — seats in the
shaded area are the least desirable.
Representative reflection distribution curves for screens of the
three types that have been discussed will be found in Part II of
this report. That part of the report also contains a much more de-
tailed discussion of the relations that exist between the reflecting
properties of the screen and the appearance of the picture as viewed
from various angles and distances.
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 31
Need for Replacement of Old Screens. — An unfortunate aspect of
the screen problem is the fact that screens deteriorate with age. If
left exposed to the air, both the matte and beaded types darken rather
rapidly from the accumulation of dust and soot. If rolled up in pro-
tective cases when not in use, they remain in good condition longer,
but tend eventually to turn yellow.
Some screen surfaces can be cleaned, but this should be attempted
only on the advice of the manufacturer.
A matte screen that has deteriorated until it appears dark in
comparison with a sheet of clean white writing paper placed against
it should be replaced, since it is wasting from Y4 to as much as l/2
of the light from the projector. A beaded screen that has become
noticeably yellowish should not be used, especially for the projection
of color films.
An especially bad practice is to continue the use of a screen after
it has acquired a mottled appearance. It is, of course, impossible
to view a motion picture with any satisfaction when a splotchy pat-
tern of lights and shades due to the screen itself is superposed on the
lights and shades of every scene. The apparent movement of the
pattern on the screen when objects in the picture move is especially
distracting.
Picture Brightness. — Another question that has been investigated
in detail by the Society of Motion Picture Engineers is that of the
proper brightness of the picture on the screen.4 It has been estab-
lished that there is _a fairly definite minimum picture brightness
necessary to permit the eye of the spectator to function at full
efficiency. When films are printed so as to obtain the best rendition
of lights and shadows that can be obtained with present photographic
materials, this desirable value of picture brightness corresponds to
a brightness of 10 foot-lamberts as measured on the illuminated
screen with the shutter of the projector running, but without film.
Values of screen brightness are customarily stated in this way, for
convenience in measurement.
It will be noticed that in this discussion the foot-lambert, a unit of
brightness, is used. The foot-candle, which has been used frequently
in the past in this type of discussion, is a unit of illumination. The
light falling upon the screen may be measured in foot-candles, but
this is only a partial and inaccurate measure of the brightness of
the picture, since it takes no account of the reflecting properties of
the screen.5'6
32 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
When the projector does not furnish sufficient light, picture quality
suffers, and those sitting farthest from the screen find it difficult to
see all the details in the picture. The medium densities of the pic-
ture merge with the blacks, and the highlights are weak and unreal.
On the other hand, it is definitely possible, though perhaps rare in
practice, to have too bright a picture in 16-mm projection. Under
this condition the shadows become light gray, and any graininess in
the film becomes unpleasantly noticeable. If the picture contains
a wide scale of tones the highlights become dazzling, and flicker may
appear.
In order to avoid these two undesirable conditions of excessive
screen brightness and inadequate screen brightness, the Committee
recommends that projectors be selected to provide, in conjunction with
the screens used, picture brightness not greater than 20 foot-lamberts and
not lower than 5 foot-lamberts.
Required Light Output of Projector. — When the size of the screen
and its reflection coefficient at the maximum viewing angle to be
used are known, and when the desired picture brightness is known,
it is easy to calculate the number of lumens (units of light flux) re-
quired from the projector. The formula is :
Desired brightness X area of screen
No. of lumens = <in foot-lamberts) (ia sq.-feet)
Screen reflection coefficient
(expressed as a decimal, not %)
Three cases have been worked out by the Committee. The
results are given in Table I. The values given in the column headed
Matte-Surface Screen, Minimum are sufficient to provide the mini-
mum brightness of 5 foot-lamberts for all spectators. As a general
rule twice this amount of light, as shown in the "Recommended"
column, should be used with the matte-surface type of screen. The
column headed Beaded Screen, Recommended assumes that this screen
will be used with a maximum viewing angle of 20 degrees. Under
this condition the spectators along the centerline of the room will see
a picture corresponding to a brightness of 19 foot-lamberts, while
those seated at the sides will see a picture corresponding to a bright-
ness of 5 foot-lamberts. Thus the numbers of lumens shown in
this column should not be increased, since the recommended maxi-
mum value of screen brightness will be exceeded for those sitting along
the centerline of the room. Whenever it is possible the values shown
in the table for the beaded screen should be adhered to.
Recommended
Lumens
Minimum
Lumens
Recommended
Lumens
106
53
46
152
76
67
238
119
104
344
172
150
468
234
204
612
306
267
774
387
338
956
478
417
1376
688
600
1872
936
816
1070
July, 1941] NON -THEATRICAL EQUIPMENT REPORT 33
TABLE I
Recommended Screen Brightness
Screen Size Matte-Surface Screen Beaded Screen
30" X 40"
3' X 4'
3' 9" X 5'
4' 6" X 6'
5' 3" X 7'
6' X 8'
6' 9" X 9'
7' 6" X 10'
9' X 12'
10' 6" X 14"
12' X 16'
It is expected that in the near future projector manufacturers
will make available tables showing the output in lumens of each model
of projector with each of the lamp and lens combinations provided.
From such tables it will be a simple matter to select projectors that
will give the right screen brightness in a given location, or to deter-
mine the limiting screen sizes for a given projector.
When screens larger than 8 or 9 feet wide are needed, as in large
auditorium projection, it will be found that projectors using incandes-
cent lamps are incapable of furnishing the amount of light required
by the table. If pictures are to be projected on more than a few
occasions in such auditoriums, 16-mm arc projectors should be in-
stalled. Such projectors are capable of furnishing approximately
1100 lumens, and thus give satisfactory pictures up to a width of
about 14 feet on matte-surface screens, which are required by the
shape of the usual large school auditorium. Obviously such pro-
jectors would not be used in classrooms, since the amount of light
they furnish would be excessive under practically all classroom con-
ditions. A lecture room seating 150 to 300 students may be a border-
line case, in which an arc-lamp type of machine would be as desirable
as the more usual incandescent type for a relatively permanent
installation.
Room Darkening. — Good tonal quality in the projected picture is
impossible if the room in which it is being viewed is not adequately
darkened. On the other hand, this does not mean that the room must
be absolutely dark. Studies have indicated that a general room light
of the order of Yio foot-candle is not harmful.7'8 This is a level of
34 NON-THEATRICAL EQUIPMENT REPORT [j. s. M. p. E.
illumination under which it is difficult but not impossible to read
ordinary newspaper type.
Aside from making provisions for excluding light from the room
until the general level of illumination is at least as low as is indicated
above, it is particularly necessary to make sure that no narrow beams
of light, especially sunlight, enter the room to produce bright spots
on walls near the screen, or to strike other objects in the room from
which dazzling reflections will be thrown. For the comfort of the
spectators the screen should be the brightest object in the room.
Classroom Acoustics and Sound Reproduction. — The volume of
sound needed for satisfactory reproduction of speech or music in a
classroom depends on several factors besides the size of the room.
The most important of these is the amount of sound-absorbing ma-
terial present.
A room in which the walls, ceiling, and floor are all of hard ma-
terials, such as plaster or wood, requires comparatively little sound
energy from the loud speaker in order to produce a loud effect, but
sound heard under this condition does not have the clearness or the
pleasing quality that is obtained in a room where there are curtains
or other materials that absorb sound. In many classrooms the only
sound-absorbing material is the clothing of the pupils and the teacher.
This is the reason why in such classrooms speech is fairly easily under-
stood when the class is present, but has a disagreeable loud, blurred,
ringing, or echoing quality when the room is nearly empty. Hard
surfaces, such as plaster or wood, reflect sound even more efficiently
than the best mirror reflects light. Unless the sound-waves meet
some soft or porous substance, they are reflected and re-reflected
many times. Thus the sound of a speaking voice builds up into a
roar, or reverberation, which takes a noticeable time to die away
after the last word has been spoken. The same effect naturally
occurs when speech is reproduced by an electrical system, as by a
radio receiver or a sound motion picture projector. The presence of
sound-absorbing material in the room causes the sound to die away
more rapidly, so that the blurred effect is absent or greatly reduced.
Classrooms that are to be used for sound motion picture pro-
jection should be provided with some sound-absorbing materials
either in the form of heavy curtains or drapes covering preferably at
least Vs of the wall space, or, better, in the form of acoustic
blocks covering the greater part of the ceiling. It is not necessary or
desirable to provide enough sound-absorbing material to produce
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 35
a "dead" effect in the room; it is necessary to provide only enough
sound absorption to make it easy for two people to converse in an
ordinary tone of voice when they are at opposite ends of the otherwise
empty room without experiencing any difficulty in understanding
each other due to the blurring caused by excessive reverberation.
Those who have funds available for special acoustic treatment of
classrooms can obtain expert advice from companies specializing in
this type of treatment, though in general they should try to err on
the side of having too little sound-absorbing material applied, rather
than too much. It is only in a room of auditorium proportions that
it is important to have exactly the right amount of sound-absorbing
material.
When only a little sound-absorbing material is present in a room,
the higher-pitched components of the speech are absorbed more than
the low-pitched components. This has the effect of making the
speech sound low-pitched, or "boomy," as well as blurred. In-
telligibility under such conditions is improved by electrically at-
tenuating, or weakening, the lower-pitched components of the
speech. At the same time the "balance" of the speech is improved,
so that it sounds more natural. One of the functions of the tone
control on the projector is to provide a means of removing some of
the low-pitched components of speech when it is necessary to use the
projector in an excessively reverberant room.
Reproduction of music in a reverberant room is sometimes quite
pleasing, though this is not true of music that is very rapid in char-
acter or of music in which solo instruments are prominent. On the
other hand, reproduction of music in a room that is too "dead"
gives an effect of inadequacy, of "thinness," lack of "fullness," or
"roundness." It is for this reason that care should be taken not
to make the room too dead.
The amount of sound energy required for adequate loudness of
reproduction in a room depends upon the volume of the room,
but different degrees of acoustical liveness make greater differences
in the amount of sound energy required than the differences in size
that are ordinarily encountered in classrooms. Because these
acoustical variations from room to room exist, and are difficult
to measure, it is not practical to give a table of required sound out-
puts for classrooms of various sizes. Fortunately the sound output
can be conveniently adjusted by the volume control on the projector,
so that all that is necessary is to make sure that the projector selected
36 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
has adequate maximum power for good music reproduction in the
room or rooms where it is to be used. In a general way it may be
said that a power output of from 5 to 10 electrical watts will be suf-
ficient for almost any classroom. For auditoriums, at least 15
electrical watts will be needed, and more should be available.
Measurement of sound power by electrical watts is not an entirely
satisfactory procedure, because loud speakers may differ considerably
in the efficiency with which they convert electrical energy into sound
energy. Unfortunately the conversion efficiency of a loud speaker
is exceedingly difficult to measure. For this reason all that can be
done in practice is to make certain that the projector selected, in
combination with its particular loud speaker, is capable of producing
adequate loudness before it overloads, that is, before it reaches the
point at which further increase of loudness causes the sound to be
just noticeably distorted.
Selection of Projector. — When the general questions of screen size
and type, projector light output and sound energy output have been
settled, the question still remains as to what particular make of pro-
jector should be purchased. The engineering specifications which
make up Part III of this report, when properly applied, will furnish
a basis for deciding whether a particular projector is or is not suit-
able for educational service within the limits corresponding to its
light output and sound output. However, these specifications are
unavoidably expressed in terms of quantities which can be measured
only by a fully equipped testing laboratory, having personnel ex-
perienced in the making of this particular class of tests. In the
absence of a certificate from a trustworthy source to the effect that
a particular projector meets these specifications, the user must
generally rely on the result of competitive demonstration of different
makes of projectors. Several observations with respect to such com-
petitive demonstrations are in order.
A competitive demonstration of two or more projectors or a test of them
by the prospective purchaser (the latter being preferable} should be con-
ducted under the exact conditions that will exist when the chosen machine
is placed in service. All machines must be tested on the same screen
and with the same film.
The test-film is the most important item. It should have both sharp
picture and good sound quality, or, preferably, two separate films
should be selected for the tests of picture and sound, since these
tests are not made simultaneously.
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 37
The projectors under test should be connected to the same power
line, so that they will be operated on the same line voltage, but they
should not be operated at the same time unless it has been ascer-
tained that the power line is capable of carrying the multiple load
without damage. If this precaution is neglected, fuses will probably
be blown.
A choice between competing projectors should be made on the basis cf
fundamental performance, and not on the basis of special features,
unless it is clearly apparent that these features are contributing to
the excellence of the fundamental performance. The main points
to be observed are sharpness and steadiness of picture, intelligibility
and naturalness of speech reproduction, naturalness and steadiness of
pitch in music reproduction, and smooth, quiet operation. Excel-
lence in these respects necessarily implies general good workmanship
and high quality of construction.
The prospective purchaser should first examine both projectors to
see that their condensing and projection lenses are clean and that
the lamps are new and are rated for the line voltage actually existing.
Then he should switch on each machine in turn without film, and
center the light on the screen. After a clean rectangular field of
light has been obtained by focusing the projection lens, the screen
should appear evenly illuminated and free from striations, or patches
of color. If it appears that one projector delivers more light than
the other, the observer should make sure that any rheostat that may
be on either machine ~f or controlling the lamp current is adjusted to
the correct value before drawing any conclusion. Too much im-
portance should not be attached to slight differences of light output,
since different lamps, even of the same type and from the same lot,
may differ enough in light output to produce a noticeable difference
on the screen in this test.
Next, thread the film selected for checking picture sharpness and
steadiness, first on one machine, then on the other, and project it,
adjusting the focus as critically as possible, and noting any dif-
ferences in sharpness between the center and sides of the screen.
If it is necessary to be very critical in order to detect a difference in
steadiness of picture between the machines, set the framing device
so that the frame line is visible on the screen. Walk directly up to
the screen and hold a ruler against it, so that the amount of frame-
line "jump" can be observed directly on the ruler. As measured
with an ordinary commercial film in good condition, this "jump"
38 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
should not be more than l/2 of 1 per cent of the width of the picture.
Make this test in the same part of the film for both machines.
Both machines should be operated at the speeds for which they are
designed, and the picture scrutinized for visible flicker. A slightly
greater amount of flicker will not indicate that one of the machines
is inferior, provided it is also observed to give greater screen bright-
ness than the comparison machine. In this connection it is helpful
to bear in mind that flicker increases as the screen brightness in-
creases, and also as the speed of the projector (number of frames per
second) decreases. It is unlikely that flicker will be noticed at sound
speed (24 frames per second) but it is likely to appear at silent speed
(16 frames per second), especially if the screen brightness is near the
upper limit.
Tests of sound-reproducing quality, to be conclusive, must be con-
ducted under the acoustical conditions under which the machine will
actually be used. For example, no final conclusions should be drawn
from a test in an empty classroom or auditorium. Arrangements
should be made to have an audience of normal size present and in
their seats.
By way of test, the same sound-film should be run first on one
machine and then on the other. During the running of the film
the prospective purchaser should experiment with the tone controls
and attempt to decide what adjustments give most satisfactory re-
production of speech and of music. On account of the variable
element introduced by the tone controls, it will be necessary to run
the sound test-film several times, first on one projector and then on
the other, in order to decide whether or not there is a clear superiority
of one over the other.
The sound-film selected for testing should contain both good speech
and good music. Projector salesmen can usually provide demonstra-
tion reels which can be assumed to be of good quality. The im-
portant point is to run the same films on all machines being com-
pared, at the same time, and under the same conditions. Demon-
strations at different times, or in different places, and with different
films, are not conclusive, and are of little value.
During the sound tests it is well to note how far the volume of
music may be increased on each machine before noticeable distortion
sets in. It is usually, but not invariably, true that the system that
permits the higher volume without distortion will give cleaner re-
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 39
production at normal volume. This point can be checked by at-
tentive listening.
Unless the observer is experienced in judging the quality of re-
produced sound, it will be found best to run the same reel of test-
film several times, rather than to project a wide variety of material.
The latter, of course, is also desirable when time permits. Re-
peated listening to the same reel of film makes it easier to fasten
attention on such points as the relative steadiness of film motion in
the sound-reproducing mechanisms of the machines under test,
as made audible by unsteady pitch of the reproduced sound or by
the absence of such unsteadiness. For critical comparisons on this
point, test-films containing musical selections of a slow character are
required.
It is necessary to be certain that the test-film itself is substantially
free from unsteadiness of pitch. One way of checking this point is
to notice whether or not changes of pitch are heard in the same
places when the film is reproduced on different projectors. If a
change of pitch is always heard on the same note, it is probably in
the film. In this case another film should be tried.
For detecting slow variations of film speed, commonly known as
"wows," records of the piano are most suitable. A violin or 'cello
record is perhaps as good a test as any for freedom from more rapid
speed variations, which manifest themselves as a sort of roughness or,
in extreme cases, a gurgling quality of the tone rather than as per-
ceptible changes of pitch.
In general, the inexperienced listener should be careful not to
form a hasty judgment as to which of two machines is superior in
the matter of speed constancy. Some of the effects of unsteady
film motion are subtle, and require experience for their correct inter-
pretation.
Two intimately connected points of performance are the tonal,
or frequency, range of the projector sound system and the amount
of background noise it produces. (We are referring here to the
noise that issues from the loud speaker, not to the noise produced
directly in the room by the running of the mechanism.)
A good 16-mm sound projector is capable of reproducing with nearly
uniform intensity all sound frequencies from 100 vibrations (or
"cycles") per second to at least 5000 per second. The presence of
this range can be checked by the playing of a frequency test-film,
or it can be checked by noting how certain components of the sound
40 NON-THEATRICAL EQUIPMENT REPORT [J. s. M. P. E.
are reproduced. A projector which has adequate high-frequency
response will reproduce in a natural manner the high-pitched hissing
sound of the letter 5 in speech. When high-frequency response is
inadequate, the sound of the 5 is much more like that of the th in
thin. One of the best tests is to listen attentively to the reproduction
of words which begin with s.
If there is good high-frequency response there should be adequate
low-frequency response, in order, in a sense, to "balance" the tonal
quality. Since this balance of the sound is affected by room acous-
tics, as has been explained, it must be adjusted, in many cases, by
the use of the tone control. Orchestral music is best for judging
balance.
The observer should be aware, in this connection, that tonal bal-
ance between high and low frequencies has been demonstrated to
be very largely a matter of personal taste; a matter which in many
instances is conditioned by the type of radio receiver to which the
listener is accustomed. It is highly desirable, therefore, that com-
parisons of tonal balance should be judged by a number of observers,
each of whom has an opportunity to experiment with the tone-
control facilities of the projectors under test, so as to determine what
range of tone qualities can be produced. It is perhaps wiser in most
cases to concentrate attention on the factor of intelligibility rather
than to depend too much on judgment of tone balance.
While making changes of tone quality by means of the tone con-
trol, it will be noticed that the background noise changes greatly
with the relative emphasis placed on the higher and lower frequencies.
The projector which reproduces a wide-frequency range, other things
being equal, will always produce more background noise than a pro-
jector having a narrow-frequency range. Reproduction of high
frequencies is almost unavoidably accompanied by reproduction of
hissing sounds from the film, from the photocell, and from other
parts of the amplifying system. Reproduction of low frequencies
necessarily increases the audibility of hum and other low-pitched
background noises. In the reproduction of any type of recorded
sound, a decision must be made as to whether it is preferable to
reproduce an extended frequency range with the accompanying
noise or to get rid of the noise at the expense of frequency range.
While there is here a certain amount of room for the exercise of
individual taste, it is the consensus of engineers that a range at
least from 100 to 5000 cycles per second is necessary for good repro-
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 41
dtiction of speech and music. It is preferable to tolerate the slight
amount of noise that accompanies this frequency range rather than
to make the sacrifice of frequency range that is needed to get rid of
the noise entirely. At the same time it is true that by superior de-
sign and construction of the amplifying equipment associated with a
projector, it is possible to reduce background noise to an appreciable
extent without sacrifice of the tonal range. The interrelation be-
tween these two aspects of reproduction has been emphasized here,
however, in order to place the non-technical observer on his guard
against drawing conclusions solely on the basis of either the tonal
range or the amount of background noise that is noticed.
Consumer Demands and Projector Engineering. — Many equipment
users do not realize the extent to which their demands for qualities
such as small size, light weight, and portability influence the design
and performance of sound motion picture projectors. Almost any
widely expressed consumer demand can be met by the engineer, but
as a rule this can be done only by some sacrifice of other desirable
qualities, usually of performance.
In particular, the demand for small size and light weight has led
to the adoption by the projector industry of certain practices in the
design of the sound-reproducing amplifier and loud speaker that are
deplored by most of the quality-conscious engineers in the industry. *
Thus, while 16-mm sound projectors have been improved in many
respects during the past few years, they could be made to perform
still better, and, in fact, much better, if they were not required to
meet this demand for light weight and extreme portability.
* A loud speaker, to be efficient and free from distortion, requires a moderately
large and massive field-magnet to produce a strong magnetic field around the
"voice-coil" through which the signal currents from the amplifier are passed.
Reducing the weight of the loud speaker necessarily means reducing the size of
this magnet and therefore reducing the strength of the magnetic field. If the
same amount of sound is to be produced as before, the amplifier must deliver more
electrical power to the loud speaker having the weak field. Within the limits of
size and weight that are customarily imposed today, this can be done only by the
employment of types of vacuum-tubes that produce more distortion than the
types in common use in radio sets and other sound-reproducing devices a few
years ago when loud speakers with massive field magnets were the rule. Thus,
because of the need for high amplifier power to offset loud speaker inefficiency, the
amplifier supplies a slightly distorted signal to a loud speaker, which, because of its
low efficiency, introduces still more distortion of its own.
42 NON-THEATRICAL EQUIPMENT REPORT {J. S. M. P. E.
The better sound projectors available today are deserving of far
better loud speakers than are usually supplied. However, at the
present time the cost of a suitable loud speaker of really high quality*
is almost as great as the cost of the projector itself. This condition
exists largely because few outside the theatrical motion picture field
have known that this class of loud speaker equipment existed, and
therefore the demand for it has not yet become great enough to
permit placing it in quantity production. If loud speakers of really
high quality were demanded for all sound motion picturejnstalla-
tions of a relatively permanent character, a market would be created
for such units in quantity, and it would become possible to sell them
at a much lower price. Loud speakers of the present very light-
weight type could still be used for services that require portability.
Two other features included in many current projector systems
because of consumer demand deserve mention because of their
adverse effect upon basic performance. The first of these is the
clutch mechanism which permits stopping the film for the projection
of single frames as still pictures. The Committee is unanimous in
recommending that this provision for still picture projection be omitted
from 16-mm motion picture equipment for use in schools. Records
of film distributors show that the subjection of individual frames of
film to excessive heat by this practice is one of the leading causes of
film damage. While it is possible to protect the film against ex-
cessive heat by the usual "safety" shutter, introduced into the
light-beam when the clutch is operated, this can only be done by
cutting down the amount of light transmitted to a point so low that
the picture on the screen is too dim to be seen properly. On the
other hand, if enough light is allowed to pass to give an acceptable
screen image, the film is sure to be damaged whenever the still pic-
ture is kept on the screen for any considerable length of time. When
it is considered that the image obtained by projecting a single frame
of motion picture film is none too sharp at best, it may be seen that
this practice of providing for still projection of individual frames of
film is of doubtful value, while as a cause of film damage it is demon-
strably harmful.
A second feature commonly demanded is a microphone input, to
permit the amplifier and loud speaker of the projector to be used for
* The reference here is to loud speakers of the dual type, in which a horn is
used for the higher frequencies and a large paper cone unit for the low frequencies.
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 43
public address work. A system designed only for sound-film repro-
duction could provide a noticeably reduced level of the hissing type
of background noise commonly heard when the projector is repro-
ducing sound at normal volume. The remedy for this situation
consists in either abandoning the demand for the microphone input
or increasing the price paid for the system.
Low selling price is quite properly an important consideration in
the production of sound motion picture equipment for educational
use, since the lower the cost of the equipment the more extensive
can be its use. In order to obtain maximum value for the amount
of money expended for such projection equipment, users would do
well to demand only basic functions in the equipment furnished them,
since these could then be provided in a more satisfactory form than
under present conditions. As matters stand at present, the engineer
must generally design not only in relation to cost but also in relation
to the demand for auxiliary and more or less unrelated functions in
the same piece of equipment. At best it is not easy to manufacture
good 16-mm sound motion picture equipment, nor can this be done
at too low cost, since the accuracy required in vital parts of the
mechanism is of the order of 2x/2 times as great as is required for
comparable results in 35-mm equipment.
Film Quality and Its Effect on Projector Performance. — It is not
always recognized that the quality of the film being projected has
as important an effect upon the final screen result as the quality of
the projection equipment. It is important, especially when judging
projector performance, to be able to recognize the difficulties at-
tributable to films rather than to apparatus.
An unsteady picture on the screen may be caused by imperfect
printing of the film, by damaged sprocket-holes, or by unsteady
mounting of the camera that took the picture. Jumping of the
picture when a splice passes through the gate of the projector gener-
ally indicates that the splice was improperly made, either with the
film edges not properly aligned or with the sprocket-holes not ac-
curately matched where the film ends were cemented. Whenever a
projector produces an unsteady picture, the user should make cer-
tain that the difficulty is not in the film being projected before con-
demning the machine.
It occasionally happens that the image on the film is not in sharp
focus. This may be caused either by lack of sharp focus in the origi-
nal photography or by improper printing.
44 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
It sometimes happens that films are received with which it is im-
possible to obtain a sharp focus all over the screen at once. It is
possible to focus the center of the picture sharply, or the sides, but
not both at the same time. On examination, such films will be
found to be buckled. Buckled film should be returned to its source;
it is usually impossible for the user to correct the condition.
A moderately common difficulty in sound-tracks is mislocation of
the track. If the track is more than a few thousandths of an inch
too near the film edge or too far from it, it is probable that part of
its width will not be scanned by the reproducing light-beam in the
projector. This can result in quite objectionable distortion of the
seund. Sound-tracks that are mislocated are also frequently noisy,
since images of improper parts of the negative film (for example,
the edges of sprocket-holes) are likely to be printed in the sound-
track area where the track itself ought to be.
Other causes of excessive noise in sound-tracks are low density of
the track itself, that is, lack of sufficient blackness in the black parts ;
scratches, dirt on the negative, and dirt on the print itself. Sound-
tracks that reproduce well will generally be found to be quite dark,
clean-looking, and free from the printed impressions of dust specks
and scratches on the negative. With a little experience it is usually
possible to tell merely by looking at a sound-track whether or not
it may be expected to reproduce with low background noise.
Some films, even though manifesting none of the difficulties de-
scribed above, give unsatisfactory reproduction on all projectors.
The difficulty usually is an absence of high frequencies in the sound,
manifested by lack of crispness and intelligibility and an absence of
the 5 sound. This condition may have been caused by the selection
of an unsuitable voice for the recording, by improper use of the
microphone in recording, or by printing on equipment lacking good
optical definition.
Another difficulty sometimes encountered is a harsh quality in the
sound similar to that obtained with a radio receiver when it is not
tuned accurately to the broadcasting station, or when the volume
has been increased until the amplifier system overloads. This may
be produced in sound-films by the attempt to record sound at too
high a volume level. In variable-density sound-tracks it may be
caused by improper printing or development.
A condition that is likely to continue to afflict the 16-mm sound
industry for some time to come is the lack of uniformity of sound
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 45
quality on otherwise good films produced by different methods.
For example, films made by optical reduction from negatives origi-
nally produced for 35-mm theatrical purposes generally have less high-
frequency response than films produced specifically for reduction to
the 16-mm size. Films produced directly in the 16-mm size can
be given almost any desired type of sound quality, but this very fact
makes it possible for producers whose tastes differ to vary the sound
quality in ways that are sometimes undesirable. In addition to
these sources of variation it will sometimes be found that different
prints of the same subject differ perceptibly in sound quality. This
may happen, for example, by the use of different printing machines
in the same film laboratory, the different machines not being in
equally good adjustment.
Sound Quality Obtainable from 16 -Mm Film. — The rather discourag-
ing picture just outlined may well lead to the question "Just what is
it possible to accomplish with 16-mm sound-film?" A definite and
encouraging answer can be given. With films that are well made in
all respects and with the best currently available projection equip-
ment, a quality of sound can be obtained that is substantially on a
par with that commonly heard in good neighborhood motion picture
houses. Background noise need not be objectionably high. Music
reproduction can be equal to that of an excellent radio receiver.
Speech reproduction can be completely intelligible and natural.
Failure to achieve this standard of performance in 16-mm sound-
film projection is aTesult of removable difficulties either in film or
equipment. Both these can be remedied by proper selection of
projection equipment, by expert servicing of the equipment when
necessary, and by careful selection of films.
REFERENCES
1 Report of the Projection Practice Committee, J. Soc. Mot. Pict. Eng., XXX
(June, 1938), p. 638.
2 HARDY, A. C., AND CONANT, F. H.: "Perspective Considerations in Taking
and Projecting Motion Pictutes," Trans. Soc. Mot. Pict. Eng., XII, 33 (1928), p.
117.
3TuTTLE, C. M.: "Distortion in the Projection and Viewing of Motion
Pictures," /. Soc. Mot. Pict. Eng., XXI (Sept., 1933), p. 204.
4 The Problem of the Projection Screen Brightness Committee, /. Soc. Mot.
Pict. Eng., XXVI (May, 1936), p. 489.
LOWRY, E. M.: "Screen Brightness and the Visual Functions," J. Soc. Mot.
Pict. Eng., XXVI (May, 1936), p. 490.
46 NON-THEATRICAL EQUIPMENT REPORT
O'BRIEN, B., AND TUTTLE, C. M. : "An Experimental Investigation of Projec-
tion Screen Brightness," /. Soc. Mot. Pict. Eng., XXVI (May, 1936), p. 505.
COOK, A. A.: "A Review of Projector and Screen Characteristics, and Their
Effects upon Screen Brightness," J. Soc. Mot. Pict. Eng., XXVI (May, 1936), p.
522.
WOLF, S. K. : "An Analysis of Theater and Screen Illumination Data," /. Soc.
Mot. Pict. Eng., XXVI (May, 1936), p. 532.
TUTTLE, C. M.: "Density Measurements of Release Prints," /. Soc. Mot. Pict.
Eng., XXVI (May, 1936), p. 548.
TEELE, R. P.: "Photometry and Brightness Measurements," /. Soc. Mot. Pict.
Eng., XXVI (May, 1936), p. 554.
LITTLE, W. F., AND WILLIAMS, A. T.: "Resume of Methods of Determining
Screen Brightness and Reflectance," /. Soc. Mot. Pict. Eng., XXVI (May, 1936),
p. 570.
LUCKIESH, M., AND Moss, F. K. : "The Motion Picture Screen as a Lighting
Problem," /. Soc. Mot. Pict. Eng., XXVI (May, 1936), p. 578.
6 HARRIS, S. : "Photometric Nomenclature and Conversions," /. Soc. Mot. Pict.
Eng., XXXV (Dec., 1940), p. 557.
6 TEELE, R. P.: "Photometry and Brightness Measurements," /. Soc. Mot.
Pict. Eng., XXVI (May, 1936), p. 567.
7 Report of the Theater Lighting Committee, /. Soc. Mot. Pict. Eng., XVI
(Feb., 1931), p. 239.
8 WOLF, S. K. : "An Analysis of Theater and Screen Illumination Data," /.
Soc. Mot. Pict. Eng., XXVI (May, 1936), p. 534.
(Continued on next page)
PART II
THE OPTICAL PROPERTIES OF COMMERCIALLY AVAILABLE SCREENS
FOR 16-MM PROJECTION
The brightness of the image in projection is determined by the
amount of light reaching the screen from the projector and by the
reflecting properties of the screen surface. Screens that reflect the
light in a highly directional manner can provide, under proper condi-
tions, pictures more than five times as bright as would be obtained
with non-directional screens in combination with the same projector.
Directional screens, however, have disadvantages which sharply
restrict their fields of proper use. They are highly efficient within
limited viewing angles, but relatively inefficient at large viewing
angles. More important is the fact that they produce serious in-
equalities of brightness between different parts of the screen for all
but a few favorably located spectators.
Because of these effects, an accurate knowledge of the properties
of different types of screens is essential for the intelligent selection of
projection equipment.
In order to be able to base its recommendations upon a precise
knowledge of the characteristics of the screens that are commercially
available at the time of this report, the Committee obtained samples
of screen materials from six manufacturers. After the exclusion of
screens perforated for sound transmission, which are not used in
l()-mm projection, this collection of samples included seven beaded
screens, six matte screens, and two aluminum-surfaced screens.
The procedure followed in determining the characteristics of these
fifteen samples was to project a beam of light perpendicularly on each
sample and measure the brightness of the center of the illuminated
spot at different angles from the axis.
The differences found between individual beaded screens were
relatively unimportant. The same was true of the matte-surfaced
screens examined. Therefore the results have been averaged for
each of these types. The two averages are plotted in Fig. 3.
Fig. 4 shows the characteristics of the two aluminum-surfaced
screens, which are obviously dissimilar. The screen that gave curve
47
48
NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
A has a relatively smooth surface, white the one that gave curve B
is rough in texture.
Some readers may be puzzled by the fact that values higher than
100 per cent appear in the curves of Fig. 3 and Fig. 4. This will be
readily understood if it is remembered that the results of such screen
reflection measurements are customarily expressed in terms of an
ideal, or theoretical, reflecting surface which, by definition, reflects
500}
400 yc
EB&IE
200%,
100%
30° 20° 10° 0° 10° 20° 30°
ANqtE FROM AXIS
FIG. 3. Apparent coefficient of reflection of matte and beaded screens at
various angles to axis.
100 per cent of the incident light, and distributes this reflected light
in such a way that the surface appears equally bright at all angles of
view. Thus, a surface that has highly directional reflecting proper-
ties may appear many times as bright as this theoretical standard
within a certain range of viewing angles. But, since no surface can
reflect more than a total of 100 per cent of the light that falls on it, a
directional reflector must necessarily be less bright than the standard
when viewed at angles outside this range.
A working standard that closely approximates the ideal is available
in the form of a freshly scraped surface of a block of pure magnesium
July, 1941]
NON-THEATRICAL EQUIPMENT REPORT
49
carbonate. This practical working standard was used in making the
measurements for the Committee. Therefore, the curves of Fig. .'i
and Fig. 4 may also be interpreted in this way, that a value of, for
example, 135 per cent at a given angle means that, when viewed from
this angle, the brightness of the screen in question was 135/ioo of
the brightness of the magnesium carbonate reference surface under
the same illumination.
There is an important difference, not appearing in these curves,
between the beaded type of screen and the two other types. The
10 0
ANiqLE. FROM
10
20°
30*
FIG. 4. Apparent coefficient of reflection of two semi-specular (metallic)
screen surfaces at various angles to axis.
beaded screen surface reflects the light most strongly in the exact
direction from which it came, even when the incident light-beam is
not perpendicular to the screen surface. The matte-surfaced and
metallic screens do not share this property. They reflect the light
most strongly along the path that would be followed by the single
reflected beam if the screen surface were replaced by a polished
mirror. This action is scarcely noticeable in the case of the matte
screens, since these are almost entirely non directional, but it is
readily noticed in the case of the aluminum-surfaced screens. It will
be shown later that this semi-specular type of reflection causes con-
50 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
siderably greater differences of brightness between different parts of
the metallic screens than are found with the beaded type.
In view of the Committee's recommendation that the average
screen brightness (for any one spectator) be held between the limits
of 5 and 20 ft.-lamberts, it is obvious that a directional type of screen
should not be used for viewing angles greater than the one at which
the apparent coefficient of reflection is 1/4 of the value along the axis.
Fig. 3 shows that, for the average beaded screen, this angle is 16
degrees. However, it will be noted that the peak is very^ sharp.
Since in practice the projector must be located far enough above the
heads of the spectators to avoid the casting of shadows on the screen,
and since all parts of the screen send this sharp maximum of reflected
light directly back to the projector, it appears reasonable to assume
that no spectators receive this maximum of reflected light. The
PROJECTOR, __ T - *(fl
n
S
SPECTATOR. ..---'.'--- —
A^-::--
FIG. 5. Method of measuring angles to determine screen brightness ratios
for beaded screen.
Committee has made this assumption, and has calculated its table
of recommended light flux from the projector (Table I of Part I of this
report) from the values of the apparent coefficient of reflection for 5
degrees and 20 degrees in the case of the beaded screen.
Fig. 4 shows that, on the basis of the 4 to 1 ratio between the maxi-
mum and the minimum brightness, the screen corresponding to the
curve marked A is suitable for use up to a viewing angle of 25 de-
grees, while the screen corresponding to the curve marked B more
than satisfies the requirement at a viewing angle of 30 degrees.
These screens are nevertheless unsatisfactory for use with groups
of more than a few spectators, because they produce large differences
of brightness between different parts of the picture.
When a spectator is located near the front of the seating area and
at one side, he views the two sides of the screen at widely different
angles. Therefore, if the screen has a highly directional reflection
July, 1941
NON-THEATRICAL EQUIPMENT REPORT
51
characteristic, the brightness of the near side of the screen for this
spectator is considerably greater than the brightness of the far side.
A good estimate of the magnitude of this effect can be obtained by
the proper use of the data in the curves of Fig. 3 and Fig. 4.
Fig. 5 is a diagram of the condition as it applies to the beaded
screen. With this type of screen, as has been stated, the strongest
beam of reflected light is sent back in the direction from which it
came, even when the incident light beam is at an acute angle to the
</>
cc
CD
ANqLE
PROJECTION AOUS)
FIG. 6. Brightness ratio of brightest part of screen to darkest part of
screen at various viewing angles. Beaded screen. (Screen, projector, and
spectators' eyes on same level.)
screen surface. Therefore the line of sight for maximum brightness
at each side of the screen is as shown by the coarse dotted line,
coinciding with the solid line which indicates the path of the incident
light.
The spectator's actual lines of sight to the two sides of the screen
are shown by the two fine dotted lines. By reading from the top
curve of Fig. 3 the brightnesses corresponding to angles A and B of
Fig. 5 we obtain a fairly accurate measure of the difference of bright-
ness of the two sides of the screen for this spectator's position.
52
NON-THEATRICAL EQUIPMENT REPORT [J. S. M. p. E.
For example, consider the spectator who sits in a row 2l/t times
the screen width in front of the screen, and a distance at the side
such that he has a viewing angle of 15 degrees. (The "viewing
angle," as used in this discussion, is the angle between the projection
axis and the spectator's line of sight to the center of the screen.)
For this spectator we find, either by calculation or by drawing a
diagram and measuring the angles with a protractor, that angle A
O
5
cr
</> 3
LLJ
z
cc
CD
O 5° 10° 15° 20° Z5° 30°
VIEWING ANqiE (FROM PROTECTION AXIS}
FIG. 7. Brightness ratio of brightest part of screen to darkest part of
screen, at various viewing angles. Beaded screen. (Screen and projector
high enough for light-beam to clear spectators' heads.)
is 19V2 degrees, while angle B is 9*/2 degrees. The corresponding
brightness values, from Fig. 3, are 95 per cent and 250 per cent.
Therefore, this spectator sees the near side of the screen as slightly
more than 2x/2 times as bright as the far side of the screen.
By applying this method to a large number of locations in the seat-
ing area, we obtain the curves of Fig. 6, which show how the bright-
ness ratio of the brightest part of the screen to the darkest part of the
screen varies with the position of the spectator.
The method by which the curves of Fig. 6 were obtained tacitly
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 53
assumes that the projection lens, the center of the screen, and the
spectator's eyes are all on the same level. With this arrangement no
spectators can be seated in the triangular space between the screen
and the lens of the projector, since anywhere in this space their
heads would cast shadows on the screen. This fact is indicated in
Fig. 6 by drawing parts of the curves as dotted lines, since these
parts correspond to parts of the seating area that are not usable.
A more practicable way of arranging matters, at least for groups
of the size common in classroom work, is to place the screen so that
its bottom edge is a few inches higher than the average level of the
spectators' eyes, and to place the projector also just high enough to
avoid interference by the spectators' heads. With this arrangement
PROJECTOR
FIG. 8. Method of measuring angles to determine screen brightness ratios
for semi-specular (metal-surfaced) screen.
the darkest part of the screen is always the upper far corner. The
brightest part varies in position; if the spectator is at 0 degrees, it
is at the center of the bottom edge of the screen ; as he moves to one
side it moves along the bottom edge of the screen until it reaches the
near bottom corner. The problem is three-dimensional, and the
mathematical calculations required to solve it are laborious. The
mathematics need not be discussed here; the results, however, have
been obtained for a typical case, and are given in Fig. 7.*
* The actual arrangement on which Fig. 7 is based is as follows: Screen size,
37Y2 X 52 inches. Height of projection lens, 55l/t inches. Bottom of screen,
52 inches high. Height of observer's eyes, 48 inches. The analysis will hold wit h
little error for any arrangement in which the lens and the bottom of the screen
are on about the same level, and the light-beam just clears the tops of the specta-
tors' heads.
54
NON-THEATRICAL EQUIPMENT REPORT [J. s. M. p. E.
ANqLE (fROM PROJECTION
FIG. 9. Brightness ratio of brightest part of screen to darkest part of screen
for various viewing angles, for average matte screen, and for the semi-specular
screens of Fig. 4.
Those members of the Committee who worked on the screen
problem reached the conclusion, by practical tests, that a brightness
ratio of 3 to 1 between the brightest part of the screen and the darkest
part of the screen was as great as should be tolerated. On this
July, 1941 ] NON-THEATRICAL EQUIPMENT REPORT 55
basis it may be seen from Fig. 7 that, in the case of the beaded screen,
most of the locations in the front of the seating area, distant from the
screen between 2 and 2x/2 times its width, are not satisfactory. It is
for this reason that this area has been marked as undesirable in
Fig. 2 of Part I of this report.
It has been stated that, while beaded screens send the strongest
reflected light back along the incident beam, metallic screens send it
in the direction determined by the usual law of specular reflection.
Accordingly, the diagram of Fig. 5 must be modified as shown in
Fig. 8 to make it applicable to the case of the metallic screen. It
will be noticed that angle A , at the far side of the screen, is increased,
while angle B, at the near side, is decreased. Because of these
changes in the angles, the brightness ratios are much higher for the
metallic than for the beaded screens, even though both of the curves
of Fig. 4 are less steep than the upper curve of Fig. 3.
The actual brightness ratios for the two aluminum-surfaced
screens of Fig. 4 are shown in Fig. 9. The values given are for the
three-dimensional arrangement, corresponding to Fig. 7. These
curves show that with either of these metallic screens there is only
one part of the room from which a picture of satisfactory brightness-
uniformity can be seen, and that is the space close to the projector.
The situation can be improved somewhat by tilting the top of the
screen slightly toward the projector, but the improvement is not
great enough to warrant the use of either of these screens for a large
group of spectators. Screens of the metallic type are needed for
stereoscopic projection with polarized light, but otherwise their use
should be avoided.
The high degree of brightness-uniformity obtained with the matte
type of screen is shown by the curves at the bottom of Fig. 9. In no
case is the brightness ratio greater than 1.2. This amount of non-
uniformity is hardly perceptible to the most critical eye. The matte
type of screen also provides nearly equal picture brightnesses for all
spectators. In the average case the brightness at a viewing angle
of 30 degrees is 85 per cent of the brightness on the axis.
Thus from the standpoint of the performance factors that con-
tribute to proper appreciation of the picture by all spectators, the
matte type of screen is far superior to the directional types. It
should be chosen in all cases where a projector of adequate illuminat-
ing power can be obtained.
56 NON-THEATRICAL EQUIPMENT REPORT
BIBLIOGRAPHY
JONES, L. A., AND FILLIUS, M. F.: "Reflection Characteristics of Projection
Screens," Trans. Soc. Mot. Pict. Eng., No. 11 (1930), p. 59.
JONES, L. A., AND TUTTLE, C. : "Reflection Characteristics of Projection
Screens," Trans. Soc. Mot. Pict. Eng., No. 28 (Feb., 1927), p. 183.
LITTLE, W. F.: "Tests of Motion Picture Screens," /. Soc. Mot. Pict. Eng.,
XVI (Jan., 1931), p. 31.
Report of the Projection Screens Committee, J. Soc. Mot. Pict. Eng., XVII
(Sept., 1931), p. 441.
LYMAN, D. F.: "Relation between Illumination and Screen Size for Non-
Theatrical Projection," /. Soc. Mot. Pict. Eng., XXV (Sept., 1935), p. 23-1.
(Continued on next page)
PART III
PERFORMANCE SPECIFICATIONS FOR 16-MM PROJECTION EQUIPMENT FOR
The following specifications attempt to give clear definitions of good
performance with respect to each of the functions of a projector or
screen. Specification in terms of mechanical or electrical design
or principles of operation has been avoided, since it is believed im-
material to the user how a result is obtained as long as the result
is satisfactory.
In many instances it has been necessary to specify the method
of measurement as well as the result required, in order that the
specifications might have precision. In connection with the specifi-
cations of sound-reproducing performance it has been necessary
to define a number of test-films. Arrangements have been made
by the Committee for the production of these test-films, which are
to be made generally available. Further announcement on this
subject will appear in an early issue of the JOURNAL.
Since there are many ways in which given aspects of performance
may be defined, notes have been added to certain of the specifica-
tions in order to make clear the thought processes that led the Com-
mittee to adopt the particular form of specification given.
The specifications are not complete in all details. Further
investigations are being conducted with respect to several of the
topics in order that definite measurement technics may be supplied
where they are now lacking, and limits in some cases may be more
precisely determined. The Committee recognizes these short-
comings in this report, but nevertheless believes that the specifica-
tions in their present form will be useful enough to justify publica-
tion at the present time rather than waiting until all uncertain points
have been clarified.
No attempt has been made to put these specifications in a form
such that they could be applied by the average user of equipment,
however intelligent or well informed he may be. The Committee
believes that the specifications must be definite if they are to be use-
ful. It has been possible to achieve definiteness only by the specifica-
57
58 NON-THEATRICAL EQUIPMENT REPORT [J. s. M. p. E.
tion of measurement procedures that in most cases can be carried
out only by a well equipped measurement laboratory.
The Committee hopes soon to be able to supplement its present re-
port by the presentation of papers by some of its members discussing
specific measurement problems in greater detail than is possible in a
report of the present character.
(A) Picture Projection
(1) Steadiness of Picture.— The Picture-Steadiness Test-Film shall
be a film carrying a readily measurable test-pattern placed^upon it
by a mechanism which positively locates each frame vertically by a
pilot-pin entering a perforation, and horizontally by pressing the
edge that is to be guided in the projector against a solid guide. The
test-pattern shall have been placed on the test-film directly, and
not by any intermediate printing process from another film. The
pattern may consist of two or more circular holes Vie inch in di-
ameter punched through each frame of the film. If the test-pattern
is produced by punching, the film used shall have been exposed
and developed previously to a density between 0.8 and 1.2. The
test-film shall not be shrunk more than 0.5 per cent.
Picture unsteadiness shall be measured by projecting the test-
film at standard speed (16 frames per second in the case of a silent
projector; 24 frames per second in the case of a sound projector).
Scales shall be placed vertically and horizontally on that part of the
screen on which the projected image of the test-pattern appears, and
the amount of unsteadiness shall be noted by observing the move-
ment of the test-pattern on these scales.
Vertical unsteadiness having a period shorter than one second shall
not exceed 0.3 per cent of the picture width.
Horizontal unsteadiness of any period shall not exceed 0.3 per cent
of the picture width.
(2) Freedom from Travel-Ghost.— The Travel-Ghost Test-Film shall
carry a pattern of small transparent areas on a dark background.
At least three of these transparent areas shall be located Vio of the
frame height from the top of the frame, and an equal number 1/10 of
the frame height from the bottom of the frame.
This test-film shall be projected at 16 frames per second on a screen
of such size that a brightness of 40 foot-lamberts is obtained with the
shutter running but with no film in the projector gate. This screen
shall be viewed from a distance equal to twice its width.
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 59
Under the above test conditions, no travel-ghost shall be visible.
(3) Freedom from Tendency to Damage Film. — The film used for
this test shall be freshly processed, having been uniformly fogged and
developed to a density between 0.5 and 0.8. It shall be used in the
form of a loop containing one splice. This loop shall be threaded
through all parts of the projector mechanism that touch the film in
normal operation. The lamp shall be turned on continuously dur-
ing the test. The room in which the test is conducted shall be closed
and otherwise well protected against avoidable dust.
Under these conditions, after 200 passages through the mecha-
nism, the film shall exhibit no perforation damage and no scratches
on either surface in either picture area or sound-track area.
(4) Take- Up Efficiency. — With reels having dimensions suitable for
the projector and with the correct take-up adjustments for these
reels, the take-up shall provide a tension on the film of not more than
10 ounces at the beginning of a reel and not less than P/2 ounces
when the reel is full.
(5) Mechanical Durability. — The projector shall be guaranteed by
the manufacturer against failure due to defective material or work-
manship for a period of one year.
This guarantee shall not, however, be required to extend to parts
that are normally subject to wear and replacement, such as lamps
and motor brushes.
•(6) Quietness in Operation.*
(7) Provision for framing. —The projector shall have conve-
niently accessible means for framing the picture through a range ex-
tending 0.015 inch above and 0.015 inch below the normal position,
measured at the film.
(8) Light Output. — The manufacturer shall state, in lumens, the
limits of light output of each model of projector with each lamp and
lens combination furnished.
For this purpose the light output shall be measured with the shutter
running, but with no film in the gate.
Measurements shall be made in the plane of a screen located a dis-
tance from the projector such that the projected image of the pic-
ture aperture is 40 inches wide by (approximately) 30 inches high.
* The Committee recognizes that quiet operation is an important feature of
good performance, but considers that the information at present available is not
sufficient to permit the writing of a satisfactory specification of allowable noise
level.
60 NON-THEATRICAL EQUIPMENT REPORT [j. s. M. p. E.
The illuminated area of the screen shall be divided into 12 equal
squares. At the center of each of these squares the illumination shall
be measured, either by a visual method or by means of a photoelec-
tric light-meter corrected by suitable color-filters to conform within a
good approximatiom to the visibility curve of the eye.
Suitable precautions shall be taken to insure that the optical
train of the projector is in normal operating condition, and that
the lamps used are representative ones, and are operated at their
design voltages.
The arithmetical average of the 12 illumination measurements,
in foot-candles, shall be multiplied by 8.33 (the area of the screen
in square-feet) to obtain the stated result in lumens.
The statement of light output shall include a complete specifica-
tion of each type of lamp used, by wattage and voltage ratings, type,
and design life.
(9) Color of Light on the Screen. — The color-temperature of the
light delivered to the screen when the source is operated at its rated
voltage shall be in the range from 3000° to 4700°K.
(10) Uniformity of Screen Illumination. — -The illumination shall be
measured on a screen not less than 40 inches wide. Measurements
shall be made at the center and at four points in the corners, located
y2o of the screen width from the top or bottom edge, and the same
distance from the side edges of the screen.
The average illumination at the four corner points shall be not
less than 55 per cent of the illumination at the center. At no corner
shall the illumination be less than 40 per cent of the illumination
at the center of the screen.
The illumination on the screen shall be free from noticeable bands
or patches differing in color or brightness from the adjacent parts of
the screen.
(11) Accuracy of Light-Source Location. — Unless the manufacturer
has provided means of adjusting the position of the light-source, the
maximum deviation of the light-source from its design position, due
to the combined tolerances of the factors that affect light-source
position, shall not be sufficient to cause the uniformity of screen
illumination to fail to satisfy requirement No. 10, above.
(12) Freedom from Flicker. — The test for flicker shall be made
by allowing the light from the projector, with no film in the gate, to
fall upon a screen of size and surface such as to give a screen bright-
ness of 3 foot-lamberts. Under this condition the projector shall
uly, 1941] NON-THEATRICAL EQUIPMENT REPORT 61
>roduce no visible flicker when running at its normal speed. If the
>rojector is designed to operate at more than one speed, the test shall
>e made at a speed of 16 frames per second.
(13) Accessibility of Optical Parts for Cleaning. — All external sur-
aces of the condensing lenses, and the front surface of the mirror
ised behind the lamp, shall be accessible for cleaning without the use
>f tools. If removable for cleaning, these parts shall be so mounted
is to be positively returnable to their proper positions.
TEST CHART
III
10 9 8 76
FIG. 10. Resolution test-chart.
(14) Location of Film in Image Plane of Lens.— As the film passes
the picture gate of the projector, it shall be so held that its plane is
perpendicular to the lens axis within limits such that there are no
differences in sharpness of focus among the four corners of the pic-
ture, visible to an observer twice the screen width distant from the
screen.
(15) Image Quality of Projection Lens— The projection lens shall
be tested by mounting it on a special test projector arranged to hold,
in proper relation to the lens axis, a glass-plate test-object made as
follows :
62 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
Copies of the test-chart shown in Ifig. 10 are arranged as shown
in Fig. 11, and photographed with a reduction such that the black
outline shown in Fig. 11 has a height of 7.21 mm (0.284 inch) and a
width of 9.65 mm (0.380 inch), with a radius of 0.5 mm in the corners.
The ratio of reduction of the test-charts is such that the nine sets
of lines in the reduced images are spaced at 20, 30, 40, 50, 60, 70, 80,
90, and 100 lines per millimeter. The sensitive coating of the glass-
plate, and the lens used in making the reduction, have resolving
= isi ,V->
«*• '' V
in
= !i
:=ill :=m
*
FIG. 11. Arrangement of test-chart images in picture frame area.
power high enough that all the lines of the test-pattern are clearly
resolved.
The test projector shall be placed at a distance from the screen
such that the projected image of the black border of the test-object
measures 30 X 40 inches. Care shall be taken to insure that the
screen is perpendicular to the projection axis. The lens under test
shall then be focused so that the central image is as sharp as possible.
The observer, standing close to the screen, shall note the finest
lines that are definitely resolved in both the tangential and radial
directions, and record the resolution figures for
uly, 1941] NON-THEATRICAL EQUIPMENT REPORT 63
(a) Center of the screen
(&) Average of mid-sides (top and bottom)
(c) Average of mid-ends (left and right)
(d) Average of the four corners
The following minimum resolving powers shall be obtained :
Lines per Millimeter
(a) Center 80
(b) Mid-sides (top and bottom) 60
(c) Mid-ends (right and left) 40
(d) Corners 30
In addition to meeting the above requirement of resolving power,
he projection lens shall be free from the following defects to a degree
uch that they are not easily noticeable when the image is viewed
rom a distance equal to twice the width of the screen:
(a) Haze. — Some lenses possess a large amount of spherical aberration, which
as the effect of covering the image with a misty haze of light, without seriously
psetting the resolution. This causes unpleasant projected images. A good pro-
action lens gives a clean, crisp image.
(b) Chromatic Aberration. — This is not a common defect. It may be detected
y the presence of a colored haze visible in the finer details over the whole of the
eld.
(c) Lateral Color. — This defect is manifested by the presence of one-sided color
ringes appearing only in the outer parts of the field and vanishing completely in
he center.
(d) Distortion. — In the presence of this aberration straight lines in the outer
>art of the field appear as curved lines. The straight boundaries of the picture
ate itself make good test objects for the detection of distortion. A lens should
iot be rejected on the ground of distortion unless the defect is bad enough to be
istracting to an average observer.
(16) Provision for Focusing. — The projection lens shall be so
nounted as to be readily brought to an exact focus. The mounting
ither shall provide means by which the lens may be locked in its
>osition of focus or shall hold the lens solidly enough to prevent dis-
urbance of the focus by the vibration of the projector.
(17) Mounting of Interchangeable Lenses. — When a projector is
lesigned to accommodate lenses of several focal lengths, the construc-
ion shall be such as to insure that each lens is centered on the optical
ixis and maintained with its axis perpendicular to the film plane
vithin close enough limits to avoid any inequalities of focus among
he four corners of the picture, visible to an observer twice the screen
vidth distant from the screen.
64 NON-THEATRICAL EQUIPMENT REPORT [j. s. M. P. E.
(18) Temperature Rise of Film. — The film, during its passage
through the projector, shall not be raised to a temperature high
enough to cause permanent distortion of the base.
The test for possible excessive temperature rise of the film shall
consist of projecting a 30-inch loop of film having a density of 2.00 or
higher, for 100 continuous passages through the gate. The test-
film shall then be wound into a roll of processed film in normal condi-
tion and allowed to remain so wound for a period of 24 hours before
being examined for distortion of the base.
(19) Temperature Rise of Projector Housing. — -During continuous
operation of the projector at the lowest speed for which it is designed,
at a room temperature of 80 °F, the temperature of no external part
of the projector except the top cover of the lamp-house shall rise above
155°F.
(20) Adequacy of Ventilation for Incandescent Lamps. — The ven-
tilation of the lamp house shall be sufficient to prevent bulging of
the lamp envelope or other damage to the lamp during continuous
operation at any available speed at any time during the life of the
lamp, provided that the lamp is operated at its rated voltage and
the ambient temperature is not higher than 80°F.
When provision is made for reverse operation of the projector
mechanism, the lamp-house ventilation during continuous reverse
operation shall be sufficient to prevent damage to the lamp.
The electrical circuits of the lamp and of the motor or motors
which drive the ventilating fan and projector mechanism shall be so
interlocked that the lamp can not be turned on at a time when the
ventilating fan is not running at a speed sufficient for proper cooling
of the lamp, provided that in cases where projectors are designed for
operation on either alternating or direct current, the user has properly
adjusted such switches or rheostat controls as may have been pro-
vided by the manufacturer in order to obtain normal film speed on
both types of current.
(21) Rewind. — Power rewind, if provided, shall be capable of re-
winding 400 feet of film under a tension of not less than 3 ounces in
not more than 2 minutes.
(22) Lubrication. — The projector mechanism shall either be
equipped with "oilless" bearings of an efficient type or provided with
easily accessible and plainly marked oiling means so constructed
that the application of oil as specified by the manufacturer will in-
sure adequate lubrication of all bearings in the machine.
|uly, 1941] NON-THEATRICAL EQUIPMENT REPORT 65
(23) Range of Line Voltage for Satisfactory Operation. — The manu-
racturer shall specify on the name-plate of the projector the range of
ine voltages on which it is designed to operate.
(24) Directional Reflection Characteristic of Screen. — The dis-
tribution of the reflected light from a screen used for 16-mm pro-
jection shall be so related to the arrangement of the spectators that
;he brightness of the screen as seen from the maximum existing view-
ng angle is not less than 25 per cent of the brightness of the screen as
seen from a position near the axis of projection.
(25) Efficiency of Screen Reflection. — The reflection coefficient of
:he screen within the angle over which requirement No. 18 is satis-
ied shall not be less than 70 per cent.
(B) Sound Reproduction
(1) Steadiness of Film Motion.— The Uniform- Motion Test- Film
ihall carry a 3000-cycle tone, recorded at a level not lower than (>
iecibels below full modulation, with a frequency deviation of not
nore than 0.2 per cent. This film shall be either an original negative
>r a direct positive, not a print.
As measured by the RCA flutter indicator, speed variations
ntroduced by the projector when reproducing this test-film shall
lot exceed 0.6 per cent.
(2) Accuracy of Length and Location of Scanning Beam. — The
Scanning-Beam Length and Location Test- Film shall be an original
legative sound-track. It shall be in two sections. The first section
ihall consist of a uniformly exposed band regularly interrupted to pro-
luce a 300-cycle tone and having its inner edge 0.017 inch from the
?dge of the film, together with a second band regularly interrupted
:o produce a 700-cycle tone and having its inner edge 0.099 inch from
:he edge of the film. The second section shall consist of a band
nterrupted to produce a 500-cycle tone and having its inner edge
).02G inch from the edge of the film, together with a band inter-
•upted to produce a 1200-cycle tone and having its inner edge 0.090
nch from the edge of the film.
The term "inner edge" in the above specification means the edge
learest the position of the centerline of the standard sound-track.
The inner edges of the exposed bands shall be free from any blur-
ing in excess of 0.0005 inch, and shall be located within 0.001 inch of
:he positions specified.
66 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.
The scanning light-beam in the projector shall be of such length
and so located that it reproduces neither the 300-cycle tone nor the
700-cycle tone, but does reproduce both the 500-cycle tone and the
1200-cycle tone.
(3) Accuracy of Azimuth Adjustment of Scanning Beam.* — The
Azimuth Test- Film shall consist of three sections of 5000-cycle vari-
able-density track, modulated 100 per cent.
The first section shall have an azimuth error of 1.0 degree, ±0.1
degree.
The second section shall have correct azimuth adjustment within
0.1 degree.
The third section shall have an azimuth error of 1.0 degree, =*=0.1
degree, in the direction opposite to that of the error in the first sec-
tion.
The test shall be made by reproducing this test-film and reading
the output levels of the three sections by means of a volume indi-
cator or output meter connected across the voice-coil of the loud
speaker or a resistance load used to simulate the loud speaker. The
output from the middle section shall be greater than the output from
either the first or the third section.
(4) Frequency Response. — The Frequency Test- Film shall consist
of at least 15 feet of 400-cycle track for level adjustment, followed
by at least 10 feet of each of the following frequencies:
* The Committee has attempted so far as possible to write these specifications
in terms of overall performance rather than performance of individual parts. It
is for this reason that scanning-beam width, for example, has not been specified,
since it is covered by the specification of overall frequency response (Require-
ment No. 4). It may appear strange, therefore, that scanning-beam azimuth
adjustment is treated separately, since the aspects of performance that it affects,
that is, overall frequency response and distortion, are covered by overall specifica-
tions.
It is necessary to measure scanning-beam azimuth error separately, however,
because the distortion it introduces lies mainly in the frequency range from 1000
to 3000 cycles. Direct distortion measurements in this frequency range are
much more difficult than the azimuth adjustment test.
The test laid down in the specification insures that the azimuth adjustment
of the projector will be correct within 0.5 degree. This limits the harmonic dis-
tortion produced to a maximum of 5 per cent total at 2000 cycles when a 1.0 mil
scanning-beam is used, as in most 16-mm projectors.
July, 1941 J NON-THEATRICAL EQUIPMENT REPORT 67
Cycles Cycles
50 2000
100 3000
200 4000
300 5000
500 6000
1000 7000
The modulation in all sections of the film shall be such that the
level of modulation imparted to a scanning light-beam of negligible
width (0.0002 inch or narrower) does not differ by more than 2 deci-
bels from the level of modulation imparted by the 400-cycle section.
When reproducing this frequency test-film, it shall be possible,
by at least one adjustment of the tone-control provided on the pro-
jector, to obtain a response curve having a variation of not more
than 10 decibels between 100 cycles and 4000 cycles, and of not more
than 14 decibels between 100 cycles and 5000 cycles. In making this
test the output voltages shall be measured across a non-inductive re-
sistance load equal to the impedance of the loud speaker at 400
cycles.
This test shall be made with a power-line voltage of 117 volts at
the amplifier terminals.
(5) Power Output Rating. — The power output of the projector
and amplifier system shall be measured by the use of a Wave- Form
Test- Film consisting of a 400-cycle symmetrically modulated variable-
area sound-track having a total amplitude of modulation of 0.048
inch ± 0.002 inch, and having a total harmonic content of not more
than 1 per cent, of which not more than 0.5 per cent is made up ot
odd-order harmonics.
Using this film as the source of signal, the output shall be mea-
sured across a non-inductive resistance load equal to the impedance
of the loud speaker at 400 cycles. The harmonic content shall be
measured either by means of a selective wave-analyzer such as the
General Radio Type 736-A or by means of band-pass filters isolating
the outputs at 800 cycles and at 1200 cycles for individual measure-
ment.
The measurement shall be made with a power-line voltage of
117 at the amplifier terminals.
At this power-line voltage, no vacuum-tube or other component
part of the amplifier shall be subjected to a higher voltage or oper-
ated at a higher rate of power dissipation than the manufacturer's
68 NON-THEATRICAL EQUIPMENT REPORT [j. s. M. P. E.
maximum ratings for the part in question. Special vacuum-tube cir-
cuits requiring special manufacturer's ratings shall be so indicated.
Using unselected vacuum-tubes which, however, perform within
the manufacturer's ratings for their types, the projector and am-
plifier shall deliver the rated power output with a total harmonic con-
tent of not more than 5 per cent, of which not more than 4 per cent
shall be made up of odd-order harmonics.
(6) System Noise.— A Standard Output Test-Film shall be pro-
vided, consisting of two sections of 400-cycle variable-area track.
The first section shall have a total amplitude of modulation of 0.48
inch ± 0.002 inch. The second section shall be recorded with an
input level 18 decibels lower than is required to produce the first sec-
tion. The print of these two tracks shall have an image density of
1.6 or higher, and a fog density between 0.03 and 0.05.
An auxiliary short length of film uniformly exposed and developed
to a density of 0.6 (transmission of 25 per cent) is required.
The test shall be made by first reproducing the higher output sec-
tion of the Standard Output Film and adjusting the volume con-
trol of the projector until the rated maximum output of the am-
plifier is being delivered across the load resistor specified for the test
under requirement No. 5. The volume control shall be left at this
setting and the film removed from the machine. A short length of
the 0.6-density film shall be placed in the path of the light-beam from
the reproducing optical system. Then, with the projector mecha-
nism running at standard speed, the output noise level shall be mea-
sured with a standard volume indicator meter across the load re-
sistor.
This test shall be made with the tone-control adjusted for the most
nearly uniform frequency response that is available in the range
from 100 to 5000 cycles.
The test shall be made at a power-line voltage of 117 volts.
Under the above conditions of test, the noise level shall be at least
30 decibels below the rated maximum power output level of the sys-
tem.
(7) Adequacy of Available Amplification. — Sufficient amplification
shall be available to develop the rated maximum output power of
the system when reproducing the lower level section of the Stand-
ard Output Test-Film specified above.
The test shall be made at a power-line voltage of 117 volts.
(8) Loud Speaker Power-Handling Capacity. — The loud speaker
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 69
supplied with a sound projector shall be capable of handling the
full rated power output of the associated amplifier without rattling
and without generating objectionable distortion.*
(9) Loud Speaker Frequency Response. — The frequency response
of the loud speaker shall effectively cover the range from 100 to 5000
cycles per second.**
(10) Accuracy of Exciter-Lamp Filament Location. — The maxi-
mum departure of the filament of the exciter lamp from its design
location, permitted by the combined effect of the lamp manufac-
turer's tolerances and the projector manufacturer's tolerances, shall
not be sufficient to cause a reduction of more than two decibels in
the level of reproduced sound, or to cause the production of har-
monics in excess of the limit specified under requirement No. .">,
above.
(IT) Safety of Electrical System. — The projector shall have been
approved for safety by the Underwriters' Laboratory.
(12) Mechanical Noise, f
* This specification expresses the general intent of the Committee, but is ob-
viously incomplete without specification of the method of test. Investigation
has shown that at the present time there is not sufficiently widespread agreement
among specialists in acoustical measurements to permit the writing of a generally
acceptable complete specification covering the power-handling capacity of the
loud speaker.
** The above note also applies to the measurement of loud speaker frequency
response.
t The Committee recognizes that mechanical noise from the projector mecha-
nism must be kept below certain limits if sound reproduction is to be satisfac-
tory but considers that the information at present available is not sufficient for
the writing of a specification.
(Continued on next page)
Supplement to the Report of the Committee on
Non-Theatrical Equipment
RESOLUTION TESTS ON 16-MM PROJECTION LENSES
R. KINGSLAKE
Since some means for the quantitative expression of thej)erform-
ance of a projection lens is very desirable, especially when attempting
to set up standards of projector quality, it is suggested that the visual
resolving power of the lens, expressed in lines per millimeter at the
film plane, be used as a criterion. The lines should be equally spaced,
with the spaces equal in width to the lines, so that a test-chart labeled
"100 lines per mm," shall consist of straight black lines Vsoo mm wide
separated by 1/2oo-mm spaces. At least three lines and two spaces
should be included in the test-chart.
It is suggested further that the performance of a projection lens
should be specified by stating the resolving power at (a) the center of
the field, (b) the average of the mid-points of the top and bottom of the
gate, (c) the average of the mid-points of the left and right sides of the
gate, and (d) the average of the four corners of the gate. The gate
dimensions should be in accordance with the SMPE standard,
namely, 7.21 X 9.65 mm with a 0.5-mm corner radius.
To facilitate this test, a glass photographic test-plate has been
constructed of the required size, and at each of the specified points is
situated a panel carrying resolution test-charts spaced 20, 30, 40,
up to 100 lines per mm, the lines lying both radial and tangential
to the field (Fig. 10). A few additional panels may be added to
complete the whole target (Fig. 11). The requirement for resolu-
tion is that both the radial and tangential lines at any spacing shal
be clearly visible as lines and not as a diffuse patch.
A suitable target was made to a greatly enlarged scale by stickinj
paper prints from a negative transparency on a wooden board. This
was then reduced photographically to the desired size on Eastmai
Spectroscopic High-Resolution plates (Type 548), using a highl;
corrected Microfile lens. This emulsion has a very high resolving
power, and it was found possible to make plates on which the 100-
line charts are clearly recorded. A photomicrograph of a corner oi
one of the test-charts is shown in Fig. 12. The writer is indebted t(
70
NON-THEATRICAL EQUIPMENT REPORT 71
Mr. B. Elle of the Eastman Kodak Company for his kindness and
skill in making these test-plates.
THE TEST PROJECTOR
A simple projector was then constructed to make the tests (Fig. 13).
It consists of a standard Kodascope lamp house with 300-w lamp,
ventilating fan, condenser, and heat-absorbing glass. The lamp was
operated at perhaps 65 per cent of its rated voltage, and as a result
BHBHBP BHHHHH
FIG. 12. A photomicrograph of the 100, 90, and 80-line sections of the
test plate (X 450).
of all these precautions, the test-plates remained cool enough not to
fracture during the test. Adapters were constructed to hold various
standard makes of projection lens, the rear of each adapter being
machined square to the axis of the lens. The glass test-plate was
then held against the rear face of the adapter, film side toward the
lens, by means of spring clips. A circular hole just larger than the
diagonal of the projector aperture was bored in the rear of each
adapter to assist in the accurate centering of the test-plate relative
to the projection lens axis.
72 NON-THEATRICAL EQUIPMENT REPORT [j. s. M. p. E.
Most projection lenses have a decidedly curved field. For this rea-
son it is necessary to adopt a standard focusing procedure to be used
in making a test. It was decided to focus the lens so that the resolu-
tion observable at the center of the field is as good as possible. This
provides a definite and repeatable criterion of focus, based upon the
assumption that most users of cine projectors are more interested in
the center of the picture than in any other part.
FIG. 13. The test projector.
The size of the projected image is not important, but it may con-
veniently be about 21 X 28 or 30 X 40 inches. The projected image
should be studied close-up when determining the resolving power.
A telescope to view the screen is practically a necessity when focusing
the projector. Care must be taken to ensure that the light in the
center of the field is falling perpendicularly on the screen.
THEORY
Assuming that the eye can resolve two lines subtending an angle
of 1 in 2000 (1.7 minutes of arc), then an observer sitting at a distance
of two picture- widths from the screen could just resolve details in
the screen-image spaced at Viooo of the picture width. Carried back
July, 1941] NON-THEATRICAL EQUIPMENT REPORT 73
into the film plane, the actual width of the gate being about 10 ram,
this least resolvable distance becomes just Yioo mm. We may thus
draw up a table of optimum eye resolutions for different viewing dis-
tances :
Viewing Distance fl- M-IUpta ot "SS?$S!£SSfc &1&? ff J£!S
Picture Width) at the Film-Gate)
2 100
2.5 80
3 67
4 50
5 40
6 33
It is therefore useless to require better resolution in our projection
lens than these figures. In practice, the finest resolution is needed
only in the central parts of the screen, where the most significant
parts of the picture will generally be found. Actual projected images
can not usually be resolved to the extent indicated in this table, be-
cause the spherical aberration of the projection lens tends to blur the
images slightly.
OTHER PROPERTIES OF THE IMAGE
Although good resolution is the principal requirement of an image-
forming system, there are other properties that should be watched.
However, if they are not easily noticeable when the image is viewed
from a distance of two screen-widths, they are not likely to be
serious.
(a) Haze. — Some lenses possess a large amount of spherical
aberration, which has the effect of covering the image with a misty
haze of light, without seriously upsetting the resolution. This causes
unpleasant projected images, and lenses showing the defect should be
avoided. A good projection lens gives a clean, crisp image.
(b) Chromatic Aberration. — This is not a common defect, and it
may be detected by the presence of a colored haze visible in the finer
details over the whole of the field.
(c) Lateral Colors — This defect is manifested by the presence of
one-sided color fringes, appearing only in the outer parts of the field
and vanishing completely in the center.
(d) Distortion. — In the presence of this aberration, straight lines
in the outer part of the field appear as curved lines on the screen.
The straight boundaries of the gate itself make good test objects for
74
NON-THEATRICAL EQUIPMENT REPORT [J. s. M. P. E.
the presence of distortion. A lens should not be rejected on the
ground of distortion unless it is bad enough to be distracting to an
average observer.
OBSERVATIONS
A number of representative 16-mm projection lenses were tested
by this method, and the results are recorded below. The positions
(a), (b), (c), and (d) refer to the center, top, side, and corner of the
frame as outlined above.
Manufac-
turer E. F.
//No.
(a)
Resolution
(d)
Remarks
(Inch)
(Lines per mm)
(a) 1
2.5
80
50
20
<20
Normal lens type
1V«
2.5
70
70
60
50
Same lens with field
flattener
IV*
2.5
80
70
60
50
2
2.5
70
70
60
50
2
1.6
=-100
60
20
<20
Normal lens type
2
1.6
90
80
60
40
Same lens with field
flattener
3
2.0
100
70
50
20
3
1.4
90
80
70
40
4
2.5
80
60
50
40
4
1.6
60
50
40
30
(6) 2
2
70
60
40
20
2
2
100
50
20
<20
iVt
2
>100
30
<20
<20
1
2
100
30
<20
<20
3
2.5
80
70
50
30
(c) 2
2
90
50
30
20
2
2
100
60
30
<20
2
1.6
60
40
30
<20
(4) 1
2.46
100
40
<20
<20
11/2
1.8
90
60
40
20
2
2.0
100
50
<20
<20
2
.,,1.6
100-
70
40
<20
3
2.3
90
60
50
40
31/2
2.7
100
80
60
40
4
2.8
80
80
70
70
(<0 2
1.65
90
40
30
20
(/) 2
1.4
70
70
60
20
July, 1941 ] NON-THEATRICAL EQUIPMENT REPORT 75
CONCLUSIONS
The most common type of projection lens is the //1. 6 or//2.0 lens,
2-inch focus, having excellent central definition, and a strongly curved
field. These commonly show resolution figures of approximately:
(<0 (b) (c) (d)
100 60 30 <20
Upon refocusing, these figures can readily be changed to something
like this:
50 70 30 20
If a 2-inch //1. 6 lens is equipped with a field flattener, the resolution
in the center is slightly diminished, but that at the corners is much
increased :
90 80 60 40
A similar case is found for a 1-inch f/2. 5, which became transformed
by the addition of a field flattener as follows :
(without) 80 50 20 <20
(with) 70 70 60 50
However, an //1. 6 lens in the 3-inch or 4-inch size is often quite satis-
factory.
As a result of these studies, it is concluded that, assuming the lens
is focused as accurately as possible for the center of the field, accept-
able resolution figures would be as follows :
Center 80 lines per mm
Top and bottom 60
Left and right sides 40
Corners 30
REPORT OF THE STANDARDS COMMITTEE*
At the Hollywood Meeting of the Society last fall a report was
given of the activities of the Standards Committee for the year 1940
to that date. Since that time the Standards mentioned as having
been reviewed during the past two years by the Committee, -the cor-
responding group in the Research Council, and other interested per-
sons, have been approved by the American Standards Association
and appeared as American Motion Picture Standards and Recom-
mended Practices in the March, 1941, issue of the JOURNAL. The
proposed SMPE Recommended Practices mentioned in that same
report for 35-mm and 16-mm raw stock cores, motion picture screen
brightness, lantern slides, and cutting and perforating specifications
for 35-mm positive and negative raw stock have been approved by
the Society and have likewise appeared in the March, 1941, JOURNAL.
The procedure for adopting SMPE Recommended Practices and
for proposing, in the name of the Society, Standards or Recommended
Practices to the ASA Sectional Committee on Motion Pictures has
been revised by the Board of Governors and has resulted in a simpler
method of adoption of SMPE Recommended Practices. This con-
sists essentially of discussion and approval by the full Standards
Committee and further approval by the Board of Governors and
publication in the JOURNAL with, of course, provision for suggestions
or changes by any member of the Society or any other interested
party. The above-mentioned Recommended Practices were handled
in this manner.
The work in progress includes proposed SMPE Recommended
Practices for edge numbering 16-mm film, and designations of wind-
ing directions for 16-mm film, which have recently received initial
approval and are being voted upon by the full Committee at the pres-
ent time.
Other subjects such as 16-mm emulsion position for printers,
specifications for 16-mm and 8-mm reels, 16-mm sound-track and
scanning area, sound-track blooping patches, standard volume indi-
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received April
22, 1941.
76
STANDARDS REPORT
77
cator, glossary, sound-track nomenclature, sound transmission of
screens, projection lenses, and 35-mm projection sprockets are either
under consideration by the Committee or have been referred to other
Committees of the Society or to the Research Council of the Academy
of Motion Picture Arts and Sciences for further information or sug-
gestions. The Committee suggested a group of papers or symposium
on sprockets to aid in clarifying the situation on this subject.
The Standards Committee wishes gratefully to acknowledge the
cooperation of the Society Officers, the other Committees in the So-
ciety, various individual members, and the Research Council of the
Academy of Motion Picture Arts and Sciences.
P. H. ARNOLD
H. BAMFORD
M. C. BATSEL
F. T. BOWDITCH
M. R. BOYER
F. E. CARLSON
T. H. CARPENTER
E. K. CARVER
H. B. CUTHBERTSON
L. W. DAVEE
J. A. DUBRAY
STANDARDS
D. B. JOY, Chairman
A. F. EDOUART
J. L. FORREST
G. FRIEDL, JR.
P. C. GOLDMARK
A. N. GOLDSMITH
H. GRIFFIN
A. C. HARDY
P. J. LARSEN
C. L. LOOTENS
J. A. MAURER
G. S. MITCHELL
K. F. MORGAN
R. MORRIS
WM. H. OFFENHAUSER
G. F. RACKETT
W. B. RAYTON
E. C. RICHARDSON
H. RUBIN
O. SANDVIK
R. E. SHELBY
J. L. SPENCE
H. E. WHITE
REPORT OF THE THEATER ENGINEERING COMMITTEE*
In the Report of this Committee, presented at the Hollywood Con-
vention last October, and published in the December, 1940, issue of
the JOURNAL, the growth of the Society's activities in the various
phases of theater engineering was described. It was pointed out also
that many phases of theater design, particularly from the projection
viewpoint, had been considered by the Committee and had resulted
in a number of recommended practices and procedures in general ac-
ceptance by the industry. Nevertheless, there were other phases
that had not yet received adequate consideration — these phases re-
ferring more particularly to the theater structure rather than to the
process of projection.
Accordingly, by action of the Board of Governors, on July 13,
1939, what was formerly known as the Projection Practice Committee
was dissolved, and a new Committee was established, known as the
Theater Engineering Committee. This new Committee originally
functioned primarily through two sub-committees, namely, the Sub-
Committee on Projection Practice and the Sub-Committee on Theater
Design.
For a long time, the original Projection Practice Committee had
been studying the question of picture brightness and its measure-
ment. Some years ago, another Committee of the Society, known
as the Screen Brightness Committee, had done considerable work on
this subject and had published a noteworthy report and accompany-
ing symposium on various features of screen brightness in the May
and August, 1936, issues of the JOURNAL. With the publication of
this material, the Screen Brightness Committee became relatively
inactive since the information then at their command did not permit
further constructive analysis.
In the interim, the study was continued to some extent by the then
existing Projection Practice Committee, and during the past year it
Lecame increasingly evident that further active work could be done
on the subject. Accordingly, it was decided to establish a third sub-
committee of the Theater Engineering Committee, to be known .as
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 1,
1941.
78
THEATER ENGINEERING REPORT
79
the Sub-Committee on Screen Brightness, which was to include in
its scope, not only the actual specifications of screen brightness in
theaters, but also the problem of devising appropriate means of
measuring screen illumination and brightness, and of discovering or
devising suitable meters for the purpose. Since this new sub-com-
mittee has been functioning only a short time, its work has not prog-
ressed to the point at which it can make definite recommendations
to the industry. However, some progress has been made during the
past few months, and the Theater Engineering Committee is pleased
to include in this report the first report of the new Sub-Committee
on Screen Brightness.
The personnel of the Theater Engineering Committee, sub-divided
into its three sub-committees is given below. Each sub-committee
also has its subordinate working committees.
THEATER ENGINEERING COMMITTEE
ALFRED N. GOLDSMITH, Chairman
Projection Practice Sub-Committee
H. RUBIN, Sub-Chairman
T. C. BARROWS E. R. GEIB
H. D. BEHR M. GESSIN
K. BRENKERT A. GOODMAN
F. E. CAHILL, JR. H. GRIFFIN
C. C. DASH S. HARRIS
A. S. DICKINSON J. J. HOPKINS
J. K. ELDERKIN C. HORSTMAN
J. FRANK, JR. L. B. ISAAC
R. R. FRENCH I. JACOBSEN
P. J. LARSEN
Theater Design Sub-Committee
B. SCHLANGER, Sub- Chairman
F. W. ALEXA C. HORSTMAN
D. EBERSON E. R. MORIN
J. FRANK, JR. K. C. MORRICAL
M. M. HARE I. L. NIXON
S. HARRIS
Screen Brightness Sub-Committee
F. E. CARLSON, Sub-Chairman
F. T. BOWDITCH S. HARRIS
F. J. DURST W. B. RAYTON
W. F. LITTLE
J. H. LlTTENBERG
E. R. MORIN
J. R. PRATER
F. H. RICHARDSON
J. J. SEFING
R. O. WALKER
V. A. WELMAN
H. E. WHITE
A. T. WILLIAMS
C. C. POTWIN
A. L. RAVEN
R. F. Ross
E. S. SEELEY
J. J. SEEING
C. TUTTLE
H. E. WHITE
A. T. WILLIAMS
SO THEATER ENGINEERING REPORT [J. s. M. p. E.
PROJECTION PRACTICE SUB-COMMITTEE
Much of the work undertaken by the Sub-Committee on projec-
tion practice is at the present time incomplete, so that definite re-
ports are not appropriate at this time. Work is continuing on the
fourth revision of the Projection Room Plans, and it is hoped that a
new report on this subject may be available in the near future.
Tools, Tolerances, and Safety Factors. — The Working Committee
on Tools, Tolerances, and Safety Factors has held a number of meet-
ings and has made a number of tests on projection equipment.
The purpose of the Committee is to conduct a study of the motion
picture projector mechanism from the servicing and operating view-
point, and to determine the degree of wear at various points that may
be tolerated with safety, and to find or devise tools or gauges
that may assist the projectionist in checking the degree of wear, and
the corresponding departure of the mechanism from suitable operat-
ing conditions. Several meetings of the Working Committee have
been held and a number of tests have been conducted on projection
equipment to determine the relation between the pressure of the film
shoe and the spacing between the shoe and the surface cf the film-gate.
This relation has been found to be linear, being approximately
0.0005 inch per gram of pressure. Slight variations in the positions
of the gates apparently make little noticeable difference in the pic-
ture jump. However, it is the intention to check this matter more
accurately and to determine the minimum pressure required for
steady operation. In addition, further tests will be made to deter-
mine the relation between shoe pressure and wear on the film per-
forations, and the relation between the shoe pressure and the wearing
of the sprocket- teeth.
This report should be regarded as preliminary, and it is hoped that
a comprehensive report will be available at the next Convention.
Sub- Committee on the Power Survey. — In the last report of the
Theater Engineering Committee was included a preliminary report
of the Working Committee on the Power Survey, in which it was
pointed out that numerous data had been accumulated through
questionnaires distributed among 1600 theaters of the country.
The purpose of these questionnaires was to secure a cross-section of
data in relation to (1) the trend in current consumption for the vari-
ous electrical units used in theaters throughout the country, (2) the
total cost of electrical current, (3) energy consumption charges, and
(4) the average proportions of power used for projection, air condi-
July, 1941] THEATER ENGINEERING REPORT 81
tioning, lighting, etc. The previous report included a brief table of
data pertaining to these factors. Insufficient time has been available
to complete the tabulation, and it is hoped that a complete report
will be available by the Fall of this year.
Carbon Arc Terminology. — It had been noted that some confusion
existed in the motion picture industry with regard to the terms ap-
plied to various types of arc. In particular, specific definitions of
the terms "high intensity" and "low intensity" were not available.
The Projection Practice Committee, therefore, submits the following
definitions of these terms :
The fundamental distinction between the high intensity and low
intensity carbon arcs is based upon the origin and character of radia-
tion. The chief contributing factors and associated characteristics
are composition of the carbons, current density, and brilliancy.
Low Intensity
The low intensity carbon arc is one in which the principal light
source is incandescent solid carbon at or near its temperature of
volatilization. In the case of the direct current low intensity arc, as
used for projection, this is the crater face of the positive carbon.
The maximum brilliancy of this crater face is limited by the vaporiz-
ing temperature of carbon to a value of about 175 candles per square
millimeter. This crater brilliancy varies but little with changes in
current within the usual operating range, but the crater area increases
considerably with increasing current. Current density in the posi-
tive carbon for the familiar commercial lamps ranges from approxi-
mately 50 to 200 amperes per square inch.
High Intensity
The high intensity carbon arc, as used for projection, is one in which,
in addition to the light from the incandescent crater surface, there is
a significant amount of light originating in the gaseous region im-
mediately in front of the carbon in an atmosphere containing flame
materials (materials which become highly luminescent when volatil-
ized in the arc stream). In the case of the direct current high inten-
sity arc this light comes from within and near the crater of the posi-
tive carbon. The maximum brilliancy of the crater obtained in
various types of direct current high intensity carbon arcs used in
common commercial lamps ranges from 350 to 1200 candles per square
82 THEATER ENGINEERING REPORT [j. s. M. p. E.
millimeter with current densities in tl\e positive carbon ranging from
about 400 to well over 1000 amperes per square inch. Increase of
current increases the crater area only slightly, but produces marked
increase in brilliancy.
Symposium on Projection Practice. — One of the aims of the Pro-
jection Practice Sub-Committee is to make available to the projec-
tionists of the country technical data in such form as it may be easily
applied in practice. With this thought in mind the Committee has
formulated a brief symposium on projection practice for presentation
at this Convention. Following the presentation of this ^Report of
the Theater Engineering Committee, there will be four papers pre-
pared by members of the Projection Practice Committee dealing
with "Projection Room Equipment Requirements," "The Projection
Room — Its Location and Its Contents," "Factors Affecting Sound
Quality," "Factors to Be Considered in a Sound Screen."
REPORT OF THE THEATER DESIGN SUB-COMMITTEE
The Glossary compiled by this Sub-Committee is intended for use
for all those interested in motion picture theater design. The Glos-
sary will be submitted to the SMPE Standards Committee for possible
inclusion in the General Glossary of Motion Picture Terms, which is
under preparation by them, and will be called to the attention of
other interested organizations or groups, including the American
Institute of Architects and various architectural periodicals and
trade papers.
One of the chief benefits which it is hoped will be derived from
this work will be to help in the writing of a uniform Code, which will
govern the functional design of motion picture theaters. The pres-
ent non-uniformity and confusion which exist in the large number
of Building Codes both as to legal requirements and terminology
has been brought to the attention of this committee through the study
of a large number of existing Building Codes throughout the United
States.
It is realized that it would be an almost impossible task to bring
about a major change in the existing codes, particularly as regards
uniformity. However, it is felt that this Committee can start with
an attempt at standardization of terminology and the fixing of uni-
form viewing and hearing requirements in auditoriums. This would
enable such authorities as are contemplating changes in existing
Codes or writing new Codes for motion picture theater construction
July, 1941]
THEATER ENGINEERING REPORT
to be guided by the important visual and auditorium requirements in
the theater.
In addition to the Glossary, the Committee is first giving considera-
tion to the lighting of theater auditoriums. It is recommended that the
wall and ceiling surfaces within the spectators' field of vision, while
viewing the picture, should appear to the spectator as a uniformly
and uninterruptedly illuminated surface. Anything in the lighting
that would tend to distract the viewer's attention from the screen
picture should be avoided if possible. It is very important for best
results in the projection of colored pictures that the color of the light-
ing and wall surfaces be neutral. No departure from uniformity
should be made unless the changes of intensity are gradual.
The Committee is not prepared
to specify actual values of illumi-
nation but does stress, for the
time being, uniformity of illumi-
nation and the elimination of
isolated islands of light in dark
surroundings or dark voids in
areas of light.
This recommendation very
definitely affects the style of
ENTRY TO
AUDITORIUM
LINE OF -
TRAFFIC
DOORS
;
y-DOORS
FIG. 1.
STREET
Light-trap.
architectural ornamentation and
the design of the auditorium
interior. The surfaces em-
ployed must be of such tex-
ture and color over large areas
as will make possible this uniform illumination. Ornamental pro-
jections or cavities which cast shadows, and painted decorations in
various colors and intensities are objectionable. The fact must not
be overlooked that the motion picture screen is a source of light and
may cause undesirable and objectionable illumination of auditorium
surfaces or ornaments if the latter are improperly designed.
In connection with illumination, it is important that the arrange-
ment of walls and doors of the outside lobby, the main lobby, the
foyer, and so forth, be so arranged as to entrap the light coming from
the street. If the line of traffic from the street to the auditorium is
straight, this problem is difficult to solve unless extra sets of doors are
used at intervals to block the light. A more efficient method of an
intimate form can be successfully evolved by so arranging doors and
84 THEATER ENGINEERING REPORT [J. S. M. p. E.
walls that the line of traffic follows a zee shape (Fig. 1). This is
helpful also in eliminating objectionable drafts and in reducing the
infiltration of street noises.
Glossary of Terms Used in Theater Design
Aisle. — A passageway in a seating area.
Center aisle. — An aisle on the longitudinal axis of the theater.
Wall aisle. — An aisle along one of the side walls of a theater.
Intermediate aisle. — Any longitudinal aisle that is not a center aisle or wall
aisle.
Cross-over. — A transverse aisle.
Balcony. — An area of seats, part, or all of which overhangs another seating area.
Orchestra Floor. — The lowest seating area of a theater.
Stadium. — An area of seats higher than and to the rear of the standee rail or parti-
tion, accessible directly from the standee space.
Stepped Platform Seating. — Stepped platforms, one above the other upon which
seats are placed. The amount of rise from one platform to another being deter-
mined by the sight clearance factor.
Uniformly Pitched Auditorium Floor. — A floor having an equal rise or fall for each
row of seats.
Variably Pitched Auditorium Floor. — A floor incline having a changing pitch for
every row, or groups of rows of seats to obtain proper sight clearance.
Auditorium Bowl Floor. — A floor incline for curved rows of seating in which the
change of pitch takes place '>v keeping all of the seats of each respective row on
one level.
Concentric Arcuated Seating Rows. — Seats placed in curved rows, the radii of
which increase for each row, placed farther from the auditorium front wall.
Down Pitch Auditorium Floor. — A floor which pitches in part or whole downward
toward the auditorium front wall to provide sight line clearances.
Reverse Pitch Auditorium Floor. — A floor which pitches upward in part or whole
toward the auditorium front wall to provide raised seating levels located near
to a motion picture screen to bring these seating levels as close to the screen
level as possible.
Combination Pitch Auditorium Floor. — A floor which pitches downward toward
and then upward toward the front wall of the auditorium.
Auditorium Lighting. — Any auditorium lighting in use when the motion picture
show is not in progress
Projection Period Lighting. — Any lighting of the auditorium that may be necessary
or desirable during the projection of the motion picture.
Transition Lighting. — The gradation of illumination from outdoors to the audi-
torium.
Light Trap. — An arrangement of wall and doors designed to exclude undesired
light from the auditorium.
Re-reflected Screen Light. — Light reflected from the screen and re-reflected from
any other surface in the auditorium.
Atmospheric Light Reflection. — Reflection of light by particles in the atmosphere
of the auditorium.
July, 1941] THEATER ENGINEERING REPORT 85
Auditorium. — The space in a theater from any point of which the performance
may be viewed.
Standee Partition (or Rail}. — A partition (or rail) separating a last row of seats
from a cross-over.
Standee Space. — A space in a theater in which patrons are permitted by law to
stand and view the performance.
Lobby. — The space between the first and second sets of doors of a theater.
Foyer. — A gathering place between the auditorium and the lobby.
Outside Lobby. — A partially enclosed space in front of the first set of entrance
doors. (Sometimes called "Vestibule.")
Soffit. — Generally used to refer to the ceiling under the balcony.
Right Side (of auditorium). — The right-hand side, looking toward the screen.
Left Side (of auditorium). — The left-hand side, looking toward the screen.
Mezzanine. — An intermediate level between seating levels.
Auditorium Front Wall. — False wall or structural wall at the front of the audi-
torium on the audience side of the screen.
Exit Court. — A space for egress open to the sky.
Exit Passage. — A space for egress entirely enclosed.
Auditorium Rear Wall. — The wall at the opposite end of the auditorium from
the screen.
Auditorium Side Walls. — Walls other than the front or rear walls of the audito-
rium.
Proscenium Opening. — The opening in the auditorium front wall through which
the screen is viewed. .j
Rear Screen Space. — The space on the side of the sci • n away from the audience.
Traffic Control. — Physical or suggestive. Any device (architectural lighting or
decoration, signs, door controls, barriers, etc.) used to control the direction of
the passage of people in the public spaces of the theater structure.
Vomitory. — A walled in passage used for circulation to seating areas usually cut
through a raised inclined seating level.
Balcony or Stadium Fascia. — The surface facing the motion picture screen which
forms part of the protective wall and rail in front of a balcony or stadium.
REPORT OF SUB-COMMITTEE ON SCREEN BRIGHTNESS
The recently appointed Sub- Committee on Screen Brightness has
held its first meeting.
Reflection characteristics of the usual screen materials and their
response under given conditions are for the most part appreciated
only in a general way, or take on only academic significance. Most
people, who have to do with the specification of screens and projec-
tors and the other factors of theater design and operation which af-
fect the basic fundamentals of motion picture exhibition, have lacked
the means to acquaint themselves with the values of brightness ac-
tually experienced by the audience. An appropriate correlation of the
86 THEATER ENGINEERING REPORT
physical factors with the physiological and psychological elements
involved has therefore been difficult.
It is axiomatic that progress on a technical problem is limited unti
one can deal with it quantitatively and do so conveniently. The
first objective of the Sub-Committee, therefore, is to develop measure
ment procedures and facilities of such low cost and convenience that
on the one hand, specialists will be encouraged to amplify the infor
mation they now have, and on the other, that knowledge and experi
ence of these matters may be widely diffused among those who contro
the conditions under which pictures are viewed.
Brightness meters presently available have limitations as to cos
or convenience in use by others than specialists. Accordingly, as it
first step, the Sub-Committee has formulated provisional specifica
tions for instruments which would facilitate attainment of its objec
tive, and is placing these before instrument manufacturers to deter
mine the feasibility of having them made available.
TELEVISION REPORT, ORDER, RULES,
AND REGULATIONS
FEDERAL COMMUNICATIONS COMMISSION
WASHINGTON, D. C.
MAY 3, 1941
The following contains such extracts from the Report of the Federal Communica-
tions Commission as are deemed of interest to the members of the Society and other
readers of the JOURNAL. The complete report deals with the following main headings:
(I) Definitions
(II) Television Transmission Standards
(III) Change or Modification of Transmission Standards
(IV) Engineering Standards of Allocation
( V) Objectionable Interference
( VI) Transmitter Location
(VII) Operating Power, Determination, and Maintenance
( VIII} Equipment
(IX) Monitors
Appendix I: Charts for Determining Service Areas and Interference
Range
Appendix II: Requirements for Contour Maps in Establishing Service
Areas
REPORT ON MARCH 20, 1941, TELEVISION HEARING
DOCKET NO. 5806
By the Commission (Fly, Chairman, and Commissioners Walker, Payne, Thompson
and Wakefield concurring — Commissioners Case and Craven not participating) :
On March 20, 1941, a hearing was held for considering when tele-
vision broadcasting "shall be placed upon a commercial basis" and
for considering rules and regulations and standards for such stations.
Upon the hearings held in January and in April of 1940, the Com-
mission found the industry divided upon the basic question whether
television was ready for commercial broadcasting, and also found the
industry divided as to transmission standards for television broad-
cast stations. Some believed that television had not reached the
point where it could offer sufficient entertainment value to justify
commercial operation and that standardization would result in the
freezing of the science at the then level of efficiency. Others were
87
88 TELEVISION REPORT [j. s. M. p. E.
determined to proceed at all costs with .the launching of television on
a large scale.
In its report of May 28, 1940, on the April hearing, the Commission
declared :
As soon as the engineering opinion of the industry is prepared to approve any one
of the competing systems of (television} broadcasting as the standard system the
Commission will consider the authorization of full commercialization. That a
single uniform system of television broadcasting is essential — so far as the basic
standards are concerned — must also be amply clear. The public should not be
inflicted with a hodge-podge of different television broadcasting and receiving sets
Because the situation was one which threatened to hold up co-
ordinated television development indefinitely and to delay public
service on a widespread basis, the Commission offered its coopera-
tion to the industry along lines in furtherance of the achievement of
higher standards by research and development.
First, it provided for new experimental television stations in various
sections of the country to engage in practical demonstration of pre-
vailing competing systems. Later, it collaborated with the Radio
Manufacturers Association (RMA) in creating the National Tele-
vision System Committee (NTSC). The RMA felt that "Because
of the inadequacy of the various suggested standards for television"
all existing systems should be explored and developed, and new
standards formulated. The NTSC was given this task.
The Commission now finds the industry entirely in agreement that
television broadcasting is ready for standardization. The stand-
ards as finally proposed by the NTSC at the March 20, 1941, hearing,
represent, with but few exceptions, the undivided engineering opinion
of the industry. Some difference of opinion exists among broad-
casters as to the date when commercial operation should begin.
The National Broadcasting Company and the Columbia Broadcast-
ing System, in effect, urged some delay in beginning commercial
television. However, the Commission is of the opinion that the
reasons advanced for the delay are not controlling. Other leading
figures in the industry that earlier opposed commercialization, such
as Philco, Zenith, and De Forest, now express the view that the
present stage of scientific development warrants prompt standardiza-
tion and commercialization.
The demonstrations conducted by different broadcasters and manu-
facturers for the benefit of the NTSC and the Commission revealed
the merits and demerits of the systems upon which standards could
July, 1941] TELEVISION REPORT 89
be based. The eleven volumes constituting the proceedings of the
Committee and its sub -committees stand as evidence of the great
volume of work done. The Commission acknowledges its apprecia-
tion of the RMA and NTSC for their cooperation in performing this
worth-while work.
The three-color television system demonstrated by the Columbia
Broadcasting System during the past few months has lifted tele-
vision broadcasting into a new realm in entertainment possibilities.
Color television has been known for years but additional research
and development were necessary to bring it out of the laboratory for
field tests. The three-color system demonstrated insures a place for
some scheme of color transmissions in the development of television
broadcasting.
The NTSC proposals provide that color television be given a six-
month field test before standardization and commercialization.
The Commission finds this requirement necessary. However, im-
mediate experimental color program transmissions are encouraged.
The standards proposed by the NTSC provide for most of the im-
provements held out as readily possible a year ago for monochrome
transmissions (black and white pictures). These standards fix the
line and frame frequencies at 525 and 30, respectively.* The 525
lines provide for greater detail in the pictures transmitted than the
441 lines advocated a year ago. They give substantially eaual
resolution and more fully exploit the possibilities of the frequency
bands allocated for television. Different line and frame frequencies
will likely be required for color transmissions. This, however, is a
matter for future consideration after color transmissions have been
adequately field tested.
A year ago one of the weakest phases of the proposed television
standards was an unreliable synchronizing pulse which frequently
caused the loss of the picture under interference conditions. A few
weeks before the March 20, 1941, hearing, developments were brought
forth for greatly intensifying the synchronizing signals transmitted.
These developments have been incorporated in the new standards.
* Certain experimental systems require variable line and frame frequencies.
However, the fixed values proposed appear to be best for monochrome trans-
missions, because only 30-frame pictures have been fully developed and as long
as the frequency band for television channels (aural and visual) is limiu-d i.» '•
megacycles not more than 525 lines can be employed to advantage with 30 frauu-x
90 TELEVISION REPORT [j. s. M. p. E.
The demonstrations witnessed by the Commission impressively
showed the tenacity with which this new form of synchronizing signals
holds the picture in place under extremely adverse interference con-
ditions.
The proposed standards require frequency modulation for sound
accompanying the pictures. Television is therefore benefited by the
recent development of frequency modulation.
The standards proposed by the NTSC reasonably satisfy the re-
quirement for advancing television to a high level of efficiency
within presently known developments. These standards are 'adopted
by the Commission and made effective immediately.
The Commission feels that this state of the science affords some
reasonable assurance against early obsolescence of equipment. At
the same time, it must explicitly recognize the advancing and neces-
sarily fluid state of the science. Accordingly, procedure has been
provided for the consideration of new developments, including,
but by no means limited to, color television.
Procedure is also provided for expediting completion of the tele-
vision stations now authorized by the Commission. Existing
licensees and permittees who can satisfy the Commission that their
station construction will meet all the engineering requirements of the
rules and regulations and standards for such stations may begin com-
mercial operation on July 1, 1941.
The Commission finds that at least six months will be required for
obtaining comparative test data on the alternative methods permitted
for transmitting synchronizing signals. Such data are necessary
for further limiting the signal synchronizing standards. The Com-
mission is requesting the industry to provide the necessary test data
as to both color transmissions and synchronizing signals within the
six-month period following the beginning of commercial operation.
The regulations require that at least 15 hours' program service per
week shall be rendered by each station.
The Commission adheres to the policy set forth in its report on the
April, 1940, television hearing regarding multiple ownership or con-
trol of television broadcast stations. Under this policy no person is
permitted to own or control more than three television broadcast
stations.
This is to preserve the public benefits of competition in the use of
the limited number of channels available for television broadcasting.
The order and appropriate regulations carrying out the principles
July, 1941
TELEVISION REPORT
91
of this report were adopted by a unanimous vote of the Commission
en bane in its meeting of April 30, 1941.
II. TELEVISION TRANSMISSION STANDARDS
The Television Channel
(1} The width of the standard television broadcast channel shall be six
megacycles per second.
(2) It shall be standard to locate the visual carrier 4.5 megacycles lower in
frequency than the unmodulated aural carrier.
g i.on
K
B
M
1
i
e
3?
a;
C_>
aj
W rH
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5
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a
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0
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Q ac
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oco
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\ 0.5 1 1
.25 2 3 4 5 5.25 5.75
S
H0.75MC
MIX.
FIG. 1 . Idealized picture transmission amplitude characteristic.
Relative field strength of picture side band not to exceed
0.0005. Drawing not to scale.
(5) It shall be standard to locate the unmodulated aural carrier 0.25 mega-
cycle lower than the upper frequency limit of the channel.
(4} The standard visual transmission amplitude characteristic shall be that
shown in Fig. 1.*
(5) The standard number of scanning lines per frame period shall be 525,
interlaced two to one.**
* In the use of any type of transmission permitted under Standards 9 and 15,
the emissions (aural and visual) must be kept strictly within the 6 megacycle
band authorized.
** The presently favored values for lines and for frame and field frequem-k-s
for experimentally field testing color transmissions are, respectively, 375, III),
and 120.
92 TELEVISION REPORT [j. s. M. p. E.
(6) The standard frame frequency shall ^be 30 per second and the standard
field frequency shall be 60 per second.**
(7) The standard aspect ratio of the transmitted television picture shall be
4 units horizontally to 3 units vertically.
(8) It shall be standard, during the active scanning intervals, to scan the
scene from left to right horizontally and from top to bottom vertically, at uniform
velocities.
(9) It shall be standard in television transmission to modulate a carrier
within a single television channel for both picture and synchronizing signals, the
two signals comprising different modulation ranges in frequency or amplitude or
both.*
(10} It shall be standard that a decrease in initial light intensity cause an
increase in radiated power.
(11) It shall be standard that the black level be represented by a definite
carrier level, independent of light and shade in the picture.
(12) It shall be standard to transmit the black level at 75 per cent (with a
tolerance of plus or minus 2.5 per cent) of the peak carrier amplitude.
Aural Signal Modulation
(13) It shall be standard to use frequency modulation for the television trans-
mission with a maximum frequency swing of 75 kilocycles.
(14) It shall be standard to preemphasize the sound transmission in accord-
ance with the impedance-frequency characteristic of a series inductance-resistance
network having a time constant of 100 microseconds.
Synchronizing Signals
(15) It shall be standard in television transmission to radiate a synchronizing
wave-form which will adequately operate a receiver which is responsive to the
synchronizing wave-form shown in appended Fig. 2.
(16) It shall be standard that the time interval between the leading edges of
successive horizontal pulses shall vary less than one-half of one per cent of the
average interval.
(^7) It shall be standard in television studio transmission that the rate of
change of the frequency of recurrence of the leading edges of the horizontal syn-
* Practical receivers of the "RA" type (those which attenuate the carrier
50 per cent before detection) designed for the synchronizing signals shown in
Fig. 2 will also receive interchangeably any of the following :
(a) Amplitude modulated synchronizing and picture signals of the 500-kilo-
cycle vertical synchronizing pulse type.
(b) Synchronizing signals of the alternate carrier type with amplitude modu-
lated picture signals.
(c) Frequency modulated picture and synchronizing signals.
Each of the above signals will be permitted over a reasonable period for trans-
mitting regularly scheduled programs as required by Sec. 4.261 (a) of the Rules
and Regulations Governing Television Broadcast Stations.
July, 1941]
TELEVISION REPORT
u
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? -%- "s - •? Sd ii= s
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ul
94 TELEVISION REPORT fj. s. M. P. E.
chronizing signals be not greater than 0.15 per cent per second, the frequency to
be determined by an averaging process carried out over a period of not less than
20, nor more than 100 lines, such lines not to include any portion of the vertical
blanking signal.
(18) It shall be standard to rate the visual transmitter in terms of its peak
power when transmitting a standard television signal.
(19) It shall be standard in the modulation of the visual transmitter that the
radio frequency signal amplitude be 15 per cent or less of the peak amplitude, for
maximum white.
(20) It shall be standard to employ an unmodulated radiated carrier power
of the aural transmission not less than 50 per cent nor more than 100 per cent
of the peak radiated power of the picture transmission.
(21) It shall be standard in television broadcasting to radiate signals having
horizontal polarization.
III. CHANGE OR MODIFICATION OF TRANSMISSION STANDARDS
The Commission will consider the question whether a proposed
change or modification of transmission standards adopted for tele-
vision would be in the public interest, convenience, and necessity,
upon petition being filed by the person proposing such change or
modification, setting forth the following:
(a) The exact character of the change or modification proposed ;
(b) The effect of the proposed change or modification upon all other trans-
mission standards that have been adopted by the Commission for tele-
vision broadcast stations;
(c) The experimentation and field tests that have been made to show that
the proposed change or modification accomplishes an improvement and
is technically feasible;
(d) The effect of the proposed change or modification in the adopted standards
upon operation and obsolescence of receivers;
(e) The change in equipment required in existing television broadcast stations
for incorporating the proposed change or modification in the adoptee
standards, and
(/) The facts and reasons upon which the petitioner bases his conclusion tha
the proposed change or modification would be in the public interest
convenience, and necessity.
Should a change or modification in the transmission standards be
adopted by the Commission, the effective date thereof will be deter
mined in the light of the considerations mentioned in sub-paragrap]
(d) above.
Following is a list of Television Broadcast Stations, at presen
operating, under construction, experimental, and relay broadcast.
July, 1941]
TELEVISION REPORT
95
SCHEDULE A
(AT PRESENT OPERATING)
Licensee and Location
Columbia Broadcasting
System, Inc., New York,
N. Y.
Don Lee Broadcasting
System, Los Angeles,
Calif. T. Hollywood,
Calif.
National Broadcasting
Co., Inc., New York,
N. Y.
Philco Radio and Tele-
vision Corporation,
Philadelphia, Pa.
Zenith Radio Corporation,
Chicago, 111.
Licensee and Location
Earle C. Anthony, Inc.,
Los Angeles, Calif.
Balaban & Katz Corp.,
Chicago, 111.
Bamberger Broadcasting
Service, Inc., New York,
N. Y.
Columbia Broadcasting
System, Inc., Chicago,
111.
Crosley Corporation, Cin-
cinnati, Ohio
Don Lee Broadcasting
System, San Francisco,
Calif.
Allen B. DuMont Labora-
tories, Inc., Washing-
ton, D. C.
Allen B. DuMont Labora-
tories, Inc., New York,
N. Y.
Call
Letters
Frequency
(Kc)
Power
Visual Aural
W2XAB
60,000-66,000
71A
kw
7»/2 kw
(Channel No. 2)
W6XAO
50,000-56,000
1
kw
150 kw
(Channel No. 1)
W2XBS
50,000-56,000
12
kw
15 kw
(Channel No. 1)
W3XE
66,000-72,000
10
kw
10 kw
(Channel No. 3)
W9XZV
50,000-56,000
1
kw
1 kw
(Channel No. 1)
SCHEDULE B
(UNDER CONSTRUCTION)
Call
Frequency
Power
Letters
(Kc)
Vis
ual
Aural
W6XEA
96,000-102,000
1
kw
1 kw
(Channel No. 6)
C. P.*
W9XBK
60,000-66,000
1
kw
1 kw
(Channel No. 2)
C. P.
W2XBB
96,000-102,000
1
kw
1 kw
-
(Channel No. 6)
C. P.
W9XCB
78,000-84,000
1
kw
1 kw
(Channel No. 4)
C. P.
W8XCT
50,000-56,000
1
kw
1 kw
(Channel No. 1)
C. P.
W6XDL
50,000-56,000
1
kw
1 kw
(Channel No. 1)
C. P.
W3XWT
50,000-56,000
1
kw
1 kw
(Channel No. 1)
C. P.
W2XWV
78,000-84,000
1
kw
1 kw
(Channel No. 4)
C. P.
* C. P. = Construction Permit.
96
TELEVISION REPORT
[J. S. M. p. E.
Licensee and Location
Call
Letters
Frequency
, (Kc)
Power
Visual Aural
Hughes Productions Divi-
W6XHH
60,000-66,000
10
kw
10
kw
sion of Hughes Tool Co.,
(Channel No. 2)
C.
P.
Los Angeles, Calif.
Hughes Productions Divi-
W6XHT
60,000-66,000
10
kw
10
kw
sion of Hughes Tool Co.,
(Channel No. 2)
C.
P.
San Francisco, Calif.
The Journal Company
W9XMJ
66,000-72,000
1
kw
1
kw
(The Milwaukee Jour-
(Channel No. 3)
C.
P.
nal), Milwaukee, Wis.
Metropolitan Television,
W2XMT
162,000-168,000
250
w__
1
kw
Inc., New York, N. Y.
(Channel No. 8)
C.
P.
National Broadcasting
W3XMB
60,000-66,000
1
kw
1
kw
Company, Inc., Wash-
(Channel No. 2)
C.
P.
ington, D. C.
National Broadcasting
W3XPP
102,000-108,000
1
kw
1
kw
Company, Inc., Phila-
(Channel No. 7)
C.
P.
delphia, Pa.
Television Productions,
W6XYZ
78,000-84,000
1
kw
1
kw
Inc. (Area of Los An-
(Channel No. 4)
C.
P.
geles, Calif.)
WCAU Braodcasting Co.,
W3XAU
84,000-90,000
1
kw
1
kw
Philadelphia, Pa.
(Channel No. 5)
C.
P.
SCHEDULE C
EXPERIMENTAL TELEVISION BROADCAST STATIONS
Call
Frequency
Power
Licensee and Location
Letters
(Kc)
Visual
Aural
Allen B. DuMont Labo-
W2XVT
78,000-84,000
5
kw
5
kw
ratories, Inc., Passaic,
(Channel No. 4)
C.
P.
N.J.
Balaban & Katz Corp.
384,000-396,000
10
w
(Area of Chicago, 111.)
C.
P.
Columbia Broadcasting
W6XCB
162,000-168,000
1
kw
1
kw
System, Inc., Los An-
(Channel No. 8)
condl.
C.
P.
geles, Calif.
Farnsworth Television &
66,000-72,000
1
kw
1
kw
Radio Corp., Ft. Wayne,
(Channel No. 3)
C.
P.
Ind.
General Electric Com-
W2XB
60,000-86,000
10
kw
3
kw
pany, Scotland, N. Y.
Kansas State College of
W9XAK
50,000-56,000
100
w
100
w
Agriculture & Applied
(Channel No. 1)
C.
p.
Science, Manhattan,
Kans.
Leroy's Jewelers, Los An-
W6XLJ
230,000-236,000
1
kw
1
kw
geles, Calif.
(Channel No. 13)
condl.
C.
p.
July, 1941]
TELEVISION REPORT
1)7
Licensee and Location
Purdue University, West
Lafayette, Ind.
RCA Manufacturing Com-
pany, Inc., Portable
(Camden, N. J.)
RCA Manufacturing Com-
pany, Inc., Camden,
N.J.
State University of Iowa,
Iowa City, Iowa
Call
Letters
Frequency
(fee)
Power
Visual Aural
W9XG 66,000-72,000 750 w 750 w
C. P.
W3XAD 321,000-327,000 500 w 500 w
W3XEP 84,000-90,000
(Channel No. 5)
30 kw 30 kw
W9XUI 50,000-56,000 100 w
(Channel No. 1)
210,000-216,000
(Channel No. 12)
EXPERIMENTAL TELEVISION RELAY BROADCAST
Balaban & Katz Corp.
(Area of Chicago, 111.)
Balaban & Katz Corp.
(Area of Chicago, 111.)
Columbia Broadcasting
System, New York,
N. Y.
Don Lee Broadcasting
System (Area of Los
Angeles, Calif.)
Allen B. DuMont Labora-
tories, Inc. (Area of
New York, N. Y.)
General Electric Com-
pany, New Scotland,
N. Y.
General
pany,
N. Y.
National Broadcasting
Co., Inc., Portable
(Camden, N. J., and
New York, N. Y.)
National Broadcasting
Co., Inc., New York,
N. Y. (Port.-Mobile)
Philco Radio and Tele-
vision Corporation,
Philadelphia, Pa.
Television Productions,
Inc. (Area of Los An-
geles, Calif.)
Electric Com-
Schenectady,
W9XBT 204,000-210,000 250 w
210,000-216,000
(Channel Nos. 11 & 12)
384,000-396,000 10 w
W2XCB 336,000-384,000 6.5 w
W6XDU 318,000-330,000 6.5 w
WIOXKT 258,000-264,000 50 w
264,000-270,000
(Channels Nos. 15 & 16)
W2XI 162,000-168,00 10 w
(Channel No. 8)
W2XD 156,000-162,000 40 w
162,000-168,000
W2XBT 162,000-168,000 400 w
W2XBU 282,000-294,000 15 w
W3XP 230,000-236,000 125 w
236,000-242,000
(Channels Nos. 13 & 14)
W6XLA 230,000-236,000 250 w
236,000-242,000
C. P.
C. P.
C. P.
C. P.
CHARACTERISTICS OF INTERMITTENT CARBON ARCS*
F. T. BOWDITCH, R. B. DULL, AND H. G. MACPHERSON**
Summary. — Although the carbon arc is usually considered as a continuous
source of light, the experiments reported in this paper show that it may be used for
the generation of light surges as well. If these surges are made to occur at a rate
so fast that the arc stream does not have time to deionize between them, then the elec-
trical circuit may be completely broken at the conclusion of each surge and closed
again to initiate the next one. For longer periods between surges, a very low main-
taining current is employed. The timing and duration of the light pulses are con-
trolled by electronic switching of half -cycle current surges from an alternating-current
supply.
For a given size of carbon, much higher brilliancy and candle-power can be obtained
in intermittent than in continuous operation; a brilliancy of 1600 candles per
sq-mm is reported for a 7 -mm carbon of the "Suprex" type. The efficiency of the
intermittent carbon arc is limited by the thermal lag in the electrodes, in that they
continue to radiate energy for a considerable period after the current is reduced to zero
at the end of each surge.
The carbon arc is usually considered as a continuous source of
light, although it is used only intermittently in motion picture photog-
raphy and projection, the particular intervals of light usage being
determined by the camera or projector shutter. Since as much as
one-half of the light generated is wasted in this way, worth-while
economies would appear to be possible by the elimination of this
waste through the intermittent generation of light as needed.
Interest in such an intermittent light-source was further stimulated
by the theoretical demands of a radically new system of motion pic-
ture photography, known as the "I-R" or "Increased Range System,"
sponsored by Dr. Alfred N. Goldsmith and others. In a typical ap-
plication of this system, very short light-pulses of only one-eighth
the duration of a single frame are required, separated by dark periods
three times as long. Such a light-cycle, supplied by a continuous
source and shutter, would necessitate the waste of 75 per cent of the
generated light.
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May 7, j
1941.
** National Carbon Company, Inc., Cleveland, Ohio.
Society is not responsible for statements by authors &
INTERMITTENT CARBON ARCS
99
In attacking the general problem of an intermittent carbon arc,
the idea was first conceived of employing an alternating-current
source of suitable frequency with switching circuits permitting the
delivery of heavy current surges to the arc during selected positive
half-cycle intervals. Thus, while the arc would be maintained be-
tween surges at a low value of alternating current, it would operate as
a direct-current high-intensity arc during the half-cycles when a
heavy surge current was permitted to flow. A simplified diagram
of the electrical circuit employed for this purpose is shown by Fig. 1.
In this figure, the carbon arc is shown in series with two ballast
resistors, RI and Rt, across an alternating-current source. One of
these resistors, R%, may be intermittently short-circuited as desired
through the switch 5 shown at the right. The combined resistors
FIG. 1
Simplified circuit diagram of the inter-
mittent carbon arc.
limit the current to a minimum value necessary to maintain the arc
between surges; the single resistor RI determines the magnitude of
the surge current which will flow while the switch 5 is closed. If
rapid flashing of only one-half cycle duration is desired, then a simple
knife-switch of the type indicated can not, of course, be used. A
mercury- vapor switch of the ignitron type, however, completely
satisfies all requirements as to capacity and speed and has been suc-
cessfully employed in a number of circuits.
The complexity of the timing circuit which tells the ignitron when
to short-circuit the ballast resistance is determined by the nature
of the light-pulses required. For instance, if a surge is to be delivered
every positive half-cycle, the ignitron and timing circuit can be dis-
pensed with entirely and a simple half-wave rectifier used. How-
100
BOWDITCH, DULL, AND MACPHERSON [J. s. M. p. E.
ever, if succeeding surges are to be separated by one or more idle
cycles, then a more complex arrangement is required. A circuit
found suited to this type of service is illustrated by Fig. 2.
In this circuit alternating voltage from the same source that sup-
plies the arc is used to charge a condenser C through the transformei
TR, the half-wave rectifier 7\, and the resistance R3. This condenser
is connected in the grid circuit of a mercury-vapor thyratron T2 with
polarity such that the condenser voltage opposes that of the negative
bias battery BI, reducing the negative grid potential of T2 as the con-
denser charge increases until this thyratron is tripped. IrTtripping,
current is permitted to flow into the ignitron firing electrode E,
vaporizing the mercury and causing the ignitron T4 to conduct,
which short-circuits the ballast ^2- As the ignitron fires, the volt-
age across it drops to a value below the extinction point of thyratron
T -r
FIG. 2. Circuit diagram of the intermittent carbon
arc with surge timing control at the beginning of any
chosen half-cycle.
T2, so that this tube is extinguished. In the meantime, the volts
developed between the electrode E and the mercury pool during
firing overcomes the bias voltage B*, tripping the small argon -filled
thyratron T3 so that it may discharge the condenser C. Finally, at
the end of one half-cycle, when the voltage across the ignitron falls
to zero, it, too, is extinguished, so that all elements are returned to
their initial condition ready to set off the next surge. The timing of
this circuit may be adjusted in a number of different ways, since firing
can not occur until the grid voltage of the thyratron T2 reaches a
specific minimum value. For instance, the secondary voltage of the
transformer TR, the magnitude of the resistor R3, and the magnitude
of the condenser C may be independently adjusted to determine the
number of half-cycle charging pulses needed to raise the condenser
voltage to the critical tripping value. Also, this critical voltage
July, 1941]
INTERMITTENT CARBON ARCS
101
value may be adjusted by changing the voltage of the battery B^
which must be overcome. By phase reversal through the trans-
former TR, the condenser is charged during half-cycles when the volt-
age is negative, so far as the main arc circuit is concerned ; thus the
condenser voltage remains steady during the positive half -cycles
when firing might occur. In practice, circuit values are so adjusted
that the voltage across condenser C is a little too low during the posi-
tive half-cycle just prior to the one when firing is desired, so that it
will be appreciably above the required minimum when wanted.
The circuit just described insures that firing will occur very early
in a predetermined half-cycle as desired. It will not, however, per-
mit adjustment of the firing time throughout the duration of a half-
cycle, and thus does not provide for a current surge lasting for only
FIG. 3. Circuit diagram of the intermittent carbon
arc with surge timing control at any time during any
chosen half-cycle.
a predetermined fraction of a half-cycle. A circuit permitting such
adjustment is shown by Fig. 3.
The left portion of this figure up to and including the thyratron
T2 is identical with that of the previous figure. Also that portion
of the circuit including the transformer TR\, the single-wave rectifier
TI, condenser C, and resistor R3 constitute the essential timing cir-
cuit as before, but now operating to raise the plate voltage of thyra-
tron T3 to its tripping point, so that the resulting discharge through
the resistor R± may trip T2. The tripping of thyratron T3, however,
is also dependent upon the grid voltage pulse received each positive
half -cycle through the transformer TR2. This transformer has a
constricted iron magnetic path giving a very peaked wave-form con-
ducive to accurate timing of the voltage pulse, and the primary is
102 BOWDITCH, DULL, AND MACPHERSON [j. s. M. p. E.
supplied through the phase-shifting network composed of the four
elements in Wheatstone bridge arrangement at the right. As with
the previous circuit, the condenser C receives its charging pulses dur-
ing negative half-cycles when the grid pulse applied to thyratron T3
is of opposite polarity to that required for firing. On positive half-
cycles, therefore, when firing might occur, the voltage across the con-
denser remains fixed, so that timing is solely controlled by the grid
pulse. In operation, then, the thyratron T3 receives a firing pulse
once each positive half-cycle, in a phase relationship with respect to
the source as determined by the setting of the phase-shifter circuit.
If, during the preceding half -cycle, the voltage of the condenser C
has risen to a sufficiently high value, the tube fires, tripping thyra-
tron T2 and ignitron T4 along with it. The act of firing discharges
the condenser, so that the circuit automatically clears itself, ready
for the next sequence.
The time required for all these things to happen is, fortunately, '•
only a matter of microseconds, from the firing of the first element in
the chain of either one of the circuits described until the light-surge
is emitted by the arc.
The choice of frequency of the alternating-current source is
governed by the light-pulse timing required for a particular service. \
For instance, in motion picture projection at 24 frames per second, a
48-cycle source might be used, with current surges every positive
half -cycle, giving a light-pulse of V%-second duration as with the pres-
ent 90-degree shutter. In the "I-R" system of motion picture
photography previously described, a 96-cycle source with a half-
cycle duration of 1/i92 second could be employed, with the timing
circuit set to give surges during alternate positive half-cycles. Sig-
nalling applications might also be conceived in which any commercial
frequency could be used, and the tripping of the ignitron controlled
through the tapping of a telegraph key or other contacting device to *
give successive bursts of light, each consisting of a series of half-cycle
surges.
The firing circuit shown in Fig. 3 is best adapted for experimental
work, since it will do everything the simpler circuit can accomplish
and, in addition, permits variation of the phase-angle at which the
discharge through the arc starts. As previously mentioned, however, '
once the discharge has started it will continue until the end of the
half-cycle, since there is no way of extinguishing the ignitron until
the voltage across it falls to zero. Using the circuit of Fig. 3 and a
July, 1941] INTERMITTENT CARBON ARCS 103
96-cycle source firing on alternate positive half-cycles, a series of
measurements was made using the same ballast resistors in series
with the arc, but starting the surge-current at different points after
the start of the half-cycle. It was found that the peak candle-power
during a surge is highest when the firing is started as soon as possible
in the cycle, because of the greater crater area obtained. The peak
intrinsic brilliancy, however, remains constant throughout a wide
variation of starting phase-angle, from 30 to 75 degrees. When the
arc is started at a large phase-angle, that is in the middle or toward
the end of the half-cycle, it emits an intense throbbing noise at the
flashing frequency, which gradually decreases to a minimum as the
phase of starting is shifted toward the beginning of the cycle.
The circuits for the intermittent arc so far described call for the
use of sustaining current between flashes, conducted through the
ballast resistor ^2 of Figs. 1, 2, and 3. It was soon found, however,
that when the flashes occur as often as every other cycle at 96 cycles
per second, this sustaining current could be reduced to zero. That
is, the resistor R2 could be omitted entirely from the circuit, and
flashes initiated from a complete open-circuit condition. The time
between flashes is so short under these conditions that the arc does
not have time to deionize completely, so that a conducting path re-
mains for energy to fire the ignitron and then reestablish the arc.
It was also found possible to operate the arc with a small sustaining
direct current, provided, of course, that the d-c sustaining source and
the a-c surge source were otherwise electrically independent.
Two considerations proved to be important in determining which
of these three arrangements was the best, i. e., an alternating or a
direct sustaining current, or none at all. In the first place, if it is
desirable that the light between flashes should be kept as low as
possible, then the arc should be operated without any sustaining cur-
rent, since a minimum light between flashes is obtained in this way.
However, this is possible only when the time between flashes is very
short.
Another consideration of importance in this connection is that of
the steadiness of the arc. One of the principal difficulties originally
encountered was an unsteadiness in the light output associated with
a wandering of the negative flame to various positions in front of
and around the positive carbon, due to wandering of the cathode spot
around the tip of the negative carbon. Apparently this spot did not
remain anchored in one place when the current was reduced between
104 BOWDITCH, DULL, AND MACPHERSON [j. s. M. P. E.
flashes, since the current-density was then too low to load the nega-
tive carbon adequately. Using regular negatives, it was impossible
to eliminate this unsteadiness so long as an alternating sustaining
current or a zero sustaining current was used. The use of a direct
sustaining current, however, held the cathode spot in one place, and
eliminated this type of unsteadiness. It was found also that the use
of a small-diameter copper-coated graphite negative was helpful in
this respect, so that a reasonably steady arc could be achieved when
no sustaining current was used. No means were found, however,
for completely steadying the cathode spot when an alternating sus-
taining current was used.
Both positive and negative carbons for use with the intermittent
arc must have sufficient current capacity to carry the rms or ef-
fective current without overheating. In cases where the surge cur-
rent is passed through the arc at frequent intervals as in the "I-R
System" application, it is desirable to use carbons of greater electrical
conductivity than those of the same diameter conventionally used
in d-c arcs. One of the best positive carbons for this purpose was a
7-mm "Suprex" with a copper coat of twice the usual thickness.
When operated with no sustaining current, this carbon gave the steadi-
est performance without rotation, and in combination with a 5.5-
mm copper-coated graphite negative at an angle of 20 to 30 degrees
with the positive. At greater angles, a lip forms on the upper edge
of the positive carbon, causing unsteadiness and a decrease in candle-
power in a forward direction, while at an angle of less than 20 degrees,
the negative flame is deflected first in one direction and then in an-
other by the positive carbon, causing corresponding fluctuations in
candle-power. The average consumption of the carbons with this
trim, when surges with a peak current of 270 amperes were timed to
occur every other positive half-cycle of a 96-cycle source, is 13 inches
per hour for the positive carbon and 1 1 inches per hour for the nega-
tive carbon.
The appearance of the intermittent arc employing this trim is in-
dicated by the photographs of Fig. 4. The first five photographs are
side views of the arc, and give the appearance of the arc (a) just be-
fore the active half -cycle (—10 degrees); (b) at the start of the cur-
rent surge (30 degrees) ; (c) at the peak surge current (90 degrees) ;
(d) as the current is dying away (160 degrees) ; and (e) after the end
of the conducting half -cycle (200 degrees). All pictures were made
with the same exposure time, employing a specially constructed syn-
July, 1941
INTERMITTENT CARBON ARCS
105
(a) Before surge
starts. (-10°)
(b) A s surge
starts. (30°)
(c) At surge peak.
(90°)
(d) Near cut-off.
(160°)
(e) After cut-off.
(200°)
(/) Before surge
starts. (-20°)
(g) At surge peak.
(90°)
(K) After cut-off.
(210°)
FIG. 4. Photographs of the intermittent carbon arc at various phase-angles:
7-mm "Suprex" positive carbon; 270-ampere peak surge current (electrical de-
grees given beneath pictures refer to zero at beginning of surge half -cycle).
106 BOWDITCH, DULL, AND MACPHERSON [j. s. M. p. E.
chronous shutter whose opening could be adjusted in phase along
the time-cycle of events. The last three photographs show the
front view of the positive crater : (/) before the start of the conducting
half-cycle ( — 20 degrees); (g) at the peak current (90 degrees); and
(h) after the end of the half-cycle (210 degrees). The photographic
exposure is the same in all three of these pictures.
One of the most interesting characteristics of the intermittent arc
is that it is possible to obtain much higher momentary values of in-
trinsic brilliancy and candle-power than can be obtained with the
same carbons operating on direct current. The 7-mm "Suprex"
carbon at 50 amperes' direct current produces 12,000 candle-power
and a brilliancy of 600 candles per sq-mm. This same carbon, oper-
ated intermittently from a 60-cycle source and flashing the arc every
fourth half-cycle, gives a peak candle-power of 70,000 to 75,000 and a
peak brilliancy of 1350 candles per sq-mm at a peak current of 350
amperes. Flashing much less frequently, a maximum brilliancy of
1600 candles per sq-mm can be obtained from this carbon using a
675-ampere peak current.
The average light emitted during a light-pulse was measured by a
photocell limiting the light reaching the active surface to a half-cycle
by means of a sector opening in a synchronously driven disk placed
in front of the cell. Measured in this way, the trim shown in Fig. 4
has an average candle-power of 26,000 during the surge half -cycle.
During the first half-cycle of the inactive period between surges, the
average candle-power is 3100, or 12 per cent of the candle-power dur-
ing the current surge. The candle-power during the second and third
half-cycles following the surge is 2400 and 2200, 9 per cent and 8 per
cent, respectively, of the average surge candle-power. The brilli-
ancy during the active half-cycle averages 660 candles per sq-mm,
while the brilliancy during the succeeding three inactive half-cycles
is 20, 12, and 10 per cent of this, respectively. When a sustaining
current is used, the light between surges is still greater. The time-
interval between current surges is evidently too short to allow the
carbons to cool below incandescence; and since, with zero current
between surges, re-ignition depends upon maintaining ionization
during the inactive period, this is obviously an inherent character-
istic of the intermittent arc on such a time-cycle of operation.
A test of the intermittent arc in an optical system was made, using
a 14-inch Fresnel lens. The lens had a focal length of 14 inches and
was placed 10V2 inches from the crater. At this distance the lens
July, 1941] INTERMITTENT CARBON ARCS 107
picks up a 70-degree cone of light from the positive carbon and pro-
jects a beam having an angular spread of 20 degrees. During the
active half-cycle, a quantity of light equal to 88 lumen-seconds was
projected in the beam per light-pulse. The light projected during
the succeeding three inactive half-cycles was 14, 10, and 9 per cent
of this, respectively.
These measurements of the light radiated during the inactive, or
dark half-cycles, as well as the photographs of Fig. 4, indicate that
the carbons do not cool to a very great extent between surges at the
frequency employed in these experiments. Although this "thermal
lag" is of use in permitting the reestablishment of the arc after short
periods with no sustaining current, it seriously reduces the efficiency
of the intermittent arc in comparison with a d-c arc with a shutter
when the duration of the light-pulses is of the same order of magni-
tude as the time between pulses. Comparisons made between the
intermittent arc of Fig. 4 and an 11 -mm high-intensity d-c arc with
a shutter giving the same light-cycle produced the following result.
Considering only the surge-light of the intermittent arc and the light
passed by the shutter from the d-c arc, the intermittent arc was 1.6
times as efficient as the shuttered d-c arc in terms of candle-power-
hours per watt-hour. Since current flowed only one-fourth of the
time for the intermittent arc, a 4:1 instead of a 1.6:1 advantage over
the continuous arc might have been anticipated, since three-fourths
of the light generated in the latter case is wasted. That this did not
prove to be the case is_due to the thermal lag of the intermittent arc,
which causes it to radiate energy between flashes.
The basis for expecting a 4:1 efficiency advantage for the intermit-
tent arc over the shuttered d-c arc in this service depends upon ob-
taining the same instantaneous light for a given instantaneous cur-
rent through the arc in both cases. This implies that the light is di-
rectly produced by the current. However, conditions in the arc
which determine the production of light are essentially thermal in
character, and the same atomic and molecular processes would take
place, giving the same light, if the carbons and their associated gases
were heated to the same temperature by any means whatsoever.
Electrical means is ordinarily used for this heating, because it is most
convenient. Light production, then, is a result of the temperature
and its distribution, in an atmosphere provided by the controlled
evaporation of core material from the positive carbon; there is no
other connection between current and light.
108 BOWDITCH, DULL, AND MACPHERSON
In operation, heat is lost from the arc by radiation, convection,
and conduction along the carbons at a rate depending upon the tem-
perature of the various parts. These losses must be supplied by the
current input in order to maintain the arc at a temperature suitable
for light emission and for the evaporation of sufficient flame material.
Calculations based upon radiation theory indicate that a black body
at the temperature of the carbon electrodes during the active period
of the intermittent arc will continue to lose radiant energy at sub-
stantially the same rate during the idle interval between flashes for
the time-cycle just described. This is confirmed both by the photo-
graphs of Fig. 4, and by actual measurements of electrode tempera-
ture vs. time taken optically with a synchronous shutter. If the
heat losses from the intermittent arc could be confined to the surge
periods, then the anticipated efficiency advantage over a shuttered
d-c source would be realized. However, the losses do continue dur-
ing the intermediate periods, and at almost the same rate as during the
surge periods. Consequently, in order to maintain the required
temperature when wanted, additional energy must be supplied during
the surge period to overcome these losses.
These remarks apply, of course, only to those applications where
the time between light-surges is very short, as in both motion picture
projection and the special photography application discussed. It is
believed that they will apply to any situation in which the time be-
tween flashes is short enough to permit restriking without a main-
taining current. As the time between flashes increases, however,
the potential economy of the intermittent arc increases at a rapid
rate, so that if and when such applications arise, the intermittent car-
bon arc may find commercial utility. In the meantime, it has pro-
vided a most interesting means for the advancement of fundamental
arc theory.
DEVELOPMENT AND CURRENT USES OF THE ACOUSTIC
ENVELOPE*
HAROLD BURRIS-MEYER**
Summary. — A technic is described by which acoustic conditions surrounding
singers and instrumentalists during performance may be made to approximate those
of a highly reverberant studio, irrespective of the normal acoustic characteristics of
the theater, stage, or studio in which the performance takes place.
The acoustic envelope is a curious and useful product of the sound
research being conducted at Stevens Institute. Its applications
have still to be thoroughly explored, but at the moment they seem
to be much wider than originally contemplated. Working with the
acoustic envelope has, moreover, led us into a most interesting series
of studies based upon the observation that, to get a satisfactory per-
formance of any sort in the theater, it is no less important that the
performer hear what he needs to hear than it is to control the audi-
tory component of the show for the audience.
Concert singers and instrumentalists perform, by choice, in small
highly reverberant rooms since, in them, they are able to hear them-
selves easily. This phenomenon is familiar to all who have indulged
in the popular pastime of "singing in the bath." With the same una-
nimity with which the artists prefer small, highly reverberant rooms,
they deplore the acoustic conditions of most large concert halls and
auditoriums.
The nature of the complaint is that the artist can not hear himself.
The results of not being able to hear are the catalog of the artist's
woes: tension, inability to relax, a feeling of being ill at ease, of low
vocal efficiency, forcing the voice in an effort to project, using a
higher key than is best for the song in an effort to get out more volume
and fill up the house. Some singers carry all the pieces in their re-
pertoire in a number of keys, and use the key that is nearest the res-
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May 16,
1941.
** Stevens Institute of Technology, Hoboken, N. J.
109
Society is not responsible for statements by authors
110 H. BURRIS-MEYER [j. s. M. p. E.
onance frequency of the hall, despite ,the fact that few singers can
sing the same piece equally well in more than one key.
It would be a boon to musicians performing in public or before the
broadcast, recording, or motion picture microphone if they could be
surrounded by acoustic conditions characteristic of the small studio,
especially if this could be accomplished without affecting the acous-
tics of any area except that occupied by the artist.
Several years ago, Mr. Paul Robeson discovered that if he stood
in front of the loud speaker of the public address system being used
in a concert, he enjoyed some of the desirable acoustic conditions
usually associated with the small studio. On the occasion of the
stereophonic recording of the first forest scene from The Emperor
Jones, we discussed the possibility of using this phenomenon to sur-
round the performer by an acoustic envelope tailored to his demands.
Experiments were conducted last August in the Maple wood (N. J.)
Theater, which has many acoustic limitations. Simple equipment
was then devised for Mr. Robeson and used in two out-of-town con-
certs and, for the first time in New York, at Carnegie Hall on October
6th, and thereafter throughout Mr. Robeson's concert tour.
When we tried to make the singer think he was in a small studio,
it was first necessary to find out what it is about the acoustics of the
small, reverberant room that is significant as far as the artist is con-
cerned. We controlled individually the intensity of the sound as the
artist heard it, and the frequency of the sound (i. e., response curve)
we varied the direction from which the sound came to the artist and
the distance from which it came to him, which resulted in a time-in-
terval at the artist's position between the original and the reproduced
sound. We found that the artist would hear himself if he could per-
ceive a difference in any characteristic of sound between the original
sound as it left him and the reproduced sound as it came back. It is
the difference which counts.
Once it was established that the artist's ability to hear himself de-
pends upon a difference, it became a simple problem to find those
differences most useful in making an acoustic envelope to surround
the artist. Obviously, it must be possible to isolate the envelope
surrounding the artist completely from the audience; that is, acous-
tic conditions in the audience part of the theater must not be affected.
It must be impossible for the audience to be aware of the presence of
any sound-control equipment; and the task must be accomplished
with simple portable equipment.
July, 1941] THE ACOUSTIC ENVELOPE 111
Intensity differences are not useful. When the reproduced sound
is less intense than the original, the artist can not hear it; when it is
more intense, the artist is satisfied but the audience hears the re-
produced sound.
Time differences are useful. If the artist hears the reproduced
sound later than the original one, he is perfectly satisfied, even
though the reproduced sound be of lower intensity than the original.
This seems logical since such time difference is a characteristic of re-
verberation or room resonance.
Present practice indicates that the total distance from singer to
microphone, and loud speaker to singer, need only exceed the dis-
tance travelled by the first reflected sound-wave, which is usually to
the floor and back again.
Experiments involving frequency control at low intensity have
shown that the presence or absence of low frequencies is not apparent
except in the case of excessively loud reproduction. Overemphasized
frequencies of 1500 cycles and up can be heard at low intensity.
Low frequencies lack directional characteristics and, even with a
highly directional speaker, will be heard in the audience if they have
to travel more than a very short distance before reaching the artist.
Moreover, they are not readily absorbed by wall surfaces or audience
so, if they do get to the audience, the audience is aware of them.
Also, when a footlight microphone is used, the system will pick up
low-frequency sounds transmitted by the floor if the system responds
to low frequencies. For-these reasons, low frequencies are not used.
High frequencies are directional enough to be kept away from the
audience and are absorbed readily enough so that they are below
background if they ever do get out. It is also interesting to note that
in judging the quality of his own voice, the singer seems to feel a
particular need for harmonics returned to him acoustically or elec-
tronically. We believe this to be so because highs are so readily ab-
sorbed. Replacing the highs for the singer then constitutes com-
pensating for the acoustic limitations of the stage.
By eliminating unwanted characteristics, we are left with a system
that (1) feeds sound back to the artist through 15 or more feet of
air, and can limit the zone it affects so the audience can not hear it;
and (2) has a response curve cut off below 500, and peaked above the
highest fundamental note which can be sung, so that the significant
harmonics are the only part of the sound that comes back.
The response curve is not particularly critical, although the one
112
H. BURRIS-MEYER
[J. S. M. p. E.
used in Fig. 1 is the result of quite thorough exploration of the fre-
quency spectrum. It is cut off below 500 cycles, has a flat peak at
2000 cycles from which it drops off slowly, and is down 10 db at 6000
cycles. The attenuation at the upper end is a characteristic of the
inexpensive equipment used and has no other significance. The
singer seems to hear himself quite as well with this response as if the
upper end of the spectrum were in. The technic is fully effective
when the sound level, at the position of the artist, is not measurably
affected by turning the system on or off, whether the measurement
be made at flat response or weighted for loudness in conformity with
the ear curve.
A single speaker can effectively cover a sharply defined stage area
3-
FIG. 1.
300 500 KX)0 6000
Frequency in Cycles
Frequency characteristic of equipment used in
the acoustic envelope.
of approximately 200 square-feet, outside of which it is impossible to
tell by ear whether the system is on or off. A single footlight micro-
phone can respond effectively to music emanating from any point
within that area. A level set well below the point of regeneration
for the empty house is safe, and more than adequate for the full house.
Equipment in use at present consists of a crystal microphone, a 5-
watt amplifier, and a moving-coil diaphragm loud speaker unit
mounted on a small exponential horn. The overall response is as
shown in Fig. 1. Further simplification of the apparatus is envi-
sioned. The apparatus is generally disposed as shown in Fig. 2.
The speaker is located at the side of the stage for convenience only.
The system has been found to work satisfactorily with the speaker
July, 1941]
THE ACOUSTIC ENVELOPE
113
hung in the flies, mounted on a lighting tower, laid on the apron floor,
hung under the fly gallery, or located onstage and pointed at a wing
or back wall so that the sound is reflected to the artist.
In exploring the uses of the acoustic envelope, we first tried it on
voices of all types in a number of theaters and concert halls, and
found that the device was satisfactory except in the case of one radio
performer whose technic is quite different from that of the concert
artist. This performer was almost inaudible at a distance of 20 feet
and could not tell whether the envelope was in operation or not.
Next we tried the envelope for a singer accompanied by full or-
FIG. 2. Diagram showing instrument placement
on concert stage for the acoustic envelope. £ 5 indi-
cates position of singer.
chestra. In the first test, the singer, Paul Robeson, was satisfied,
though the conductor, Eugene Ormandy, standing next to him, did
not know there was any electronic device in operation. Radio
pick-up was not affected and apparently can not be, so long as the
microphone is outside the envelope area.
In January the acoustic envelope was tried with a violinist at Town
Hall, and later in the Stevens Theater. The artist's comment was
that it made the violin sound better than that violin could sound,
and that he felt the same added ease and freedom that singers ex-
perience.
Last Fall, when we undertook the problem of controlling back-
114 H. BURRIS-MEYER
stage acoustics at the Metropolitan, Opera House, we employed,
among other devices, the acoustic envelope. Other phases of our
work prevented a test covering the whole acting area, but, turned on
individual artists during performance, the device performed so well
that it has been written into the specifications for the permanent
backstage sound-control system for the Metropolitan.
The only test made of the acoustic envelope in which performers
were unaware of the fact that it would be used was in a joint concert
of the Stevens and Barnard Glee clubs. The singers wondered why
they sang so well.
Mr. Robeson suggested using the envelope as an aid to the actor
in the spoken play, particularly in instances where the house was
dead and the actor's role trying. Only one brief test of this use has
been made at a rehearsal of The Emperor Jones in the Stevens Theater,
but the results were sufficiently satisfactory to warrant further ex-
periments which are contemplated.
Recording and broadcast studios generally have provision for the
use of varying amounts of sound-absorbing or reflecting-wall sur-
faces, to control the brilliance of the music as the microphone gets it.
Where the demands of the musician vary from what is wanted on
the record or on the air, the acoustic envelope may prove helpful.
REFERENCE
This paper contains extensive quotations from "The Control of Acoustic Con-
ditions on the Concert Stage," H. Burris-Meyer, /. Acoust. Soc. Amer., 12 (Jan.,
1941), p. 335, published through the American Institute of Physics. Figs. 1 and
2 are reproduced from the same source.
CURRENT LITERATURE OF INTEREST TO THE MOTION PICTURE
ENGINEER
The editors present for convenient reference a list of articles dealing with subjects
cognate to motion picture engineering published in a number of selected journals.
Photostatic^copies may be obtained from the Library of Congress, Washington, D. C.,
or from the New York Public Library, New York, N. Y. Micro copies of articles
in magazines that are available may be obtained from the Bibliofilm Service, Depart-
ment of Agriculture, Washington, D. C. at prevailing rates.
American Cinematographer
22 (May, 1941), No. 5
Breaking the Bottleneck of Fine-Grain Positive (pp. 210-
211, 236)
Russia's Third Dimensional Movies (pp. 212-213)
Filming Infra-Red Night Effects in the Air (pp. 214, 236,
238)
Hollywood's First Art Director (pp. 219, 238, 240)
Educational Screen
19 (May, 1941), No. 5
Motion Pictures— Not for Theaters (pp. 198-199), Pt. 27
Electronics
14 (May, 1941), No. 5
A Ruler for Record Patterns (p. 47)
International Projectionist
16 (March, 1941), No. 3
Some Common Projection Troubles Due to Power Line
Deficiencies (pp. 7-8, 11)
Effect of Abnormally High Filament Temperature on
Tube Life (p. 12)
A.S.A. "Recommended Practice" for Motion Picture
Projection (p. 14)
RCA Theater Television: Program, Cast and Effect on
Film Industry (pp. 15-16)
Coated Lenses in Photography: Their Effect upon the
Screen Image (pp. 17-18)
Journal of Physics
4 (1941), No. 3
A New Optical Mechanical System of Television (pp. 227-
234)
W. STULL
S. IVANOV
E. G. DYER
J. GRANT
A. E. KROWS
D. R. KING
L. CHADBOURNE
C. W. SCOTT
J. J. FINN
C. G. CLARKE
O. B. LURYE
115
116
CURRENT LITERATURE
Kinematograph Weekly
291 (May 15, 1941), No. 177g
New Apparatus Demonstrated at B. K. S. Meeting (p. 26)
Kinotechnik
23 (March, 1941), No. 3
Die Messung der Bildwandleuchtdichte (Measurement of
Screen Brightness) (pp. 33-37)
Untersuchung auslandischer Schmalfilmprojektoren in
der Filmtechnischen Prufstelle der Reichsnlmkammer
(Investigation of Foreign Substandard Film Projectors
at the German Government Technical Film Testing
Laboratory) (pp. 37-40)
Die Toneinrichtungen (Sound Equipment) (pp. 40-47)
Motion Picture Herald (Better Theaters Section)
143 (May 31, 1941), No. 9
Sealing Projection Room Ports Against Fire and Noise
(pp. 36)
A Theater System for Television (pp. 41-42)
Philips Technical Review
5 (December, 1940), No. 12
The Blended-Light Lamp and Other Mercury Lamps
with Improved Colour Rendering (pp. 341-347)
H. ETZOLD
J. J. SEFING
E. L. J. MATTHEWS
FIFTIETH SEMI-ANNUAL CONVENTION
OF THE
SOCIETY OF MOTION PICTURE ENGINEERS
HOTEL PENNSYLVANIA, NEW YORK, N. Y.
OCTOBER 20TH-23RD, INCLUSIVE
OFFICERS AND COMMITTEES IN CHARGE
Program and Facilities
E. HUSE, President
E. A. WILLIFORD, Past-President
H. GRIFFIN, Executive Vice-President
W. C. KUNZMANN, Convention Vice- President
A. C. DOWNES, Editorial Vice-President
R. O. STROCK, Chairman, Local Arrangements
S. HARRIS, Chairman, Papers Committee
J. HABER, Chairman, Publicity Committee
J. FRANK, JR., Chairman, Membership Committee
H. F. HEIDEGGER, Chairman, Convention Projection Committee
Reception and Local Arrangements
R. O. STROCK, Chairman
P. J. LARSEN
F. E. CAHILL, JR.
H. RUBIN
E. I. SPONABLE
P. C. GOLDMARK
W. H. OFFENHAUSER, JR.
A. S. DICKINSON
W. E. GREEN
R. O. WALKER
T. E. SHEA
J. A. HAMMOND
O. F. NEU
V. B. SEASE
H. E. WHITE
L. W. DAVEE
L. A. BONN
J. H. SPRAY
J. J. FINN
A. N. GOLDSMITH
J. A. MAURER
L. B. ISAAC
E. W. KELLOGG
M. HOB ART
J. A. NORLING
H. B. CUTHBERTSON
J. H. KURLANDER
C. F. HORSTMAN
E. R. GEIB
P. SLEEMAN
E. S. SEELEY
C. Ross
P. D. RIES
Registration and Information
W. C. KUNZMANN, Chairman
J. FRANK, JR.
Hotel and Transportation
G. FRIEDL, JR., Chairman
R. B. AUSTRIAN
R. F. MITCHELL
P. A. McGuiRE
M. W. PALMER
F. HOHMEISTER
H. MCLEAN
F. C. SCHMID
F. M. HALL
J. A. SCHEICK
117
118
FALL CONVENTION
[J. S. M. p. E.
H. A. GILBERT
G. A. CHAMBERS
Publicity Committee
J. HABER, Chairman
P. SLEEMAN
S. HARRIS
C. R. KEITH
W. R. GREENE
H. MCLEAN
D. E. HYNDMAN
L. A. BONN
E. G. HINES
A. S. DICKINSON
Banquet
O. F. NEU, Chairman
R. O. STROCK
J. C. BURNETT
J. A. SPRAY
J. A. NORLING
W. H. OFFENHAUSER, JR. M. HOBART
P. J. LARSEN
E. C. WENTE
A. GOODMAN
M. R. BOYER
J. A. HAMMOND
MRS. D. E. HYNDMAN
MRS. E. I. SPONABLE
MRS. E. S. SEELEY
MRS. A. S. DICKINSON
Ladies' Reception Committee
MRS. R. O. STROCK, Hostess
MRS. O. F. NEU, Hostess
MRS. H. GRIFFIN MRS. E. A. WILLIFORD
MRS. P. J. LARSEN MRS. J. FRANK, JR.
MRS. J. A. HAMMOND MRS. H. E. WHITE
MRS. G. FRIEDL, JR.
MRS. F. C. SCHMID
Convention Projection
H. F. HEIDEGGER, Chairman
F. H. RICHARDSON T. H. CARPENTER J. J. SEFING
L. B. ISAAC P. D. RIES H. RUBIN
A. L. RAVEN J. J. HOPKINS F. E. CAHILL, JR.
G. E. EDWARDS W. W. HENNESSY C. F. HORSTMAN
J. K. ELDERKIN L. W. DAVEE R. O. WALKER
Officers and Members of New York Projectionists Local No. 306
Hotel Reservations and Rates
Reservations. — Early in September, room-reservation cards will be mailed to
members of the Society. These cards should be returned as promptly as possible
in order to be assured of satisfactory accommodations. Reservations are subject
to cancellation if it is later found impossible to attend the Convention.
Hotel Rates. — Special per diem rates have been guaranteed by the Hotel Penn-
sylvania to SMPE delegates and their guests. These rates, European plan, will
be as follows:
Room for one person
Room for two persons, double bed
Room for two persons, twin beds
Parlor suites: living room, bedroom, and bath for
one or two persons
$3. 50 to $8.00
$5. 00 to $8.00
$6. 00 to $10. 00
$12.00, $14.00, and
$15.00
July, 1941] FALL CONVENTION 119
Parking.— Parking accommodations will be available to those motoring to the
Convention at the Hotel fireproof garage, at the rate of $1.25 for 24 hours, and
$1.00 for 12 hours, including pick-up and delivery at the door of the Hotel.
Convention Registration. — The registration desk will be located on the 18th
floor of the Hotel at the entrance of the Salle Moderne where the technical sessions
will be held. All members and guests attending the Convention are expected to
register and receive their badges and identification cards required for admission
to all the sessions of the Convention, as well as to several de luxe motion picture
theaters in the vicinity of the Hotel.
Technical Sessions
The technical sessions of the Convention will be held in the Salle Moderne on
the 18th floor of the Hotel Pennsylvania. The Papers Committee plans to have
a very attractive program of papers and presentations, the details of which will
be published in a later issue of the JOURNAL.
Fiftieth Semi- Annual Banquet and Informal Get -Together Luncheon
The usual Informal Get-Together Luncheon of the Convention will be held in
the Roof Garden of the Hotel on Monday, October 20th.
On Wednesday evening, October 22nd, will be held the Silver Anniversary
Jubilee and Fiftieth Semi-Annual Banquet at the Hotel Pennsylvania. The
annual presentations of the SMPE Progress Medal and the SMPE Journal
Award will be made and officers-elect for 1942 will be introduced. The proceed-
ings will conclude with entertainment and dancing.
Entertainment
Motion Pictures. — At the time of registering, passes will be issued to the dele-
gates of the Convention admitting them to several de luxe motion picture theaters
in the vicinity of the Hotel. The names of the theaters will be announced later.
Golf. — Golfing privileges at country clubs in the New York area may be ar-
ranged at the Convention headquarters. In the Lobby of the Hotel Pennsylvania
will be a General Information Desk where information may be obtained regarding
transportation to various points of interest.
Miscellaneous. — Many entertainment attractions are available in New York to
the out-of-town visitor, information concerning which may be obtained at the
General Information Desk in the Lobby of the Hotel. Other details of the enter-
tainment program of the Convention will be announced in a later issue of the
JOURNAL.
Ladies' Program
A specially attractive program for the ladies attending the Convention is be-
ing arranged by Mrs. O. F. Neu and Mrs. R. O. Strock, Hostesses, and the Ladies'
Committee. A suite will be provided in the Hotel where the ladies will register
and meet for the various events upon their program. Further details will be pub-
lished in a succeeding issue of the JOURNAL.
120 FALL CONVENTION
PROGRAM
Monday, October 20th
9:00 a. m. Hotel Roof; Registration.
10:00 a. m. Salle Moderne; Technical session.
12:30 p. m. Roof Garden; Informal Get-Together Luncheon for members, their
families, and guests. Brief addresses by prominent members of
the industry.
2:00 p. m. Salle Moderne; Technical session.
8:00 p. m. Salle Moderne; Technical session.
Tuesday, October 21st
9:00 a. m. Hotel Roof; Registration.
9:30 a. m. Salle Moderne; Technical session.
2:00 p. m. Salle Moderne; Technical session.
Open evening.
Wednesday, October 22nd
9: 00 a.m. Hotel Roof; Registration.
9:30 a. m. Salle Moderne; Technical session.
Open afternoon.
8:30 p. m. Fiftieth Semi- Annual Banquet and Dance.
Introduction of officers-elect for 1942.
Presentation of the SMPE Progress Medal.
Presentation of the SMPE Journal Award.
Entertainment and dancing.
Thursday, October 23rd
10:00 a. m. Salle Moderne; Technical session.
2 : 00 p. m. Salle Moderne; Technical and business session.
Adjournment
W. C. KUNZMANN,
Convention Vice- President
BACK NUMBERS OF THE TRANSACTIONS AND JOURNALS
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Price
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Beginning with the January, 1930, issue, the JOURNAL of the Society has been
issued monthly, in two volumes per year, of six issues each. Back numbers of
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Orders for back numbers of Transactions and JOURNALS should be placed through
the General Office of the Society and should be accompanied by check or money-
order.
SOCIETY SUPPLIES
The following are available from the General Office of the Society, at the prices
noted. Orders should be accompanied by remittances.
Aims and Accomplishments. — An index of the Transactions from October,
1916, to December, 1929, containing summaries of all articles, and author and
classified indexes. One dollar each.
Journal Index. — An index of the JOURNAL from January, 1930, to December,
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Motion Picture Standards. — Reprints of the American Standards and Recom-
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Test-Films. — See advertisement in this issue of the JOURNAL.
S. M. P. E. TEST-FILMS
C ^ ;>
These films have been prepared under the supervision of the Projection
Practice Committee of the Society of Motion Picture Engineers, and are
designed to be used in theaters, review rooms, exchanges, laboratories,
factories, and the like for testing the performance of projectors.
Only complete reels, as described below, are available (no short sections
or single frequencies). The prices given include shipping charges to all
points within the United States; shipping charges to other countries are
additional.
35-Mm. Visual Film
Approximately 500 feet long, consisting of special targets with the aid
of which travel-ghost, marginal and radial lens aberrations, definition,
picture jump, and film we' ~e may be detected and corrected.
Price $37.50 each.
16-Mm. Sound-Film
Approximately 400 feet long, consisting of recordings of several speak-
ing voices, piano, and orchestra; buzz-track; fixed frequencies for focus-
ing sound optical system; fixed frequencies at constant level, for de-
termining reproducer characteristics, frequency range, flutter, sound-
track adjustment, 60- or 96-cycle modulation, etc.
The recorded frequency range of the voice and music extends to 6000
cps. ; the constant-amplitude frequencies are in 11 steps from 50 cps. to
6000 cps.
Price $25.00 each.
16-Mm. Visual Film
An optical reduction of the 35-mm. visual test-film, identical as to
contents and approximately 225 feet long.
Price $25.00 each.
JCIETY OF MOTION PICTURE ENGINEERS
HOTEL PENNSYLVANIA
NEW YORK, N. Y.
JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
Volume XXXVII August, 1941
CONTENTS
Page
Fantasound ............ W. E. GARITY AND J. N. A. HAWKINS 127
Vitasound ............... N. LEVINSON A>T-> L. T. GOLDSMITH 147
Multiple-Speaker Reproducing Systems foi Motion Pictures
H. I. REISKIND 154
Some Theoretical Considerations in the Design of Sprockets for
Continuous Film Movement ............... J. S. CHANDLER 164
A Method for Designing Film Sprockets ................... ..
W. G. HILL AND C. L. SCHAEFER 177
Improved Motor Drive for Self-Phasing of Process Projection
Equipment ................................. H. TASKER 187
Black Light for Theater Auditoriums .......................
H. J. CHANON AND F. M. FALGE 197
Current Literature .......................................
1941 Fall Convention at New York, October 20th-23rd. . 216
Society Announcements .................................
JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
SYLVAN HARRIS, EDITOR
BOARD OF EDITORS
ARTHUR C. DOWNES, Chairman
JOHN I. CRABTREE ALFRED N. GOLDSMITH EDWARD W. KELLOGG
CLYDE R. KEITH ALAN M. GUNDELFINGER CARLETON R. SAWYER
ARTHUR C. HARDY
Subscription to non-members, $8.00 per annum; to members, $5.00 per annum,
included in their annual membership dues; single copies, $1.00. A discount
on subscription or single copies of 15 per cent is allowed to accredited agencies.
Order from the Society of Motion Picture Engineers, Inc., 20th and Northampton
Sts., Easton, Pa., or Hotel Pennsylvania, New York, N. Y.
Published monthly at Easton, Pa., by the Society of Motion Picture Engineers.
Publication Office, 20th & Northampton Sts., Easton, Pa.
General and Editorial Office, Hotel Pennsylvania, New York, N. Y.
West Coast Office, Suite 226, Equitable Bldg., Hollywood, Calif.
Entered as second class matter January 15, 1930, at the Post Office at Easton,
Pa., under the Act of March 3, 1879. Copyrighted, 1941, by the Society of
Motion Picture Engineers, Inc.
OFFICERS OF THE SOCIETY
**President: EMERY HUSE, 6706 Santa Monica Blvd., Hollywood, Calif.
** Past-President: E. ALLAN WILLIFORD, 30 E. 42nd St., New York, N. Y.
**Executive V ice-President: HERBERT GRIFFIN, 90 Gold St., New York, N. Y.
*Engineering Vice-President: DONALD E. HYNDMAN, 350 Madison Ave., New
York, N. Y.
** Editorial Vice-President: ARTHUR C. DOWNES, Box 6087, Cleveland, Ohio.
* Financial Vice-President: ARTHUR S. DICKINSON, 28 W. 44th St., New York,
N. Y.
** Convention Vice-President: WILLIAM C. KUNZMANN, Box 6087, Cleveland, Ohio.
* Secretary: PAUL J. LARSEN, 44 Beverly Rd., Summit, N. J.
* Treasurer: GEORGE FRIEDL, JR., 90 Gold St., New York, N. Y.
GOVERNORS
**MAX C. BATSEL, 501 N. LaSalle St., Indianapolis, Ind.
*JOSEPH A. DUBRAY, 1801 Larchmont Ave., Chicago, 111.
*JOHN G. FRAYNE, 6601 Romaine St., Hollywood, Calif.
*ALFRED N. GOLDSMITH, 580 Fifth Ave., New York, N. Y.
*ARTHUR C. HARDY, Massachusetts Institute of Technology, Cambridge,
Mass.
**LOREN L. RYDER, 5451 Marathon St., Hollywood, Calif.
*TIMOTHY E. SHEA, 195 Broadway, New York, N. Y.
*REEVE O. STROCK, 35-11 35th St., Astoria, L. I., N. Y.
*Term expires December 31, 1941.
**Term expires December 31, 1942.
FANTASOUND*
WM. E. GARITY AND J. N. A. HAWKINS**
Summary. — This paper discusses the multiple-speaker system known as "Fanta-
sound," currently used with Walt Disney's "Fantasia."
First are discussed some of the deficiencies of conventional sound-picture reproduc-
tion, and then a very complete history of the Fantasound development. In addition,
are described in considerable detail the various important elements of the system.
The art of sound-picture reproduction is about 15 years old. While
an engineer familiar with the complications of sound reproduction
may be amazed at the tens of thousands of trouble-free performances
given daily, the public takes our efforts for granted and sees nothing
remarkable about it.
Therefore, we must take large steps forward, rather than small
ones, if we are to inveigle the public away from Softball games, bowling
alleys, nightspots, or rapidly improving radio reproduction.
The public has to hear the difference and then be thrilled by it, if our
efforts toward the improvement of sound-picture quality are to be re-
flected at the box-office. Improvements perceptible only through
direct A-B comparisons-have little box-office value.
While dialog is intelligible and music is satisfactory, no one can
claim that we have even approached perfect simulation of concert hall
or live entertainment. It might be emphasized that perfect simula-
tion of live entertainment is not our objective. Motion picture en-
tertainment can evolve far beyond the inherent limitations of live
entertainment.
Before discussing the operation of the Fantasound equipment, some
deficiencies of conventional sound-picture reproduction may be
summarized :
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received April
25, 1941.
** Walt Disney Studios, Burbank, Calif.
127
4>The Society is not responsible for statements by authors^
128 W. E. GARITY AND J. N. A. HAWKINS [J. S. M. p. E.
(a) Limited Volume Range. — The limited volume range of conventional record-
ings is reasonably satisfactory for the reproduction of ordinary dialog and inci-
dental music, under average theater conditions. However, symphonic music and
dramatic effects are noticeably impaired by excessive ground-noise and amplitude
distortion.
(6) Point-Source of Sound. — A point-source of sound has certain advantages for
mon-aural dialog reproduction with action confined to the center of the screen, but
music and effects suffer from a form of acoustic phase distortion that is absent
when the sound comes from a broad source.
(c) Fixed Localization of the Sound-Source at Screen Center. — The limitations of
single-channel dialog have forced the development of a camera and cutting technic
built around action at the center of the screen, or more strictly, the center of the
conventional high-frequency horn. A three-channel system, allowing localization
away from screen center, removes this single-channel limitation, and this in-
creases the flexibility of the sound medium.
(d) Fixed Source of Sound. — In live entertainment practically all sound-sources
are fixed in space. Any movements that do occur, occur slowly. It has been
found that by artificially causing the source of sound to move rapidly in space the
result can be highly dramatic and desirable.
It is felt that Fantasound provides a desirable alternative to the
four major deficiencies just described.
There have been other attempts to provide increased volume range
and a broad sound-source. It appears that three separate program
channels are an essential part of any solution to these sound problems.
The matter of maximum usable loudness in the theater is closely
related to the number of separate program channels used.
Three channels sound louder than one channel of three times the
power-handling capacity. In addition, three channels allow more
loudness to be used before the sound becomes offensive, because the
multiple source and multiple standing-wave pattern prevents sharp
peaks of loudness of long duration.
Three tracks and program channels have other advantages over a
single-channel system. Cross-modulation between different sounds
can be greatly minimized. Dialog, music, and effects could conceiv-
ably be placed upon separate tracks. It should be pointed out that
single-frequency steady-state measurements of amplitude distortion
do not necessarily give an indication of the amount of cross-modula-
tion that may be present in a single channel. It has been found that
low-frequency transients, caused by even-order overtones, can cause
objectionable cross-modulation at levels somewhat below the nominal
peak overload point of the amplifier.
For economic reasons, it is almost impossible to eliminate this
source of cross-modulation from single-channel reproducing systems.
Aug., 1941]
FANTASOUND
129
It is a simple matter to isolate conflicting program material on a three-
channel system.
The use of three program channels allows phase differentiation to
supplement amplitude differentiation in obtaining directional per-
spective. The phase differentiation also minimizes trouble with
acoustic interference in the theater, which often accompanies attempts
to use a multiplicity of horns on a single program channel.
THE DIFFERENTIAL JUNCTION NETWORK
The first step toward Fantasound occurred when we were asked to
make a sound move back and forth across the screen. It was found
ia
3 10
75'
150'
IN Df<*EE5
FIG. 1. Curves showing loss vs. shaft rotation of a
typical two-circuit differential junction network. The
cross-over point for the two losses is at —3 db.
that by fading between two speakers, located about 20 feet apart, we
could simulate a moving sound-source, provided that the total level in
the room remained constant. It became obvious at once that simple
mechanical ganging of the volume controls feeding the two loud
speaker circuits was not capable of producing the desired effect.
A special two -gang volume-control was then designed with comple-
mentary attenuations in the two circuits such that the sum of the
attenuations, expressed as power ratios, equalled a constant. The
formula for the relationship between the two attenuations is :
B - 20 log (2 sinh Q.115B)
2
A =
130
W. E. GARITY AND J. N. A. HAWKINS [J. S. M. P. E.
where A and B represent the two attenuations, expressed in decibels.
Typical attenuation curves are shown in Fig. 1.
Many uses have been found for this type of network. It is exten-
sively used in our Fantasound re-recording system to make constant
output fades possible. A special 3-circuit differential junction net-
work, nicknamed "The Panpot," is used to dub one original track
onto one, any two, or all three of our Fantasound program tracks
with smooth transitions and any desired level difference. Thus we
simulate a moving sound-source by starting on either side-track and
PEC
PEC
PEC
PEC
FIG. 2. Simplified block diagram of the Fantasound
reproducing equipment.
progressively moving the program material through the center-track
to the other side-track. This move through three tracks, and thus
three horns, is made smoothly by maintaining constant the total out-
put of the three tracks and horns, regardless of the distribution among
the three program circuits.
The simple 2-circuit differential junction network has been used to
make smooth, constant-level fades between two sound-sources. It
also has been used to vary the ratio of close to reverberant microphone
pick-up without affecting the output level. It was found to be a
convenient means of controlling reverberation.
Aug., 19411 FANTASOUND 131
FANTASOUND REPRODUCING EQUIPMENT
A simplified block diagram of the reproducing equipment is shown
in Fig. 2. On the left are shown the four photocells which scan three
program tracks and a pilot control- track. Each program photocell
feeds a variable-gain amplifier, then, through power amplifiers, the
three-stage horns.
Associated with each variable-gain amplifier is a tone rectifier,
which selects one of the three pilot tones on the control-track, rectifies
it, and applies the resulting d-c control bias to the grids of the vari-
FIG. 3. Circuit diagram of the variable-gain amplifier.
able-gain stage. Thus the output from each loud speaker varies with
the amplitude of its associated control tone.
TOGAD
The heart, or perhaps we should call it the brain, of the Fanta-
sound reproducing equipment is the tone-operated gain-adjusting de-
vice, abbreviated, Togad.
The Togad equipment is composed of two units — the variable-gain
amplifier and the tone rectifier. A sine-wave control-tone is applied
to the input of the tone rectifier, where it is transformed into a d-c bias
voltage. This d-c bias voltage is then applied to the variable-gain
amplifier to vary its transmission. The equipment is arranged so
132
W. E. GARITY AND J. N. A. HAWKINS [J. S. M. P. E.
that a 1-db change in tone level causes a 1-db change in program trans-
mission through the variable-gain amplifier.
Variable-Gain Amplifier. — The variable-gain amplifier, abbreviated,
VGA, is a single stage of transformer-coupled push-pull pentode
voltage amplification (Fig. 3). Its transmission is a function of the
d-c bias applied to its grid circuit. A variation of 50 db in the trans-
mission through the VGA can be effected by changing the bias.
A two-stage, single-ended voltage amplifier follows the variable-
gain stage and the three-stage unit has a maximum gain of 58 db and
maximum power output of +6 db above 6 milliwatts.
The circuit features of the variable-gain stage include a balancing
FIG. 4. Circuit diagram of the tone rectifier.
potentiometer in the plate circuit to balance out tone cross -talk; a
loaded cathode resistor to provide high initial bias and low transmis-
sion in the absence of tone; and switches and bias potentiometers
to test and adjust the bias-gain characteristic of the 6K7 variable-
gain stage.
Normally, the maximum level applied to the VGA input termi-
nals is about -45/0.006w, although up to about -30/0.006)
the distortion is not excessive. Hum and tone cross- talk at this
point are well below tube hiss.
The change in transmission, with bias, is the result of ti
effects occurring simultaneously. Raising the bias lowers the m\
of the tubes, thus reducing the ability of the tubes to amplify. Rais
ing the bias also raises the internal plate resistance, which increj
Aug., 1941]
FANTASOUND
133
the ratio of mismatch between the plate circuits of the tubes and the
relatively low load resistance into which the tubes look. The com-
bination of these two effects makes
the transmission a complex inverse
function of the bias.
It might be noted that screen
and bias regulation have a marked
effect upon the bias- transmission
characteristic. The external con-
trol bias, obtained from the tone
rectifier, is used to "buck out" a
semi-fixed bias obtained from a
cathode tap on the plate supply
bleeder.
The Tone Rectifier. — The tone
rectifier (Fig. 4) contains four im-
portant elements:
(a) A band-pass filter in the input
circuit designed to select the proper con-
trol tone and reject noise and the un-
wanted tones.
(6) A compressing amplifier, using a
6H6 and a 1620 tube. The output of
this amplier varies approximately as the
logarithm of the logarithm of its input.
The 6H6 half -wave rectifier'cuts off the
negative half-cycles of tone and the re-
maining positive half -cycles are applied
to the grid of a 1620 triode functioning
as a grid current compressor. Contact
potential and gas current in both 6H6
and 1620 tubes are balanced out by the
variable cathode-bias resistor in the
1620. This particular log-log amplifier
was devised by Kurt Singer.
(c) A 1620 triode amplifier, trans-
former coupled.
(d) A 6H6 full-wave rectifier, whose
d-c bias output is fed to the variable-
gain amplifier.
There are many time-constants in the VGA and tone rectifier which
contribute to the total "operate" and "restore" time-constants of the
combination. However, all but the time-constants associated with
FIG. 5. Program rack with front
cover removed.
134
W. E. GARITY AND J. N. A. HAWKINS [J. s. M. p. E.
the 6H6 rectifier ripple filter are so small, relatively, that they may be
neglected. The RC products of both charge and discharge circuits
are approximately equal and the "operate" and "restore" times are
about 15 milliseconds.
Fig. 6. Film-phonograph.
Fig. 5 shows most of the equipment used in one program chann(
The topmost panel contains a pilot light. Below that is shown th<
tone rectifier unit. Next below is the variable-gain amplifier, which
has two volume-control knobs in the center. Immediately below the
VGA is an equalizer panel. Below that is a volume-control panel, and
Aug., 1941] FANTASOUND 135
next below is a 20-watt power-amplifier. The lowest shelf contains a
regulated plate supply.
In addition to the equipment shown in this rack, a program channel
normally includes a single stage of preamplification ahead of the VGA
and a 60-watt power-amplifier following the 20-watt amplifier. The
front cover, normally used on this rack, is not shown in Fig. 5.
MULTIPLE-TRACK FILM-PHONOGRAPH
This film-phonograph, shown in Fig. 6, scans four 200-mil push-pull
sound tracks simultaneously on one 35-mm print. It is driven in
synchronism with a picture projector by means of a selsyn interlock
system. The lamp and film compartments are shown in Fig. 7.
Film Drive. — The sound-tracks are scanned on a curved film-gate.
Constancy of film movement is obtained by the use of a magnetically
driven drum which draws the film down over the gate. Flutter
measurements indicate that this is a highly satisfactory driving and
scanning arrangement.
Optical System. — A single 10- volt, 5-ampere exciter lamp mounted
in a double holder in the left compartment of the sound-head provides
the illumination. All four sound-tracks are scanned simultaneously
by a single optical system of the slitless type. The optical train con-
sists of a light-collecting optical system which images the lamp fila-
ment as a long beam of light I1/* mils high across the four sound-
tracks. The illuminated image of the sound-tracks is then projected
by a camera and cylindrical lens system onto four multiple beam-
splitter lenses which, in turn, focus each half of the push-pull sound-
tracks upon the respective cathodes of four push-pull phototubes
(Fig. 8).
OPTICAL PRINTER
The Fantasound optical printer is designed to print four double-
width sound-tracks side by side across the useful area of a 35-mm film,
from negatives having a standard width sound-track in the standard
location. The way in which this is accomplished may most easily be
explained with reference to Fig. 9, which shows the printer threaded
for operation in one direction. The negative passes around under
the upper drum and is illuminated by an illuminating system enclosed
in the lamp-house. Below the upper drum is the optical system which
projects an image of the sound negative upon the positive raw-stock
134
W. E. GARITY AND J. N. A. HAWKINS [J. S. M. P.
•
the 6H6 rectifier ripple filter are so small, relatively, that they may be
neglected. The RC products of both charge and discharge circuits
are approximately equal and the "operate" and "restore" times are
about 15 milliseconds.
Fig. 6. Film-phonograph.
Fig. 5 shows most of the equipment used in one program channc
The topmost panel contains a pilot light. Below that is shown the
tone rectifier unit. Next below is the variable-gain amplifier, which
has two volume-control knobs in the center. Immediately below the
VGA is an equalizer panel. Below that is a volume-control panel, and
Aug., 1941] FANTASOUND 135
next below is a 20- watt power-amplifier. The lowest shelf contains a
regulated plate supply.
In addition to the equipment shown in this rack, a program channel
normally includes a single stage of preamplification ahead of the VGA
and a 60-watt power- amplifier following the 20-watt amplifier. The
front cover, normally used on this rack, is not shown in Fig. 5.
MULTIPLE-TRACK FILM-PHONOGRAPH
This film-phonograph, shown in Fig. 6, scans four 200-mil push-pull
sound tracks simultaneously on one 35-mm print. It is driven in
synchronism with a picture projector by means of a selsyn interlock
system. The lamp and film compartments are shown in Fig. 7.
Film Drive. — The sound-tracks are scanned on a curved film-gate.
Constancy of film movement is obtained by the use of a magnetically
driven drum which draws the film down over the gate. Flutter
measurements indicate that this is a highly satisfactory driving and
scanning arrangement.
Optical System. — A single 10- volt, 5-ampere exciter lamp mounted
in a double holder in the left compartment of the sound-head provides
the illumination. All four sound-tracks are scanned simultaneously
by a single optical system of the slitless type. The optical train con-
sists of a light-collecting optical system which images the lamp fila-
ment as a long beam of light I1/ \ mils high across the four sound-
tracks. The illuminated image of the sound-tracks is then projected
by a camera and cylindrical lens system onto four multiple beam-
splitter lenses which, in turn, focus each half of the push-pull sound-
tracks upon the respective cathodes of four push-pull phototubes
(Fig. 8).
OPTICAL PRINTER
The Fantasound optical printer is designed to print four double-
width sound-tracks side by side across the useful area of a 35-mm film,
from negatives having a standard width sound-track in the standard
location. The way in which this is accomplished may most easily be
explained with reference to Fig. 9, which shows the printer threaded
for operation in one direction. The negative passes around under
the upper drum and is illuminated by an illuminating system enclosed
in the lamp-house. Below the upper drum is the optical system which
projects an image of the sound negative upon the positive raw-stock
136
W. E. GARITY AND J. N. A. HAWKINS [J. S. M. P. E.
running over the lower drum. Each < scanning drum is driven by a
magnetic drive. The optical system projects an image of the sound
negative upon the positive film travelling on the lower drum. This
image is enlarged in the lateral plane but not enlarged in the longi-
tudinal plane of the film.
Traversing Mechanism. — On the left of the upper mechanism (Fig.
9) is the traversing lever which controls the position of the image on
the positive raw-stock. By raising this lever and moving it forward
and backward, the entire upper mechanism and optical system are
FIG. 7. Lamp and film compartments of film-phonograph.
moved forward or backward across the film to be printed. The
traversing mechanism provides four locking positions for the upper
mechanism spaced 0.200 inch apart so that the resulting sound-tracks
are spaced 0.200 inch apart on the print.
Reversing Mechanism. — The printer is designed to print negatives
either forward or backward; i. e., either "heads" or "tails" out, at
feet per minute. It incorporates simplified threading in that regard-
less of the direction of printing, the threading is always done in on<
standard manner with tight film loops. Then by operating one lever,
Aug., 1941]
FANTASOUND
137
the correct film paths and loops are formed for either direction of film
travel.
Fig. 10 shows the threading position. In this view, the arrow-
shaped lever is shown in a vertical position. With the reversing lever
in this position, the four loop-forming rollers, guide-rollers, and pres-
sure-rollers assume the positions shown. The negative and raw-stock
are threaded as shown, over the sprockets, loop-forming rollers, and
drums. It will be noted that the film loops are fairly tight when the
FIG. 8. Phototube compartment of film-phonograph.
sprocket pad-rollers are closed. Also, on each side of each drum, the
film lies between the flanges of a guide-roller.
Automatic Blooping.— Automatic blooping of splices is provided on
the printer. Two blooping switches are shown in Fig. 9. These
switches are designed to close when a double thickness of negative at a
splice passes the switch rollers. When printing to the left, the right-
hand blooper switch operates; whereas, when printing to the right,
the left-hand switch operates. In either direction, when the portion
138
W. E. GARITY AND J. N. A. HAWKINS [J. S. M. p. E.
of the printing stock on which the image of the splice will fall, passes
the end of the blooper tube (Fig. 9), the light in the blooper tube
lights and blacks out the image of the splice. A time-delay me-
chanism synchronizes the bloop and the splice. In series with the
blooper lamp is the blooper indicator lamp, also shown on Fig. 9.
Direction Indicator. — On the lower right-hand sprocket is mounted
a small lamp-house for exposing an arrow-shaped image on the
U30P
FORMING
ROLLER IN
OPERATING
POSITION
TRAVERSING
LEVCR
-
LEFT
PRESSURE
ROLLER !N
OPERATING
LAMP
HOUSE
FIG. 9. Optical printer in operating position for printing to the left.
guiding edge of the print. An arrow-shaped detent is ground on the
outer edge of the sprocket and is so arranged that an arrow will
appear on the print every 32 sprocket-holes, pointing toward the
start, or in the direction of travel of the print when being reproduced.
Lamp-House and Optical System. — The lamp used is the standard
10-volt, 7.5-ampere, curved-filament exposure lamp, and is housed
in the light-proof lamp-house above the upper drum.
The functions of the various lenses will be explained, starting at the
Aug., 1941]
FANTASOUND
139
lamp end of the optical train. The first is a plain window which
keeps dust out of the illuminating system. Next is a reversing prism
which rotates the image of the filament 90 degrees in a horizontal
plane.
The next item in the optical train is the ultraviolet filter, followed
by a piano-cylindrical lens which focuses the image of the filament in
one plane only upon the negative.
Below the negative is the optical system for projecting the image
FIG. 10.
Front view of MI-3817 optical printer showing
threading position
of the negative upon the printing stock. The top lens system is
used to project an enlarged (5 to 1) image of the negative on a plane
approximately in the center of the optical system. At the center of
the optical system is a condenser lens followed by an aperture.
This aperture limits the illumination in both planes and is 0.471 inch
long by 0.050 inch wide. The lower lens system images this enlarged
image at the center of the optical system upon the printing stock
and has a reduction ratio of 5 to 1 in the direction of travel of the
140 W. E. GARITY AND J. N. A. HAWKINS [J. s. M. P. E.
film and a little less than 2.5 to 1 across the film. Thus the image oi
the negative on the printing stock has a ratio of 1 to 1 in the din
tion of motion of the film and a ratio of one to slightly over two ac
the film.
This printer has proved to be very free from flutter. The 9000-
cycle loss 'from negative to print (corrected for 1000-cycle loss)
averages less than one decibel.
HISTORY OF THE FANTASOUND DEVELOPMENT
Fantasound reproduction differs markedly in both results and
equipment from standard theater reproduction. It may be of inter-
est to follow the history of the development step by step.
A great many equipment combinations were explored on paper,
probably several hundred. Of these, ten different systems have been
built up and tried out, up to the time this paper was written. Even
though Fantasia has been released, development has not stopped.
The Mark I system used three widely separated horns across the
stage and horns in each rear corner of the house. Two tracks were
used, one feeding the screen horn, or center-stage horn, while the other
fed the remaining four horns selectively by means of a four-circuit
differential junction network. By manipulating a manual control,
the sound could be moved smoothly around the theater. Experi-
ments with this system brought out the advantages of a broad sound-
source.
The Mark II system was a simple expansion of the Mark I systei
adding three horns; one on each side- wall about halfway back froi
the stage, and one in the ceiling at about the center of the houj
These were in addition to the screen horn and four corner horns us
in the Mark I system. This system used three tracks and a 6-circuil
manually controlled differential junction network. In addition to
creating the effect of moving the sound around the theater, the con-
trols allowed side to side movements in any plane between the
and rear wall of the house. Simultaneous fore and aft control VK
also available.
Up to this time it was felt that the Fantasia roadshow equipmei
could be manually operated by a mixer who would go along with each
show. He would provide manual volume range expansion as well
as control the perspective effects. However, two objections to
manual operation appeared. The five controls became rather com-
plex for one man operation and the studio felt that it would be
Aug., 1941] FANTASOUND 141
difficult to keep all shows alike, due to the large human element in-
volved.
The use of a pilot tone-control arrangement was suggested to
avoid these difficulties, and the Mark III system came into existence
to study the advantages and difficulties of a pilot tone-control track.
This Mark III system was a single-channel Togad expander, con-
trolled by either an oscillator or a tone track. Problems of cross-
talk balance, tone-program amplitude characteristic, time-constants,
distortion and noise compromise, and amount of range expansion
desirable, etc., were attacked.
The Mark IV system was identical with the 8-horn, 3-track Mark
II system, except that Togad control replaced manual control. This
system used 8 control-tones on the control track logarithmically
spaced from 250 to 6300 cycles, using a preferred number series.
This Mark IV system was installed in our Hyperion studios in the
summer of 1939 and was used for sound and music department re-
search until we moved to Burbank in 1940.
The equipment racks and sound-heads for this system required a
floor space about 35 feet long by 4 feet wide. It used nearly 400
vacuum-tubes. All equipment appeared on jacks and almost any
conceivable combination could be patched up in a few minutes.
The Mark V system, first installed at Burbank, was similar to the
Mark IV system in that 8 horns, 3 program tracks, and an 8-tone
control-track were used. However, by using 8 hybrid coils in the
program circuits we obtained a still more flexible system. This
system was in operation only one day. The equipment operated
satisfactorily and no technical difficulties were encountered. The
system failed only because the musical director, the music cutter, and
the "enhancing mixer," could no longer remember from one rehearsal
to the next, "What should come out where?"
From this extreme of complication, the pendulum swung to the
Mark VI system, which used 3 stage horns, 3 program tracks, and a
3- tone control- track.
Our first serious dubbing of Fantasia was attempted on this system.
Our original Fantasound dubbing set-up required 10 program mixers,
each with 3 pots, designated "Left, Center, and Right" positional
controls. In addition, 3 mixers with one pot each were used to handle
the left, center, and right pilot tones. We soon found that the
tremendous number of positional mixing cues made it nearly im-
possible for a mixer to handle 3 positional controls in such a way as
142
W. E. GARITY AND J. N. A. HAWKINS [J. S. M. P. E.
to avoid undesirable discontinuties during moves. We then de-
signed some differentially ganged 3-circuit pots, based on the differ-
ential junction network principle, which greatly simplified the mixing
problem. This change allowed 6 mixers to satisfactorily control 24
program circuits.
The Mark VII system was the first of the RCA-manufactured
systems. Functionally, this system closely resembled the Mark VI
system. The only important difference lay in the use of a linear tone
1
FIG. 11.
View of eight recording channels at the Philadelphia Academy
of Music.
rectifier in place of the log-log rectifier used in our earlier systems.
This changed the tone-program amplitude characteristic.
The Mark VIII system consisted of the Mark VII equipment re-
arranged physically. An ingenious log-log tone rectifier, designed by
RCA, replaced the linear tone rectifier used in the Mark VII set-up.
The second dubbing of Fantasia was done through this system.
After adding a stand-by channel, this equipment was installed in the
Broadway Theater in New York for Fantasia's World Premiere.
The Mark IX equipment closely resembled the Mark VIII system.
Aug., 1941]
FANTASOUND
143
The physical layout was again modified, a few minor changes were
made, and two sets of rear-house horns were manually switched in to
supplement or replace the left and right screen horns at several points
in the picture. This system is operating in eight of the roadshows.
The Mark X system is identical with the Mark IX equipment,
except that the switching and level changes in the rear horn circuits
are done automatically instead of manually. The control arrange-
ment uses a thyratron and mechanical relay system operated by means
of notches on the edge of the film. This ingenious arrangement
FIG. 12. View of some of the mixer positions at the
Philadelphia Academy of Music.
was developed by Messrs. Hisserich and Tickner of our engineering
department. The Mark X system is installed at the Carthay Circle
Theater in Los Angeles.
SCORING AND DUBBING
Scoring.— All the numbers, except The Sorcerer's Apprentice and
the vocal portions of Ave Maria, were scored at the Philadelphia
Academy of Music. Eight push-pull variable-area recording chan-
nels were used (Fig. 11).
144
W. E. GARITY AND J. N. A. HAWKINS [J. s. M. p. E.
Separate channels recorded close pick-ups of violins, cellos and
basses, violas, brass, woodwinds, and tympani. The seventh channel
recorded a mixture of the first six channels and the eighth channel
recorded a distant pick-up of the entire orchestra. The mixer han-
dling the distant pick-up used horn monitoring, while the other mixers
used headphone monitoring. Cathode-ray oscilloscopes were used
as level indicators (Fig. 12).
The Sorcerer's Apprentice number was done in Hollywood on a
somewhat similar multi-channel system. The Ave Maria vocal
numbers were recorded on three channels : two close channels, sepa
FIG. 13. View of the 3-channel mixing position used in scoring
the Fantasia vocal numbers at Burbank. (Messrs. Hawkins, Hisse-
rich, and Marr.)
rating male and female voices, with a distant overall channel for
added reverberation.
Fig. 13 shows the mixer arrangement used in recording the 3-
channel vocal numbers. Three- channel horn monitoring was pro-
vided in our theater and the level-indicating oscilloscopes again
proved valuable in avoiding overloads.
The necessity for checking the range compression on all channels
during scoring and dubbing caused the development of a meam
whereby one man could visually monitor three oscilloscopes. B]
using color differentiation at the overload and underload points, eye
fatigue was minimized. This was accomplished by masks on the
face of the cathode-ray tube. An opaque mask eliminated every-
Aug., 1941] FANTASOUND 145
thing below about 3 per cent modulation, including the complete
negative, or downward, half-cycles. A translucent red mask covered
the range from 3 to 100 per cent modulation on the positive half-
cycles. Above 100 per cent modulation, the trace on the tube was
not masked, and so was highly visible. Program material below 3
per cent modulation (100 per cent - 30 db) produced no visible in-
dication. Material between 3 and 100 per cent modulation appeared
as a white series of half -cycles, and modulation in excess of 100 per
•••n I ! I ,flBBBBB
FIG. 14. View of the program dubbing console in operation.
(Tone console not shown.) (Left to right, at console, Messrs. Blinn,
Steck, Marr, Perry, Moss, Hawkins, Slyfield, and Hisserich. At
rear, Ed Plumb, Musical Director; Luisa Fiels, Asst. Music Cutter,
and Stephen Csillag, Music Cutter.)
cent appeared as a brilliant green series of peaks. The recording, re-
recording, and monitoring systems were poled so that the com-
pression wave, referred to the original microphone, gave positive
peaks on both oscilloscopes and galvanometers. This adaptation of
the oscilloscope was devised by C. O. Slyfield. Over half a million
feet of sound negative was exposed on our scoring channels on this
picture.
Dubbing. — Our re-recording process used 8 to 10 tracks, depending
upon the sequence. Fig. 14 shows the re-recording console in opera-
146
W. E. GARITY AND J. N. A. HAWKINS
tion. The output of the mixing panels fed three recorders, one for
each horn channel, left, center, and right. Another channel recorded
the tone track. These four re-recorded negatives were then printed
on the composite quad print. The Mark VIII Fantasound reproducer
was used for dubbing monitoring (Fig. 15). Including everything
but release prints, about five million feet of film was used for this
picture.
This history of Fantasound is far from complete. Another year
7
FIG. 15.
View of part of the dubbing monitoring equipment.
(Messrs. Hawkins and Garity.)
and we shall know a great deal more about theater operating and
maintenance problems on this type of equipment. To date, our
operating and maintenance experience has been quite satisfactory.
We should like to acknowledge the suggestions and assistance of
C. O. Slyfield, W. C. Lamb, Jr., C. A. Hisserich, H. M. Tremaine,
P. J. Holmes, Melville Poche, H. J. Steck, and E. A. Freitas in the
development of this system. We wish to express our appreciation to
Walt Disney, whose vision and willingness to encourage technical
development made this system possible.
VITASOUND*
NATHAN LEVINSON AND L. T. GOLDSMITH**
Summary. — Two features that would add to the enjoyment and realism of sound
in motion picture theaters are an increased volume range and a more widespread
source of sound for music and effects reproduction. A method of accomplishing
these aims is described which employs a control-track printed in the sprocket-hole
area of the release print to operate a variable-gain amplifier and loud speaker control
equipment.
It has been long recognized in the motion picture industry that an
increased dramatic use of sound in the theater would add to the
enjoyment and realism of sound pictures. Two features that would
contribute to this realism of music and sound effects are an increased
volume range and a spreading of the source of sound.
W. A. Mueller1 has pointed out that an increased volume range is
neither necessary nor desirable for dialog reproduction, but that for
music and sound effects an increase in the effective volume range of
signal to auditorium or film noise of approximately 10 decibels is
both practicable and desirable. As the volume range on the sound-
track is limited by the available volume range of the film itself, an
effective increased range can be secured by automatically raising
and lowering the gain of the reproducing amplifiers in the theater.
The spreading of the source of sound can be accomplished by
adding loud speaker systems outside the screen area. These added
speakers, however, can reduce the illusion of the dialog coming from
the screen if not properly placed and operated. The additional
loud speakers may be automatically cut in the circuit for music and
sound-effects and cut out of the circuit for dialog.
Additional reproducing equipment to accomplish these aims for
the majority of feature films must be readily adaptable to the modern
types of sound equipment found in the well equipped theaters.
Also, the cost of the modification to the exhibitor must be a reason-
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received April 20,
1941.
** Warner Bros. Pictures, Inc., Burbank, Calif.
147
OThe Society is not responsible for statements by author sO
148
N. LEVINSON AND L. T. GOLDSMITH [J. s. M. p. E.
able one and the costs of operation, maintenance, and service must
not be increased.
The Vitasound system was developed with the foregoing considera-
tions in mind. A control- track printed in the sprocket-hole area of
standard release prints is employed to operate a variable-gain ampli-
fier to secure the increased effective volume
range and to operate a loud speaker switch-
ing relay for extending the source of sound
to loud speakers beyond the screen.
Fig. 1 shows three different sample widths
of control-track as it appears on a standard
composite release print. The two top frames
have no operable control-track because the
clear portion between the sprocket-holes is
110 mils wide, or as wide as the sprocket-
holes themselves. The two central frames
have a control-track 40 mils wide, which
serves to cut in the side speakers automati-
cally by means of the relay control. The
two bottom frames have an almost com-
pletely closed or zero-width track which, in
addition to operating the side speaker relay,
is used also to increase the gain of the vari-
able-gain amplifier by 10 db. Any inter-
mediate width of control- track between 40
mils and zero may be printed to secure gain
increases from zero to 10 db.
The control-track is scanned in the sound-
head by a separate photocell at a point
14 frames ahead of the sound-scanning
point. The point on the control-track cor-
responding to the sound on the sound-track
is therefore printed 14 frames nearer the
head end of the reel.
Fig. 2 shows the scanning bracket mounted
in a sound-head around the hold-back sprocket. The scanning aper-
ture is a 90 by 90-mil square opening cut in a shoe on the bracket
which also supports a small 6-8-volt, 0.4-ampere lamp and a type
927 photocell. No optical system is necessary and the film is
threaded over the sprocket in the normal manner.
FIG. 1 . Composite print
with sprocket-hole con-
trol-track.
Aug., 1941]
VlTASOUND
149
FIG. 2. Sound-head scanning bracket for sprocket-hole control
track.
The control frequency is 96 cycles and varies in amplitude with
the width of the clear portion of the film between the sprocket-holes.
The output of the control-track photocell of each projection machine
©—
SOUND
PHOTO-CELL
CONTROL.
PHOTO- CELL
FIG. 3. Block diagram of control- track apparatus.
150
N. LEVINSON AND L. T. GOLDSMITH [j. s. M. P. E.
is connected by a low-capacity cable ^ to a combination control-tone
amplifier and variable-gain amplifier.
Fig. 3 is a block diagram of a typical sound-reproducing system
modified for control- track operation. The heavy lines indicate the
equipment added for Vitasound. The variable-gain amplifier has
SWITCH PANEL
VOLTAGE -
AMPLIFIER
LOW- PASS FILTER
* SWITCH PANEL
CONTROL AND
VARIABLE -
GAIN
AMPLI FIER
RELAY PANEL
POWER-
AMPLIFIER
ADDED
EQUIPMENT
FIG. 4. Rack mounting for typical sound-reproducing system.
a normal zero insertion gain and is electrically connected in a 500-ohm
link circuit between stages of the voltage amplifier. A speaker relay
panel also operates from the control amplifier and closes the side horn
circuit at the output of the power amplifier. The power amplifier
must have sufficient power capacity for a 10-db increased output
when maximum control is utilized.
Aug., 1941]
VlTASOUND
151
The side horns are each equal to one-half of the screen horn sys-
tem in power-handling capacity and are of the same type so that the
same amplifier equalization serves for both horn systems. The
additional horns may be located at or near the sides of the proscenium
arch in a line with the center horns.
Fig. 4 shows how the relay panel and combined control-tone
and variable-gain amplifier are added to the existing rack of a typical
sound system, in this case an RCA PG-92. In the case of cabinet-
mounted amplifier systems, the same control-track equipment can
be furnished mounted in wall cabinets. Both units of the control-
track equipment have self-contained power supplies which are so
AMPLIFIER GAIN
CONTROL TRACK WIDTH
VARIABLE-GAIN AMP _
OR
SYSTEM GAIN
( DECIBELS)
WIDTH OF CLEAR PORTION
OF CONTROL TRACK (MILS)
90 80 7O 6O SO 4O 30 20 10
FIG. 5. Amplifier gain vs. control-track width.
regulated as to be independent of line-voltage variations from 90 to
130 volts. The variable-gain amplifier has two screwdriver adjust-
ments; one for the point of gain increase, and one for the degree of
maximum gain. A "normal-control" key serves to by-pass the
amplifier if no control is desired. The relay panel has one screw-
driver control for sensitivity. This adjustment is necessary only at
the time of installation in order that the relay may operate to cut in
the side horns at a control-track width of 40 mils or less.
Fig. 5 shows how the system gain is increased almost linearly over
a 10-db range with a 40 to zero-mil change in width of the control-
track. At present, the scanned width from 90 to 40 mils is not used.
The operating time of the control equipment is of the order of 60
152 N. LEVINSON AND L. T. GOLDSMITH [J. s. M. P. E.
milliseconds, which is fast enough to ajlow full control on effects and
music sections of short duration.
In practice the control-track is prepared on the film in the follow-
ing manner: A control-track print is made up by splicing together
prints of various widths appropriate to the degree of control desired.
These tracks are available in the re-recording department in steps of
1 db. This cut control-track print is used for the production of a
separate control-track negative, or is printed onto the undeveloped
sound-track negative in order that release prints can be made from a
separate control negative or from a composite control-souncT negative
as desired. The release composite print is then made in the normal
manner except that the sound-track printer is equipped to print
both the sound-track and control-track in one operation. There is
no change in technic or increase in operating cost in the release
laboratory in the case of composite control-sound negatives. The
greater cost involved in the separate control-track method is due to
the one additional printing operation. The cut control-track print is
returned to the sound department after the negative is made, where
it is broken down and used again to make other cut control
prints.
Release prints with control-tracks reproduce normally in theaters
where the equipment has not been modified to take advantage of the
control feature. Conversely, standard release prints without control-
tracks reproduce normally on modified theater equipment provided
the film is threaded over the control lamp to miss the control-track
attachment or that the print is clear in the area between the sprocket-
holes. In either case the control equipment is inoperative. Oil, dirt,
and scratches incurred during normal print life have no appreciable
effect upon the operation because of the relatively large scanning
aperture employed. Track misalignment in printing, and projector
weave up to 10 mils have no effect for the same reason.
The equipment has been in use experimentally in the Warner Bros.
Hollywood Theater in Hollywood, and the Strand Theater in New York
for several months and has proved to be effective, reliable, and
trouble-free in operation.
REFERENCE
1 MUELLER, W. A., "Audience Noise as a Limitation to the Permissible Volume
Range of Dialog in Sound Motion Pictures," /. Soc. Mot. Pict. Eng.,
(July, 1940), p. 48.
Aug., 1941] VlTASOUND 153
DISCUSSION
MR. REISKIND : The various methods being tried out by the industry all have
merit and the problem facing us is that of picking the method that offers the best
engineering and commercial compromise. It is essential that we remember that
the scheme adopted must not be very expensive, must be relatively simple to op-
erate and maintain, and must not require special prints which can be played only
on the new equipment.
It seems to me that the single-sound-track scheme using the sprocket-hole
control-track that I discussed in my paper and which was described in detail by
Messrs. Levinson and Goldsmith, offers the best compromise. While the three-
track system will provide greater flexibility in obtaining dramatic effects, it does
not seem that this advantage will compensate for the great increase in equipment
complexity. The three-sound-track system requires three reproducing channels,
complete from photocell to loud speakers, each including a variable-gain amplifier.
The system requires three control tones, and it is proposed to record these as a 5-
mil track in the narrow space between the sound-track and the picture. The 5-
mil track provides very low output, which will necessitate the use of additional
amplification in the control system. The three band-pass filters required to sepa-
rate the tones will further increase the cost of the system. Very extensive modi-
fication will be required in the sound-head to provide reproduction of the four
tracks.
The possibility of employing high-speed compression and expansion will provide
additional noise reduction which must first make up for the loss caused by the
reduction in track width to one-third normal and then may provide increased
volume range. However, it appears to me that such a compressed print could
not be satisfactorily reproduced on standard equipment. In the past we have
seen several instances of the impracticability of expecting the exchanges to handle
two types of prints.
I should like again to stress the factor of cost. Regardless of the improvement
obtained, expensive modification will be practicable for only the largest theaters.
This is directly contrary to the basic idea of the industry which aims to provide
essentially the same entertainment for all audiences whether they attend large
or small theaters.
MULTIPLE-SPEAKER REPRODUCING SYSTEMS FOR
MOTION PICTURES*
H. I. REISKIND**
Summary. — Several types of multiple speaker reproducing systems have been
demonstrated and used during the past two years. For general theater use such a
system must be simple and must employ a release print that is interchangeable with
standard release prints.
The use of a number of loud speaker systems spread across the front of the theater
and operating in parallel will effect a material improvement in the reproduction of
music and "sound effects.1' By providing supplementary speakers well to the sides
of the screen, operated by a control track so that they are faded out during dialog, an
improvement in music and effects reproduction is obtained without harming dialog.
The sprocket-hole area may be used for the control track, thus eliminating the
necessity of changing existing film standards or obsoleting reproducer equipments.
During the past two years a great deal of interest has grown up in
the industry with regard to the improvements in reproduction which
may be obtained by the use of multiple-speaker systems for sound
motion picture reproduction. Not only have there been demonstra-
tions of such systems1- 2- 3 but several pictures have been released for
multiple-speaker reproduction. One of these, Walt Disney's Fantasia,4
makes use of special road-show prints and reproducing equipment,
while several Warner Bros, pictures have been released as standard
type prints including a sprocket-hole control track and shown on
standard reproducing equipment modified to provide multiple-speaker
reproduction.
In general two methods are employed and all the systems make use
of either one or some combination of the two. One, the stereo-
phonic method, uses two or three channels to produce motion of the
sound source and thus allows the sound to follow the picture within
the confines of the screen and in some cases to produce "off-screen"
effects. The other method makes no attempt to provide sound
motion within the screen area. Instead, the sound source for music
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May 1,
1941.
** RCA Manufacturing Co., Indianapolis, Ind.
154
•0- The Society is not responsible for statements by authors «>
REPRODUCING SYSTEMS 155
and sound effects, which generally are not localized on the screen,
is broadened beyond the screen area by the use of multipled groups
of loud speakers.
A committee composed of members of the various Hollywood
studios has been set up under the Academy of Motion Picture Arts
and Sciences and is engaged in studying the various systems with the
view of standardizing one of them for general industry use.
With this amount of activity in the field it was felt that a discussion
of some of the aspects of multiple-speaker reproduction and of one of
the proposed systems would be of interest to the members of the
Society.
It is of course understood that everyone is interested in standard-
izing a system that will be practicable for the majority of motion
picture theaters. In order that it be generally acceptable the system
should satisfy these five requirements :
(1) It should make an improvement in the dramatic quality of the motion
picture presentation that will justify the cost of the change.
(2} The cost of the additional equipment should be low enough to make it
practicable for the smaller as well as the larger theaters.
(5) Present standards of film, picture, and sound-track dimensions should
not be changed in any way that will require modification of existing equipment
except to provide the improved reproducing characteristics.
(4) Existing theater equipment of modern types should not be rendered obso-
lete. The improved reproduction characteristics should be obtainable by addi-
tions to the installed equipment.
(5) The modified equipment must reproduce sound from the present standard
release films without any Deterioration in quality over that which would be
obtained from existing standard equipment. Release prints prepared for the im-
proved type of reproduction should be reproduced on standard equipment with
quality as good as would be obtained from a standard print.
It is possible, even within the limits of these requirements, to make
a noticeable improvement in reproduction by taking advantage of
the differences between what constitutes the most favorable condi-
tions for reproducing dialog and music. It has been recognized
almost from the earliest days of motion picture recording, that
dialog represents a recording and reproducing problem that is en-
tirely different from that presented by music, choruses, or sound-
effect scenes. We might distinguish between the two types by say-
ing that the original speech is produced by approximately a point-
source while the original source of music and the sounds of most
spectacular effect-scenes is one of large area.
156 H. I. REISKIND [j. s. M. P. E.
The motion picture technic used for dialog scenes is one that plays
almost all the action in medium or cl6se shots. In order to improve
both illusion and intelligibility we are interested in obtaining a
high degree of "presence" ; that is, we should like to get the effect of
the sound coming from just in front of the screen.
Because of the limited size of the screen the technic is one of always
bringing the action in front of the viewer rather than having him
look toward the action; and in general it can be said that there is
comparatively little motion in the scene with respect to the viewer.
It must be recognized also that even when there is motion on the
screen the angle subtended by the screen at the eyes of most of the
viewers is quite small, and consequently the viewers are seldom con-
scious of such motion. On the basis of SMPE Recommended Prac-
tice6 the screen subtends an angle of about I6l/z degrees at the eye
of an observer in the middle of the theater. Particularly in a theater
with a balcony, the angle at the majority of seats is even smaller.
Music and sound effects present an entirely different problem.
Not only are they generally produced over a large area but in most
instances the source of the music is not pictured on the screen, and
we are more interested in obtaining a spatial effect than in localizing
the source of the sound. A very similar condition applies in the
spectacular type of sound-effect scene, such as the earthquake of
San Francisco, the avalanche of Lost Horizon, or the battle in The Sea
Hawk. Here we are interested in obtaining the illusion of sound
coming from an area much greater than that pictured on the screen
and the effectiveness of the scene would be enhanced by having the
sound come from the entire front of the theater or in certain cases
even have the audience entirely surrounded by the sound source.
We are able to differentiate between the various types of scenes in
the recording operation and provide the microphone placement and
acoustic environment best suited to each scene. However, in repro-
duction it has been the practice to use the same loud speaker system
at all times. Because of the importance of the dialog in telling the
story, our speaker systems have evolved into a form that is par-
ticularly well adapted to give a high degree of intelligibility and
presence. The single set of speakers located back of the screen tends
to approach a point-source, and while this is exactly what is needed to
give maximum clarity and presence for dialog, it tends to give
music a "squeezed" effect; particularly in a large theater. The
comparison is especially striking when it is made between an_actual
Aug., 1941] REPRODUCING SYSTEMS 157
orchestra and music reproduced through a single speaker system in
the same theater.
Methods have been developed for overcoming this "squeezed"
effect by the use of multiple sound-tracks and reproducing channels.
The possibilities of auditory perspective have been demonstrated by
the Bell Telephone Laboratories, on several occasions. L 3
Another method of attacking the problem is based upon the idea
that exact imitation of the original may not be our goal in the re-
production of music. Many persons, including musicians, feel that
the sound from a real orchestra may not be the ultimate in impres-
siveness. The belief has been expressed that a better effect might be
obtained from an orchestra if the violins, for example, instead of
being seated in a group, were intermingled with the other instru-
ments. In some ways reverberation produces a little of this effect
in that it brings the sound to the listener from many directions and
thus reduces the effect of definite location. It is accepted that a
large amount of reverberation is necessary to make music pleasing.
However, intermingling the instruments is impracticable from the
players' standpoint, since it is largely because of the grouping that
each section of instruments is able to play together, both as to tempo
and pitch. Such an arrangement, however, can be accomplished in
a reproducing system by several groups of speakers in multiple spread
across the front of the theater. The effectiveness of this method of
reproducing music was demonstrated in 1937 in the RCA sound re-
producing system installed for the production The Eternal Road,
where individual sound-tracks, reproducing channels, and speaker
systems were employed for the orchestra, choruses, and soloists.4
The individual solo and chorus channels allowed speaker placements
giving the desired illusion of location. The orchestra music, which
had been recorded on a single sound-track, was reproduced through
a number of loud speaker systems spread across the front of the
theater. This use of multipled speaker systems provided a large
sound-emitting area more nearly approximating the original source
than did the single-speaker system. This system did not localize
the position of each instrument, but added a "spread" or spatial
effect and gave the impression that the music actually filled the
auditorium rather than that it came from a definite source at the
center of the stage.
The effectiveness ot the multiple-speaker system for music and
effects has also been demonstrated by the sound-reinforcing system
158 H. I. REISKIND [j. s. M. P. E.
installed in the Radio City Music Hall. This system consists of
three individual amplifier channels feeding three banks of speakers.
One bank is located to the right, one to the left, and one above the
center of the stage, and the system is arranged so that the three
channels may be used individually or in parallel. Comparative tests
almost six years ago convinced both the Music and Sound depart-
ments of the Radio City Music Hall that their multiple-speaker
method gave more effective and pleasing reinforcement and more
nearly simulated the effect of a large orchestra playing in a large
auditorium than the use of three separate discrete channels.
The "Fantasound" system of reproduction is an example of the
possibilities of combining both stereophonic reproduction and the
principle of extending fie source. Both methods are used in this
picture, depending up- \ the effect desired. The "Ave Maria"
number, which many c nsider the most impressive part of the per-
formance, is an examj of the results that may be obtained with
multiple-speaker reprq otion. For this selection a large number
of speakers were installec ajong the sides and back of the theater and
multiplied to the corresj ending set of side speakers on the stage.
In this way the sound irom each side sound-track was reproduced
along the entire corresponding side of the house rather than from the
stage alone.4
Another example of the effectiveness of surrounding the audience
by the sound source is the reproducing equipment installed for the
RCA large-screen television demonstration at the New Yorker Thea-
ter. Loud speakers in multiple, located on all the walls and on the
ceiling, as well as on the stage, materially improved the sound illu-
sion, and in one scene were used to make the audience teel that they
were actually being subjected to a bombing attack.
The improvement in music and effect reproduction obtainable
through the use of multipled groups of speakers, and the development
by C. M. Burrill of a method of using the sprocket-hole area of the
film to provide a control signal, led M. C. Batsel to propose that a
commercially practicable multiple-speaker reproducing system be
developed for motion picture theaters.
This could be done by equipping theaters with additional speakers
located well to the sides of the screen and arranging the control
equipment so that these supplementary speakers would operate in
parallel with the screen speakers during all music and effect sequences,
but be off during dialog. This arrangement would provide a spatial
Aug., 1941]
REPRODUCING SYSTEMS
159
effect or "acoustic spread" for the music and effects reproduction and
still maintain the intelligibility and "presence" of the dialog, since
dialog will be reproduced exactly as at present.
In addition to "acoustic spread," consideration was given to the
desirability of providing increased volume range. It was recently
pointed out by W. A. Mueller,7 that the permissible volume range of
reproduced dialog is limited by theater and audience noise at one
end, and at the other by the maximum loudness to which the audience
can comfortably listen. This range is less than the volume range of
SIDE
SPEAKERS
)
:K
sc
SPE
]
S
SPE
>— <
DL
<
- n
1
SIDE
FIG. 1. Simplified block diagram
of a multiple-speaker reproducing
system.
50% 1002
CONTROL TONE AMPLITUDE
FIG. 2. Output characteristics
of loud speaker groups in a mul-
tiple-speaker system.
existing film recording methods, and it was therefore not considered
necessary to have any volume control for dialog scenes.
Mr. Mueller pointed out also that with standard reproduction,
audiences generally object to music reproduced at levels much higher
than those used for dialog. However, tests made of music reproduc-
tion with acoustic spread indicated that higher levels could be used
without discomfort and with consequent improvement in the ef-
fectiveness of the music.
The spectacular type of effect sequences, hurricanes, battles, and
160
H. I. REISKIND
[J. S. M. P. E.
so forth, which of course call for increased reproducer gain, are also
improved by acoustic spread.
Since it appears that all those sequences that may require in-
creased reproducer volume are also benefited by acoustic spread, it
was decided that the system would be arranged so that as the control
tone was increased it would first provide acoustic spread by fading
in the supplementary side speakers and then control the volume of
the entire system.
a b c
(Courtesy Warner Bros. Pictures, Inc.)
FIG. 3. Composite release print with sprocket-hole control track:
(a) Minimum modulation, (b) Intermediate modulation, (c} Maxi-
mum modulation.
An elementary block diagram of such a system is shown in Fig. 1.
The control circuits are designed so that with the minimum control
signal, unit / has a gain that is less than its maximum gain, and unit
71 is off. This represents the dialog reproducing condition (screen
speaker operating, side speakers off). For music or effect reproduc-
tion at normal levels, the control signal amplitude may be increased
to about 50 per cent, operating unit // and turning on the side
speakers. Any further increase in control tone amplitude has no
effect upon the gain of unit //. Unit / is designed so that its gain
Aug., 1941]
REPRODUCING SYSTEMS
161
(which represents the overall system gain) is unchanged by the in-
crease of the control signal to 50 per cent, but a further increase
(from 50 to 100 per cent) will increase its gain, and thus increase the
loudness of both screen and side speakers. The relation between
speaker outputs and control tone amplitude is shown in Fig. 2.
This system requires a single-frequency control tone variable only
in amplitude. Such a tone might be recorded on the portion of the
film outside the sprocket-holes, between the sound-track and the
picture, or standards could be changed and a portion of the sound-
track area utilized. However, the proposal ot C. M. Burrill to use
FIG. 4. Sprocket-hole control track scanning system.
the sprocket-hole area appears to be the most practicable since it
requires no changes in existing standards. Such a track can be re-
corded and printed very easily, and can be reproduced by a very
simple and inexpensive attachment to the sound-head.
When the sprocket-hole area is scanned, a 96-cycle tone is generated.
The positive half-wave will have an amplitude dependent upon the
amount of light passing through the hole, while the negative half-
wave amplitude will be determined by the light passing through the
"lands" (the spaces between the sprocket-holes). Accordingly, the
96-cycle control tone may be varied in amplitude simply by changing
the transmission of the "lands." Maximum signal is obtained when
162 H. I. REISKIND [j. S. M. P. E.
the "lands" are all black, and minimum when they are clear. Fig. 3
is a photograph of three portions of a composite release prints show-
ing the track for minimum, maximum, and intermediate values of
control tone.
Since the track occupies the entire width of the sprocket-hole area
and the frequency to be reproduced is low, a very large aperture may
be used. This makes possible the very simple scanning system shown
in Fig. 4. The lamp is rated at 6.5 volts, 0.43 ampere, and because
of the large aperture, furnishes more than sufficient light without the
use of any lenses. The signal output of this simple scanning system
is higher than that from a normal sound-track scanned by a standard
FIG. 5. Sprocket-hole control track scanning system mounted in a
standard sound-head.
optical system, thus allowing the use of a reasonably low-gain
amplifier.
A further advantage of this track lies in the large tolerances allow-
able in recording, processing, and reproducing. It will be noted that
it has not been necessary to provide any lateral guide adjustment of
the scanning assembly. Fig. 5 shows the system mounted in a stand-
ard sound-head. The unit mounts around the hold-back sprocket
and requires practically no modification to the sound-head.
In any reproducing system it is desirable that the signal amplitude
be independent of exciter lamp output or photocell sensitivity. For-
tunately this result can be obtained in the reproduction of the con-
Aug., 1941] REPRODUCING SYSTEMS 163
trol tone by a very simple method which makes use of the logarithmic
relation existing between the grid current and the plate voltage of
any vacuum tube.8 With a circuit using this characteristic it is pos-
sible to vary the exciter lamp intensity by more than 5 to 1 with a
change in output of less than 1.5 db. With a linear amplifier, this
same variation in exciter lamp intensity would produce a change in
output of over 14 db.
The system described above meets all the requirements laid down
at the beginning of this discussion. By requiring only that all prints
not recorded for control track reproduction have a clear sprocket-
hole area, complete interchangeability of prints is obtained and it will
not be necessary for exchanges to carry two types of prints for any
picture. In addition, the system is simple; any modern system can
be modified to provide multiple-speaker reproduction, and the im-
provement obtained is a real one.
Thanks and credit are due to Messrs. M. C. Batsel, C. M. Burrill,
and A. R. Morgan for the ideas and original work upon which this
system is based; to Messrs. J. L. Underbill, R. Bierwirth, L. Biber-
man, and J. Lehman for their help in the design and construction of
the original equipment; to Messrs. E. W. Kellogg and J. E. Volkman
for their assistance and ideas throughout the work; and to Warner
Bros. Pictures, and the Hollywood staff of RCA Manufacturing
Company, Inc., for the field testing of the system.
REFERENCES
1 MAXFIELD, J. P.: "Demonstration of Stereophonic Recording with Motion
Pictures/' /. Soc. Mot. Pict. Eng., XXX (Feb., 1938), No. 2, p. 131.
2 OFFENHAUSER, W. H., JR., AND ISRAEL, J. J.: "Some Production Aspects
of Binaural Recording for Sound Motion Pictures," /. Soc. Mot. Pict. Eng.,
XXXII (Feb., 1939), No. 2, p. 139.
3 FLETCHER, HARVEY: "Stereophonic Reproduction from Film," Bell Labora-
tories Record, XVIII (May, 1940), No. 9, p. 260.
4 KPWALSKI, R. J.: "RCA's 'Fantasound' System as Used for Disney's
'Fantasia/ " Internal. Project. (Nov., 1940), p. 20.
5 "Revision of SMPE Standards," /. Soc. Mot. Pict. Eng., XXX (March, 1938),
No. 3, p. 249.
6 DIMMICK, G. L.: "The Eternal Road," Electronics, 10 (April, 1937), No. 4,
p. 28.
7 MUELLER, W. A.: "Audience Noise as a Limitation to the Permissible
Volume Range of Dialog in Sound Motion Pictures," /. Soc. Mot. Pict. Eng.,
XXXV (July, 1940), p. 48.
8 PAYNE, E. L., AND STORY, J. G.: "A Portable Programme Meter," The
Wireless Engineer, XII (Nov., 1935), No. 146, p. 588.
SOME THEORETICAL CONSIDERATIONS IN THE DESIGN
OF SPROCKETS FOR CONTINUOUS FILM MOVEMENT*
J. S. CHANDLER**
Summary. — After a brief introduction the paper gives a discussion of the steps
of sprocket design with the ultimate aim of keeping the flutter to a minimum.
Firs4 the selection of the proper sprocket-tooth pitch is considered, then the steps
required in arriving at the proper basic tooth profile, and finally the modified tooth
profile are illustrated by an example.
Curves of theoretical flutter versus per cent of film shrinkage are given for several
cases for a 24-tooth sprocket. The effect of number of teeth is also shown by curves.
An analysis of film and friction forces gives a clue to proper film guide design.
A word about sprocket-tooth shapers and results obtained from an experimental
sprocket conclude the paper.
The degree of accuracy and the directness of the method, as well as the resulting opti-
mum performance, are noteworthy.
This paper gives an analysis of the mechanism of film engagement
with sprocket- teeth, and proposes a method of determining the cor-
rect tooth profile for a given set-up.
In order to avoid confusion the convention represented by Fig. 1
will be adopted. An external net force, F, is exerted on the film to-
ward the left. This is balanced by the force of the sprocket-teeth
against the film. The direction of rotation is counter-clockwise.
In other words, the sprocket under consideration is a "hold-back"
sprocket. (The analysis will hold equally well for a "drive" sprocket
by reversing the direction of rotation.) The film comes in contact
with the base circle, B, of the sprocket at the point of tangency, P.
Only the right-hand faces of the teeth come in contact with the film.
FILM PATH
As shown by Fig. 1 the lower surface of the film travels along path
AP during tooth engagement. This may be any suitable curve,
either convex upward or downward, or it may be a straight line.
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May
12, 1941. Communication No. 806 from the Kodak Research Laboratories.
** Eastman Kodak Co., Rochester, N. Y.
164
^ The Society is not responsible for statements by authors •$•
CONSIDERATION IN SPROCKETS DESIGN 165
The film path, in general, is fixed by the design of film shoe, stripper,
roller, or other means, and in some cases may be determined wholly
or in part by the film tension and stiffness. In order to simplify the
analysis and the shoe construction, the film path, AP, is taken to be
the arc of a circle (or a straight line) tangent to the base circle at P.
The sprocket-base circle is a convenient reference circle and may not
exist on the actual finished sprocket if other means of film support are
provided.
The film may remain in contact with the base circle any arbitrary
distance, say, PC, and then disengage from the teeth along any suit-
able path similar to or different from AP. The special case of PC
= 0, as well as the general case, will be treated later in the paper.
In general, only one sprocket-tooth will touch the film at any given
time except at the transition instant when two teeth will touch.
SPROCKET TOOTH
FIG. 1. Sketch of film and sprocket.
FLUTTER
The aim of the designer is to produce a sprocket that will move the
film along as steadily as possible. Unsteady movement will produce
density variations in the case of picture printers or "flutter" in the
case of sound apparatus. For purposes of this paper, flutter will be
considered as a measure of the unsteadiness and will be defined as
the ratio between the unsteady film velocity component and the
steady velocity component. If the film speed is 1 per cent higher
than the average at a given instant, the flutter is 1 per cent at that
instant. The flutter frequency will be that of the sprocket teeth, 24
cycles per second for 16-mm film and 96 cycles per second for 35-mm
film. As a rough basis of comparison, the time average of the in-
stantaneous flutter may be considered but is not to be taken as a
measure of the subjective effect.
The determination of flutter is facilitated if we consider only ve-
locities relative to the base circle of the sprocket, which is assumed to
rotate at constant velocity. For this reason we will consider that
166
J. S. CHANDLER
[J. S. M. P. E.
the sprocket is stationary while the ; film moves around, winding on
at the right and off at the left.
SPROCKET-TOOTH PITCH
The sprocket-tooth pitch is defined as the distance measured along
the base circle between corresponding points on consecutive teeth.
The sprocket must be designed to accommodate a certain range of
film pitch caused by shrinkage or stretch after the perforating opera-
tion.
Let us consider two cases: In Case /, the tooth pitch equals the
maximum film pitch expected. The tooth shape can be so designed
that the film of minimum expected pitch first begins tooth contact
at point A (Fig. 1). Thus, the entire path from A to P is available
PERCENT AVERAGE FLUTTER
0 o P C
0 M Ifr <J> 6
\
%
\
\
\
V
f
^* """
\
0.6 0.8
PER CENT SHRINKAGE
1.0
FlG. 2.
Flutter vs. Shrinkage for two cases of
a 24-tooth sprocket.
for accommodating the pitch range. The teeth act to slide the film
to the right over the base circle.
In Case II the tooth pitch is made equal to the minimum film
pitch expected. The film of more than minimum pitch is permitted
to slide to the left over the base circle as the perforations leave the
teeth to the left of point C (Fig. 1). Because of the abrupt manner
in which the film leaves the teeth, however, the pitch range must be
accommodated during a relatively short time.
Fig. 2 gives the average theoretical flutter as a function of film
shrinkage for a 16-mm, 24-tooth sprocket accommodating 1 per cent
of shrinkage. Optimum tooth design is assumed in each case. Not
only is the maximum average flutter 3.85 times greater for Case //,
Aug., 1941] CONSIDERATIONS IN SPROCKET DESIGN 167
as the curves show, but owing to sharper peaks in Case //, the maxi-
mum instantaneous flutters are in the ratio of 5.20 to 1. Further-
more, the design of Case // will not work well as a drive sprocket
(by reversal of rotation), since the film will tend to ride off or produce
excessive friction against the guides.
It will be noted that the sprocket of Case / will serve as either a
"hold-back" or a "drive" sprocket. If used as a "hold-back"
sprocket, a guide above the film or additional balanced film tension
is required to move the film down the tooth face. This may be
avoided by using Case // for "hold-back" sprockets, but more un-
steadiness of film movement results.
Hence, for flutter considerations, most, if not all, of the pitch range
should be accommodated by the method of Case /. The tooth
pitch should be equal to or just a little less than the maximum film
pitch contemplated.
The first step in the sprocket design is to determine the base circle
radius that will make the sprocket pitch equal to maximum film
pitch for the number of teeth desired. The film thickness must be
taken into consideration.
BASIC TOOTH SHAPE
For the second step in the design it is necessary to find the curve
generated by a point on the film path when this path is rolled with-
out slipping on the base circle. This is the well known epicycloid
(or involute, in case the film path is a straight line). It is recom-
mended that this curve be plotted from the parametric equations
given in Appendix / for the three possible cases. The graphical
method requires an awkwardly large scale to obtain sufficient ac-
curacy.
The parameter in each case is the angle rolled through on the base
circle. When angle 0 is multiplied by a suitable constant, it gives
the time required for the film to travel over the path from point
(x, y) to P (Figs. 3, 4, and 5).
It is evident that one degree of freedom at the designer's command
is the radius of curvature of the film path. In plotting the above
curves it will be discovered that the second case of Appendix / gives
the longest time of film engagement for a given tooth height. This is
desirable in reducing flutter.
The value of a = 0.5c for this. case has been found to be a good
basic assumption. A much larger value reduces the engagement
168
J. S. CHANDLER
U. S. M. P. E.
time while a much smaller value causes the tooth to "lie down" and
gives an impracticable shape. Appendix II gives values of x and y
from equations 3 and 4 of Appendix I for c = 1 and a = 0.5.
The epicycloid curve gives the path traveled by a point on the
OF FILM
EPICYCLOID
FIG. 3. Generation of epicycloid, Case I.
Y
S
PATH OF FILM
EPICYCLOID
FIG. 4. Generation of epicycloid, Case //.
<a$£-
cg
PATH OF FILM
INVOLUTE
FIG. 5. Generation of involute.
film (such as the edge of a perforation) relative to the sprocket if the
film does not slip over the base circle. But, as we have seen, all film
but that of one particular pitch must slip over the base circle. The
tooth shape, therefore, can not be that of the generated epicycloid
but must be suitably modified to produce the desired slipping.
Aug., 1941]
CONSIDERATIONS IN SPROCKET DESIGN
MODIFIED TOOTH SHAPE
The designer may exercise his ingenuity in modifying the tooth
shape ; one film pitch may be favored more than the others if he so
desires.
In order to illustrate the entire design procedure, as well as to give
one method of modifying the tooth form, the following example will
be used: Let it be desired to design a 24-tooth sprocket for 16-mm
film, the film pitch to vary from 0.300 inch to 0.297 inch, or from 0
per cent to 1 per cent shrinkage. (The shrinkage range will vary
1.20
R= 0.2000
CENTER 0.0212
BELOW BASE CIRC
0.04
1.14
FIG. 6. Tooth layout.
in practice according to equipment and film.)
Step I. Radius of Base Circle.— Ej aUowing 0.006-inch film thick-
ness and matching to maximum film pitch, the base circle radius, c,
0.300(24)
comes out as
27T
- 0.003 = 1.143 inches.
Step II. Basic Tooth Shape.— By multiplying the coordinates
given in Appendix // by 1.143 and plotting them, we obtain the
epicycloid curve AB of Fig. 6 as the basic tooth shape. The values
of 0 are marked on the curve. A scale of about 100:1 or larger may
170 J. S. CHANDLER [j. s. M. P. E.
conveniently be used. The picture is more complete if the base circle
is drawn in from equation 7 or from equations 8 and 9.
x* + y* = c* (7)
x = c(cos 0) (8}
y = c(sin 0) (9)
The film may also be sketched in in various positions, noting that
it makes the angle <£ with the Faxis, where
0 = ad/(a + c) (10}
Step III. Modification of Tooth Shape. — First, the permissible
working height of the tooth is determined by noting its base width
and the general shape. The tooth width is dependent upon the
number of teeth in "mesh." For this example the width at base is
taken as 0.034 inch and the working height as 0.040 inch.
This tooth height is found to correspond to 0 = 26.7 degrees by
reading from a curve of x plotted against 6 (not shown). In other
words, we have available 26.7 degrees of rotation in which to take
care of films having from 0 per cent to 1 per cent shrinkage.
We must now prepare a schedule of deductions, D, or distances
which the basic tooth shape must be moved to the left, as a function
of e.
The following procedure is recommended but is not the only one
which may be followed. Let the curve of D plotted against 0 take
the form of a perabola with its nose at the origin, or
D = Ae* (11)
The value of A is determined by the fact that the distance measured
along the film path between the tip of one tooth and the next tooth,
which is 15 degrees to the left, must equal the minimum film pitch or
0.297 inch. In other words, D at 26.7 degrees is 0.003 inch more than
D at 11.7 degrees. This places the value of A at 5.21( 10~6). Fig.
7 gives the schedule of deductions thus determined.
These deductions are laid off along the film to the left of curve AB
to determine curve AB' of Fig. 6.
The left-hand face of the tooth should give uniform film clearance
along its length. This is dependent upon the film-disengaging path.
The tooth of Fig. 6 is symmetrical.
Step IV. Radius of Tooth face. — The design of the sprocket has
been completed in the previous three steps. However, since in
Aug., 1941]
CONSIDERATIONS IN SPROCKET DESIGN
171
actual construction the tooth profile must conform to the arc of a
circle, the designer must establish the radius of this arc and its center
location. This may be done either graphically from Fig. 6 or an-
alytically. An arc can be found which fits the curve AB' remarkably
well. Any deviation of the arc from the curve simply means that a
slightly different schedule of deductions is enforced. The perform-
ance of the sprocket is but slightly altered. For the example at
hand, a radius of 0.2000 inch with a center located 0.0212 inch below
the base circle was found suitable.
It may be desirable in some cases to specify the radius of the gen-
erating circle for the involute that best approximates the desired
tooth profile. This can be done by trial and error and, for our ex-
ample, this radius is 1.118 inches. The involute approximation is
0.004
Q003
- 0.002
0.001
12 16
6, DEGREES
Ficr7. Schedule of deductions.
not as good as the circular arc approximation because the radius of
curvature of the involute is about 80 per cent too great.
FLUTTER DETERMINATIONS
It is of considerable interest to investigate the flutter that might
theoretically be expected from the sprocket as a function of the film
shrinkage.
If the curve of Fig. 7 is replotted on a time base, remembering that
15 degrees is equivalent to l/u second, we have a displacement curve.
The slope of the displacement curve gives the velocity. Determined
mathematically we find that velocity
v = 1.350 /,
(12)
where t = time in seconds.
172
J. S. CHANDLER
[J. S. M. P. E.
Film of any given shrinkage is under the influence of one tooth for
y24 second when the next perforation engages the next tooth and the
same cycle of velocities is repeated. If we substitute t = l/u in
equation 12, we obtain v = 0.0563 inch per second, one-half of which
is 0.0281 inch per second. Therefore, the film may be considered as
having an unsteady velocity component which varies linearly from
plus 0.0281 inch per second to minus 0.0281 inch per second every x/24
second. The arithmetical time average of such a component is
0.01405 inch per second. Since the steady component of film veloc-
ity is 7.2 inches per second, the average flutter is 0.195 per cent.
0.4
0.3
O.I
8 TEETH
24
40
64
0.2 0.4 0.6 0.8
PER CENT SHRINKAGE
1.0
FIG. 8. Flutter vs. Shrinkage for sprockets of
different numbers of teeth.
From Fig. 7, it will be seen that 6 = 15 degrees the deduction
D is 0.00118 inch, which corresponds to a shrinkage of 0.394 per
cent. Film of this shrinkage will rest against the teeth during rota-
tion from B = 15 degrees to 6 = 0 degrees. For film of 0.394 per cent
to 1 per cent shrinkage the flutter will be 0.195 per cent as calculated
above and as shown by Fig. 8. However, for film from 0 per cent to
0.394 per cent shrinkage, the tooth contact is carried to the left of
point P (Fig. 1) for part of the cycle. If PC (Fig. 1) is equal to or
greater than the film pitch, the velocity during this part of the cycle
is zero relative to the base circle. This determines the portion of the
flutter curve as shown from A to B in Fig. 8. If PC (Fig. 1) = 0,
the film begins to move up the tooth for the part of its cycle to the
Lug., 1941] CONSIDERATIONS IN SPROCKET DESIGN 173
left of P. This permits the film to slide to the left over the base circle
during this time; for the rest of the cycle it slides to the right under
the action of the tooth to the right of P. Thus, the flutter can not
become zero but is that shown by line AC of Fig. 8. The maximum
instantaneous flutter is no longer just twice the average flutter but is
higher. The sprocket may be designed for a greater range of shrink-
age so that this portion of the curve is avoided if so desired.
EFFECT OF NUMBER OF TEETH
Fig. 8 gives the "theoretical flutter" curves for sprockets of 8, 12,
40, and 64 teeth as well as the 24-tooth sprocket. These curves were
SHOE FRICTION
FORCE- 0.120 F
FORCE OF SHOE
AGAINST FIL.M«0.480
SPROCKET TOOTH
A
TAN."0.2&/
FORCE OF TOOTH
AGAmSTF.LMxO.995F ^/ \Q^Qf
TOOTH FRICTION
FORCE «0 248 F
^
FIG. 9b
FIG. 9. Tooth and shoe friction force diagram.
calculated for PC greater than the film pitch, for a working tooth
height of 0.040 inch, and for a shoe radius =1.5 times the base circle
radius. The curves apply to 16-mm film only. In some cases a
better design might result by reducing the shoe radius to give more
angle of contact. With more teeth in mesh the working height might
have to be reduced because of narrower teeth.
An analysis of the curves shows that the maximum average flutter
varies nearly inversely as the 0.7 power of the number of teeth. In
other words, increasing the number of teeth for a small sprocket gives
more improvement than for a large sprocket.
174 J. S. CHANDLER [j. s. M. P. E.
FRICTION FORCES
The term "theoretical flutter" was used in the preceding section
because it is recognized that other sources of flutter exist and the
calculated values may not be attained in practice. Two additional
sources of flutter will be considered here: (1) friction of film against
tooth and (2) friction of film against guide shoe. Fig. 9 (a) shows a
portion of the film as it is just beginning tooth engagement at the
top of the tooth of Fig. 6. If we assume a coefficient of friction of
0.25, the forces of tooth against film and of shoe against film make an
angle of tan"1 0.25 with their respective normals. The magnitudes
of these forces relative to the film force, F, are determined by the
force triangle of Fig. 9(6).
If such a force analysis is made for the different film positions it
will be found that the tooth force remains substantially equal to F,
while the shoe force varies from 0.480 .F to 0.4347^. As the tooth face
slopes more to the left with a decrease in the ratio of a to c (Appendix
/), the film shoe force increases.
The force analysis also shows that the film guide must be above the
film to hold it down. If the sprocket is to act as a drive sprocket,
the tooth and shoe forces fall on the other side of their respective
normals, and the guide must be below the film to support it.
In calculating the force of the film against the shoe, it is well to
note that there is an additional radial force toward the center owing
to the curvature of the film path. Neglecting the stiffness of the
film this radial force = F/r per unit length of film, where F is the
film tension and r is the radius of curvature. For the 24-tooth
sprocket example, the radial force = 0.17 oF for a film length equal
to the pitch.
It is the varying nature of friction which imposes a varying load
on the sprocket or causes the film to proceed by jerks and thus intro-
duces flutter.
SPROCKET-TOOTH SHAPER
The usual method of hobbing sprocket teeth leaves tool marks
across the face of the tooth. This may cause the film to catch as it
moves up or down the tooth. For some experimental work, we have
used a tooth shaper in which the cutting stroke is downward from the
top of the tooth. With this machine a very smooth tooth surface
can be obtained, and the remaining tool marks offer the least resis-
tance to film movement.
Aug., 1941] CONSIDERATIONS IN SPROCKET DESIGN
CONCLUSIONS
175
An 8-tooth experimental 16-mm sprocket was designed according
to the procedure just described. It was possible to obtain consis-
tently a flutter meter reading as low as 0.7 per cent for a film of 0.5
per cent shrinkage. The flutter meter was calibrated to read ir/2
times the average flutter. The average flutter, therefore, was 0.45
per cent, which is in reasonable agreement with Fig. 8.
It is hoped that the method will be more thoroughly tested by ex-
perimentation and that it will prove helpful in tackling the problem
of sprocket design.
APPENDIX I
Case I. Film path is an arc or circle convex downward (see Fig. 3).
x = (a + c) cos 6 — a cos I ( 1 +
y = (a + c} sin 0 — a sinl ( 1 + M0J
Case II. Film path is an arc of circle convex upward (see Fig. 4).
a8
x = (a + c) cos — - -- a cos 6
a + c
(2}
(3)
(a -f- c] sin
a -\- c
— a sin 6
Case III. Film path is a straight line (see Fig. 5).
x = c(cos 6 + rad 6 sin 6}
y = c(sin 6 — rad 8 cos 6)
where rad 6 = 6 measured in radians.
(5)
APPENDIX II
0, deg.
x
y
i
0
6
.00183
.00008
1.
0
12
00726
. 00068
1
0
18
.01625
.00229
1.
0,
24
02863
00539
30
1.04420
0.01048
36
1.06272
0.01797
0,deg
x
y
i
0
42
.08388
.02831
1
0
48
. 10732
.04189
1
0
54
. 13269
.05902
1,
0
60
15954
.08001
176 J. S. CHANDLER
DISCUSSION
MR. FRIEDL, JR.: Is there any established figure for the optimum shrinkage
for 16 and 35-mm positive prints as circulated for general use?
MR. CHANDLER : I do not know about the shrinkage to be expected from 16 and
35-mm film. There will undoubtedly exist individual cases of extreme shrinkage.
I believe that the sprocket should be designed for the great bulk of the cases falling
within a moderate shrinkage range, reserving a small portion of the tooth height
to be rounded off so that the extreme cases can be handled without regard to the
flutter, in these cases.
DR. CARVER : The average shrinkage of 35-mm cellulose nitrate film is about
0.3 per cent, of 16-mm cellulose acetate film about 0.6 per cent.
MR. FRIEDL, JR. : In 35-mm sound-film reproduction we are not very conscious
of flutter, but sprockets are not used today to pull film past the sound-gate.
Various devices using damped rotary inertia gates like the Rotary Stabilizer have
made the matching of the perforation pitch and tooth pitch less critical.
MR. KELLOGG: I am surprised at the small amount of flutter shown on the
curves, especially in cases where large numbers of teeth were used.
MR. CHANDLER: The flutter curves of Fig. 8 represent the unsteady velocity
of the film owing to the action of the teeth against it. The film has a certain veloc-
ity of slip relative to the base circle. This velocity of slip varies between two ex-
treme values in a definite periodic manner, thus determining the flutter. The
velocity of slip may become zero for a portion of the cycle for films of certain
shrinkage. For the example of the 24-tooth sprocket, the velocity of slip over the
the base circle is zero for a portion of each cycle for films of less than 0.394 per cent
shrinkage (curve AB, Fig. 8), while for films of more than 0.394 per cent shrinkage
the velocity of slip never becomes zero.
A METHOD FOR DESIGNING FILM SPROCKETS
W. G. HILL AND C. L. SCHAEFER**
Summary. — A method is described for determining the sprocket -tooth pitch and
consequently the base diameter, the tooth profile shape and tooth dimensions of film
moving sprockets together with the tooth location transverse to the direction of film travel.
The method assumes that the film dimensions are known or can be determined. Com-
putations are given for a 35-mm 32-tooth sprocket and data showing the allowable film
stretch or shrink for various numbers of teeth in mesh.
Film moving sprockets can be classified into three groups accord-
ing to their use and extent of film shrinkage which they must accom-
modate.
(7) Sprockets where little or no shrinkage is encountered. Associated with
this group are: film manufacturing machines, such as perforators, examining
machines, etc.
(II) Sprockets for use with aged undeveloped film, where moderate shrinkage
must be accommodated: cameras, printers, recorders, etc.
(IIP) Sprockets for use with developed film, where considerable shrinkage may
be present: printers, projectors, etc.
For any particular group the main considerations in sprocket de-
sign are: the pitch or distance between teeth, which of course de-
termines the base diameter of the sprocket wheel; the tooth shape;
and the distance between the rows of teeth. Dimensions referring
to relieved areas for pictures and sound-track, also reference to flanges
for guiding the film edges, should not in our opinion be included in
the design method. These points are not related to sprockets in
general, but apply to special cases. Furthermore, factors that de-
termine such dimensions are not definite, and the introduction of a
new process might result in entirely different requirements. It is
our belief that such construction details should be left to the discre-
tion of the designer.
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received April
14, 1941.
** Agfa Ansco, Binghamton, N. Y.
177
Society is not responsible for statements by authors $
178
W. G. HILL AND C. L. SCHAEFER
[J. S. M. P. E.
Since the perforating standards for film are well established and
there is reason to believe that no changes will be made in the principal
dimensions, we have expressed the base diameter for the sprocket in
terms of film pitch. It Will be recognized, then, that as the shrinkage
characteristics of motion picture film change, and they no doubt will
through improvements in manufacturing, the method of sprocket de-
sign is not altered. Another point that should be mentioned is that
for any motion picture film, for instance 16-mm, classified in group
/, and a sprocket of a definite number of teeth and with anywhere
from 1 to 10 teeth in mesh, the same sprocket is used. This means
that the same cutter can be used in forming the teeth of such sprock-
py-r.
FIG. 1. Hold-back sprockets
B, base sprocket dia. - (Dl ~ G}N
- F
3.1416
DI, film perforation pitch = D — DS
D, nominal pitch of freshly perforated film
S, shrinkage
G, clearance of second tooth in mesh
N, number of teeth on sprocket
F, film thickness
F, number of teeth in mesh minus one
P, sprocket-tooth pitch
ets. American Sprocket Standard Z22.6-1941 for 16-mm film calls
for different tooth thicknesses for each different number of teeth in
mesh, which means that different cutters must be used.
Fig. 1 represents a hold-back or take-up sprocket; the sprocket is
shown rotating counter-clockwise, the film feeding from a loop onto
the sprocket at the right, and under tension as it leaves engagement
at the left. It will be noticed that the clearance G has been so chosen
that the tooth pitch is less than the film perforation pitch. This is
because for best operation it is deemed advisable to relate the film
and sprocket pitch in such a manner that the film is always free to
pass onto the sprocket teeth and have the disengaging tooth carry
the load.
Aug., 1941]
DESIGNING FILM SPROCKETS
179
DI is the perforation pitch of the film that is to operate on the
sprocket and is equal to D, the nominal pitch of freshly perforated
film minus D X 5, where S represents film shrinkage. Assuming
that the film bends on a neutral plane midway between the base and
emulsion surfaces, the tooth pitch P at this plane then equals (Di—G).
If N represents the number of teeth on the sprocket and F is the film
thickness, then the sprocket base diameter as shown equals (Di — G)N/
TT — F. The actual value of G should, of course, be determined by the
use for which the sprocket is intended but should be as small as
practicable to insure that the film will run smoothly over the sprocket
and reduce what is known as "sprocket-tooth frequency." For 10
or less teeth in mesh we have taken G equal to 0.001 inch and for 11
NO. OT TEETUINHCSU
FIG. 2. 35-mm hold-back sprocket.
to 20 teeth, 0.0008 inch. The tooth thickness T is also reduced for
more than 10 teeth in mesh.
Although in general practice 20 teeth or more in mesh will seldom
be used, some means of design for such cases should be considered.
It will be recognized that for large numbers of teeth in mesh, the
wide limits of shrink and stretch will not be permissible. This is
true because the thickness of teeth T, for which there must be a
practical minimum, has a direct bearing on the allowable film stretch
or shrinkage. In view of these facts, we have, for these cases of more
than 20 teeth in mesh, set what we believe is a minimum tooth thick-
ness of W- 0.028 inch by fixing C as 0.020 inch and E as 0.008 inch,
and allowed G to vary, G being equal to the constant C divided by
180
W. G. HILL AND C. L. SCHAEFER
LT. S. M. P. E.
Y. This means, for instance, that for, 16-mm sprockets with more
than 20 teeth in mesh, the tooth thickness becomes 0.022 inch. The
value of G, determined by the number of teeth in mesh, can then be
substituted in the equation and the base diameter determined.
If now for the moment, the film as shown on the sprocket is as-
sumed to be freshly perforated stock, the amount of film shrinkage
accommodated is the ratio of D— P to D and the film stretch accom-
modated is equal to the quantity (D Y— W) — (P Y— T) divided by
(D Y— W), where Y equals one minus the number of teeth in mesh.
Fig. 2 is a graphic representation for allowable film shrinkage vs.
teeth in mesh for 35-mm hold-back sprockets. For 2 to 10 teeth in
FIG. 3. 35-mm hold-back sprocket designed for 0.70%
shrinkage.
mesh G = 0.001 inch and the tooth thickness T = 0.055 inch; for
11 to 20, G = 0.0008 inch and the tooth thickness T = 0.049 inch;
for more than 20 teeth in mesh T = 0.045 inch and G varies. The
tooth thickness has been reduced for the larger numbers of teeth in
mesh so as to increase the range between the shrinkage and stretch
curves. By decreasing the clearance G for 11 to 20 teeth in mesh
both curves have been lowered somewhat and thus the clearance in-
creased at E for the teeth entering engagement. For a definite
known number of teeth in mesh, the curves show the amount of
stretch or shrinkage which will be accommodated before interference
occurs. For instance, regardless of the total number of teeth on the
sprocket, a sprocket with 6 teeth in mesh will take film which has
Aug., 1941 J
DESIGNING FILM SPROCKETS
181
stretched 1.51 per cent or shrunk 0.54 per cent. These shrinkage
factors, of course, refer to freshly perforated stock.
If we examine the curves (Fig. 3) for sprockets designed to take
film that has shrunk 0.70 per cent, we find the curves the same as
the previous set but displaced. The stretch curve SB now crosses
the zero line and becomes positive between 8 and 9 teeth in mesh.
This may be interpreted, for example, for 10 teeth in mesh, simply
that the film must shrink at least 0.17 per cent from freshly per-
forated film dimensions if interference is not to be encountered but
may shrink as much as 1.23 per cent. To show the amount of film
slip on the sprocket for each tooth leaving engagement, let it be as-
FIG. 4. Feed sprockets.
- F
B, base sprocket dia. = — Ti^ia
DI, film perforation pitch = D — DS
D, nominal pitch of freshly perforated film
S, shrinkage
G, clearance of second tooth in mesh
N, number of teeth on sprocket
F, film thickness
Y, number of teeth in mesh minus one
P, sprocket-tooth pitch
sumed that film which has shrunk 1.0 per cent is operating on the
sprocket. Then the vertical distance from this point (1.0 per cent)
to the Sc curve represents the slip which in this case is 0.23 per cent.
For films with less shrinkage the slip of course will be greater.
Fig. 4, for feed sprockets, shows the sprocket rotating counter-
clockwise, receiving film from the right under tension and feeding
into a loose loop on the left. In order that the film may pass freely
onto the teeth, the sprocket-tooth pitch is greater than the perfora-
tion pitch. The sprocket base diameter then, as shown by the equa-
tion, equals (Dl + G)N/r-F. In this case G, for 10 or less teeth in
mesh, is taken equal to 0.0015 inch as against 0.0010 inch for hold-
back sprockets.
182
W. G. HILL AND C. L. SCHAEFER
\J. S. M. P. E.
The difference in the numerical value of G for holdback and feed
sprockets is partially explained by the fact that for feed sprockets
there is in general more tension on the film as it is wound on the
sprocket and it is believed that instead of bending on a plane midway
between the emulsion and base surfaces, it actually bends near the
surface of the sprocket-wheel and in effect reduces G. Since it is
difficult to ascertain or predict the exact location of the plane of
bending, the values of G for feed sprockets have been increased as
added assurance that the sprockets will function properly as feed
members.
It will be noticed that in this case the percentage of film stretch
FIG. 5. 35-mm feed-sprocket.
accommodated, (D—P)/D, is independent of the number of teeth in
mesh and the per cent of shrinkage accommodated varies with the
number of teeth in mesh and the tooth thickness.
In Fig. 5 the allowable film shrinkage curve Sc for feed sprockets
now takes a shape similar to the stretch curve for hold-back sprockets.
Decreasing the tooth thickness at 11 teeth in mesh increases the range
between the two curves, and decreasing the clearance G permits
greater film shrinkage. The amount of slip for each tooth leaving
engagement for this case is represented by the vertical distance to the
SE curve.
The dimensions in a direction transverse to the film are given in
Fig. 6. Again the dimensioning is related directly to the size of
Aug., 1941]
DESIGNING FILM SPROCKETS
183
freshly perforated film, V being the distance between rows of per-
forations and U being the length of the perforations, both as taken
from the film Standards. The clearance between perforations and
teeth has been taken as 0.020 inch at the tooth base and 0.035 inch at
the tooth tip. No effort has been made to show how the film might
be guided by the sprocket-teeth or a combination of teeth and flange
because in the opinion of the writers such practice is not advisable.
Figs. 7 and 8 show the relation of tooth base shapes to film per-
forations. In Fig. 7, for 16-mm, the tooth-base shape has been taken
similar to the film perforation because only the one type of perfora-
FIG. 6. Feed and hold-back sprockets.
S, per cent shrinkage from freshly perforated film
r, round corners with approximately 0.010-inch radius
for 35-mm; 0.005-inch radius for 16- and 8-mm
V, center distance of freshly perforated film
U, length of perforation
tion is used. The 0.020-inch clearance between the perforation
edges and the tooth allows a 0.042-inch flat as a bearing edge. In
Fig. 8, for 35-mm sprockets, the form has been made to correspond
more to the shape of the negative perforation. The reason for this
is that 35-mm sprockets must accommodate both positive and nega-
tive types of perforation and the proposed shape satisfies this condi-
tion to a reasonable extent. The bearing edge of the tooth face in
this case is approximately 0.060 inch.
For determining the tooth shape (Fig. 9), the method indicated is
used for all types of sprockets. The values of X and G have been so
chosen that the perforation leaving engagement is normally free of
the tooth face after it is stripped about half of the tooth height.
184
W. G. HILL AND C. L. SCHAEFER
[J. S. M. P. E.
Since the base diameter has already been determined the involute
curve may be generated, and using the proper values of T, H, and X,
the tooth face may be described by radius Q with its center at O, the
point O being found by erecting a perpendicular at the midpoint of a
.072'
.052
/
FIG. 7. Tooth base shape for 16-mm film sprockets.
FIG. 8. Tooth base shape for 35-mm positive and
negative film sprockets.
straight line through MK and projecting this line to intersect the
periphery of the base circle.
By means of the above-described method now let us follow through
Aug., 1941]
DESIGNING FILM SPROCKETS
185
FIG. 9. Sprocket tooth shape
Film Size T* H X
35-mm 0.055 0.050 0.006
16-mm 0.032 0.032 0.004
8-mm 0.032 0.032 0.004
Radius Q to have its center at 0 on the base diameter with the arc
passing through K and M.
* For 10 or less teeth in mesh. (Note. — Dimensions are given in inches)
a typical example for a sprocket running under the following condi-
tions :
Sprocket: 32 teeth with 6 teeth in mesh to act as hold-back member
Film: 35-mm negative, 0.0055 inch thick; average film perforation pitch 0.1857
inch; perforation pitch of freshly perforated film 0.187 inch
B, base dia.
3.1416
- P =
- 0.001)82
3.1416
D - P 0.187 - 0.1847
SCt per cent shrinkage accommodated = — - — X 100 = - — ^T^ — "
100 - 1.23
/ T\ Y _ w") _ (p Y _ T}
SE, per cent stretch accommodated = - - - — — ~ X 100 =
(0.187 X 5 - 0.073) - (0.1847 X 5 - 0.055)
- — — - x iuu =
(0.187 X 5 - 0.073)
— u./o
186 W. G. HILL AND C. L. SCHAEFER
S, per cent shrinkage computed from freshly perforated stock as a basis =
0.187 - 0.1857
0.187
X 100 = 0.70
Dist. between rows of teeth = V - VS = 1.109 - 1.109 X 0.007 = 1.1012 inch
Tooth base width = U - 0.020 = 0.110 - 0.020 = 0.090 inch
Tooth tip width = U - 0.035 = 0.110 - 0.035 = 0.075 inch
Corner tooth radius at base, r = 0.010 inch; for tooth profile shape refer to Fig. 9.
Other dimensions referring to items such as picture and sound areas will be de-
termined by the particular use of the sprocket; discussion of these items is beyond
the scope of this paper.
In conclusion it should be pointed out that by classifying sprockets
in groups according to their use, by agreement on certain values for
optimum performance, and by applying a method as outlined in this
paper, a practical solution to the sprocket dimensioning problem and,
ultimately, standards for film sprockets might be reached.
DISCUSSION
MR. FRIEDL, JR. : It is noted by the author that the matter of film guidance
was not considered in the paper. In establishing the SMPE Recommended Prac-
tices for film dimensions, the subject of film guidance was taken into account.
The standards are based on edge guiding.
MR. HILL: That question was considered beyond the scope of the paper, be-
cause the subject is very involved and deserves considerable attention. A com-
plete investigation should be made of the means employed in guiding film through
all stages from the time of perforating to projection, or sound reproduction.
MR. MAURER : The method of design presented in this paper leads to a varying
range of shrinkage accommodation, depending on the number of teeth in mesh.
Is it not preferable, when designing a sprocket for a given application, to take
into account the actual shrinkage range that will be encountered and the number
of teeth that will be in mesh, and design the sprocket in accordance with these
specific conditions? In my experience this practice generally leads to a thicker
and higher tooth than Mr. Hill and Mr. Schaeffer have proposed. I have con-
sidered this desirable because the larger tooth makes film threading easier, and the
stronger tooth can perhaps be machined more accurately.
MR. HILL: Although it is theoretically possible to design sprockets in such a
way, the changing of the tooth thickness for different numbers of teeth in mesh
seems to complicate the problem unnecessarily. If, for a definite number of teeth
in mesh, the sprocket can be designed with a tooth thickness so as to accommo-
date the desired film shrinkage, then there is no advantage in increasing the tooth
thickness for a lesser number of teeth in mesh. The question of tooth thickness in
connection with tooth strength and wear is not a major consideration, the ordi-
nary tooth strength being more than sufficient.
MR. FRIEDL, JR. : How much film shrinkage might be expected in practice?
DR. CARVER : The average film shrinkage of cine positive as measured in the
theaters will be about 0.3 per cent for a nitrate film and about 0.6 per cent for
safety film.
IMPROVED MOTOR DRIVE FOR SELF-PHASING OF
PROCESS PROJECTION EQUIPMENT*
HOMER TASKER**
Summary. — Process projection photography requires that the shutter of the pro-
jector and that of the camera open and close simultaneously. The relation between
the shutter speeds and the pole frequencies of normal motion picture motor systems is
such that there may be one, four, or five incorrect shutter relationships for each correct
one, if the motors are interlocked at random. Earlier methods of insuring correct
phasing between camera and projector shutters did not take proper account of the
economic importance of fast and reliable operation. This paper presents the results
of a time and economic study indicating savings of many thousands of dollars annually
per studio, accruing from the use of a motor system which automatically phases the
shutters of camera and projector, and which has a very high degree of reliability. The
design and performance features of such a motor system are described in their relation
to earlier efforts along this same line, together with a report on production use of the
new system.
The economic and dramatic importance of process projection photog-
raphy has been so great that motion picture managements have been
inclined to overlook its cumbersome operation, even though wasteful
of company time valued at $500 per hour and more. This is easily
understood. Economically, the process often avoids the high cost of
sending production units to locations and, dramatically, it often per-
mits a story scope otherwise unachievable by microphone and camera
at any price.
On the other hand, no amount of advantage can justify continued
toleration of time-consuming features of this process if they can be
improved upon. Among important past offenders is the motor sys-
tem, and careful study has shown that the motor system which is the
subject of this paper will effect economies of the order of $20,000 per
year at this studio alone.
Although the system to be described here is an outgrowth of
earlier work by Mr. Olin Dupy at MGM and Mr. Roy Otto at RKO,
these earlier applications have not been described in print; hence this
* Presented at the 1940 Spring Meeting at Hollywood, Calif.
** Paramount Pictures, Inc., Hollywood, Calif.
187
OThe Society is not responsible for statements by authors <>
188 H. TASKER [j. s. M. P. E.
paper will describe the basic principle as well as the special features of
the system as now being installed at Paramount.
A brief statement of the problem is necessary. Process projection
photography normally involves the projection of a moving back-
ground scene upon a translucent screen in front of which are placed
the actors involved, together with whatever foreground setting is
needful to permit the whole to be welded into an effective motion
picture scene.
The relationship of projector, screen, and camera is shown dia-
grammatically in the right-hand portion of Fig. 1. It is obvious that
the shutter on the camera which sees both the foreground and the
projected background must be open at the same time as that of the
background projector and experience has proved that this relation-
ship must be maintained with high accuracy. Random variations at
rates which can be appreciated by the eye as flicker must not exceed
=±=6 degrees, while a fixed displacement of more than 15 degrees may
cause serious loss of light.
Salient-pole synchronous motors, used in several studios for syn-
chronous operation of recording machines and cameras, do not meet
the requirements of synchronizing camera and projector in process
projection because, for the currently used 60-cycle and 48-cycle
motors there are, respectively, 10 and 4 different shutter relation-
ships possible when the motors fall into step; only one of which is
correct. Even though the shutters of camera and projector were care-
fully pre-aligned before starting, the accelerating times of the two
machines would almost certainly be different so that phasing errors
could not be avoided, except by provision of rather elaborate means
for automatically phasing the shutters while running.
A 12-cycle synchronous motor system would provide the desired
reliability of phasing but with serious penalties of bulk and weight of
camera motor, plus the cost of generating a special frequency for this
particular service. Use of these motors would necessitate extending
the system to all studio camera operation or else changing motors on
the camera as between process projection and normal motion picture
service.
The normal a-c interlock motor system likewise has four incorrect
interlocking positions for each right one but because it may be inter-
locked while at rest, shutter phasing is more controllable and hence
this motor system has been universally used for process projection
until the work of Dupy and Otto, mentioned above.
Aug., 1941]
IMPROVED MOTOR DRIVE
189
As applied to process projection photography this normal or
"regular" interlock system (see Fig. 1) consists of a three-phase
selsyn motor applied to each of the film-driving mechanisms plus
another and larger three-phase selsyn unit direct-coupled to a suitable
constant-speed driving source to form a "distributor" by which the
whole system is rotated. As indicated in Fig. 1, the interlock con-
nections are made by six-wire cables. The usual or normal speeds for
such a system as used in the past are 1200 rpm for four-pole motors
and 2400 rpm for two-pole motors, as indicated in this figure. Such
motors interlock on every second pole. This condition gives rise to
the fact that there are four incorrect interlocking positions for the
camera motor, with respect to that of the projector, for each right
one. Accordingly, with such a system it is necessary that the shutters
be phased by hand at camera and projector before the motors are
started.
1 INTERLOCK
4 POLE Z POLS
1200 f?.f?M. Z400R.RM
=\ 1 1 \
' I .. J *£ <;<,BO< 'y { \
1/440 R.PH.
6 c
X
/^40«'>V
1
4 ;
RECORDER
Cw»
,m
PKOJ. H
h
. i.
1
/20ORf>M.
FIG. 1. Interlock connections.
If clutches are used to facilitate this phasing, the motor system
must be interlocked (at rest) during this interval. If instead the
drives are solidly pinned, the motors at camera and projector must be
rotated from one to as many as five revolutions by hand to get motor
and shutter into the proper mutual relationship. In either event,
valuable time is lost because the phasing operation usually occurs
when the director and the cast are ready and waiting to begin the
next take. Furthermore, errors occur all too frequently, causing loss
of complete takes.
These difficulties may be avoided by changing the speed of the in-
terlock motors at camera and projector, and also the gearing between
motor and shutter in each case, to such a value that there is exactly
one motor interlock position for each complete revolution of the
shutter. This requirement is met by two-pole a-c interlock motors
190 H. TASKER [j. s. M. P. E.
geared 1 : 1 to the shutter shafts, and hence operated at 1440 rpm.
It is also met by four-pole motors geared 1:2 and operated at 720
rpm. From the standpoint of operating speed and reliability this is
the ideal arrangement for process projection because it eliminates all
delays of pre-alignment and all hazard of poorly tightened clutches,
etc.
This system requires, however, what amounts to an abnormal
motor speed, viz., 720 rpm for four-pole units of the system and 1440
rpm for two-pole units. There is also required a corresponding dis-
tributor or driving motor which is customarily four-pole -and must,
therefore, operate at 720 rpm. In this and other studios, the recording
machine and cameras have heretofore been driven by a 1200-rpm
(four-pole) motor. After weighing the comparative merits of intro-
ducing gears on the recording machine to accommodate the new
speed against the addition of another distributor motor unit geared
to 720 rpm, the latter was felt most desirable. This affords the basic
arrangement shown in Fig. 2. The 1200-rpm distributor unit drives
the recording machine and the 720-rpm distributor unit drives the
camera and projector on process projection shots or the camera only
on regular production. The numerals associated with the inter-
connecting cables indicate the number of wires in each. It will be
noted that electrically there are two independent selsyn systems with
a common three-wire stator energizing source. The rotor coupling
of the two systems resides entirely in the mechanical connection af-
forded by the distributor gears.
In making the above change in the speed of the motors driving
camera and projector, it is necessary that these motors deliver ad-
ditional torque because of the higher gear ratio between motor and
load.
Still further increase of torque was greatly desired because of past
difficulty with flicker which was at times traceable to inadequate in-
terlock between camera and projector. The steps taken to rewind
these motors for very much higher torques than ever heretofore ob-
tained in these particular frames are of sufficient interest to discuss
here because they bring to light design factors bearing on motors for
motion picture services which seem to have been overlooked.
The importance of small size and weight in motion picture camera
motors is so great that full advantage is taken of their extremely in-
termittent duty. A motor which is satisfactory for the average
"take" length of one minute, with two-minute intervals between, as
Aug., 1941] IMPROVED MOTOR DRIVE 191
in some danger of burning insulation if operated continuously for as
much as ten or fifteen minutes on a camera which is more than nor-
mally stiff. Owing, however, to the rarity of scenes exceeding two or
three minutes in length it was felt that there was sufficient margin of
safety to permit increasing the torque enough to offset the proposed
reduction in speed, but grave doubt was expressed as to the possibility
of going beyond this point.
In order to examine this question experimentally without a series of
expensive and time-consuming rewinds, one of these motors was
operated on a test load equal to that of a normal camera and supplied
with various excess voltages from a three-phase tapped transformer.
Upon operating this motor at a constant speed of 1440 rpm and vary-
ing the input voltage (and hence the available interlock torque)
we were immediately reminded of the rather elementary considera-
" SELF PHASING* INTERLOCK
FIG. 2. Basic arrangement, employing additional distributive
motor unit geared to 720 rpm.
tion that the mechanical power delivered by such a motor is repre-
sented by substantially in-phase electrical input power. In other
words, as the voltage was increased the current decreased so long as
the mechanical load remained constant. In consequence of this fact
the copper losses of the motor actually reduced as the interlock was
improved by increasing voltage; probably more than offsetting any
increase in iron losses. The indications are that increased interlock
tightness may be obtained in this way without any increase in heat-
ing so long as the load remains constant. The limit of such increase
should occur when saturation sets in before the maximum torque
actually required in service is obtained.
It is true that with the higher applied voltage the motor is capable
of delivering a great deal more power without falling out of step and
that, if it were called upon to deliver this new maximum power, it
would promptly overheat and burn the insulation. Fortunately, in
192 H. TASKER [j. s. M. P. E.
the problems here considered, power of such excessive value can be
demanded only in the case of a film buckle, and in this event the motor
is instantly protected by the film buckle trip switch.
It is apparent that the results obtained by applying excessive volt-
age to the motor as described above are substantially duplicated
upon rewinding the motor with fewer turns of heavier wire. In this
case there is an increase hi the exciting current but no increase in
that portion of the current which represents mechanical power de-
livered to the load. Since both these currents pass through heavier
wire of shorter length the total copper loss is substantially reduced,
and so long as saturation is avoided within the normal requirements of
the new motor it may be expected that heating will not increase.
Based on the above test data several motors were rewound for
MINUTES
FIG. 3. Heat tests.
double their former torque. Heat tests on these motors not only
bore out but somewhat exceeded expectations, as may be observed in
Fig. 3. Of the three smooth curves arising from the origin, the upper
one gives the performance of an unrewound motor on a continuous-
1000-ft "take" at the normal motor speed of 2400 rpm. Tempera-
ture rise was measured by the copper resistance method.
The lowermost of these three curves is an identical motor except
for rewinding and indicates that the temperature rise is only half as
great as before rewinding despite the fact that the latent power ca-
pacity of the motor is doubled. In consequence the rewound motors
are far superior to their predecessors for process projection service
for the dual reasons that they provide a much tighter interlock and
operate at lower temperatures.
Aug., 1941] IMPROVED MOTOR DRIVE 193
The reduction in speed required to accommodate the motor to the
self-phasing system here described results in approximately 40 per
cent increase in heating, as shown by the intermediate curve, but the
end-result is still conservative as compared to the unrewound motors
operating under the original conditions.
One of these rewound motors, operating at the new 1440-rpm speed,
was put through one of the most severe production cycles ever en-
countered in practice. This is shown as the long, irregular line on
Fig. 3. Operation through six 200-ft takes with two-minute rest
periods between was followed by a series of twelve 60-ft closeups
with one and one-half minute rest periods between. A ten-minute
set-up period followed after which four additional 200-ft takes
were made. Maximum temperature rise on this production run was
34:1/2°C as measured by the copper resistance method. This is very
satisfactory as the motor is still well below a dangerous heating point
and production requirements are seldom as severe as those given in
this example.
Thus far, an elementary system has been described which has great
advantages of rapidity and certainty in operation because it will al-
ways interlock with shutters in proper phase whenever the power is
applied. To be a thoroughly adequate production tool, however,
it must have other important features :
(1) There must be provision for running the projector alone for rehearsals.
(2) There must be provision for running the film on out at the end of a take or
for rewinding it by back-tracking in the projector at the end of each rehearsal or
take because unthreading the projector at the point where the take stops may
scratch the film and spoil a subsequent take which may run a few feet farther.
(5) There must be simple provision for slating each production take.
(4) For silent shots there should be simple means for interlock operation of
camera and projector alone, independent of the sound system.
(5) For such silent operation speeds 20 per cent above and 50 per cent below
normal should be readily available and positively controllable by the projection-
ist. Nevertheless the normal 24-frame speed should be instantly available without
supervision on his part, and without attention on his part.
(6) There should be a minimum number of switches or patches to change when
changing from silent operation to operation from the sound recording system.
(7) There should be a minimum of special equipments to be set up when the
process projection booth is moved from one stage to another.
(8) The entire system, including the necessary controls and switches, should
involve a minimum of maintenance.
(£>) Control of the system should be available at the camera position, both for
rehearsals and for silent takes.
194
H. TASKER
[J.S. M. P. E.
A system is shown in Fig. 2 which meets all these requirements
when it is associated with the controls shown in the schematic dia-,
gram of Fig. 4.
It will be seen that the process projector is equipped with two
motors. One is a d-c motor, equipped with a governor to control its
speed at the standard 24 frames per second, plus a rheostat and
tachometer for control of its speed above and below normal. The
other is an interlock motor of the type described above. In the pres-
ent application, the space available dictated that the two^ motors be
FIG. 4. System of Fig. 2 with asscx^iated controls.
mounted one above the other. In order to relieve the interlock motor
of the burden of dragging the d-c motor when making normal sound
shots an over-riding clutch is used.
Formerly rehearsals and silent shots involved the use of a separate
local distributor equipment which was dolly-mounted, moved from
stage to stage with the projection booth, and plugged in as required.
This awkward and bulky apparatus is replaced by the new dual motor
arrangement mounted directly and permanently on the projector.
Operation is then as follows :
Aug., 1941] IMPROVED MOTOR DRIVE 195
•
(a) Rehearsals.— The projectionist (or the stage crew via the remote control)
throws the "Run" switch which applies power to the d-c motor which picks up the
interlock motor through the overrunning clutch and drives the projector at the
governor controlled speed of 24 frames per second. If some other speed is required
it is obtained by the projectionist with the aid of field rheostat and tachometer
after throwing the "Speed" switch to "Variable."
(6) Run Outs, Etc. — The projectionist throws the "Run" switch to run out at
the end of a take. When fully reversible projectors become available at this stu-
dio, he will be able to rewind the film in the projector by merely throwing this
switch to the reverse position. At that time, a centrifugal or magnetic clutch will
be substituted for the overrunning clutch.
(c) When silent takes involving only the camera and projector (and not the
sound recorder) are to be made, the interlock motors of projector and camera,
taken together, become a complete independent interlock system through the
application of local three-phase supply. This is accomplished with the utmost
convenience and without any preliminary changes merely by throwing first the
"Interlock" and then the "Run" switch, either at the remote position or in the
projection booth. In other words, the difference between a rehearsal and a take is
simply that for the latter, the "Interlock" switch was operated in addition to and
before operating the "Run" switch.
(d) When a take is to be made which involves sound, the recordist operates
the interlock controls in the recording building in the usual way, and as he does so,
relays in the projection booth operate to connect this interlock voltage on through
to projector and camera motors. The purpose of these relays is to isolate re-
corder and sound department distributor from the system during silent shots yet
to instantly interconnect the entire system for sound shots. The relays used are
idential with those employed at this studio for a number of years in connection
with our previous method of slating. They involve very moderate maintenance.
(e) Slating no longer requires separate operation of the camera between takes
as it is adequately cared for by the new camera slating device described by Mr.
F. C. Gilbert.1 This device performs its function while the system is coming up
to speed.
Three months of experience with a trial installation of this equip-
ment has proved its reliability, speed, and efficiency. The convenient
controls speed up rehearsals. The complete absence of pre-phasing
speeds up takes and reduces strain on directors and talent. The in-
creased certainty of good results reduces the number of takes made,
and it appears likely that the complete time studies made in the
course of considering this motor system will result in subsequent im-
provements of equipment and technic which will permit additional
savings of from $20,000 to $30,000 per year.
Finally, if process projection photography is to become a
thoroughly efficient studio tool, it must not only be capable of swift,
easy, and reliable operation in itself, but changes necessary to under-
take this type of photography or return to normal photography
196 H. TASKER
•
without projection must be so simple that the two may be inter-
mingled several times in a given production day. To this end it is
very desirable that the same camera motor remain on the camera at
all times; hence we are arranging to use only 1440-rpm camera motors
and to supply only 720-rpm distributor service to the stages at all
times. This means, of course, that such auxiliary motor-driven
facilities as playback machines must also employ the new motor
speeds. The costs of such changes, however, are negligible as com-
pared to the benefits to be derived from a single standardized system
for all studio production services.
REFERENCE
1 GILBERT, F. C.: "Scene-Slating Attachment for Motion Picture Cameras,"
/. Soc. Mot. Pict. Eng., XXXVI (April, 1941), p. 355.
BLACK LIGHT FOR THEATER AUDITORIUMS*
H. J. CHANON** AND F. M. FALGEf
Summary. — The demand for near-ultraviolet radiation, commonly called "black
light," in the production of luminescent effects has shown the need of a technical ap-
proach to the problem. New technics of measurement, design information, and data
on sources and materials are necessary to insure most effective use of these new media.
This paper covers suggestions for energizing fluorescent carpet, decorative wall and
ceiling murals, and other decorative applications. Data are presented on sources of
radiation, standard filters for absorbing the visible light emitted by the sources, and
the relative response characteristics of various types of luminescent materials. The
effect of extraneous visible light in masking the brightness produced by fluorescence is
discussed. Methods for measuring the near-ultraviolet energy from mercury light
sources in the field as well as in the laboratory are explained.
"Black light" is the popular term applied to the phenomenon of
luminescence or the conversion of invisible near-ultraviolet energy
to radiation in the visible portion of the spectrum by means of fluores-
cent or phosphorescent materials. Although theaters have em-
ployed this phenomenon for many years in spectacular stage produc-
tions, the lack of convenient sources and materials prevented its ap-
plication in theaters primarily engaged in the presentation of motion
pictures. New lamps and materials have made possible decorative
and utilitarian applications as well as the spectacular, and the field is
now of interest to all types of theaters. Theater operators, archi-
tects, designers and decorators, lighting engineers, and technicians
require design information, new technics of measurement, as well as
data on sources and materials to enable them to design objectively
for a definite brightness and pattern in theater interiors.
Luminescence occurs when fluorescent or phosphorescent materials
are exposed to near-ultraviolet radiation in the region of 3200 to 4000
A. Luminescence may likewise be stimulated by energy in the
visible range, that is, above 4000 A. However, in this case, the
* Presented at the 1940 Fall Meeting at Hollywood, Calif. ; received Novem-
ber 20, 1940.
** General Electric Co., Cleveland, Ohio.
t General Electric Co., Los Angeles, Calif.
197
Society is not responsible for statements by author sO
198
H. J. CHANON AND F. M. FALGE [j. s. M. P. E.
brightness produced by fluorescence is, not apparent as it is masked
by the visible light from the exciting source and the effect is of little
or no value in the theater.
When luminescence exists only during the period of excitation by
the near-ultraviolet source, the effect is called fluorescence and when
it persists for a period after the exciting energy is removed it is called
phosphorescence. Materials having phosphorescent characteristics
are sometimes more desirable for certain effects, although in general,
either type is suitable for use in the theater.
»-
Z Oft
X
Per Cent
u
CL 7Q
3255.
3322.
....0.2
5
»
1 60
3394.
•*472
0
4
>-
O
tt 50
3556.
•>RAC*
9
AA 7
UJ
Z
UJ4O
3745.
...4.0
UJ
>
— 3Q
3852.
4047.
... .8
5
5_30
-J
UJ 90
£Tt0
0
1 . •
3200
3400 3600 3800 4000
WAVE-LENGTH - ANGSTROMS
FIG. 1. Near-ultraviolet energy radiation
normal to a 100- watt mercury lamp of the
capillary type equipped with a Corning No. 587
filter. (Energy measurements by B. T.
Barnes.)
SOURCES AND FILTERS
There are many sources of near-ultraviolet radiating energy in
varying degrees. The 100- watt and 250- watt capillary mercury
lamps supply the need for small relatively potent sources of low wat-
tage. Where long throws are required, the carbon arc, because of its
high intensity, is generally used. The 1000- watt water-cooled cap-
illary mercury lamp may also be employed. Filament lamps, particu-
larly the photoflood types having high filament temperatures, like-
wise produce moderate amounts of near-ultraviolet energy. Unlike
mercury sources, which can not be turned on and off quickly, filament
lamps can be used on flashers or dimmers in applications where this
Aug., 1941] BLACK LIGHT FOR AUDITORIUMS 199
service may be desirable and high output in near-ultraviolet energy
is not required. The blue fluorescent lamp, which has considerable
energy below 4000 A, has been found very satisfactory for some ap-
plications. The 2V2-watt argon glow lamp produces near-ultra-
violet energy in small amounts suitable for some uses.
The energy radiation from a 100-watt mercury vapor lamp of the
capillary type equipped with a Corning No. 587 filter is shown in
Fig. 1. Most of the energy emitted is in the region of 3650 A, the
band of maximum response for the majority of the commercial ma-
terials employed for black-light effects.
Data on the transmission characteristics of several Corning filters
which transmit near-ultraviolet radiation are shown in Table I. All
these filters have a maximum transmission at 3650 A and practically
zero transmission below 3100 A. The values in the table represent
the percentage of normally incident radiation transmitted by repre-
sentative filters of 5-mm thickness.
TABLE I
Per Cent Transmission of Corning Glass Filters 5 mm. Thick
Wavelength
in Angstroms Type of Filter
584 585 586 587 588 597
Near
ultraviolet
Visible
3020
0
0
0
0.5
0
0
3130
3
2
0
4
5
3
3340
42
48
5
31
40
44
3650*
65
82
27
59
72
80
4050
0
70
0
1
18
14
4358
0
37
0
0
0
0
5461
0
0
0
0
0
0
5770
0
0
0
0
0
0
5961
0
0
0
0
0
0
6908
0
2
0
0
6
1
Maximum response of most luminescent materials.
For most black-light applications the best filter is one which has
high transmission in the region of 3650 A but absorbs most of the
visible radiation. Table I shows that Corning filters Nos. 584, 587,
and 597 have these desirable transmission characteristics. Filter
No. 585 transmits considerable blue and some red light and may,
therefore, be suitable for special effects where visible light in this re-
gion is not detrimental to the fluorescent pattern obtained. Where
200
H. J. CHANON AND F. M. FALGE [J. s. M. p. E.
fluorescent materials must be observed in complete darkness it has
been found that filter No. 586 is most applicable. Filter No. 587,
known as Heat Resisting Red Purple Ultra, has been employed in the
majority of theater black-light applications.
The visible radiation transmitted by some of these filters is of a
predominant color, usually violet in character, and is therefore of un-
certain value in providing seeing at the low levels of general illumina-
tion in theater auditoriums. For such illumination it is better to
employ visible light, essentially white in character, from other sources.
Good control of this lighting is required for least impairment of the
fluorescent decorative patterns.
2500 2700 2900 3100 3300 3500
WAVE-LENGTH - ANGSTROMS
FIG. 2. Relative luminous response of luminescent ma-
terials to radiation of different wavelengths. The curves
for the sulfides and synthetic organic preparations show
typical average values for all colors. The canary (ura-
nium) glass is Corning No. 375 with vaporized aluminum
backing and with the face of the glass ground.
LUMINESCENT MATERIALS
Fluorescent materials for black light are usually synthetic organic
materials — the majority of sulfides are phosphorescent. Fig. 2 shows
the average response for these two groups as compiled from data for
materials having different color responses. It is not unusual to have
a pronounced resonant effect at some of the principal mercury lines;
this effect has been eliminated for simplicity. Fig. 2 shows also the
response characteristic of fluorescent Canary glass, the material
selected in this study as a reference standard. Although this glass
has a low response at 3650 A, its permanence and availability counter-
balance this objection. The section of this paper "Technic of Mea-
Aug., 1941] BLACK LIGHT FOR AUDITORIUMS 201
surement" deals with the use of this material as a reference standard.
Measurements taken on about fifty different samples of fluorescent
lacquer enamels indicate that the average relative efficiency of pro-
ducing color by fluorescence is as shown in Table II.
TABLE II
Relative Response of Representative Fluorescent Lacquer Enamels
Color Relative Brightness
White 100
Red 38
Orange 55
Yellow 88
Green 42
Blue 34
Violet 18
The colors in Table II were classified according to hues, as each of
them was made up in various shades and tints. With the exception
yellow is the brightest color; orange and green are next highest in
efficiency; and red, blue, and violet are lowest. This characteristic
is similar in shape to the response curve of the human eye. The eye
is most sensitive to radiation in the green, yellow, and orange regions
of the spectrum.
Lacquer materials, both opaque and transparent, are available for
black-light effects. The majority of the former appear colored under
visible light and this color is enhanced and may appear as another
shade when energized ^with near-ultraviolet. A few appear colorless
under the visible and glow in saturated color when energized. In
applying the transparent materials it is important to provide light-
reflecting backgrounds which increase the brightness of the resulting
effect considerably.
In addition to lacquer enamels, many other fluorescent materials
are now available. Some of these are plastics, papers, fabrics, inks,
dyes, and water colors, each of which has specific applications. The
choice of material depends on the use for which it is to be employed.
The brightness of the material depends on three factors :
(1) The amount of near-ultraviolet energy falling on the material; this is
affected by the energy distribution of the light source and by the transmission
characteristics of the filter.
(2) The efficiency of the material in converting near-ultraviolet energy into
visible light.
(5) The response of the eye to the color produced.
202 H. J. CHANON AND F. M. FALGE [j. s. M. p. E.
TECHNIC OF MEASUREMENT
Requisites common to all types of measuring equipment used in
the field are simplicity, portability, and the ability to maintain cali-
bration. For field as well as laboratory measurements of black-
light sources and their effects, a method incorporating the use of a
brightness meter and a foot-candle meter was found to be practicable
when used in conjunction with a reproducible fluorescent material
having unchanging characteristics. The material chosen as a refer-
ence standard (Fig. 3) was a two-inch square of Corning No. 375
fluorescent Canary glass 5 mm thick. This glass, which^ contains
uranium, has particularly stable characteristics. It was found that
when a piece of this glass is half covered with an opaque material and
the whole is exposed for a long period of time to ultraviolet radiation,
fluorescent brightness measurements on the two portions show no
difference. Being a glass of considerable thickness it is sturdy and
will withstand handling. The response of this material to ultraviolet
energy is shown in Fig. 2.
Because of the low response of fluorescent Canary glass to energy
in the 3650 A band, it was found desirable to increase its brightness
by means of an aluminized back and edge coating. To protect the
aluminum surface, a black coating of protective lacquer can be ap-
plied if desirable. To minimize specular reflection the front surface
of the glass was ground with No. 100 carborundum. This reference
standard was used in laboratory and field measurements as a means
of determining the radiation upon luminescent materials and the dis-
tribution of the near-ultraviolet radiation of sources.
The full-range brightness of the reference standard can be mea-
sured by means of the Luckiesh-Taylor brightness meter shown in
Fig. 3. The advantage of this combination lies in the ability to
measure the low levels of radiation often used for exciting fluorescent
and phosphorescent materials in dimly lighted interiors.
The reference standard can be calibrated by exposing it to a stand-
ardized source of near-ultraviolet radiation equipped with a filter to
absorb the visible light. The energy from the source transmitted by
the filter can be calibrated in microwatts per square-inch or in milli-
watts per steradian. With a reference standard prepared as described
above, and a capillary type of mercury light-source equipped with a
Corning No. 587 filter, the energy to produce a brightness of one foot-
lambert was found to be 240 microwatts per square-inch. Knowing
the near-ultraviolet energy distribution of the mercury source, and
Aug., 1941] BLACK LIGHT FOR AUDITORIUMS 203
measuring the brightness of the reference standard with the Luckiesh-
Taylor brightness meter, it is possible to compute any other value of
incident near-ultraviolet energy within an accuracy of =*=5 per cent.
An investigation showed that within the range of zero to 40 foot-
lamberts, or an energy value equal to approximately 10,000 micro-
watts per square-inch, there is no saturation; that is, for every in-
crease in incident energy there is a corresponding increase in bright-
ness of the reference standard.
The barrier type of foot-candle meter, such as the General Electric
light meter, may likewise be utilized to measure near-ultraviolet
radiation. A 5-mm piece of Corning No. 587 glass ground on the
outside surface is clipped over the cell (Fig. 3). A deflection of one
foot-candle on the meter is produced by an energy value of 54 micro-
watts per square-inch. The deflection is directly proportional up to
75 foot-candles, the highest value checked, which is the full range of
the meter without multipliers.
In a darkened room, where all the ultraviolet sources are equipped
with filters and practically no visible light is present, the sensitivity
of the light-meter to near-ultraviolet energy can be increased by re-
moving the clip-on filter. Under these conditions one foot-candle de-
flection is produced by an energy value of 27 microwatts per square-
inch.
Where mercury sources are employed with filters such as the Corn-
ing No. 587 type, the energy in the incident visible light is negligible
as compared to the near-ultraviolet energy. However, when the
energy from filament lamp sources is being measured an error is in-
troduced because of the greater percentage of visible light passed by
the filter. For this type of measurement, the brightness produced by
visible light can be conveniently obtained by measuring the bright-
ness of a magnesium oxide disk (Fig. 3), or any other non-fluorescent
material of known reflection factor. This value, subtracted from the
brightness of the reference standard, is an approximate measure of the
brightness produced by fluorescence.
APPLICATIONS
General. — The use of fluorescent-treated murals, medallions, drapes,
etc., introduces a new type of decorative treatment as well as a new
form of utilitarian lighting for theater interiors. This approach
makes possible the use of large expanses which may embrace the en-
tire ceiling or walls, or the use of localized treatment. In either ap-
204
H. J. CHANON AND F. M. FALGE [j. s. M. P. E.
proach, the near-ultraviolet energy, which is invisible to the eye be-
fore it strikes the fluorescent material, 'is converted into visible light
and follows the same laws as ordinary light. The radiation may be
controlled by properly selected materials, and shielded in the same
way as the energy from the familiar filament type of light-source.
Since black-light sources are relatively inconspicuous, the problem
of shielding and locating them is generally simpler. While the beam
or flood of black light may overlap a particular pattern or area, the
light emission is confined to the surfaces upon which the fluorescent
material is applied.
LUCK1ESH -TAYLOR
BRIGHTNESS METER
CLIP-ON FILTER
(CORNt'WJ "587}
L O
MAGNESIUM OXIDE
LIGHT METER
FIG. 3. Equipment for making measurements in the application of near-
ultraviolet radiation for luminescence.
During the process of treating surfaces with fluorescent material
it is well to observe the colors and then- relative effect under black
light in surroundings as nearly similar as possible to the auditorium.
This procedure is often helpful in determining satisfactory placement
for the black-light units, as well as in determining the amount of
energy which will be required. Because of the variation in response
of various fluorescent materials and the different efficiencies of equip-
ment, only a general range of energy can be suggested here. For
the 100- watt type H-4 and the 250- watt type H-5 mercury sources
equipped with Corning No. 587 filters, the order of 0.5 to 1.0 watts per
Aug., 1941] BLACK LIGHT FOR AUDITORIUMS 205
sq-ft gives a basis for estimating the approximate number of units re-
quired for average throws in the theater.
There is another factor which influences the effectiveness of lumi-
nescence called "masking." Masking light is any visible light which
reaches the luminescent material and nullifies the luminescent effect.
This is a subjective matter and will vary with individuals and various
field conditions. The fluorescent materials which are now available
FIG. 4. Fluorescent mural over exit door at front of
auditorium. Black-light unit below mural in architec-
tural element over door.
have high response, and with reasonable care the masking light should
not be a problem. This is particularly true in theater interiors and
locations where low levels of general illumination are present. In
theater foyers and in advertising the masking light becomes an im-
portant factor.
Treatment of Large Areas— Large fluorescent-treated wall or ceil-
ing areas of fairly uniform low brightness, which are not broken up
by designs that produce high contrast, create an atmosphere under
206
H. J. CHANON AND F. M. FALGE [J. s. M. P. E.
RCE
FLUORESCENT
MATERIAL APPLIED
which pictures may be viewed advantageously. The utilization of
black light is generally the best under such conditions. From a com-
plete blue sky ceiling, for example, enough near-ultraviolet energy
may be converted into visible light by the fluorescent material to re-
sult in a low order of illumination throughout the entire auditorium.
In this case the color of the illumination will predominate in blue,
and if a more natural color for the appearance of clothes and people is
desired, it may be necessary to offset the blue by well controlled down-
lighting from a separate system of filament sources. The quality of
the illumination is dependent upon the colors of fluorescent-materials
used and their distribution throughout the auditorium. In general,
the warmer, more flattering tones
are preferred.
The placement of the black-
light sources under these condi-
tions is similar to that of general
auditorium lighting. The sources
can be concealed in coves below
the ceiling line and aimed for
most uniform distribution of
energy. More uniform distribu-
tion is obtained as the distance
of the source from the ceiling is
increased. Wall urns, pilasters,
and other architectural elements
offer suitable locations.
Light-Emitting Decorations —
Individual wall murals or patterns
can be rendered in sharp or graded outline with fluorescent materials.
Variations in brightness as well as in color can be obtained readily.
There is no need to control the black-light beam to fit the decoration
exactly. It is therefore particularly applicable to intricate designs.
Light-emitting murals on the proscenium wall can be utilized to
relieve the high contrast between the screen and surrounding areas
and at the same time produce a decorative effect. A simple arrange-
ment for energizing fluorescent-treated murals is shown in Fig. 4.
Here the space above the exit doors has been utilized to conceal a 100-
watt mercury lamp and suitable equipment.
Where the proscenium wall does not lend itself so readily to place-
ment of the energizing source below the mural, a simple installation
FIG. 5. One method of energizing
wall panels treated with fluorescent
material. See Table III for coverage
with equipment having beam spread
of approximately 30 degrees.
Aug., 1941
BLACK LIGHT FOR AUDITORIUMS
207
of a unit recessed in the ceiling can be made as shown in Fig. 5.
More uniform illumination of the panel is possible by this method as
more latitude in dimension A is usually practicable. Table III in-
dicates coverage for different dimensions from a single unit having a
30-degree beam-spread.
FIG. 6. A striking effect simulating an outdoor scene is obtained with these
wall murals due to the illusion of depth imparted by a blue background. The
highlights on the gazelles are in warmer colors, amber and gold, and although
the mural is flat, the appearance is similar to that of a three-dimensional object
in a niche.
TABLE III
Approximate Coverage for Reflector of 30-Degree Spread, Used as in Fig. 5.
Values Are in Feet)
(All
A
B
H
W
Radiated Area
(Sq-Ft)
10
13
7
64
8
15
26
9
185
20
56
12
510
10
12
8
76
12
15
19
10
180
20
31
13
310
Fig. 6 shows an installation of side-wall murals energized as sug-
gested in Fig. 5. Fluorescent colors having high response were
selected for use on the gazelles; thus they were accentuated in the
208
H. J. CHANON AND F. M. FALGE [j. s. M. p. E.
higher brightness even though they were farthest removed from the
energizing source.
Continuous decorative bands or panels (Fig. 7) may be energized
by units of the type shown for carpet illumination (Fig. 10). The
rectangular shape distribution with a beam-spread of approximately
CONTINUOUS MURAL OF
WALL DECORATION
FIG. 7. Rectangular beam of black light more nearly
fits panels of the same proportion. Downlighting unit
such as shown in Fig. 10 is particularly applicable for con-
tinuous murals. See Table IV for coverage with equip-
ment having beam spread of 10 degrees X 80 degrees.
10 X 80 degrees is desirable for this application. The coverage ob-
tained for various sizes of murals and for different locations of the
unit from the wall are shown in Table IV.
TABLE IV
Approximate Coverage for Reflector with Rectangle Beam-Spread of 10° X 80°,
Used as in Fig. 7. (All Values Are in Feet)
A
B
H
W
Radiated Area
(Sq-Ft)
10
5
14
70
4
15
12
20
240
20
23
26
600
10
4
16
64
8
15
7
22
154
20
10
28
280
10
4
20
80
12
15
6
25
150
20
8
30
240
Aug., 1941]
BLACK LIGHT FOR AUDITORIUMS
209
Energizing Fluorescent Carpet.— Self-luminous carpet, in addition
to its decorative effect, is of value as a means of traffic or directional
lighting. In contrast to the successive spots of light as produced by
aisle illumination with low-wattage filament lamps recessed or at-
tached to the seats, fluorescent carpet can be made to glow uniformly
in well designed black-light installations.
In order to use the energy most efficiently, it is desirable to confine
it as much as practicable to the fluorescent carpet. An effective
method is to use downlighting equipment recessed in or suspended
from the ceiling of an auditorium. Fig. 8 shows a cross-section of a
FIG. 8. It is desirable to confine the near-ultraviolet
energy as much as possible to the aisle. Maximum effi-
ciency can be thus assured and the spill of near-ultra-
violet energy is minimized.
typical theater auditorium with equipment placed directly over the
aisles. In confining the near-ultraviolet to the aisle, annoying fluo-
rescence of eyeballs and tinted spectacles is minimized. In either
case the individual may not be able to see clearly because of the re-
sulting haze. Some cosmetics and fabrics, as well as teeth, nails, and
skin blemishes likewise fluoresce, and this occasionally may have em-
barrassing aspects. All the above-mentioned effects can be con-
trolled by the introduction of some masking light.
A reflector of parabolic contour has the property of collecting light
from a source at the focus and redirecting it into a concentrated beam
210
H. J. CHANON AND F. M. FALGE [J. s. M. P. E.
in the case of the paraboloid, or into a thin wedge in the case of a
parabolic trough. A concentrated beam would be applicable from
above if units were installed at very close spacing. This spacing can
be increased by directing the beam through a prismatic lens which
AOMEDlUM SOCKET
MAZDA, H(MERCURy)LAMP
' TYPE CH4
CORNING 587 FlLTER-
OSITION AT LEA.ST
i" FROM LAMP
FIG. 9. A downlighting unit employing an in-
tegral projector type 100-watt mercury lamp. Rec-
ommended for the lower ceilings where a high de-
gree of control is not necessary and where a low
order of general illumination exists in the audi-
torium.
ADMEDiUM SOCKET
.NTiLATED
TAL HOUSING- BLACK. INSIDE ANB OUT
SHIELD FOP REDUOf
- STRAY LIGHT
PARABOLIC TROUGH
FOCAL LENGTH
POLISHED SURFACE
NINO *587 FILTER
TAL 5CREE.N
FIG. 10. A parabolic trough downlighting unit
employing a 100-watt mercury lamp, type AH-4.
This unit directs a ribbon of near-ultraviolet energy to
the fluorescent carpet.
spreads the light along the aisle but not crosswise. Such a prism
glass is included in the unit in Fig. 9. A parabolic trough redirects
the light to a rectangular area of greater relative length without the
intervention of any spreading glass (Fig. 10).
The lamp in Fig. 9 is of the capillary mercury type (100-watt CH-4)
Aug., 1941 J
BLACK LIGHT FOR AUDITORIUMS
211
with integral reflector of paraboloidal form producing a relatively
concentrated distribution. A Corning No. 587 filter screens out the
major part of the visible radiation. Considerable energy is absorbed
in the filter and it is therefore recommended that a coarse wire screen
be included below the filter as protection in the event of glass failure.
For fanning out the radiation along the aisle it is desirable to select
a prism glass which directs more of the rays toward the front of the
auditorium than to the back.
FIG. 11. Fluorescent carpet patterns, shown as illu-
minated with (top row} visible light and (bottom row)
black light. Pattern B has approximately 50 per cent
more brightness than A or C.
In Fig. 10 the light-source is identical with that in Fig. 9. How-
ever, it is incorporated in a tubular instead of a projector bulb. The
lamp is placed in a separate parabolic trough so that it lies in the
focal axis and off-center with respect to the opening of the reflector
This directs a greater proportion of the energy toward the front of
the auditorium, as in the unit in Fig. 9, thus producing greater shield-
ing for the audience from both the near-ultraviolet and the small
amount of visible violet light transmitted by the filter.
212 H. J. CHANON AND F. M. FALGE [j. s. M. P. E.
Tests of response have been made on three patterns of fluorescent
carpet. In Fig. 11 their appearance is indicated when illuminated
with visible light and with black light. Since different colors are(
employed to make up the patterns, certain parts of the carpet will be
brighter than others. Patterns A and C have approximately the
same general brightness, whereas pattern B is fully 50 per cent
brighter. Experiments in an auditorium, with equipments of the
type suggested in Figs. 9 and 10, and with carpet types A and C,
furnished the data on spacing for the different mounting heights
given in Table V.
TABLE V
Spacing Requirements for Fluorescent Carpet Downlighting Units
Ceiling Height Spacing
Unit (Feet) (Feet)
Fig. 9 or Fig. 10 20 18-20
Same 25 22-26
Fig. 10 30 28-32
Same 35 24-28
Same 40 20-24
Same 50 16-18*
Same 60 14-16*
* Spacings can be doubled if twin units are employed at each location. This
may be desirable in some architectural and decorative treatments.
The use of a 2V2-watt argon glow-lamp in regular aisle lights which
operate on standard lighting circuits without a transformer has a
number of shortcomings. The near-ultraviolet energy is of a low
order and accurate control of the energy is not practicable. In addi-
tion, the effect is likely to appear as spotty, as the conventional
method using visible light, unless one or more lamps are placed in
every aisle seat.
The above suggestions for energizing fluorescent carpet do not
cover all effective equipments and methods. Satisfactory carpet
brightness has been reported where the only energy striking the car-
pet came, merely incidentally, from mercury units directed at side-
wall or ceiling murals treated with fluorescent paints. However,
where no use of fluorescence is contemplated other than on carpet,
the downlighting systems suggested, or other designs which incorpo-
rate close control, will be found most effective.
The authors acknowledge their indebtedness to Prof. John O.
Aug., 1941] BLACK LIGHT FOR AUDITORIUMS 213
Kraehenbuehl of the University of Illinois and to Mr. C. M. Cutler of
the General Electric Company, respectively, for their collaboration
in the technical and application aspects discussed herein.
BIBLIOGRAPHY
KRAEHENBUEHL, JOHN O., AND CHANON, H. J.: "The Technology of Bright-
ness Production by Near-Ultraviolet Radiation," Trans. I. E. S., 36 (Feb., 1941),
p. 151.
CUTLER, C. M., AND CHANON, H. J. : "What Theater Operators Should Know
About Black Light," Better Theatres (June, July, Sept., 1940).
PORTER, L. C., AND DITCHMAN, J. P.: "Black Light and Fluorescent Ma-
terials," Magazine of Light, General Electric Company (May, 1936) p. 25.
FALGE, F. M.: "And Now 'Black Light': Another Tool of Showmanship,"
Better Theatres (April, 1938).
CURRENT LITERATURE OF INTEREST TO THE MOTION PICTURE
ENGINEER
The editors present for convenient reference a list of articles dealing with subjects
cognate to motion picture engineering published in a number of selected journals.
Photostatic copies may be obtained from the Library of Congress, Washington, D. C.,
or from the New York Public Library, New York, N. Y. Micro copies of articles
in magazines that are available may be obtained from the Bibliofilm Service, Depart-
ment of Agriculture, Washington, D. C., at prevailing rates.
Academy of Motion Picture Arts & Sciences, Technical Bulletin
(May 31, 1941)
Theater Acoustic Recommendations. Prepared by The
Theater Sound Standardization Committee of The
Academy Research Council.
American Cinematographer
22 (June, 1941), No. 6
Controlling Color for Dramatic Effect (pp. 262-263, 288,
290) R. MAMOULIAN
22 (July, 1941), No. 7
An Artist Looks at Technicolor Cinematography (pp. 318,
346) D. MACGURRIN
Using Arcs as Boosters (pp. 319, 346) M. KRASNER
The "Inkie's" Place in Technicolor Lighting (pp. 323,
348-349) E. PALMER
"Synchro-Sunlight" Movies with Reflectors (pp. 328-
329, 350-351) G. GAUDIO
Composition and Continuity for Natural- Color Filming
(pp. 334-335, 353-354) J. A. SHERLOCK
Educational Screen
20 (June, 1941), No. 6
Motion Pictures — Not for Theaters, Pt. 28 (pp. 241-242) A. E. KROWS
Electronic Engineering
14 (June, 1941), No. 160
The Preparation of Sound-Film Track (pp. 255-257, 281) R. H. CRICKS
The Design of Wide-Band Video Frequency Amplifiers.
Pt. I — High-Frequency Correction by Series Inductance
(pp. 258-261, 266) C. E. LOCKHART
214
CURRENT LITERATURE
215
International Photographer
13 (July, 1941), No. 6
Animated Cartoon Photography (pp. 10, 25)
The Kodatron (pp. 13-16)
International Projectionist
16 (April, 1941), No. 4
More Data on Control-Track Sound (pp. 7-8, 10-11)
Russia's Three-Dimensional Motion Pictures (pp. 12-13,
30)
National Defense and Its Effect upon Projection Room
Supplies (pp. 15-16, 27-30)
Fatal Theater Fires in Iowa Reveal Lack of Regulation
(P. 18)
Kinematograph Weekly
291 (May 22, 1941), No. 1779
Increased Brilliance in Motion Pictures. Colour-Former
Positive Process Results (pp. 23, 26)
Kinotechnik
23 (April, 1941), No. 4
Ein neues Doppelspalt-Mikrophotometer zur Ausmessung
von Lichttonaufzeichungen (A New Double Slit Micro-
photometer for Measuring Sound Film Densities)
(pp. 54-57)
Die Sucher-Parallaxe bei Normalfilm-Aufnahmeapparaten
(Finder Parallaz in Standard Film Cameras) (pp. 57-
65)
Motion Picture Herald
143 (June 28, 1941), No. 13
Commercial Television Starts, but without Films from
Majors (pp. 65-66)
J. W. BURTON
A. NADELL
S. IVANOV
L. CHADBOURNE
G. HARTNETT
T. THORNE BAKER
A» NARATH AND
K. SCHWARZ
H. WEISE
FIFTIETH SEMI-ANNUAL CONVENTION
OF THE
SOCIETY OF MOTION PICTURE ENGINEERS
HOTEL PENNSYLVANIA, NEW YORK, N. Y.
OCTOBER 20TH-23RD, INCLUSIVE
OFFICERS AND COMMITTEES IN CHARGED
Program and Facilities
E. HUSE, President
E. A. WILLIFORD, Past-President
H. GRIFFIN, Executive Vice-President
W. C. KUNZMANN, Convention Vice-P resident
A. C. DOWNES, Editorial Vice-President
R. O. STROCK, Chairman, Local Arrangements
S. HARRIS, Chairman, Papers Committee
J. HABER, Chairman, Publicity Committee
J. FRANK, JR., Chairman, Membership Committee
H. F. HEIDEGGER, Chairman, Convention Projection Committee
Reception and Local Arrangements
R. O. STROCK, Chairman
P. J. LARSEN T. E. SHEA A. N. GOLDSMITH
F. E. CAHILL, JR. J. A. HAMMOND J. A. MAURER
H. RUBIN O. F. NEU L. B. ISAAC
E. I. SPONABLE V. B. SEASE E. W. KELLOGG
P. C. GOLDMARK H. E. WHITE M. HOBART
W. H. OFFENHAUSER, JR. L. W. DAVEE J. A. NORLING
A. S. DICKINSON L. A. BONN H. B. CUTHBERTSON
W. E. GREEN J. H. SPRAY J. H. KURLANDER
R. O. WALKER J. J. FINN C. F. HORSTMAN
Registration and Information
W. C. KUNZMANN, Chairman
E. R. GEIB J. FRANK, JR. F. HOHMEISTER
P. SLEEMAN H. MCLEAN
Hotel and Transportation
G. FRIEDL, JR., Chairman
E. S. SEELEY R. B. AUSTRIAN F. C. SCHMID
C. Ross R. F. MITCHELL F. M. HALL
P. D. RIES P. A. McGuiRE J. A. SCHEICK
M. W. PALMER
216
FALL CONVENTION
217
H. A. GILBERT
G. A. CHAMBERS
D. E. HYNDMAN
L. A. BONN
E. G. HINES
A. S. DICKINSON
Publicity Committee
J. HABER, Chairman
P. SLEEMAN
S. HARRIS
C. R. KEITH
Banquet
O. F. NEU, Chairman
R. O. STROCK
J. C. BURNETT
J. A. SPRAY
J. A. NORLING
W. H. OFFENHAUSER, JR. M. HOBART
W. R. GREENE
H. MCLEAN
P. J. LARSEN
E. C. WENTE
A. GOODMAN
M. R. BOYER
J. A. HAMMOND
MRS. D. E. HYNDMAN
MRS. E. I. SPONABLE
MRS. E. S. SEELEY
MRS. A. S. DICKINSON
F. H. RICHARDSON
L. B. ISAAC
A. L. RAVEN
G. E. EDWARDS
i| J. K. ELDERKIN
Ladies' Reception Committee
MRS. R. O. STROCK, Hostess
MRS. O. F. NEU, Hostess
MRS. H. GRIFFIN
MRS. P. J. LARSEN
MRS. J. A. HAMMOND
MRS. G. FRIEDL, JR.
Convention Projection
H. F. HEIDEGGER, Chairman
T. H. CARPENTER
P. D. RIES
J. J. HOPKINS
~W. W. HENNESSY
L. W. DAVEE
MRS. E. A. WILLIFORD
MRS. J. FRANK, JR.
MRS. H. E. WHITE
MRS. F. C. SCHMID
J. J. SEFING
H. RUBIN
F. E. CAHILL, JR.
C. F. HORSTMAN
R. O. WALKER
Officers and Members of New York Projectionists Local No. 306
Hotel Reservations and Rates
Reservations. — Early in September, room-reservation cards will be mailed to
members of the Society. These cards should be returned as promptly as possible
in order to be assured of satisfactory accommodations. Reservations are subject
to cancellation if it is later found impossible to attend the Convention.
Hotel Rates. — Special per diem rates have been guaranteed by the Hotel Penn-
sylvania to SMPE delegates and their guests. These rates, European plan, will
be as follows :
i Room for one person
Room for two persons, double bed
Room for two persons, twin beds
Parlor suites: living room, bedroom, and bath for
one or two persons
$3. 50 to $8.00
$5. 00 to $8.00
$6.00 to $10. 00
$12.00, $14.00, and
$15.00
218 FALL CONVENTION [j. s. M. P. E.
Parking. — Parking accommodations will' be available to those motoring to the
Convention at the Hotel fireproof garage, at the rate of $1.25 for 24 hours, and.
$1.00 for 12 hours, including pick-up and delivery at the door of the Hotel.
Convention Registration. — The registration desk will be located on the 18th
floor of the Hotel at the entrance of the Salle Moderne where the technical sessions
will be held. All members and guests attending the Convention are expected to
register and receive their badges and identification cards required for admission
to all the sessions of the Convention, as well as to several de luxe motion picture
theaters in the vicinity of the Hotel.
Technical Sessions
The technical sessions of the Convention will be held in the Salle Moderne on
the 18th floor of the Hotel Pennsylvania. The Papers Committee plans to have
a very attractive program of papers and presentations, the details of which will
be published in a later issue of the JOURNAL.
Fiftieth Semi-Annual Banquet and Informal Get-Together Luncheon
The usual Informal Get-Together Luncheon of the Convention will be held in
the Roof Garden of the Hotel on Monday, October 20th.
On Wednesday evening, October 22nd, will be held the Silver Anniversary
Jubilee and Fiftieth Semi- Annual Banquet at the Hotel Pennsylvania. The
annual presentations of the SMPE Progress Medal and the SMPE Journal
Award will be made and officers-elect for 1942 will be introduced. The proceed-
ings will conclude with entertainment and dancing.
Entertainment
Motion Pictures. — At the time of registering, passes will be issued to the dele-
gates of the Convention admitting them to several de luxe motion picture theaters
in the vicinity of the Hotel. The names of the theaters will be announced later.
Golf. — Golfing privileges at country clubs in the New York area may be ar-
ranged at the Convention headquarters. In the Lobby of the Hotel Pennsylvania
will be a General Information Desk where information may be obtained regarding
transportation to various points of interest.
Miscellaneous. — Many entertainment attractions are available in New York to
the out-of-town visitor, information concerning which may be obtained at the
General Information Desk in the Lobby of the Hotel. Other details of the enter-
tainment program of the Convention will be announced in a later issue of the
JOURNAL.
Ladies' Program
A specially attractive program for the ladies attending the Convention is be-
ing arranged by Mrs. O. F. Neu and Mrs. R. O. Strock, Hostesses, and the Ladies'
Committee. A suite will be provided in the Hotel where the ladies will register
and meet for the various events upon their program. Further details will be pub-
lished in a succeeding issue of the JOURNAL.
Aug., 1941] FALL CONVENTION 219
PROGRAM
Monday, October 20th
9:00 a. m. Hotel Roof; Registration.
10:00 a. m. Salle Moderne; Technical session.
12:30 p. m. Roof Garden; Informal Get-Together Luncheon for members, their
families, and guests. Brief addresses by prominent members of
the industry.
2:00 p. m. Salle Moderne; Technical session.
8:00 p. m. Salle Moderne; Technical session.
Tuesday, October 21st
9: 00 a.m. Hotel Roof; Registration.
9:30 a. m. Salle Moderne; Technical session.
2: 00 p.m. Salle Moderne; Technical session.
Open evening.
Wednesday, October 22nd
9:00 a. m. Hotel Roof; Registration.
9:30 a. m. Salle Moderne; Technical and Business session.
Open afternoon.
8:30 p. m. Fiftieth Semi- Annual Banquet and Dance.
Introduction of officers-elect for 1942.
Presentation of the SMPE Progress Medal.
Presentation of the SMPE Journal Award.
Entertainment and dancing.
Thursday, October 23rd
10:00 a. m. Salle Moderne; "Technical session.
2: 00 p.m. Salle Moderne; Technical and business session.
Adjournment
W. C. KUNZMANN,
Convention Vice- President
SOCIETY ANNOUNCEMENTS
1941 FALL CONVENTION
NEW YORK, N. Y.
OCTOBER 20TH-23RD, INCLUSIVE
The 1941 Fall Convention will be held at New York, N. Y., with headquarters
at the Hotel Pennsylvania.
Members are urged to make every effort to attend the Convention, as a very
interesting program of papers and presentations is being arranged.
Details of the Convention will be found elsewhere in this issue of the JOURNAL.
ADMISSION COMMITTEE
At a recent meeting of the Admissions Committee, the following applicants
for membership were admitted into the Society in the Associate grade :
ALDOR, H. H.
22 Rambam St.,
Tel Aviv, Palestine
BUTTERFIELD, A.
210-14, 94th Ave.,
Queens Village, N. Y.
CADENAS, G.
Photographic Committee of the
Mutual Aid Society,
Consolidated Edison Co.,
4 Irving Place,
New York, N. Y.
DEHOFF, H. A.
1224 South Hope Street,
Los Angeles, Calif.
FIELDS, J. L.,
12643 Hortense St.,
North Hollywood, Calif .
GODQUIN, PIERRE
c/o American Consulate,
Casablanca, French Morocco
GREENHALGH, P. J.
1225 Vine St.,
Philadelphia, Pa.
GREGORY, J. R.
1109 Douglas Ave.,
Urbana, 111.
220
KALLMANN, H. E.
417 Riverside Dr.,
New York, N. Y.
KATZ, J. E.
Boyertown Inn,
Boyertown, Pa.
KONISHI, RYO
c/o Far East Laboratories, Ltd.,
3, Uzumasa Yasu Nishuramachi,
Ukyo-ku,
Kyoto, Japan
LAUB, J. H.
Hanovia Chemical & Mfg. Co.,
Chestnut St.,
Newark, N. J.
LEBEL, C. J.
370 Riverside Dr.,
New York, N. Y.
MADISON, H. L.
1521 So. Curson Ave.,
Los Angeles, Calif.
MALOFF, I. G.
RCA Manufacturing Co., Inc.,
Camden, N. J.
MARSEY, J. S.
110 Norton St.,
Rochester, N. Y.
SOCIETY ANNOUNCEMENTS
221
MERCIER, E. G.
405 Comstock Ave.,
Syracuse, N. Y.
MURRAY, C. G.
1132 Bell Bldg.,
1365 Cass Ave.,
Detroit, Mich.
POPPELE, J. R.
Bamberger Broadcasting
Inc.,
1440 Broadway,
New York, N. Y.
PFEIFF, F. J.
P. O. Box 82,
Hamden, Conn.
PRYER, CARL
3807 N.W. Military Rd.,
Washington, D. C.
ROCKLIN, D.
Ft. Monmouth, N. J.
TONOIKE, K.
No. 415, Koenji 2-Chome, Sugin-
amiku,
Tokyo, Japan
VAN DER HOEF, G. T.
903— 16th St., N.W.,
Service, Washington, D. C.
VAN DYKE, W.
515 Madison Ave.,
New York, N. Y.
VICKERS, J. H.
P. O. Box 1145,
Charlotte, N. C.
ZlEGLER, J.
101 Court St.,
Syracuse, N. Y.
In addition, the f ollowing applicant has been admitted to the Active grade :
OWEN, R. L.
1021— 23rd St.,
Santa Monica, Calif.
BACK NUMBERS OF THE TRANSACTIONS AND JOURNALS
Prior to January, 1930, the Transactions of the Society were published quar-
terly. A limited number of these Transactions are still available and will be
sold at the prices listed below. Those who wish to avail themselves of the op-
portunity of acquiring these back numbers should do so quickly, as the supply
will soon be exhausted, especially of the earlier numbers. It will be impossible
to secure them later on as they will not be reprinted.
1924
1925
No.
19
20
21
22
23
24
Price
$1.25
1.25
1.25
1.25
1.25
1.25
1926
1927
No.
25
26
27
28
29
32
Price
$1.25
1.25
1.25
1.25
1.25
1.25
1928
1929
No.
33
34
35
36
37
38
Price
$2.50
2.50
2.50
2.50
3.00
3.00
Beginning with the January, 1930, issue, the JOURNAL of the Society has been
issued monthly, in two volumes per year, of six issues each. Back numbers of
all issues are available at the price of $1.00 each, a complete yearly issue totalling
$12.00. Single copies of the current issue may be obtained for $1.00 each.
Orders for back numbers of Transactions and JOURNALS should be placed through
the General Office of the Society and should be accompanied by check or money-
order.
SOCIETY SUPPLIES
The following are available from the General Office of the Society, at the prices
noted. Orders should be accompanied by remittances.
Aims and Accomplishments. — An index of the Transactions from October,
1916, to December, 1929, containing summaries of all articles, and author and
classified indexes. One dollar each.
Journal Index. — An index of the JOURNAL from January, 1930, to December,
1935, containing author and classified indexes. One dollar each.
Motion Picture Standards. — Reprints of the American Standards and Recom-
mended Practices as published in the March, 1941, issue of the JOURNAL; 50 cents
each.
Membership Certificates. — Engrossed, for framing, containing member's name,
grade of membership, and date of admission. One dollar each.
Journal Binders. — Black fabrikoid binders, lettered in gold, holding a year's
issue of the JOURNAL. Two dollars each. Member's name and the volume
number lettered in gold upon the backbone at an additional charge of fifty cents
each.
Test-Films. — See advertisement in this issue of the JOURNAL.
JOURNAL .
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
Volume XXXVII September, 1941
CONTENTS
Page
New and Old Aspects of the Origins of 96-Cycle Distortion. . .
J. O. BAKER AND R. O. DREW 227
Some Properties of Polished Glass Surfaces F. L. JONES 256
Recent Improvements in Non-Reflective Lens Coating
W. C. MILLER 265
New Gadgets for the Film Laboratory
B. ROBINSON AND M. LESHING 274
M-G-M's NEW Camera Boom J. ARNOLD 278
An Improved Mixer Potentiometer K. B. LAMBERT 283
Report on the Activities of the Inter-Society Color Council . . . 292
Air-Conditioning Safety Device for Theaters E. R. MORIN 307
New Motion Picture Apparatus
Five New Models of 16-Mm Sound Kodascope
W. E. MERRIMAN AND H. C. WELLMAN 313
High Fidelity Headphones L. J. ANDERSON 319
1941 Fall Convention at New York, October 20th-23rd 324
Society Announcements 328
JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
SYLVAN HARRIS, EDITOR
BOARD OF EDITORS
ARTHUR C. DOWNES, Chairman
JOHN I. CRABTREE ALFRED N. GOLDSMITH EDWARD W. KELLOGG
CLYDE R. KEITH ALAN M. GUNDELFINGER CARLETON R. SAWYER
ARTHUR C. HARDY
Subscription to non-members, $8.00 per annum; to members, $5.00 per annum,
included in their annual membership dues; single copies, $1.00. A discount
on subscription or single copies of 15 per cent is allowed to accredited agencies.
Order from the Society of Motion Picture Engineers, Inc., 20th and Northampton
Sts., Easton, Pa., or Hotel Pennsylvania, New York, N. Y.
Published monthly at Easton, Pa., by the Society of Motion Picture Engineers.
Publication Office, 20th & Northampton Sts., Easton, Pa.
General and Editorial Office, Hotel Pennsylvania, New York, N. Y.
West Coast Office, Suite 928, Equitable Bldg., Hollywood, Calif.
Entered as second class matter January 15, 1930, at the Post Office at Easton,
Pa., under the Act of March 3, 1879. Copyrighted, 1941, by the Society of
Motion Picture Engineers, Inc.
OFFICERS OF THE SOCIETY
* *President: EMERY HUSE, 6706 Santa Monica Blvd., Hollywood, Calif.
"Past-President: E. ALLAN WILLIFORD, 30 E. 42nd St., New York, N. Y.
** Executive Vice-President: HERBERT GRIFFIN, 90 Gold St., New York, N. Y.
* Engineering Vice-President: DONALD E. HYNDMAN, 350 Madison Ave., New
York, N. Y.
** Editorial Vice-President: ARTHUR C. DOWNES, Box 6087, Cleveland, Ohio.
* Financial Vice-President: ARTHUR S. DICKINSON, 28 W. 44th St., New York
N. Y.
"Convention Vice-President: WILLIAM C. KUNZMANN, Box 6087, Cleveland, Ohio
^Secretary: PAUL J. LARSEN, 44 Beverly Rd., Summit, N. J.
^Treasurer: GEORGE FRIEDL, JR., 90 Gold St., New York, N. Y.
GOVERNORS
**MAX C. BATSEL, 501 N. LaSalle St., Indianapolis, Ind.
*JOSEPH A. DUBRAY, 1801 Larchmont Ave., Chicago, 111.
*JOHN G. FRAYNE, 6601 Romaine St., Hollywood, Calif.
*ALFRED N. GOLDSMITH, 580 Fifth Ave., New York, N. Y.
*ARTHUR C. HARDY, Massachusetts Institute of Technology, Cambridge,
Mass.
**LOREN L. RYDER, 5451 Marathon St., Hollywood, Calif.
*TIMOTHY E. SHEA, 195 Broadway, New York, N. Y.
*REEVE O. STROCK, 35-11 35th St., Astoria, L. L, N. Y.
*Tenn expires December 31, 1941.
**Term expires December 31, 1942.
NEW AND OLD ASPECTS OF THE ORIGINS OF 96-CYCLE
DISTORTION*
J. O. BAKER AND R. O. DREW**
Summary. — The work of previous investigations is reviewed and correlated with
the results obtained in a comprehensive study of 96-cycle distortion due to the presence
of sprocket-holes adjacent to the sound-track. This distortion has been known for
some time. Much improvement has been made by the adoption of the magnetic-drive
recorder, the non-slip printer, and the rotary stabilizer sound-head for the purpose of
overcoming the problem of slippage.
Recording sound on doubly perforated film will introduce 96-cycle disturbances of
both amplitude and frequency modulation because of the film flexure and possible
variations of film speed at the sprocket-hole rate.
Processing sound records on doubly perforated film will introduce a 96-cycle hum
and amplitude modulation depending upon the processing technic.
Printing sound records on doubly perforated film introduces 96-cycle hum and dis-
turbances of both amplitude and frequency modulation, due to film flexure and varia-
tions of film speed at sprocket-hole rate.
Reproducing sound records on doubly perforated film introduces 96-cycle dis-
turbances because of film flexure.
Since it has been proved that the presence of the sprocket-holes adjacent to the sound-
track is the source of all 96-cycle distortion, and the omission of the sprocket-holes
entirely eliminates this distortion, it becomes obvious that singly perforated film
should be used throughout all phases of sound recording and reproduction if complete
freedom from 96-cycle distortion is to be obtained.
Substantial improvement can be realized if the singly perforated film is employed
only for the original negative, master positive, and re-recorded negative, and doubly
perforated film for the release prints. The use of singly perforated film throughout
all phases has a decided advantage of providing additional space, without affecting the
picture dimensions for a double-width sound-track or two sound-tracks, one for control
or other purposes.
Types of 96-Cycle Distortion. — If a film is given a uniform exposure,
as, for example, in the recording of an unmodulated density track, it
is not uncommon when the film is run through a reproducing machine
to hear a faint tone of 96-cycle pitch. This means that there are
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received June 5,
1941.
** RCA Manufacturing Co., Indianapolis, Ind.
227
•$• The Society is not responsible for statements by authors &
228 J. O. BAKER AND R. 0. DREW [j. s. M. P. E.
fluctuations in the density at the rate of 96 per second. They may
be too small to measure with an ordinary densitometer, but they can I
be measured by means of a wave analyzer. This 96-cycle tone is
absent if the exposure is so low that the film is almost transparent, and
with very high densities there is practically no 96-cycle output, since
the total amount of light transmitted is small. The maximum tone
is produced if the film is a light gray, such as to transmit approxi-
mately half of the incident light. Since variable- width recordings
consist of clear areas and substantially black areas, they are not
ordinarily subject to 96-cycle hum, except when the film is improperly
guided.
If a constant tone (for example, 1000 cps) is recorded on a variable-
density track and means provided for measuring rapid fluctuations in
the amplitude of the reproduced tone, it will usually be found that the
1000-cycle tone rises and falls in amplitude at the rate of 96 times per
second. This effect, like the density modulation, is generally negli-
gible in variable-area recordings.
A "flutter bridge" is a device which measures departures from
normal frequency of a reproduced tone. For example, a constant
3000-cycle tone may be recorded, and when it is reproduced its pitch
or frequency may vary periodically between 2994 and 3006 cycles per
second. Flutter-bridge measurements show that recordings on 35-
mm film are- rarely entirely free from some flutter or frequency modu-
lation at the rate of 96 cycles per second. This is equally true of
variable-density and variable-area recordings.
The density modulation, amplitude modulation, and frequency
modulation are present in the sound-tracks as first recorded, or in
the sound negatives. The operation of duplicating by printing intro-
duces further causes of 96-cycle distortion and while these may
occasionally cause some compensating or neutralizing effect, so that
the print film has less of the 96-cycle distortion than the negative,
ordinarily the effects are cumulative, and prints have more distortion
than the negatives.
REVIEW OF PREVIOUS INVESTIGATIONS
The influence of sprocket-holes upon the recorded sound in 35-mm
films has been known for some time. One effect, which was early
observed and steps taken to correct, was the slippage of the film
when passing over a toothed sprocket. While a sprocket could be
designed to operate without slippage for a film with a given sprocket-
Sept., 1941] ORIGINS OF 96-CvcLE DISTORTION 229
hole pitch, it would not be satisfactory for all films due to shrinkage
which varies with the age and the condition of the film. Some of the
steps taken to overcome the problem of shrinkage were the magnetic
drive recorder1 which utilized a drum without sprocket-teeth, and
suitable damping to insure uniform passage of the film past the re-
cording point; the non-slip printer2 which also employed a smooth
drum, whose diameter is calculated so that with suitable compen-
sating loop-formers, the negative and positive film could contact each
other at the printing point without slippage ; and the rotary stabilizer
sound-head3 which likewise carried the film past the scanning point
over a rotating drum whose motion was smoothed by the rotary
stabilizer.
The adoption of the smooth drum instead of a sprocket for obtain-
ing uniform film motion in the recording, printing, and reproducing of
sound made considerable improvement in the quality, thus eliminat-
ing the 96-cycle hum. The presence of the sprocket-holes adjacent
to the sound-track still introduces 96-cycle flutter in variable-area
sound. Both the 96-cycle hum and 96-cycle flutter are present in
variable-density sound.
Effect of Sprocket-Hole Pitch in Printing. — J. Crabtree4 studied the
production of sound-film prints from variable-density negatives by a
sprocket printer from the viewpoint of high-frequency response and
uniformity of product.
96-Cycle Distortion by Film Processing. — The influence of sprocket-
holes upon the development of a variable-density sound-track was
observed and reported upon by Frayne and Pagliarulo5 in 1936.
They summarize their work as follows :
An unmodulated sound-track shows 96-cycle modulation on development.
The effect is a maximum at the edge of the sprocket-holes and diminishes ex-
ponentially for a distance of approximately 30 mils into the sound-track. A film
modulated by a constant frequency shows 96-cycle amplitude and frequency
modulation over the same area. Both effects are introduced principally during
processing of the film. A film having no sprocket-holes on the sound-track side is
entirely free of these effects. The conclusion is that processing standards in
many laboratories require improvement to eliminate distortions of this type.
96- Cycle Distortion Due to Deformation Next to Sprocket-Holes. —
Crabtree and Herriott6 made a study of the film distortion at the
sprocket-holes when the film is flexed around a curved surface.
They presented a series of photographs of the image of a parallel-line
228 J. 0. BAKER AND R. 0. DREW [j. s. M. P. E,
fluctuations in the density at the rate of 96 per second. They may
be too small to measure with an ordinary densitometer, but they can
be measured by means of a wave analyzer. This 96-cycle tone is
absent if the exposure is so low that the film is almost transparent, and
with very high densities there is practically no 96-cycle output, since
the total amount of light transmitted is small. The maximum tone
is produced if the film is a light gray, such as to transmit approxi-
mately half of the incident light. Since variable- width recordings
consist of clear areas and substantially black areas, they are not
ordinarily subject to 96-cycle hum, except when the film is improperly
guided.
If a constant tone (for example, 1000 cps) is recorded on a variable-
density track and means provided for measuring rapid fluctuations in
the amplitude of the reproduced tone, it will usually be found that the
1000-cycle tone rises and falls in amplitude at the rate of 96 times per
second. This effect, like the density modulation, is generally negli-
gible in variable-area recordings.
A "flutter bridge" is a device which measures departures from
normal frequency of a reproduced tone. For example, a constant
3000-cycle tone may be recorded, and when it is reproduced its pitch
or frequency may vary periodically between 2994 and 3006 cycles per
second. Flutter-bridge measurements show that recordings on 35-
mm film are' rarely entirely free from some flutter or frequency modu-
lation at the rate of 96 cycles per second. This is equally true of
variable-density and variable-area recordings.
The density modulation, amplitude modulation, and frequency
modulation are present in the sound-tracks as first recorded, or in
the sound negatives. The operation of duplicating by printing intro-
duces further causes of 96-cycle distortion and while these may
occasionally cause some compensating or neutralizing effect, so that
the print film has less of the 96-cycle distortion than the negative,
ordinarily the effects are cumulative, and prints have more distortion
than the negatives.
REVIEW OF PREVIOUS INVESTIGATIONS
The influence of sprocket-holes upon the recorded sound in 35-mm
films has been known for some time. One effect, which was early
observed and steps taken to correct, was the slippage of the film
when passing over a toothed sprocket. While a sprocket could be
designed to operate without slippage for a film with a given sprocket-
Sept., 1941 ] ORIGINS OF 96-CvcLE DISTORTION 229
hole pitch, it would not be satisfactory for all films due to shrinkage
which varies with the age and the condition of the film. Some of the
steps taken to overcome the problem of shrinkage were the magnetic
drive recorder1 which utilized a drum without sprocket-teeth, and
suitable damping to insure uniform passage of the film past the re-
cording point; the non-slip printer2 which also employed a smooth
drum, whose diameter is calculated so that with suitable compen-
sating loop-formers, the negative and positive film could contact each
other at the printing point without slippage; and the rotary stabilizer
sound-head3 which likewise carried the film past the scanning point
over a rotating drum whose motion was smoothed by the rotary
stabilizer.
The adoption of the smooth drum instead of a sprocket for obtain-
ing uniform film motion in the recording, printing, and reproducing of
sound made considerable improvement in the quality, thus eliminat-
ing the 96-cycle hum. The presence of the sprocket-holes adjacent
to the sound-track still introduces 96-cycle flutter in variable-area
sound. Both the 96-cycle hum and 96-cycle flutter are present in
variable-density sound.
Effect of Sprocket-Hole Pitch in Printing. — J. Crabtree4 studied the
production of sound-film prints from variable-density negatives by a
sprocket printer from the viewpoint of high-frequency response and
uniformity of product.
96-Cycle Distortion by Film Processing. — The influence of sprocket-
holes upon the development of a variable-density sound-track was
observed and reported upon by Frayne and Pagliarulo5 in 1936.
They summarize their work as follows :
An unmodulated sound-track shows 96-cycle modulation on development.
The effect is a maximum at the edge of the sprocket-holes and diminishes ex-
ponentially for a distance of approximately 30 mils into the sound-track. A film
modulated by a constant frequency shows 96-cycle amplitude and frequency
modulation over the same area. Both effects are introduced principally during
processing of the film. A film having no sprocket-holes on the sound-track side is
entirely free of these effects. The conclusion is that processing standards in
many laboratories require improvement to eliminate distortions of this type.
96-Cycle Distortion Due to Deformation Next to Sprocket-Holes. —
Crabtree and Herriott6 made a study of the film distortion at the
sprocket-holes when the film is flexed around a curved surface.
They presented a series of photographs of the image of a parallel-line
230 J. O. BAKER AND R. O. DREW [J. S. M. P. E.
screen reflected from the emulsion surface of 35-mm film when flexed
around drums of different diameters and sprockets of various sizes.
Measures for Reducing 96-Cycle Distortion. — Steps have been taken
to minimize the distortion, for instance :
(a) Constant-speed drum for recording, printing, and reproducing.
(&) Standardization of sprocket-hole dimensions for sound negative and positive
film stock.
(c) The reduction of the maximum film shrinkage from 1.5 per cent to 0.4 per
cent.
(d ) More thorough agitation of developer solutions to reduce the density varia-
tion during processing.
There still remains the problem of flexure of the film and the mis-
matching of negative and positive perforations in non-slip printing.
ANALYSIS
Flutter Is Not Due to Changes in Speed of the Film Drum. — Tests by
the authors completely confirm the conclusions reported by Frayne
and Pagliarulo, namely, that if the sprocket-holes on the sound-track
side are omitted, none of the 96-cycle distortions appear. Our tests,
which are described in more detail later, were made using singly per-
forated film, the recording being done on a machine of the magnetic
drive type in which average film speed is controlled by sprockets but
the film is carried past the optical system on a smooth drum. Such a
test proves that there are no 96-cycle fluctuations in the speed of the
film as a whole (or, in other words, in the speed of the drum) but that
the 96-cycle flutter which the doubly perforated film exhibits must be
a purely local effect of the proximity of the perforations.
Since there is no mystery in the appearance of 96-cycle flutter
when the recording or reproducing machines are of the sprocket-
propulsion type, it will be assumed throughout the remainder of this
paper that there is no 96-cycle flutter in the speed of the drum which
carries the film, and our investigation is of the other possible causes of
the flutter.
Density Variations. — Variations in density are produced, first, by
exposure variations due to the polygoning effect of the film when
flexed around a curved surface, and, second, by the increased agitation
of the developer at the sprocket-holes in processing.
Density variations at the sprocket-hole rate results in hum or a 96-
cycle tone which is superimposed on the recorded modulation. In
the case of variable-density recordings, the increased agitation at the
Sept., 1941]
ORIGINS OF 96-CvcLE DISTORTION
231
sprocket-holes also increases the gamma which produces an amplitude
variation of the recorded modulation. Density variations or changes
in gamma will, of course, have little effect upon variable-area record-
ings.
In discussing the causes of density fluctuations consideration needs
to be given only to unmodulated sound records. The increased agita-
tion of the developer at the sprocket-holes will increase the developer
speed at that region and, assuming the exposure to be uniform, micro-
SPROCKET
REGION
FIG. 1.
Polygoning of film when flexed around a
drum.
densitometer measurements would show a maximum density opposite
the center of the sprocket-holes, provided there were no directional
effects. If directional effects are in evidence, then the maximum
density will be shifted to one side of the center of the sprocket-hole.
The exposure, however, may not be uniform. The film stiffness
varies along its length depending upon the cross-section, and will be
greater in the region between successive sprocket-holes than it will be
at the sprocket-holes. Hence, when flexed around a curved surface,
the film will be closer to that surface where the film stiffness is greatest
(Fig. 1). The effect of this is a shorter radius from the center of
rotation of the drum at this point and a longer radius at the sprocket-
232 J. O. BAKER AND R. O. DREW [J. S. M. P. E.
holes. With the drum rotating at, a constant angular velocity the
film is undergoing a speed variation at the sprocket-hole rate propor-~
tional to the variations in its distance from the axis. Since exposure
of the film is dependent upon the product of the intensity of exposing
light and exposure time, it is easily seen that the exposure of the film
will be less at the sprocket-holes than between the sprocket-holes.
The effect of variations in negative exposure is probably small com-
pared to the developer effect, but whatever there is it will tend to
counteract the latter.
The exposure and developing effects will hold for eithefnegative or
positive. However, in the case of the positive there is the additional
effect of the varying transparency of the negative.
Density Variations in Printing. — As outlined in the preceding sec-
tion a negative will have density variations along the sound-track at
the sprocket-hole rate of 96 cycles. For the sake of simplicity, as-
sume the variation to be symmetrical with respect to the sprocket-
holes, that is, a maximum density at the center of the hole and a
minimum at the center of the region between two adjacent sprocket-
holes. Neglecting for the moment the effect of polygoning in the
printer, the positive exposure will vary in inverse relationship to the
density of the negative. Thus, for the case of negative and print
sprocket-holes in register, such as printing on a sprocket printer, the
exposure of the positive will be least at the center of its sprocket-holes
and greatest at the center of the region between sprocket-holes.
Upon development of the positive, the sprocket-hole region will de-
velop faster and the increased development will tend to compensate
for the underexposure. The net result will be that the regions of
maximum density in the print may be opposite the sprocket-holes, or
between the sprocket-holes, depending upon whether the effect of
exposure variations or development variations predominates; or
there may be no measurable 96-cycle hum at all in the print, if the
effects balance.
In the case of negative and positive sprocket-holes out of register,
as occurs part of the time in printing on a non-slip printer, the positive
exposure is greatest at the sprocket-holes and least in between. The
result in this case is a 96-cycle component in the positive which is the
cumulative effect of variations in the print exposure, and the greater
development which the print receives next the sprocket-holes.
When prints are made on a non-slip printer the relative positions of
the negative and positive sprocket-holes can not be predicted, and
Sept., 1941] ORIGINS OF 96-CvcLE DISTORTION 233
may slowly change from in register to out of register. Therefore,
prints made on non-slip printers have been more frequently criticized
on the score of hum than prints made on sprocket printers.
The explanations just given do not take account of directional
effects in development. These may tend to obscure the relation-
ships, so that the advantages of printing with sprocket-holes in
registration are not as definite as would be expected.
The disadvantage of the non-slip printer for making variable-
density prints on the score of giving higher hum levels, would dis-
appear if singly perforated film were used for negatives, since a print
made on it would have only the 96-cycle distortion introduced by
print processing while the print made on a sprocket printer would
have a 96-cycle distortion produced by slippage as well as that due to
processing.
Amplitude Modulation. — When a tone is recorded on a variable-
density system and reproduced, the amplitude of the reproduced
tone is proportional to the difference in transmissions at the peak and
valley of the wave. High gamma, by increasing contrast, produces a
greater difference between maximum and minimum density, but not
necessarily a greater difference between maximum and minimum
transmission. The density difference corresponds to the ratio of
maximum to minimum transmissions, but a high ratio means a large
absolute difference only when the average transmission is high. Thus
it is quite common to reduce the output level of a variable-density
print by printing it darker than normal.
In the case of a negative the increased development next to the
sprocket-holes tends to give greater contrast but since it also makes
the negative denser at these places the output of recorded tone may be
either greater or less opposite the sprocket-holes than in between, de-
pending upon which effect predominates. When this same negative
is printed, however, on a printer which keeps the sprocket-holes in
registration, the high contrast in the negative is passed on to the
print, and the denser negative area, which goes with the high con-
trasts, produces a lighter print. Both effects then tend to increase
the amplitude of the output tone opposite the sprocket-holes. It has
already been explained that if a print is made with sprocket-holes in
registration the development effects in negative and print may neu-
tralize so far as average print density is concerned. If this occurs
such a print may show little or no hum, but the additive effects of in-
creased contrast in negative and print would tend to produce con-
234 J. O. BAKER AND R. O. DREW [J. S. M. P. E.
siderably greater amplitude of recorded tone opposite the sprocket-
holes.
In the case of variable-area recording similar factors are at play,
but their effects will be negligibly small except at very high frequen-
cies, where fogging between waves becomes a factor of importance.
Thus hi recording a 9000-cycle wave considerable amplitude modula-
tion as well as hum may result from unequal development of negative
and print around the sprocket-holes.
96-Cycle Distortion by Frequency Modulation. — It has been shown
how 96-cycle density modulation can appear in an unmodulated
sound-track due to density variations. If a signal frequency is re-
corded on a sound-track adjacent to the sprocket-holes, a 96-cycle fre-
quency modulation is introduced. Frequency modulation will occur
whenever the speed of the film is not constant in its travel past the
recording point. Constant speed is not attainable when recording on
a sprocket or on a skid or drum where the film is propelled by a
sprocket unless the sprocket-teeth and the sprocket-holes of the film
match perfectly as shown by previous investigation.1'4 Frequency
modulation, however, may occur when recording on a constant-speed
drum, due to the unequal flexing of the film previously discussed.
The frequency modulation does not result in a hum or 96-cycle tone,
but in distortion of the recorded waves, and becomes of considerable
consequence especially when considering high frequencies.
Varying Contact in Printing. — Printing from a negative containing
96-cycle frequency modulation will transfer this modulation from the
negative to the print and add more 96-cycle distortion, for the reasons
that the positive stock polygons in a manner similar to that of the
negative. The polygoning of the two films hinders the contacting of
the two emulsions at every point along the length of the sound-track.
As a result, wherever the two emulsions are not in contact a spreading
of the negative image on the positive reduces the resolution and, con-
sequently, the amplitude, particularly of high-frequency waves.
Thus an amplitude modulation is introduced which is yet different
from that due to density variations. In addition to the amplitude
variations, there will be a filling in of the clear areas between the high-
frequency waves, which, since it gets better and worse at the rate of
96 cycles per second, will produce a hum.
96-Cycle Flutter in Printing. — A doubly perforated negative con-
taining a 1000-cycle note with a known amount of 96-cycle frequency
modulation was printed on a non-synchronous non-slip printer. In
Sept., 1941] ORIGINS OF 96-CvcLE DISTORTION 235
this printer the raw stock and the negative were carried on a smooth
drum, with the negative outside. Since this arrangement is the
opposite of that which would tend to compensate for negative shrink-
age, the negative slowly crept ahead of the print, causing the sprocket-
holes to be alternately in and out of registration. The 96-cycle fre-
quency modulation component of the print was observed on a flutter
bridge. The amplitude of the 96-cycle flutter varied between
maximum and minimum values. The observations showed that the
maximum occurred when the sprocket-holes of negative and print
were in register, and the minimum when the sprocket-holes were out
of register.
It must be borne in mind that there is a difference between the 96-
cycle flutter obtained in this experiment and that which would be
obtained with a standard non-slip printer. In the case of the non-
synchronous non-slip printer just described the variation of 96-cycle
print output is of a periodic nature varying between definite maximum
and minimum values. In a standard non-slip printer the flexure of
the print stock takes a certain form depending upon the amount of
shrinkage of the negative. This form will be maintained so long as
the shrinkage is uniform along the length of film. Therefore, the
amount of 96-cycle flutter introduced in the printing process will
assume a certain value and will maintain that value at least for
considerable periods of time. Thus, the 96-cycle flutter of the print
may be of any value ranging between the maximum and minimum
depending upon the amount of negative shrinkage.
96- Cycle Distortion in Reproduction. — Both hum and frequency
modulation are generated in the reproducing. This distortion origi-
nates from several sources, namely :
(1) Stray light through sprocket-holes.
(2} Reflections within the emulsion and film base, which are affected by the
perforations.
(3} The polygoning of the film which produces unequal film speed past the
scanning point. There is also a stretching of the emulsion where the curvature is
greatest, or, in other words, opposite the perforations.
EXPERIMENTAL PROCEDURE
The experimental work reported here was directed to the :
(1) Determination of the effects of exposure and development upon the magni-
tude of the several types of 96-cycle distortion, using standard (doubly perforated)
film, for both density and area tracks,
236 J. O. BAKER AND R. O. DREW [j. s. M. P. E.
(2) Determination of whether any of the 96-cycle flutter is due to imperfect
mechanical filtering of the disturbances occurring at the sprocket, or whether the I
effects are entirely local and due to the proximity of the sprocket-holes in the ^
sound-track.
(5) Effects of printing with sprocket-holes in register or out of register upon the
hum, amplitude modulation, and the flutter.
(4) Determination of the hum and flutter introduced in reproduction, using a
machine having no 96-cycle flutter in its drum rotation.
Recording. — Recordings of both variable-area and variable-density
records were made of 1000-cycle and 9000-cycle tones and of unmodu-
lated tracks.
The variable-density records were recorded with white light and
the variable-area records with ultraviolet light.
The recorder was of the magnetic-drive type, adjusted to produce a
minimum of flutter in the recording.
The unmodulated tracks were used to study density modulation
especially in the case of the variable-density records.
A 1000-cycle tone was recorded for the study of frequency modula-
tion, this being the frequency for which our flutter bridge is designed.
A 9000-cycle frequency was recorded to study the effect of the 96-
cycle disturbance upon very high frequencies.
Recordings were made on both double perforated and single per-
forated stock.
Processing. — The various recordings were then processed in a small
developing machine having a 40-gallon developing tank with a
circulation of the developer of 4 gallons per minute and a film speed
through the developer of 10 feet per minute. Those are not ideal
developing conditions, as they are conducive to exaggeration of
sprocket-hole turbidity and directional effects. They are, however,
quite suitable for the purpose of this investigation.
The variable-density negatives were processed in D-76 developer
for values of density ranging from 0.08 to 1.15 for gammas of 0.37,
0.60, and 0.75.
The variable-area negatives were processed in D-16 developer to
various densities ranging from 2.0 to 2.53 at a gamma of 2.3.
Printing. — The prints, with a few indicated exceptions, were made
on the special non-slip laboratory printer already described, employ-
ing a three-inch drum and a heavy flywheel for constant film speed.
The variable-density records were printed with white light and the
variable-area records with ultraviolet light.
Sept., 1941] ORIGINS OF 96-CYCLE DISTORTION 237
The prints were printed for various values of density and all proc-
essed for a gamma of 2.13 in standard D-16 developer.
Reproducing. — Both negatives and prints were reproduced on a
special sprocketless laboratory reproducer utilizing a 2-inch diameter
drum and a heavy flywheel for imparting constant speed to the film.
The film is pulled by the drum. An optical system, which gives a
particularly uniform 1-mil scanning beam, was employed.
Wave Analysis. — All recordings were measured on a General Radio
Type 7 36 A Wave- Analyzer for 96-cycle output and the sum and
difference frequencies of the recorded frequency and 96 cycles.
Assurance was made in every case that the readings observed were of
the frequencies under consideration and not simply noise. The
noise was measured by tuning the analyzer a few cycles off the desired
frequency.
A 4-cycle band-pass filter was used for measurements of the un-
modulated tracks and of the 1000-cycle records. A 20-cycle band-
pass filter was used for the measurements of the 9000-cycle record.
The speed of the reproducer was sufficiently constant to obtain prac-
tically constant readings from the records in question.
Flutter Analysis. — The 1000-cycle records were the only ones which
could be utilized for the detection of 96 cycles on the flutter bridge.
The particular flutter bridge used employs two tuned circuits, one of
which shows maximum at 950 cycles and the other at 1050 cycles.
The deviations of the input frequency from 1000 cycles are measured
by the difference between the voltages across the two tunings. This
arrangement is relatively insensitive to small fluctuations in the
amplitude of the input current, but to reduce still further errors due to
such variations, a limiter was placed ahead of the flutter bridge to
insure constant amplitude input to the bridge. A 96-cycle band-pass
filter was employed also in conjunction with the bridge to suppress
flutter of other frequencies, and thus make it easier to estimate the 96-
cycle flutter.
Density Variations. — For the determination of the variations in
density along the sound-track, the reproducer described above was
used and rotated by hand to bring various sections of the track under
the scanning beam. The transmission of the film was determined
from readings of photocell output as read by means of an ultra-
sensitive d-c meter.
96- Cycle Hum Due to Optical Conditions of Reproduction. — An un-
exposed film taken fresh from its original container was processed
J. O. BAKER AND R. O. DREW [J. s. M. P. E.
according to standard variable-area, technic, and except for a fog
value of 0.02, was entirely free of any other density. This film was
TABLE I
Variable Density Negatives
Rows of
Perforations Gamma
Density
Mod.
Freq.
Densitometric Level — Decibels
90 Cps 96 Cps 102 Cps 1000 Cp
Double 0.37
0.13
1000
-60
-46
-60
-9
0.13
0
-60
-44
-60
0
0.46
1000
-60
-39.5
-60 _
-2.5
0.46
0
-61
-38.8
-61
0
0.61
1000
-61
-38.4
-61
-5
0.61
0
-62
-41
-62
0
Single 0.37
0.08
1000
-64.5
-64.5
-64.5
-7.8
0.08
0
-64.5
-64.5
-64.5
0
0.38
1000
-64.5
-64.5
-64.5
-2.4
0.38
0
-65
-65
-65
0
0.56
1000
-66
-66
-66
-4.2
0.56
0
-68
-68
-68
0
Double 0.60
0.20
1000
-65
-45
-65
-3
0.20
0
-65
-43
-65
0.56
1000
-65
-45
-65
-2
0.56
0
-64.5
-44
-64.5
0.78
1000
-66
-45
-66
A
0.78
0
-67
-46.8
-67
Single 0 . 60
0.18
1000
-70.3
-70.3
-70.3
-3.2
0.18
0
-68
-68
-68
. . .
0.53
1000
-70.3
-70.3
-70.3
-1.8
0.53
0
-70.3
-70.3
-70.3
0.76
1000
-70.3
-70.3
-70.3
-4.2
0.76
0
-74
-74
—74
Double 0.75
0.26
1000
-63
-44.4
-63
-2.0
0.26
0
-61
-41.4
-61
0.66
1000
-64.4
-46
-64.4
-1.3
0.66
0
-68
-44
-68
0.92
1000
-68
-46
-68
-3.0
0.92
0
-69
-46
-69
1.15
1000
-68
-47
-68
-8.0
1.15
0
-74
-50.3
-74
Single 0.75
0.26
1000
-68
-68
-68
-1.0
0.26
0
-70
-70
-70
0
0.64
1000
-71
-71
-71
-1.0
0.64
0
-74
-74
-74
0
0.86
1000
-71
-71
-71
-2.2
0.86
0
-74
-74
-74
0
1.09
1000
-74
-74
-74
-7.2
1.09
0
-80
-80
-80
0
Sept., 1941]
ORIGINS OF 96-CvcLE DISTORTION
239
«. £
s
240 J. O. BAKER AND R. O. DREW [J. s. M. P. E.
measured for 96 cycles and used as a negative for making prints
which were also measured.
To be certain that the fog density was not being modulated, a piece
of film was fixed without being developed and measurements of 96
cycles made thereon.
Some of the singly perforated negatives, after having been mea-
sured and used for the necessary prints, were returned to the Eastman
Kodak Company and perforated on the unperforated side. They
were then again measured for their 96-cycle content on both the wave
analyzer and the flutter bridge. In this test, all sources of 96-cycle
distortion except those which occur in reproduction were eliminated.
RESULTS OF INVESTIGATION
Variable-Density Negatives. — A series of variable-density negatives
using the penumbra-galvanometer system were recorded with 1000-
cycle modulation and without modulation. The same film emulsion
was used throughout, but differed in the respect that part of the nega-
tives was made on the standard sound-recording stock with a double
row of perforations, and part on a stock with the perforations on the
sound-track side omitted. Both stocks were processed simultane-
ously to three values of gamma, 0.37, 0.60, and 0.75, in D-76 negative
developer at 65°F. The densities obtained varied from 0.08 to 1.15.
The amplitude of the 1000-cycle recording was adjusted for approxi-
mately 90 per cent.
The results obtained for the negatives from readings made on a
General Radio, Wave- Analyzer are given in Table I. The analyzer
readings are converted to terms of "densitometric level" which is the
name we have adopted to express the light-modulating ability of the
film itself, independently of any reproducing system. Zero level is
the output of an ideal variable-density film carrying a track of 0.70-
inch amplitude, which according to present standards is 100 per cent
modulation.
The following points should be noted :
(i) With the singly perforated film the 96-cycle output is the same as that at 92
or 100 cycles, or, in other words, when the analyzer is set for 96 cycles there is only
the slight response due to general film noise. This is true whether a 1000-cycle
tone is recorded or not.
(2} With double perforations the 96-cycle hum is from 20 to 23 db above the
noise.
. (5) Differences in density and gamma make only a minor difference in the hum.
As would be expected, high density gives reduced hum, but it also gives low output
of the recorded tone.
Sept., 1941] ORIGINS OF 96-CvcLE DISTORTION 241
Fig. 2 is a set of curves of the 96-cycle values of Table I plotted
against negative density for both doubly perforated and singly per-
forated unmodulated negatives.
Fig. 3 is a similar set of curves for the 1000-cycle negatives.
FIG. 4. Comparison of doubly and singly perforated film for 96-cycle fre-
quency modulation for variable-density 1000-cycle records developed to vari-
ous densities at a gamma of 0.37.
FIG. 5. Comparison of doubly and singly perforated film for 96-cycle fre-
quency modulation for variable-density 1000-cycle records developed to vari-
ous densities and gammas.
Figs. 4 and 5 are oscillograms of the 1000-cycle variable-density
negatives for both the doubly and singly perforated films. It will be
noted that the traces obtained from the singly perforated film differ
from the static trace of the "wow-meter" light-beam only by the
noise present in the film.
Variable-Area Negatives. — Recordings were made on an unmodu-
lated full- width exposed track, 1000 cycles and 9000 cycles, using the
galvanometer bilateral system on doubly perforated stock and proc-
242 J. O. BAKER AND R. O. DREW Ij. s. M. P. E.
essed for three values of negative density of 2.0, 2.3, and 2.53 at a
gamma of 2.3 in D-16 positive -developer at 68°F. These negatives
were measured with the wave-analyzer and also with the flutter bridge.
The results obtained for the negatives from readings on a Type
7 36- A General Radio Wave- Analyzer are given in Table II.
TABLE H
Variable Area Negative — Gamma 2.3 Double Perforated Stock
Mod.
Density
2.0
2.3
2.53
0.02
2.0
Freq. 90 Cps
0 -66
0 -76.4
0 -70.5
0 -56.5
1000 -62
96 Cps
-55
-54.4
-52
-51
-50.8
102 Cps
-66
-70.5
-70.5
-58.4
-63
904 Cps
-46
1000 Cps
-4.0
1096 Cps
-47.3
2
.3
-62
-50.8
-62
-45
.2
-3,
.0
-46.8
2
.53
-64
.5
-49.3
-64.
5
-46
.8
—4
.5
-47.3
8904
Cps
9000 Cps
9096 Cps
2
.0
9000 -59
-35
-60
-32
.5
-17.
.6
-32.5
2.
3
-58
.4
-36
-59
-37
-20
.4
-37
2
.53
-56
.5
-37
-53.
6
-40
-25
-40
The values of Table II above are shown graphically in Fig. 6.
With zero or 1000-cycle modulation the 96-cycle hum is from 5 to 8 db
less than in the case of the variable-density negatives. The 904 and
1096-cycle sidebands of the 1000-cycle recording could be due either
to amplitude or frequency modulation. Variable area tracks are not
subject to much amplitude modulation (except at very high fre-
quencies) and the magnitude of the sidebands measured (about 43 db
below the recorded tone) is what would result from a frequency modu-
lation of about 0.03 per cent which is about that indicated in the flut-
ter record of Fig. 4. Hence the sidebands may be ascribed primarily
to frequency modulation, although there may be a nearly equal
amount (in terms of sideband amplitude) of amplitude modulation.
In the case of the 9000-cycle recording, the tone level is from 14 to 20
below that of the 1000-cycle recording, while the sidebands are about
9 db higher. This is practically pure amplitude modulation. There
is also a large density modulation (96-cycle output) in the 9000-cycle
recording, more than in the case of the unmodulated density track.
This is due for the most part to the higher development of an area
negative. In the case of prints made from these negatives there
might be little difference between the hum from the 9000-cycle area
track and the unmodulated density track, but it should be remem-
Sept., 1941]
ORIGINS OF 96-CvcLE DISTORTION
243
5,
244
J. O. BAKER AND R. O. DREW
[J. S. M. "P. E.
Sept., 1941] ORIGINS OF 96-CvcLE DISTORTION 245
bered that the strong hum in the area track occurs only in case of con-
tinuous recording of high frequencies.
Variable-Density Prints. — A portion of the unmodulated variable-
density negatives was printed with white light on the non-slip printer
and processed to a gamma of 2.13 in standard D-16 positive developer
at 65°F. Both doubly and singly perforated negatives were printed
and measured on the General Radio Type 736-A Wave-Analyzer for
the 96-cycle amplitude modulation component. The ground-noise
was checked and found to be considerably lower than the 96-cycle
readings, and it was thought unnecessary to repeat the readings.
Fig. 7 is a set of curves showing the results of these measurements for
those negatives having a density which had previously been found to
be the best value for the least amount of harmonic distortion when
printed to an average density of approximately 0.6. Within the
limits of experimental error, these curves indicate that the choice of
negative density and gamma has little effect upon the 96-cycle distor-
tion due to density modulation. The prints made from the doubly
perforated negatives have approximately 12 db more 96-cycle dis-
tortion than those made from the singly perforated negatives.
Effect of Printing upon Flutter. — We have seen that whenever a film
is bent around a circular support it bends more opposite the holes and
less between, forming what we might describe as a "polygon." This
is illustrated in Fig. 1. This has some effect on the linear speed and
the exposure. The effect of the linear speed variations upon flutter
is, however, probably less than the effect of stretching or compressing
the emulsion by the bending.
For the purpose of discussing the effects in printing, we may assume
that the 1000-cycle waves on the negative are of uniform pitch when
the film is straight. Fig. 8 (a) shows the conditions during printing
when the sprocket-holes are in register. In this case we have as-
sumed that the negative is on the outside, which was the condition in
our experimental work. It will be seen in Fig. 8 (a) that where the
curvature is sharpest the emulsion side of the negative will be com-
pressed and the emulsion of the print stretched. When the print is
developed and held straight, waves opposite the sprocket-holes will be
compressed, as compared with their pitch during printing, and this
will add to the effect of the compressed waves upon the negative dur-
ing the printing operation. Thus, printing in register would tend to
result in higher reproduced frequency when scanning the track oppo-
site the perforations, than when scanning the part in between.
246
J. O. BAKER AND R. O. DREW
[J. S. M. P. E.
Fig. 8(b) shows the conditions with the sprocket-hole staggered
during printing. Here the compressed waves of the negative are
opposite the unstretched emulsion of the raw stock and the uncom-
pressed negative waves are opposite the stretched raw-stock emulsion.
This tends to neutralize the flutter due to polygon bending during
printing.
Fig. 9 shows an oscillogram taken on the flutter bridge with the
negative slowly progressing with respect to the print so that the
sprocket-holes are alternately in and out of register. The print was
carefully examined for the location of in-register and out-6f-register
points of the negative and print sprocket-holes and inked lines
were drawn across the track for reference. For the in-register con-
FIG. 9. Variations in 96-cycle frequency modulation with negative and
print sprocket-holes passing in and out of register.
1000
0
TABLE HI
Doubly Perforated Negative
Gamma = 0.60, Density = 0.56
Freq.
Print
Den.
Printer
Print
Perfora-
tions 90 Cps
96 Cps 102 Cps
904 Cps 1000 Cps
1096 Cps
1000
0.64
B &H
Double
-60
-43
-60
-38.5
-8
.5
-38.
5
0.60
Non-Slip
Double
-60
37-46
-60
38-54
-8
,0
38-54
0.53
Non-Slip
Single
-67
-43
-67
-39
-6
8
-39
0
0.64
B&H
Double
-60
-40
-60
0
0.60
Non-Slip
Double
-62
39-50
-62
0
0.53
Non-Slip
Single
-66
-42
-66
Data for the Negative
-65 -45 -65 -44 -2 -44
-65 -44 -65
Sept., 1941]
ORIGINS OF 96-CYCLE DISTORTION
247
dition, a single line was used and for the out-of-register condition,
two lines. These lines produce a disturbance in the flutter bridge of
the in-register and out-of-register condition on the oscillograms. It
will be noted that printing with the sprocket-holes in register is best
for minimizing hum while printing them out of register is best for
minimizing flutter.
Two of the variable-density doubly perforated negatives were
printed on a Bell & Howell printer, with the row of teeth next the
sound-track removed from the printing sprocket. A comparison of
the 96-cycle distortion occurring in prints made on the Bell & Howell
and on the non-slip printer is given in Table III.
TABLE IV
Variable-Area Doubly Perforated Negatiies
Variable-Area Doubly Perforated Prints
Gamma =
Gamma =
= 2.30
= 2.13
Neg.
Print
Freq.
Den.
Den.
90 Cps
96 Cps
102 Cps
904 Cps
1000 Cps
1096 Cps
1000
2.0
0.92
-63
-41-
-63
44 db
1000
2.3
0.92
-63
42-45
-64
42-45
A
42-45
1000
2.53
0.92
-62
43-46
-63
41-45
-3.8
41-45
0
0.02
0.92
-66
44-48
-66
1000
2.0
1.17
-63
43-47
-63
41-45
-3.8
41-45
1000
2.3
1.17
-68
44-47
-68
42-46
-3.9
42-46
1000
2.53
1.17
-70
43-48
-70
41-47
-4.0
41-47
0
0.02
1.17
-68
49-52
-68
1000
2.0
1.32
-65
43-46
-65
39-45
-3.7
39-45
1000
2.3
1.32
-65
43-48
-65
40-45
-3.7
40-45
1000
2.53
1.32
-65
44-48
-65
39-45
-3.0
30-45
0
0.02
1.32
-70
50-54
-70
1000
2.0
1.57
-66
42-46
-66
39-44
-3.4
39-44
1000
2.3
1.57
-66
42-46
-66
37-45
-3.3
37-45
1000
2.53
1.57
-66
42-46
-66
38-44
-2.7
38-44
0
0.02
1.57
-75
54-56
-74
1000
2.0
1.68
-68
41-46
-68
41-46
-3.3
41-46
1000
2.3
1.68
-68
41-46
-68
38-45
-2.9
38-45
1000
2.53
1.68
-69
42-46
-69
39-45
-2.8
39-45
0
0.02
1.68
-74
57-59
-74
1000
2.0
1.83
-66
41-46
-66
39-45
-3.4
39-45
1000
2.3
1.83
-66
42-46
-66
39-45
-2.8
39-45
1000
2.53
1.83
-66
41-46
-66
38-46
-2.7
38-46
0
0.02
1.83
-80
58-60
-80
1000
2.0
2.04
-62
39-48
-62
39-45
-3.5
39^5
1000
2.3
2.04
-62
31-46
-62
40-46
-4.5
40^6
1000
2.53
2.04
-63
39^5
-63
38-45
-2.7
38-45
0
0.02
2.04
-80
59-62
-80
248
J. O. BAKER AND R. O. DREW
[J. S. M. P. E.
Freq.
Neg.
Den.
Print
Den.
90 Cps
96 Cps
^02 Cps
8904 Cps
9000 Cps
9096
Cps
9000
2.0
0.92
-57
33-39
-57
30-37
-18
30-37
9000
2.3
0.92
-50
32-37
-50
30-36
-17
30-36
9000
2.53
0.92
-50
31-37
-30
31-38
-19
31-38
9000
2.0
1.17
-50
30-37
-50
29-38
-16
29-38
9000
2.3
1.17
-50
30-36
-50
28-34
-16
28-34
9000
2.53
1.17
-53
20-36
-53
29-35
-17
29-35
9000
2.0
1.32
-56
30-35
-56
29-35
-15
29-34
9000
2.3
1.32
-55.5
30-36
-55.5
28-36
-15
28-36
9000
2.53
1.32
-55
29-34
-55
29-35
-16.2
29-35
9000
2.0
1.57
-56
28-35
-55
29-35
-15-
29-35
9000
2.3
1.57
-56
28-35
-56
27-33
-15
27-33
9000
2.53
1.57
-55
27-34
-55
30-37
-16
30-37
9000
2.0
1.68
-58
28-35
-58
28-34
-15
28-34
9000
2.3
1.68
-56.5
28-34
-56.5
29-33
-15
29-33
9000
2.53
1.68
-54
28-34
-54
29-33
-16.2
29-33
9000
2.0
1.83
-53
29-36
-53
30-35
-16
30-35
9000
2.3
1.83
-56
29-36
-56
29-34
-15
29-34
9000
2.53
1.83
-53
27-34
-53
30-34
-15
30-34
9000
2.0
2.04
-49
28-37
-49
29-36
-17
29-36
9000
2.3
2.04
-51
28-36
-51
30-35
-14
30-35
9000
2.53
2.04
-52
28-35
-52
30-35
-15
30-35
Variable-Area Prints. — The variable-area negatives were printed
on the non-slip printer to a series of print densities ranging irom 0.92
to 2.04. These prints were made on doubly perforated positive stock
for determining the effect of negative and print densities upon the 96-
cycle distortion. The results of the wave-analyzer measurements are
given in Table IV.
The results of these measurements show that there is very little
change in the amount of 96-cycle distortion with changes in either
the negative or print densities for any given frequency.
The 96-cycle density modulation is approximately 25 db above the
ground noise as measured at 90 and 102 cycles, and about 40 db below
the 1000-cycle output.
The 904 and 1096-cycle sidebands are of about the magnitude to be
expected from the frequency modulation shown by the flutter bridge.
This means that the amplitude modulation is at least lower than that
corresponding to the measured sidebands.
With a recorded frequency of 9000 cycles, the amplitude modulation
component of the 96-cycle distortion is 10 db greater than that for
1000 cycles and 25 db greater than that for the unmodulated track.
The magnitude of the 8904 and 9096-cycle sidebands is somewhat
Sept., 1941] ORIGINS OF 96-CvcLE DISTORTION 249
greater than that which corresponds to the measured flutter, which
means that there is considerable amplitude modulation. The signal
has been reduced considerably so that its magnitude is only 15 db
greater than the 96-cycle distortion. It is well to point out at this
time that these results are pessimistic because of the processing condi-
tions which are conducive to producing 96-cycle amplitude modula-
tion. This increase of 96-cycle distortion and ground noise with
frequency is clearly indicated in Fig. 10.
Other data of Table IV are plotted in Figs. 11, 12, 13, 14. The
curves in these figures show the 96-cycle density modulation for the
two conditions of the negative and print sprocket-holes in and out of
register and the ground-noise level.
96-Cycle Distortion in Reproduction. — To show the introduction of
96-cycle distortion in the output of a perfect film, two procedures
were adopted: (a) a film was fixed but not developed in order to
clear it and to insure its freedom from even a fog density; and (b)
singly perforated film was recorded, processed, measured, and re-
turned to the Eastman Kodak Company for adding perforations to
the sound-track side.
The clear film exhibited measurable 96-cycle hum in the reproduc-
tion. In order to obtain an idea of the source of the hum, a flat
barrier 2 mils thick, 10 mils wide, and 90 mils long was interposed in
the light-beam at two positions adjacent the film : (a) in front of the
film between the objective lens of the optical system and the sound-
track; and (b) behind the film, between the sound-track and the
photocell. By careful placement of this barrier it was possible to
intercept all the useful scanning portion of the light-beam.
The results of the three measurements, with the barrier in the two
positions described, and with no barrier, are given in Table V.
TABLE V
96-Cycle Distortion Introduced by Clear Film in Reproduction
Position of Photocell Current
Amps Per Cent
3.80 100
0.14 3.7
0.18 4.7
There is, of course, some stray light from the optical system around
the scanning point. This is responsible for the slight hum when the
main beam is cut off in front. The greater hum when the barrier is
Barrier
90Cps
96 Cps
102 Cps
None
-62
-52
-62
Front of Film
-80
-63
-80
Rear of Film
-80
-56
-80
250
J. O. BAKER AND R. O. DREW
[J. S. M. P. E.
Sept., 1941]
ORIGINS OF 96-CvcLE DISTORTION
251
. «
n
oj
W.J?
co'C
rH CX
o
£
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w
'&"
«*H "C
oa
sS
.S
,&
252
J. O. BAKER AND R. O. DREW
[J. S. M. P. E.
between the film and the photocell is, probably due to light reflected
within the film base. The barrier must have cut off a portion of this
reflected light, for the hum is still higher when the barrier is removed.
In any case the hum in these tests is very low, i. e., some 50 db below
full modulation.
GAMMA-2.3
£>EH3,TY 2.0 —3.
. 3-2. S2>
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X
X.3
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i
i
e
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i 1 '
i
-70
°f?INT DEfJlIT*
<7.7 /.i i.i. l.O a. *
FIG. 14. ' Hum levels for 1000-cycle variable-area
prints.
The wave-analyzer measurements on the unperforated film showed
that it was entirely free from 96-cycle distortion before the sound-
track was perforated. After the perforations were added, 96-cycle
distortion was observed. Table VI shows the results of the measure-
ments before and after perforating.
The data given in the Tables V and VI show that perforating the
film before reproduction raised the 96-cycle output from a level of
— 66 to —80 (which was the same as the ground-noise readings) to
about —56 level. This was true whether or not the film carried a
Sept., 1941]
ORIGINS OF 96-CvcLE DISTORTION
253'
Gamma Density Freq.
TABLE VI
90 Cps 96 Cps 102 Cps
Before Perforating
0.37
0.56
1000
-66
-66
-66
0.60
0.76
1000
-70.3
-70.3
-70.3
0.75
1.09
1000
-74
-74
-74
0.37
0.56
0
-68
-68
-68
0.60
0.76
0
-74
-74
-74
0.75
1.09
0
-80
-80
-80
After Perforating
0.37
0.56
1000
-68
-55
-68
0.60
0.76
1000
-66
-55
-66
0.75
1.09
1000
-69
-56
-69
0.37
0.56
0
-68
-58
-68
0.60
0.76
0
-68
-58
-68
0.75
1.09
0
-76
-56
-76
904
Cps
-65
-71
1000
Cps
-5.0
-4.3
-7.2
1096
Cps
-65
-68
-71
-57 -5.0 -56
-58 -4.1 -58
-58 -7.0 -58
1000-cycle tone. The 904-cycle and
1096-cycle sidebands in the 1000-
cycle recordings which in this case
are a measure of flutter, were raised
from levels of —65 to —71 (which
again is substantially ground-noise
level) to about — 57 db.
These tests show that the per-
forating of the sound-track side of
a perfect film produces both 96-cycle
hum and flutter. The hum is pro-
duced in the same way as demon-
strated in the barrier experiment.
The 96-cycle flutter is produced by
the flexure of the film causing a
local stretching of the emulsion at
the sprocket-holes.
Film Flexure. — Crab tree6 shows
how the surface of the film is dis-
torted in the region of the sprocket-
holes and also shows in a sketch
how the film will polygon when
flexed around a curved surface. As
a verification of this polygoning ef-
FIG. 15. Photomicrograph
showing polygoning of 35-mm
film when flexed around a 2-inch
diameter drum (magnification
11X).
254 J. O. BAKER AND R. O. DREW [J. S. M. P. E.
feet, a photomicrograph was made showing that the film contacts
the drum between sprocket-holes and curves away from the drum at
the sprocket-holes. Fig. 15 illustrates this effect when only a slight
tension is applied to the film. When the tension is increased enough
to insure contact at all points of the film, stresses are set up in the
film which may be more detrimental to the sound-track and repro-
duced sound unless the same tension is employed in the recording,
FIG. 16. Photomicrograph showing stresses set up in
35-mm film. Tension is increased enough to insure con-
tact at all points of the film.
printing, and reproducing processes. Fig. 16 shows the photoelastic
effect upon film when under tension.
It is apparent that the amount of flexure of the film has a direct
bearing upon the quantity of 96-cycle distortion which may be intro-
duced. The larger the radius of the curved surface around which the
film is flexed, the less will be the amount of 96-cycle distortion. It is
conceivable that a drum of infinite radius might introduce almost
negligible 96-cycle distortion. This would point to a skid gate, but
here another difficulty would be encountered: namely, uneven film
motion due to friction between the film and the gate shoes.
Sept., 1941] ORIGINS OF 96-CvcLE DISTORTION 255
CONCLUSION
Where doubly perforated film was used, 96-cycle disturbance was
encountered in every step of the investigation from the recording
through processing and printing to the reproduction. The variable-
density system is much more subject to density and amplitude modu-
lation than the variable-area system.
The tests did not separate the effect of polygon-bending in recording
from the unequal development around the sprocket-holes, but the
flutter measurements indicate variations too small to account for the
observed density variations — whence development is unquestionably
the major offender.
There is practically no 96-cycle flutter in the motion of the film as a
whole, when the recording or reproducing machines employ well-
designed mechanical filtering systems.
Printing is not ordinarily a direct cause of any but a very small
amount of density and amplitude modulation, but printing condi-
tions may frequently accentuate the distortions due to development.
Although we recognize that the adoption of singly perforated film
can hardly be considered for release prints, the authors suggest that
serious consideration be given to the possibility of employing it for
original recordings, master prints, and for the sound-track negatives
from which release prints are printed. Many printers now use only
one row of sprocket- teeth on the sound-sprocket, and non-slip printers
make no use of the perforations next to the sound-track.
REFERENCES
1 KELLOGG, E. W.: "A New Recorder for Variable Area Recording," J. Soc.
Mot. Pict. Eng., XV (Nov., 1930), p. 653.
2 BATSEL, C. N. : "A Non-Slip Sound Printer," J. Soc. Mot. Pict. Eng., XXIII
(Aug., 1934), No. 2, p. 100.
BEDFORD, A. V. : U. S. pat. No. 1,754,187.
3 LOOMIS, F. J., AND REYNOLDS. E. W. : "A New High-Fidelity Sound-Head."
/. Soc. Mot. Pict. Eng., XXV (Nov., 1935), No. 5, p. 449.
4 CRABTREE, J.: "Sound Film Printing," J. Soc. Mot. Pict. Eng., XXII (Feb..
1934), No. 2, p. 98.
6 FRAYNE, J. G., AND PAGLIARULO, V. : "The Influence of Sprocket-Holes upon
the Development of Adjacent Sound-Track Areas," /. Soc. Mot. Pict. Eng.,
XXVIII (March, 1937), No. 3, p. 235.
6 CRABTREE, J., AND HERRIOTT, W.: "Film Perforation and 96-Cycle Fre-
quency Modulation in Sound-Film Records," /. Soc. Mot. Pict. Eng., XXX (Jan.
1938), No. l,p.25.
SOME PROPERTIES OF POLISHED GLASS SURFACES*
FRANK L. JONES**
Summary. — The optical glasses made by combining silica with various other ox-
ides are similar in that the silica will not dissolve in water or weak acids at the same
rate as the other materials contained in the glass. This property of silicate glasses is
involved in the accidental formation of colored stains on the surface of dense lead or
barium glasses exposed to the weather, in the formation of surface haze on lenses ex-
posed to tropical humidity, and in the formation of silica low-reflection films on glass
by chemical treatment.
Quantitative data have been collected on the tendency to form surface stains and on
the rate of dimming for all the common types of optical glass. Surface stains do not
harm a lens and may increase its efficiency. Any haze that forms on a lens exposed
to a humid climate should be removed by careful cleaning.
Purposely formed silica surface films for increasing the transparency of glass are
identical with the stains that form accidentally on dense lead and barium glasses except
that they are of controlled thickness and may be stabilized to prevent any further in-
crease in thickness. Chemical methods are now available for forming low-reflectivity
surfaces on all of the common optical glasses. Proper heat treatment of these silica
films will lower the rate of solution of the glass. The durability of a lens is greatly
improved by this process.
The gain in light transmission that results when a silica surface film is formed by
chemical treatment is less than that produced by films of low-refractive-index fluorides
evaporated by the Cartwright and Turner method. There is no doubt but that both
the evaporation process and the chemical process will be used in the optical industry.
Each process has advantages depending on the circumstances of use.
The motion picture engineer manages and controls light by means
of glass optical systems. Glass is thus one of his basic materials.
The physical properties of glass are well known to the engineer.
This paper is presented with the idea that the chemical properties
of glass surfaces are less thoroughly understood but of equal interest
and importance.
Optical glass used in projection lenses and camera lenses is prac-
tically all made by heating sand that is over 99 per cent silicon di-
oxide with other metal oxides until they unite to form a glass. The
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received April
14,1941.
** Bausch & Lomb Optical Co., Rochester, N. Y.
256
<>- The Society is not responsible for statements by authors 4*
POLISHED GLASS SURFACES 257
finished glass consists of a random network of strongly bonded silicon
and oxygen atoms with other elements more loosely joined to the
basic network. The glass compositions employed in making bottles,
windows, and other commercial products usually contain calcium and
sodium combined with silica. Optical glasses are more varied in com-
position. They may contain lead, barium, zinc, calcium, sodium,
potassium, aluminum, boron, magnesium, antimony, or other ele-
ments. The basic ingredient, however, is generally silica. The large
variety of compositions are the result of the optical engineers' need
for glasses that vary in refractive index ; in other words, in the speed
limit they impose upon light as it enters the glass. As long as the
basic material is silica, the glasses will have one characteristic chemi-
cal property. The silica will not dissolve in water at the same rate
as the other materials contained in the glass. This characteristic of
silicate glasses may seem unimportant but it is involved in the dim-
ming of glass surfaces in tropical climates, it is involved in the for-
mation of colored stains on lead and barium glasses, and in the chemi-
cal method for increasing the transparency of lenses. This paper
will show the relationship between these seemingly different phe-
nomena.
There are other methods by which elements originally in a glass
surface can be removed or replaced but the action of water or water
solutions is involved in most of those changes that occur in normal
use. Water is present in the air as a vapor. It comes into contact
with lenses when they -are washed, when condensation of moisture
takes place due to changes in temperature, or when a lens is touched
with the fingers. Every time a lens comes into contact with water
some reaction takes place. The glass acts as if the silica were a porous
sponge with the more soluble oxides distributed through the pores.
Under most circumstances contact with water does not disturb the
basic silica network but does release the other constituents so that
they are free to move out of the glass. Different elements vary in their
speed of solution. Eventually a very thin surface layer becomes dif-
ferent in composition and refractive index from the body of the glass.
As long as the silica network is not disturbed, the surface of the lens is
not etched or roughened. Hydrogen ions from the water replace the
metal atoms that leave the glass so the changed surface layer is not
porous when examined with a microscope and does not scatter light.
Since the silica network is the strong part of a glass structure, changes
that do not damage the network do not noticeably reduce the hard-
258 F. L. JONES [J. S. M. P. E.
ness of a glass surface. If the surface silica layer is very thin, there is
no visible change in the appearance of the glass. If the thickness of
the silica film is greater than 0.05 micron interference effects between
the light reflected at the top and bottom of the silica layer will cause
the light reflected to be colored like that reflected from a layer of oil
on water.
If the color develops accidentally, the user decides that the sur-
face is stained and he may protest to the manufacturer of the lens.
If the surface layer is formed purposely by the manufacturer, the lens
is sold as chemically filmed for increased light transmission, and an
extra charge is made. The Bausch & Lomb Optical Company has
been interested in the study of these surface films so that they may
be prevented from forming when not wanted and may be made to
form quickly and evenly when wanted. It is hoped that an under-
standing of these modified surface layers will prevent the dissatisfac-
tion of lens users who find that a thin film has formed accidentally
and will show the value of purposely applied films.
SPONTANEOUS SILICATE FILM FORMATION ON GLASS IN CONTACT WITH WATER
Glass selected for lens systems is chosen primarily for its optical
properties. Chemical stability is a secondary factor, but a limiting
one, since no matter how useful the glass is in* regard to index and
dispersion, it can not be employed in an optical system if its polished
surface will corrode and roughen in the environment in which it is
used. All glass compositions used in optical work are selected from
those capable of normal exposure to water without surface rough-
ening. On the other hand, all types of glass, optical or otherwise,
will give up a small portion of their more soluble components to water
and those containing substantial amounts of lead or barium are likely
to lose material to such a depth that they develop surface interfer-
ence colors. There is ordinarily no damage to the surface polish
since only part of the glass dissolves and the hard silica structure is
not affected. The amount of liquid required is so small that finger-
prints or moisture droplets due to condensation may start the process.
The presence of an acid such as dissolved carbon dioxide or the acids
found in perspiration greatly speeds the solution. Under normal
conditions of use only dense barium crown glasses and the dense
flint glasses are subject to such staining in use but any silicate glass
will do so if kept in contact with acid water for a long enough period
of time. Tests on optical glass have indicated that the most durable
Sept., 1941] POLISHED GLASS SURFACES 259
soda-lime-silica crown may require five million times as long as the
least durable dense barium crown glass for a thick enough silica layer
to form to produce a visible stain under a given set of conditions.
It is easy to see why the soda-lime-silica crown never stains in actual
use. The light transmission of a stained surface is equal to or greater
than that of an unstained lens. The amount of light reflected by the
surface is reduced so that the staining results in better contrast and
less likelihood of ghost images. A stained lens that does not show
surface pitting and corrosion is thus more efficient than a new un-
stained lens. This fact was discovered by H. D. Taylor, the English
optical designer, in 1892 and has been general knowledge among op-
ticians since that time. It has not been known by many users of
lenses, however, and much argument has resulted whenever an at-
tempt has been made to sell lenses with surface films or when the
stains developed in use. The Germans have been more willing than
others to recognize the good points of stained lenses, possibly be-
cause they call such stains "beauty marks" (Schonheits fehler). A
lens should never be rejected merely because it has acquired a silica
surface film.
SURFACE HAZE ON GLASS
If highly polished glass surfaces are exposed to humid air for long
periods of time, a faint surface haze will develop. All glasses are
subject to such dimming if exposed to a damp atmosphere but pro-
tected from contact with liquid water. The rate of dimming varies
with different glasses. The chemical reaction is basically similar to
that involved in the formation of surface films but the effect is very
different. As in silica film formation one or more elements migrate
out of the glass but, since there is not enough water present to dis-
solve and remove the elements released by the glass, they remain on
the glass surface as microscopic crystals that scatter and reflect light.
The efficiency of the lens is thus reduced by surface haze. Since the
reaction with humid air is slow, the glass is rarely leached to sufficient
depth for any decrease in surface reflection to result. The surface
haze is easily removed with a damp cloth during the early stages of
its formation with no damage to the lens surface.
In tropical climates conditions sometimes are so balanced that just
the right amount of water collects on a lens surface to form a con-
centrated solution of the haze material. Such a solution may attack
the silica of the glass and pit the surface permanently. Under tropi-
260
F. L. JONES
[J. S. M. P. E.
cal conditions frequent cleaning of t^ie lens surfaces is necessary to
prevent such surface damage. Anyone using lenses under conditions r
of high humidity should wipe the haze from exposed surfaces with a
soft damp cloth at frequent intervals.
TABLE I
Durability Test Results for Optical Glass
Type of Glass
Type
Refrac-
tive
Index
Type
PD
Value
Dim-
ming
Test
Class
Stain
Test
-Class
Borosilicate crown
1.511
63
1.
1
1.516
64
1.5
1.516
64
1.
1.517
64
1.
1.518
64
1.5
Crown
1.523
59
1.5
1.512
60
1.5
Light barium crown
1.572
1.572
57
57
2.
2.
3
4
1.573
57
1.5
3
1.574
57
1.
1
Dense barium crown
1.608
59
3.5
5
1.609
59
3.
5
1.611
59
3.
5
1.611
60
3.
5
1.610
57
1.
4.5
1.611
57
2.
5
1.611
59
3.
5
1.612
57
2.
5
Crown flint
1.526
51
3.
1
1.528
52
1.
1
1.528
52
1.
1
1.529
52
1.
1
Barium flint
1.581
46
1.5
3
1.583
47
1.
1
1.584
46
1.
1
1.588
46
2.
2.5
1.605
44
2.
2
Extra dense flint
1.717
29
3.
3
1.717
29
2.
3.5
1.720
29
1.5
3
1.721
29
2.
5
1.648
34
3.
2
1.649
34
2.
2
1.650
34
2.
2
1.650
34
2.
2
Sept., 1941] POLISHED GLASS SURFACES 261
RATE OF STAINING AND DIMMING FOR GLASSES OF DIFFERENT TYPES
Laboratory tests for evaluating the tendency of a glass to form
low-reflecting stains in contact with weakly acid water and to collect
haze when exposed to humid air have been developed. In Table I
most of the common types of optical glasses are classified in regard to
their staining or dimming tendencies. The glasses are scored from
1 to 5 with class 1 being very resistant and class 5 being easily affected.
SILICATE FILM FORMATION BY CHEMICAL TREATMENT OF POLISHED GLASS
SURFACES
Purposely formed silica films for increasing the transparency of
glass are identical with the stains that form accidentally on dense
lead and barium glasses except that they are of controlled thickness
and may be stabilized against any further increase in thickness.
A lens treated to provide the maximum transmission for white light
will reflect so little green light that the surface of the lens will appear
purple.
The silica surface has a hardness and scratch resistance com-
parable to that of the original glass. A lens that has been given a
surface curvature to fit a test glass will pass the same inspection
after processing.
The gain in light transmission that results when a silica surface is
formed by chemical treatment is somewhat less than that produced
by films of low-refractive-index fluorides evaporated by the Cart-
wright and Turner method. When the greatest possible gain in trans-
mission is required, the evaporated fluoride films are preferable.
When hardness and durability are of primary importance, silica films
produced by chemical treatment are superior. The evaporated
fluorides have little effect upon the ability of glass to withstand the
attack of moisture. Properly heat-treated silica films lower the
solubility of glass surfaces so that durability is greatly increased over
that of untreated surfaces. There is little doubt that both the evap-
oration process and the chemical process will be used in the optical
industry, with each having advantages in certain circumstances.
Since the possibility of increasing the light transmission of lenses
by chemical treatment has been known for many years, it is difficult
for most persons to understand why the process has not been more
widely used. There were three main reasons for the delay: (1) cus-
tomers would not accept lenses that showed surface color; (2) the
chemical and physical principles involved in the process were not
262 F. L. JONES [j. s. M. p. E.
completely understood, so that the gain in transmission varied from
one piece to another; (3) many types of optical glass could not be
treated. Miss Blodgett of the General Electric Company's research
laboratory worked out the relationship between film thickness,
color, reflectivity, and light transmission for all types of surface
films, and the publicity that followed her discoveries led to the pres-
ent demand for treated lenses. Chemical study of the process at
the Bausch & Lomb plant and at the Mellon Institute has resulted in
practical plant processes for treating all the glasses made by the
Company and for making the treated surfaces less subject to
weathering than the original glass.
SOLUTION COMPOSITION USED TO PRODUCE SILICA SURFACE FILMS
Dilute solutions of the common strong acids will produce silica films
on most glasses. Nitric acid is often used since its salts are soluble.
Weaker acids, including phosphoric or boric acid, are used with those
glasses that rate class 5 in the staining test, such as the dense barium
crown types. Different solutions produce varying increases in light
transmission of a given type of glass, indicating that a greater per-
centage of the soluble elements is removed by some solutions than
by others.
All the commonly used types of optical glass containing silica
can be given a silica surface film by chemical treatment. The im-
provement in light transmission produced by the process varies ac-
cording to the refractive index of the base glass. Using a Mazda lamp
as a light-source and a Martens type visual photometer to measure
the light, a sheet of lead glass with a refractive index of 1.72 was
found to transmit 86 per cent of the light striking the sheet normal
to the surface. After the formation of a purple surface layer by acid
treatment, the light transmission was 97 per cent. A borosilicate
glass or a crown glass sheet with a refractive index of 1.52 will trans-
mit 92 per cent of the light before treatment and 95 per cent after
formation of a purple silica film. The gain in light transmission for
optical glasses of intermediate index will fall between these two val-
ues. Soda-lime-silica crown glasses are bothersome to treat because
of the long time required to form a film of the required thickness.
STABILIZATION OF SILICA FILMS
The initial rate of reaction between a clean glass surface and a
water solution is governed by the composition of the glass, the com-
Sept., 1941] POLISHED GLASS SURFACES 263
position of the solution, and the temperature. When a silica film has
formed due to the removal of the more soluble constituents of the
glass, the rate of reaction between the glass and the solution is more
and more limited by the rate at which the solution can penetrate the
silica layer already formed. In general, the protective effect of a silica
layer is very roughly proportional to the percentage of silica in the
original glass. The film produced on the glass containing a large
amount of lead or barium has little protective effect, and the amount
of lead or barium dissolved is practically proportional to time. The
protective effect of the silica film is increased enormously, however,
if the treated glass is removed from the solution and heated to densify
the silica. The rate of solution can be reduced by this process to such
a point that no further change in thickness of the silica film will occur
in normal use. Thin silica films will react similarly to those thick
enough to produce high transmission, so that, if desired, the surface
stabilization may be obtained with films too thin to show interference
color. There is a shrinkage of the surface layer during baking, so
in practice a film slightly thicker than that desired is applied in the
chemical treatment and the baking operation is controlled so that
the finished lens has the correct film thickness.
Stabilization of the film by baking is necessary for two reasons. A
film with an optical thickness J/4 the wavelength for which maxi-
mum light transmission is desired is most efficient, and any increase
in film thickness with use would be detrimental. If dense barium
crown glass is exposed constantly to outdoor weather and to tropical
rains, there is some danger that the films may become many wave-
lengths in thickness and subsequent drying may cause the film to
crack and shrink away from the base glass.
There are several points in the process of manufacturing lenses
where invisibly thin silica films may be formed in an uneven pattern.
It is therefore necessary to treat and stabilize lenses immediately
after they are polished. A scientific detective could reconstruct the
past history of a lens and its contacts with moisture, including finger-
prints, water marks, and tray marks, by chemically treating glass
that is not freshly polished. It is not ordinarily possible to produce
uniform silica films on lenses that have been in use or storage.
Lenses to be treated to form silica films require a more perfect polish
than other lenses. If the glass surface is ground and then polished
only the minimum time required to remove all visible scratches, the
chemical treatment will open up invisible scratches and make the
264 F. L. JONES
scratch pattern again visible. If the glass is polished for a sufficient
time after all visible scratches are removed, the treatment will not
harm the polished surface.
PRACTICAL APPLICATION OF SILICA FILMS
The first Bausch & Lomb product on which lenses coated with
silica films were used in commercial quantities was the f/2 Super
Cinephor projection lens. The totally enclosed elements of this lens
are coated with evaporated fluoride films, while the front and rear
elements are given silica films by chemical treatment. These ele-
ments are baked after treatment so that the exposed surfaces will
have the improved durability and permanence characteristic of a
dense silica film.
Theoretically almost all optical instruments could be improved
if made from elements coated to reduce surface reflection. The cost
of treatment and the complications introduced when established manu-
facturing routines are changed make it necessary that each instru-
ment be carefully studied to see whether coated lenses produce a
noticeable and valuable improvement that will justify increasing pro-
duction costs and selling prices. Investigations are now in progress
on many lines of optical equipment. There is little doubt that more
uses for coated lens systems will develop within the coming five years.
It is hoped that this paper will help to spread the idea that a colored
surface on a polished lens is not necessarily a defect.
RECENT IMPROVEMENTS IN NON-REFLECTIVE LENS
COATING*
WILLIAM C. MILLER**
Summary.— As early as 1892 it was known that the reflectivity of polished glass
surfaces was reduced and the light transmission increased when a suitable film was
present on the surface of the glass. Many efforts to produce such a film artificially
met with only partial success. In the past five years, two different methods have been
discovered that achieve the desired results. Only one of the processes, however, was
satisfactory for commercial application. Great improvements have been made in the
durability and weather resistance of the thin films deposited upon the lens surface by
this method. Lenses coated by this improved process require no more careful han-
dling than any good lens is entitled to; fingerprints and dust can be removed without
detrimental effects to the coating. The thin films can not be scratched with anything
less hard than a metal point. By this process, reflectivity can be reduced from an
average of 5 per cent for untreated polished surfaces to as low as 0.5 per cent for treated
ones. Experiments show that even greater reductions are possible and should be avail-
able in the near future.
The general application of the lens-coating process to studio op-
tical equipment is now just one year old. In view of the wide inter-
est and attention that this process has aroused, a discussion of the
results and a report of the improvements made in the process will be
of interest. Unfortunately, time has not permitted the accumulation
of exhaustive data. However, those that are available show that the
new process is of vital importance in many fields and is already quite
indispensable.
HISTORICAL
Although it had been known for many years that certain types
of glass developed a tarnish after prolonged exposure to the air, it
apparently was not until 1892 that any careful study of the effects of
such tarnish was made. At that time H. Dennis Taylor, famous lens
designer, made careful measurements upon several tarnished lenses
that had come to his attention. The tarnish had the appearance of a
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 1,
1941.
:* Yard Mechanical Laboratory, Pasadena, Calif.
265
•^ The Society is not responsible for statements by authors &
266 W. C. MILLER [j. s. M. P. E.
metallic sheen and had always been, considered to be highly detri-
mental. The results of Taylor's measurements and tests, however,
showed that the tarnished lenses reflected less light from their polished
surfaces than did identical new ones. This of itself was of great im-
portance, but of still greater importance was the fact that the light
that was no longer reflected by the polished surfaces was transmitted
by the lenses. The tarnished lenses produced images measurably
brighter than did identical new and untarnished lenses.
Taylor was so impressed with the potentialities of the discovery
that he made extensive experiments to find means of producing this
tarnish artificially on the surfaces of new lens elements. Unfortu-
nately he met with only partial success, for the types of glass that he
was able to treat proved to be limited. Furthermore, the reduction in
reflectivity obtainable with many of the glasses was too slight to be
of commercial value.
Many efforts were made in subsequent years to discover methods of
artificially producing the desired results, but with only moderate
success. Kollmorgen, Kellner, Wright, and Ferguson all made con-
tributions to the art, but certain types of glass resisted all attempts
to produce a tarnish of the desired nature.
All the processes developed up to that time were of the chemical
type; that is, they depended upon the action of chemical solutions
or concentrated salts upon the surface of the glass to produce the de-
sired tarnish. Since this reaction took place with the glass itself, it
was impossible to remove the effects of the treatment without com-
pletely refinishing the optical surface, a costly and time-consuming
procedure. The greatest care was therefore necessary in the treat-
ment of optical elements to insure satisfactory results, since an error
meant refinishing the surface or making a new element. This treat-
ment could not be safely attempted by anyone other than the makers
of the original optical parts.
Since many varieties of glass are employed in the lenses in common
use, and many of these glasses either could not be treated at all or
could be treated with only moderate success, the application of the
process was not widespread.
What was required to make the theory universally practical and
applicable was a method of producing the tarnish upon lens surfaces
irrespective of the type of glass from which the lenses were made and
would yield reductions in reflectivity sufficiently great to justify the
trouble and expense of application.
Sept., 1941] NON-REFLECTIVE LENS COATING 267
In view of the many years that elapsed with little or no successful
development of the art, it is remarkable that two independent proc-
esses of quite a different nature should be announced within the short
period of three years. The first announcement came in 1936 of a
process discovered by Dr. John Strong1 of the California Institute
of Technology. Strong's process consisted of the deposition of a thin
film of suitable material upon the surface of optical elements in a high
^acuum. This thin film, when deposited under the correct condi-
tions and to a specified thickness, effected reductions in the surface
•eflectivity as great as 85 per cent. The second announcement came
n 1939 of a process discovered by Miss Katherine Blodgett2 of the
General Electric Laboratories. Miss Blodgett's process consisted of
.he formation of a soapy film of the required characteristics upon the
urface of optical elements. Although the reductions in reflectivity
achieved by this process were great, the extreme fragility of the film
made the process impracticable for general use.
THEORETICAL
The theory of the reduction of surface reflection has been dealt
with so thoroughly and competently by others in the literature1' 2» 3 4
;hat it will be necessary to give only the general principles of the
>henomenon here. The quantity of light reflected from the polished
urface of a transparent material and, therefore, lost from the trans-
mitted beam, depends upon such factors as the index of refraction of
he material and the angk at which the light strikes the surface. If
he angle of incidence is kept constant, then the index of refraction is
he determining factor, and the higher the index the greater is the
icrcentage of light reflected.
Light can be considered as traveling in a wave form. When a beam
f light is reflected from two parallel polished surfaces of a trans-
iarent material, the light-waves can be made to supplement or op-
•ose each other in the reflected beams by suitable adjustment of the
eparation of the reflecting surfaces. When these have an optical
eparation of V4 of a wavelength, the waves in the two reflected beams
)ppose each other and cause destructive interference. The total
ntensity of the reflected beam will be zero when, and only when, the
wo components are of equal intensity.
If we wish to reduce the reflectivity of the polished surfaces of an
)ptical element and thereby increase their transmission, it can, there-
ore, be done by providing over the entire element two reflective sur-
268 W. C. MILLER [j. s. M. P. E.
faces separated by Y4 wavelength, both surfaces reflecting an equal
amount of light. Under these conditions, the two beams will cancel
each other. Although it was not clearly understood until the time of
Dr. Strong's work, it was this interference phenomenon that ac-
counted for the effects observed by Taylor and the others.
The most satisfactory method of producing the two reflective sur-
faces separated by the correct distance is to form upon the surface
of an optical element a film of transparent material of such nature and
of such refractive index that the light reflected from the contact
surface where the film touches the glass equals that reflected from
the upper surface. This index can be found with little trouble to be
equal to about 1.25.
The effects that Taylor observed first were due to the formation of
a film of approximately the required characteristics by the chemical
action of the air with some of the constituents of the glass. The
chemical methods that were subsequently developed all aimed at
the artificial stimulation of such a film. The failure of the methods
to produce more satisfactory results was due to the fact that a film
of the required index could not be formed on all types of glass. Even
the process developed by Strong missed perfection in that particular
respect, for there is no suitable substance that can be applied in the
form of a film having an index as low as the required 1.25.
All the processes — the chemical by Taylor, Kollmorgen, Kellner,
Wright, and Ferguson ; the evaporation by Strong ; and the one by
Miss Blodgett — fail in one other important respect which offers such
natural obstacles that it may never be surmounted ; that is, the thick-
ness requirement. The film can be made of the required thickness for
only one wavelength at a time and is, therefore, wrong for all others.
Consequently, when white light is used, the reduction of reflectivity
can be made a minimum for only one color; all others suffer greater
amounts of reflection. Fortunately, the difference for other colors
is not great, but it is sufficient to give treated surfaces a colored hue
when viewed by reflected light. If all colors were reduced equally,
the remaining small amount of reflected light would not display any
predominant color.
Optical systems designed to work with light of some certain wave-
length should be treated to give maximum transmission for that
wavelength. Complying with this rule there are in use in the studios
many violet recording systems that have been treated for maximum
transmission at about 4000 A.
Sept., 1941] NON-REFLECTIVE LENS COATING 269
At the writing of the previous paper5 on this subject in April, 1940,
the process had been in use experimentally for only a few months,
but such great interest was shown in the possibilities of the process
that a report was considered desirable at that time. Due to the new-
ness of the process, however, little definite information based on
actual production results could be given. At the present writing, how-
ever, some very interesting data are at hand, supplied through the
courtesy of several of the studios in Hollywood.
Sound-recording systems consisting of ten air-glass surfaces have
been treated both for violet and unfiltered light. A gain in trans-
mission of 50 per cent was measured in nearly all cases. Since the
tungsten recorder lamps are of necessity burned at or near their peak
capacity, this 50 per cent increase in transmission in the optical
train has made it possible to relieve the load on the lamps and thereby
considerably increase the lamp life. In some instances the gains ob-
tained by treatment of the lenses have been utilized, not to save cur-
rent or lamp life, but to make possible the use of slower, finer-grained
films.
A large number of motion picture camera lenses has been treated
during the past year. Careful measurements made at one of the
major studios on a 3-inch focus Cooke Speed Panchro lens at //2.0
showed the transmission of the untreated lens to be 69.5 per cent.
The transmission of the lens when treated was 95.1 per cent. In other
words, the light loss had been reduced from nearly 30 per cent to less
than 5 per cent. Another studio reports measurements showing a
gain of 32 per cent due to treatment of another type of lens.
Of even greater interest than the increase in transmission is the
improvement in the image quality due to this treatment. The in-
crease in contrast and brilliance of pictures made with treated lenses
is very noticeable. In work where the utmost in image quality is re-
quired, such as in process projection keys, the treatment is of great
value and is widely used in several studios.
Due to the number of steps involved in the production of a finished
process shot, it is necessary to apply every known means of reducing
the losses of picture quality to a minimum. Since these are primarily
losses of brilliance and contrast, the very features that treated lenses
enhance, the application of this treatment to both the projection
and camera lenses used in process work is of great value. Reports of
the results obtained in this field are definitely satisfactory and grati-
fying.
270 W. C. MILLER [j. s. M. P. E.
Another of the major problems encountered in the process work is
that of screen illumination. Constant efforts are being made to in-
crease the light output of the projection systems used in this work.
Gains of 10 to 20 per cent have been the subject for loud rejoicing.
Yet actual tests by the studios have shown that by treating the pro-
jection lenses, gains as high as 30 per cent are to be had. One studio
had a peak screen illumination of 24,000 lumens with untreated pro-
jection lenses. After treatment 30,000 lumens were obtained. This is
an increase of 6000 lumens.
In straight production work the results are no less interesting.
Treatment of lenses has so reduced ghosts and flares that it is now
possible to apply hitherto unusable methods of set illumination.
This is particularly true of low-key sets.
There have been several successful pictures made during the past
year in which low-key lighting greatly enhanced the atmosphere of
the picture. No small part of the success of these low-key scenes was
due to the clarity and brilliance with which they were reproduced
through the use of treated lenses. Intense local lighting did not mask
out shadow detail or cause ghosts of any sort.
Of particular interest was one shot, made in a dark hallway, of two
characters approaching cautiously with a flashlight. Quite by
accident the flashlight was turned full into the lens of the camera.
But contrary to expectation the shot was not ruined, for no flares
appeared and the dimly lighted faces of the two characters could still
be clearly seen over the brilliant image of the flashlight.
The reduction of flares or ghosts is so great that tests with treated
and untreated Astro lenses shooting straight into the sun show a bare
trace of one ghost with the treated lens which before treatment gave
thirteen conspicuous ghosts.
As the results obtained with the treated lenses became available and
comments and criticisms from the users drifted in, the need for more
research work on the process became obvious. A harder and more
durable treatment was definitely needed. The research program
that was undertaken in our laboratories had for its objectives four
primary aims. First, it was desired to produce films that were much
harder than anything available at that time. The aim in this respect
was to produce films which were just sufficiently softer than the
underlying glass to permit the removal of the film without damage
to the lens element should the removal be required. Second, this
hardness must be obtained by means other than baking, for it was
Sept., 1941] NON-REFLECTIVE LENS COATING 271
felt that to subject precision optical parts to high temperatures was
decidedly detrimental. Third, the films must be sufficiently re-
sistant to vapors to eliminate any tendency to fog in normal use.
Fourth, the efficiency of the films must not be impaired while obtain-
ing this increase in hardness.
Many months of intensive experimental work were devoted to this
program. Several methods of improving the process were discovered,
but the final method was so superior to any of the others that it was
made the subject of patent application.
Where previously the removal of dust from a treated lens with
a soft camel-hair brush had often resulted in damage to the coats,
the new hard ones could be handled with no more care than any good
optical element deserves. Test samples were subjected to very severe
treatment. Finger marks were repeatedly placed on them and suc-
cessfully wiped off. They were allowed to lie around the laboratory
for long periods where they accumulated dust and dirt, which was
then removed without damage to the treated surfaces.
Those acquainted with the fragility of the early coats would be
astonished to witness demonstrations of the hardness of the coats
when they are jabbed and scraped with wooden sticks, breathed
upon, and wiped with cloths without damage. One of the most
popular tests is to rub a sample through the hair to coat it with oil
and then to return it to its original efficiency and unblemished state
by rubbing it with a cleaning pad.
This welcome durability was obtained without loss of efficiency of
the films, as was the intention. Coated surfaces reduce the reflectivity
of polished glass to 1/8 or less of the original value. Sample glass
disks coated on both sides, but only in the center, give a most inter-
esting demonstration of the efficiency of the films. When held be-
tween the observer and the sky, the treated central portion is de-
cidedly brighter than the surrounding untreated area, due to the
increased transmission. These same samples, held between the eye
and some dark background such as black pavement, show a brilliant
ring around the untreated edge where the bright sky is reflected in
undiminished intensity. The treated center, however, appears quite
dark and the pavement beyond can be seen without difficulty, whereas
it is seen only indistinctly elsewhere through the glare of the reflected
skylight.
A camera lens was treated for demonstration purposes so that only
one-half of each element was coated, and the treated halves were
272 W. C. MILLER [j. s. M. p. E.
lined up so that they were all on tl^same side when mounted in the
barrel of the lens. Either by reflected or transmitted light the effect
is most impressive. By reflected light, the iris diaphragm is barely
visible through the glare of the light reflected from the untreated
halves of the first four surfaces; while through the treated half it is
clearly visible, as well as interior details of the lens mounting as far
back as the last element. When viewed against the sky, the treated
side of the lens is markedly brighter than the untreated half. When a
dark object surrounded by a bright background is viewed through the
lens by an observer in the dark, a good demonstration is obtained of
the benefits of this treatment. The untreated half of the lens is seen
illuminated by light from the bright background reflected and re-
reflected between the untreated surfaces. The treated half is dark,
however, since any light that reaches the eye has suffered at least two
reflections from treated surfaces, and is, therefore, reduced to Vw
of the intensity of the light from the untreated surfaces. This
demonstrates perfectly the reason for the improvement in picture
quality obtained with treated lenses. THe photographic film is no
longer confronted with the glare of light reflected to it from the
several surfaces of the lens.
A result of the research program not as yet made available to the
public is an improvement in efficiency that has been found possible.
The reflectivity of surfaces can be reduced from the present low value
of 12.5 per cent (counting untreated surfaces as reflecting 100 per
cent) to as low as 9 or 10 per cent. This may seem at first to be triv-
ial, but actually it is relatively important. Samples with this new
low reflectivity can be distinguished instantly from the others. It
appears that this low reflectivity can be supplied with a film hardness
as great as that described above. As soon as more searching tests
have been made and the results found satisfactory, this improved
coating will also be made available.
With such satisfactory results as these appearing in the short space
of one year from only one laboratory, the future of the lens-coating
process should be very promising. Certainly other improvements
will be made from time to time. Still greater efficiency will be ob-
tained, methods of treating larger and larger surfaces will be de-
veloped, and in the space of a few more years uncoated lenses will
probably be things of the past. However, although the ultimate is
not yet achieved, the process is so much improved over what it was
a year ago it should find wide application in a multitude of fields.
Sept., 1941] NON-REFLECTIVE LENS COATING 273
REFERENCES
1 STRONG, J. : "On a Method of Decreasing the Reflection from Non-Metallic
Surfaces," /. Opt. Soc. Amer., 26 (Jan., 1936), p. 73.
2 BLODGETT, K. B.: "The Use of Interference to Extinguish the Reflection of
Light from Glass," Phys. Rev., 55 (April, 1939), p. 391.
3 CARTWRIGHT, C. H., AND TURNER, A. F.: Phys. Rev., 55 (1939), p. 595(A).
4 CARTWRIGHT, C. H., AND TURNER, A. F.: "Treatment of Camera Lenses
with Low-Reflecting Films," /. Opt. Soc. Amer., 30 (Feb., 1940), p. 110.
6 MILLER, W. C.: "Speed Up Your Lens Systems," /. Soc. Mot. Pict. Eng.,
XXXV (July, 1940), p. 3.
DISCUSSION
DR. CARVER : At a demonstration last year in New York, of motion pictures
projected with lenses coated with non-reflective layers, the most obvious effect
was that of an increase in contrast. Now, the processing laboratories have
worked out their methods of processing to give a contrast that they believe to be
the most pleasing, using standard equipment. Do you know whether the labora-
tories have found it necessary to change their processing conditions in order to
compensate for the increased contrast obtained with the treated lenses?
DR. TURNER: In some cases a change in processing methods was necessary,
but it could be very easily accomplished.
MR. JOY: Has moisture any effect upon these treated surfaces?
MR. COOK: Not on the outside surfaces of the lenses, which are treated by a
method that produces a very durable film on the glass.
NEW GADGETS FOR THE FILM LABORATORY*
B. ROBINSON AND M. LESHING**
Summary. — A description of an air squeegee for use on a continuous film proc-
essing machine is given. This squeegee was designed to eliminate waters pots on the
processed film and the design is such as to give ready access for cleaning and inspection.
A waterproof tape splicer for patching leader to be used on a machine equipped with
the squeegees mentioned is described. Patches made by this method have been found
to be longer-lived than the conventional ones with metal clips and are responsible also
for the use of less leader over a period of time.
Before the advent of fine-grain material, our developing machines
were equipped with chamois-covered rollers to take off excess mois-
ture from the film before it entered the dry-box. These rollers were
never very satisfactory, especially as to the cost of maintenance.
In chamois alone the cost was about $1000 a year. There was always
the possible danger that grit would stick to the chamois and ruin all
the film passing over it. But, until the advent of the fine-grain
materials, the lack of tune and the usual inertia kept us from im-
proving the unsatisfactory condition.
The very first week of using fine-grain film (in this instance we have
in mind the master positive type) we ran across a situation which had
to be solved immediately. The chamois rollers now and then left
innumerable very tiny circular spots of moisture on the base side
which were very hard to polish off and which were very hard to dis-
cover before the duplicate negative was made. During one of his
visits Mr. J. G. Capstaff of the Eastman Kodak Company mentioned
a very satisfactory air squeegee he had designed. After the drawings
arrived from Kodak Park we could readily see that some changes were
necessary to suit the conditions in our laboratory. From our point of
view the main difficulty was the necessity of taking the whole squeegee
apart for inspection and cleaning. While retaining the main design
of the squeegee, we split it in half to make the threading-up, cleaning,
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 5,
1941.
** Twentieth-Century Fox Film Corp., Hollywood, Calif.
274
<>The Society is not responsible for statements by authors^
NEW GADGETS FOR FILM LABORATORY
275
and inspection a very simple matter. Fig. 1 shows the simple design.
The air is supplied by a Nash Hythor compressor at eight pounds'
pressure. After passing through two separators to take out water,
the pressure is reduced by a Reliance regulator to two pounds for
picture negative and to about two and one-half pounds for sound
negative. No filters or screens are used in the airlines as we have
never noticed any need for them. The air reaches the film through
0.038-inch apertures shown in the drawing. The results obtained are
excellent.
FIG. 1. Simple design of air squeegee.
With the introduction of this new air squeegee we were forced to
look for a different method of splicing film for the developing machines
due to the fact that the metal patches we had been using produced too
big a disturbance in the rubber rollers of the squeegees, making the
work unsatisfactory. We remembered seeing in the drawings of an
Eastman Kodak developing machine mention of a waterproof tape
splicer, which we proceeded to adopt for our needs. The first splicer
built by us required too many manual operations, but as the work done
by the splicer showed us that we were on the right track and that the
waterproof tape was very satisfactory as a splicing medium, we pro-
ceeded to build an improved model. Fig. 2 is a photograph of the
improved model, which has been in use now for a number of months
in our laboratory. The number of operations has been decreased
considerably and that is the reason for the complicated appearance
of this piece of equipment.
276 B. ROBINSON AND M. LESHING [J. S. M. p. E.
It can be seen from the photograph' that a splicer of this kind can
not be produced very cheaply — ours cost a few hundred dollars.
But when we say that during the first six months after installation of
the waterproof tape splicer we used 39,000 feet less leader than during
a corresponding period preceding the use of splices of this type, it
will be seen that the money was very well spent. This saving may be
surprising to some. Splices made with the metal patches took about
eight inches of film. We inspected the leader and cut out every in-
dication of damage every night before starting our night's work.
FIG. 2. Improved splicer.
Damaged portions were quite numerous, due to the prongs of the
metal splices catching in the perforations of adjacent convolutions.
The waterproof tape removed this difficulty, and we have leader with
splices several months old that are just as good today as they were
when they were made.
The patcher is centrally located in the machine room on a bench or
table, flanked on the left by a spindle bracket to hold the reel of leader
to be inspected and on the right by a rewind on which the inspected
and patched leader is wound. This rewinding is done by hand, any
damaged or weakened portions being cut out and the ends joined on
the patcher. All existing splices are examined and any that have
deteriorated are replaced.
Sept., 1941] NEW GADGETS FOR FILM LABORATORY 277
The splicer is mounted on a cast-iron base which is recessed so
that shielded space is provided between the table top on which it is
placed and the top of the casting, to accommodate the major part of
the operating mechanism. A horizontal groove is provided through
the length of the casting to receive two sliding plates. This groove is
interrupted by a centrally located stationary anvil. The sliding
plates are grooved for film width and each is provided with a single-
perforation or index pin. The bed of the plates and that of the anvil
are on a common plane. A lever projecting from the front of the
base controls the reciprocating motion of these two plates.
The first operation in making a patch is to throw the two plates
outward to a stop. The two leader ends to be joined are placed in
the groove of the plates and over the perforation pins, with the film
ends extending over the edges of the anvil. The plates are provided
with hinged covers, which are lowered and spring-latched to lock the
film ends in place. Two hinged and connected shearing cutters are
now lowered manually. They straddle the anvil and shear the film
ends, establishing a definite gap between the ends to be spliced.
The operator now picks up the adhesive tape end, which is located
centrally and back of the anvil, between thumb and forefinger, and
operating a lever with the other hand, feeds the required length of
tape over the anvil with the adhesive side up. The two film-holding
plates are now moved inwardly toward the anvil by means of the
same lever that located them for the shearing operation. The two
film ends travelling toward each other are automatically raised to
clear the tape. As the tables carrying the film ends approach their
inward station, the film ends are dropped to the adhesive tape, leav-
ing a gap of approximately Vie inch.
A pedal-operated cutter parts the tape over the rear of the anvil.
This operation includes pressing a rubber pad against the film, forcing
au- from between the tape and film, and the breaking of the tape
around the film edges. The two tape ends are then folded manually
to overlap. The pedal is again depressed, the rubber-padded head
irons out the splice ; and eight manually operated punches, spaced to
match four perforations on each film end, strike the waterproof tape
through the perforations to bring the adhesive back to back. The
patch is now complete. The latched covers holding the leader are
released and raised, and the leader is drawn from the index pins.
The operations described are performed in approximately ten seconds.
M-G-M'S NEW CAMERA BOOM*
JOHN ARNOLD**
Summary. — A new type of intermediate-size boom incorporating a number of
very desirable features has been placed in serivice ai the M-G-M Studio. —The device
is of the crane-arm, or boom, type, with a boom 9 feet in length carrying an underslung
camera mounting. The camera may literally be laid upon the stage floor, or lifted
to a maximum height of 16 feet. The entire boom-arm may be raised or lowered bodily
by means of a motor-driven helical hoist.
With the popularization of the modern moving-camera technic
there has been an increasing trend toward the development of camera-
supporting units capable of serving as virtually universal camera
carriages for use not only in stationary but in most types of moving-
camera shots. Obviously, questions of bulk and weight have con-
sistently been limiting factors, as have those of operational facility.
Accordingly we have seen the evolution of two principal types.
On the one hand, there is a variety of small, mobile camera carriages
such as the "rotambulator" and the "velocilator." On the opposite
extreme are the much larger crane or boom-type units capable of
lifting a camera and its crew twenty or thirty feet into the air.
In some instances, intermediate-size cranes have been built; but,
in general, various conditions of design and operation have limited
their usefulness.
Nonetheless, it has been admitted generally that if some single
device had been available, capable of fulfilling all the camera-carriage
requirements of modern technic, with the exception of those few de-
manding the use of the largest cranes, production would have gained
a valuable tool.
A new type of intermediate-size boom, apparently incorporating
most of these desirable features, has been placed in service at the
Metro-Gold wyn-Mayer studio. It features not only unusual versa-
tility, but in many respects it differs radically from all accepted
practice.
* Presented at the 1940 Fall Meeting at Hollywood, Calif.
** Metro-Goldwyn-Mayer Studios, Culver City, Calif.
278
<» The Society is not responsible for statements by authors <"
M-G-M's NEW CAMERA BOOM
279
The device is of the crane-arm or boom type, with a boom 9 feet in
length carrying an underslung camera mounting. The camera may
literally be laid on the stage floor, or lifted to a maximum height of
16 feet. The entire boom arm may be raised or lowered bodily, by
means of a motor-driven, helical hoist.
The boom arm rotates freely through a full 360-degree horizontal
circle, while, in addition, the camera-head may, by an independent,
FIG. 1. The new M-G-M camera boom.
extra quick-action pan movement, be panned through a full 360-
degree circle. The tilt-head likewise operates through a 360-degree
vertical circle. The device is considerably lighter, and may be op-
erated much easier than any comparable unit.
Radically new principles of construction have been employed
throughout, and full use has been made of the modern, lightweight,
high-tensile alloys and stainless steels.
The chassis is of unusually simple tubular construction. Instead
280 J. ARNOLD [J. s. M. P. E.
of the usual channel sections conventionally employed for this pur-
pose, the main frame consists of a single tube of high- tensile steel.
Welded to this, at right-angles, are two smaller tubes forming the
axles. No springs are employed, as these devices invariably are used
on special plank or metal tracks, and it has long since been found
that any form of springing introduces an undesirable unsteadiness,
especially with the camera at the end of a long boom.
All four wheels are mounted in conventional steering knuckles.
The rear wheels, however, are at present locked in a non-steerable
position, though the design makes provision for rendering them
steer able if any future need should arise.
The front wheels are steerable, being controlled from an auto-
mobile-type steering wheel mounted before an underslung seat on
the left side. The design is such that the steering wheels may be
turned almost parallel to their axle, for sharp maneuvering.
A fifth wheel is provided at the rear of the tubular main frame.
This may be dropped down to raise the rear end from the rear wheels,
so that the device can be turned in its own length, or moved sidewise
into position. All four service wheels are ball-bearing equipped.
Extending upward from this tubular frame is a tubular vertical
member. Upon this is mounted a power-driven helical hoist strik-
ingly similar to that employed in the "rotambulator."
The mounting of the crane arm slides up and down this main
shaft in a friction mount. It is propelled upward or downward by a
suitably proportioned screw paralleling the main shaft.
This screw or helix is rotated by a 3A~hp motor controlled through
a d-c reversing circuit and controller. Automatic stop switches
limit the upward and downward travel of this unit.
This hoist is not intended primarily for changing the height of the
camera during a scene, but instead for more accurate positioning,
after which the boom arm raises or lowers the camera. The drive,
therefore, while quiet, is not noiseless. In addition, it is low-geared,
to simplify construction.
The crane arm itself embodies a type of construction not hitherto
applied to this type of studio equipment. Instead of the
conventional girder or box-truss construction, this arm employs a
stressed-skin or "monococque" construction combining unusual ri-
gidity with extremely light weight.
The arm is constructed of four 10-gauge sheets of high-tensile
steel, welded together to form a long, tapering box girder. This
Sept., 1941] M-G-M's NEW CAMERA BOOM 281
boxlike construction is reinforced at approximately 6-inch intervals
with transverse bulkheads of the same alloy, welded into place.
The result is a boom of unusually light weight, yet of remarkable
strength. From an engineering viewpoint it is strikingly similar to
the monocoque fuselages of the most modern transport and racing
airplanes, in which the bulk and weight of longitudinal girders are
eliminated by a skin strong enough to withstand the stresses normally
taken by longitudinal beams, and reinforced with stiffening trans-
verse bulkheads.
The outer end of the boom arm curves upward to afford increased
clearance. At this end is the camera mount, which is of the under-
slung type.
In this the camera is slung beneath the panning mechanism,
though of course the pan and tilt controls are in their usual places,
beside and slightly under the camera. Each gives the camera a full
360-degree rotation in its plane; the crank- wheel controls favored at
M-G-M are used.
The panoramic movement is geared to unusually high speed : only
14 revolutions of the control wheel are required to revolve the
camera through a full 360-degree circle.
A single, well-upholstered seat, of tubular metal construction, is
provided for the operative cameraman. This seat is quickly remov-
able when not needed. Ordinarily no seat is needed for the assistant,
as the camera is focused by an adaptation of a new d-c remote-
control method.
Provision is made for mounting a second camera above the crane
arm. This has a conventional M-G-M type pan-and-tilt head, and
pans and tilts wholly independently of the lower camera.
A source of constant irritation, and in some cases even of danger,
in conventional crane designs is the system of counterbalancing the
weight of camera and crew, which is usually done by means of remov-
able lead weights placed in a box at the opposite end of the arm.
The counterbalance is built permanently into the arm. Compen-
sation for the varying weights of equipment and crew is made by
turning a large control wheel at the inner end of the arm. This
moves the counterweight toward or away from the fulcrum, accord-
ingly decreasing or increasing its leverage.
By this means it is possible to counterbalance the boom so accu-
rately that it may literally be raised or lowered with one finger. A
set-screw type of friction lock, operating on a quadrant, permits
282 J. ARNOLD
locking the arm in any position. A similar lock is provided to limit
the boom's horizontal rotation, and brakes of the automotive type
are provided on the rear wheels.
A full circular catwalk is provided for the boom-operator. This is
made in four sections, all of which are demountable. At the front
end are two telescopic tubular members, either or both of which can
be extended — one on either side — for the stage crew to use in pushing
the crane for dolly-shots.
A non-extensible, curved bumper is fixed at the rear for the same
purpose, and also as a guard-rail. All these units — catwalk, pushing
arms, and bumper — are instantly demountable.
The degree to which the unique construction employed saves
weight may be judged by the fact that while booms of comparable
size and of conventional construction have an average of over 7600
pounds, the new M-G-M boom weighs but 3100 pounds. Yet there
appears to be no sacrifice in either strength or rigidity.
This new boom is as nearly as possible a completely universal
camera carriage. Its rigidity is such that it can be employed, except
in the most cramped quarters, as a stationary camera support in
place of conventional tripods and the like.
In this service, the elevated crane arm and underslung camera
mount give the camera crew more clear working space about the
camera than any conventional type of tripod or boom. At the same
time the crane arm, together with the power-driven hoist and free-
rolling chassis, makes accurate positioning of the camera quicker and
easier.
The suitability of the unit for the majority of moving- earner a
shots will of course be obvious. The precise controllability of the
counterbalancing facilitates one-man operation in scenes where the
camera must quickly follow an actor from a low position to a normal
or high one, or the reverse.
In addition, the underslung camera mount will permit the boom
arm to be extended completely over such a prop as a cafe table or
even an automobile, and, with the boom extended to the side of the
chassis, to dolly from or to such a position without interfering with
the use of the prop in the wider angles of the same shot.
Altogether the unit appears unusually versatile and represents a
distinct forward step in the evolution of mobile camera platforms.
The application of advanced materials and engineering principles to
its construction are also noteworthy.
AN IMPROVED MIXER POTENTIOMETER*
K. B. LAMBERT**
Summary. — A type of mixing control is described that permits unusual
efficiency of operation. It is applicable to all types of mixing, particularly the com-
plicated operations such as recording music, re-recording, and radio broadcasting.
In re-recording, or in multiple-channel recording of original music,
or in multiple-microphone set-ups in radio broadcasting, the conven-
tional mixing potentiometer having a rotary motion has serious dis-
advantages. In 1930, M-G-M Studios adopted a potentiometer control
for re-recording having a linear motion, to and from the operator, in-
stead of rotary motion. The operating philosophies that prompted
this move were important then and have become increasingly so since.
We believed that a certain amount of re-recording was inevitable
and that more improvement than harm would result to the remainder
of our product if it were completely re-recorded for release. We also
believed that the control of the release quality should be concen-
trated largely in the re-recording operation, and that the group which
did this operation should be as small as practicable, for ease in co-
ordinating their operations to achieve a consistent product. Re-
recording has become increasingly complex, but progressive experience
has enabled our four re-recorders to continue to handle nearly all our
work without other assistance. Even within this group, each man
prefers to work alone on a reel when he can, for then he can concen-
trate upon his own concept of the work and not have to coordinate
his concept with that of someone else assisting him. The writer be-
I lieves that some studios make composite records of a number of ef-
fects when a sequence is very complex, to simplify the final re-record-
ing operation. We have not found this practicable, as the use of
any individual effect is influenced by each of the others. We there-
fore mix all component tracks together at the same time. For these
* Presented at the 1940 Fall Meeting at Hollywood, Calif. ; received October
1, 1940.
** Metro-Goldwyn-Mayer Pictures, Inc., Culver City, Calif.
283
O The Society is not responsible for statements by authors <>
284 K. B. LAMBERT [j. s. M. P. E.
reasons, it is desirable to make the mechanical handling of the con-
trols as simple as possible.
With linear movement, several potentiometers can be operated si-
multaneously with each hand and the control of a number of channels
becomes much easier. The early forms of these mixers were some-
what inconvenient to maintain and keep clean, but having them, we
did not redesign them until they needed major repairs. Then a new
design was produced jointly by M-G-M and Audio Products Co. engi-
neers, which corrected the disadvantages of the earlier types to such a
degree that we now recommend them to the industry's attention.
FIG. 1. Mixer table operated by one re-recorder.
As employed in re-recording at M-G-M, each mixer table contains
ten similar potentiometers, eight of which are used for mixing, and
two for extra controls that may be required, as for the simultaneous
control of a group of tracks (Fig. 1).
The mixer potentiometers are connected to the channel through a
group of coils, each of which has five windings. Each coil combines
four circuits into one having the same impedance, with a loss of 6.5
db. If only four potentiometers are used, one coil is required. Our
circuits are normally set up for eight potentiometers, which require
three coils, as shown in Fig. 2. If four or eight additional potentiome-
ters are required, they may be added by the addition of one or two
coils as shown in dotted lines in Fig. 3. These are high-quality coils
Sept., 1941] AN IMPROVED MIXER POTENTIOMETER
285
having very small losses and phase-shifts. The coil circuits are made
to have a uniform resistance of 200 ohms in all circuit directions by
300-ohm terminating resistances, as shown in Fig. 2. These resis-
«-2oou.-» f Zoouj Zoocu V.C.
FIG. 2. Four positions of an M-G-M re-recording mixer.
To 8 K.Vf>roduCer-& Cortfi1fC/t</ -f-
Oncf
rtixer Tabte as r-fyuir*<j
FIG. 3. M-C-M re-reco/ding mixer tables.
tances are included in the 6.5-db loss mentioned. Each potentiometer
is of approximately constant 200-ohm impedance in both circuit di-
rections, and all equalizers are of this impedance. A truly constant
impedance at all degrees of attenuation is not practicable because the
potentiometers are not of the step-type but are continuously wire
286
K. B. LAMBERT
[J. S. M. P. E.
wound. The large change of attenuation in one step of a conven-
tional step-type potentiometer causes a click in the presence of loud
signals. The change of attenuation in these potentiometers as the
slider moves from wire to wire is about 0.1 db in the region where loud
signals are mixed, and they therefore operate quietly. Quietness is
apparently further aided by the use of graphite from a very soft lead-
pencil as lubricant for the slider against the wires. This has also
greatly reduced maintenance as compared to that required with the
vaseline previously used. With the slider operating dry, or lubricated
with vaseline, we averaged about one noisy potentiometer per day,
with twenty in use; whereas with graphite we now have only about
• Attenuation in DB
FIG. 4. Attenuation vs. movement, M-G-M re-recording mixers.
one per month. The minimum loss of each potentiometer is 6 db,
and the minimum loss from the input of a potentiometer to the output
of the 16-mixer coil is 19 db. The potentiometer is designed and
wired to avoid internal cross-talk, having a maximum attenuation of
105 db at 1000 cps, and 90 at 7000 cps. This range appears to be
sufficient in service. A key is provided for disconnecting and short-
circuiting the reproducing machine and terminating the mixer with
200 ohms when it is not in use. The wiring of the table and jacks
has cross-talk of less than 110 db. The mixers do not have a uniform
linear rate of attenuation, as it has been found more desirable in op-
eration to be able to change the attenuation rapidly at high degrees of
attenuation where the signal is low, and more gradually at small at-
tenuations where the signal is loud and the ear is more sensitive to
Sept., 1941] AN IMPROVED MIXER POTENTIOMETER
287
FIG. 5. Mixer construction.
'
FIG. 6. Mixer table open, showing jacks,
288
K. B. LAMBERT
[J. S. M. p. E.
small level changes. The curve of -attenuation vs. movement is
shown in Fig. 4.
The 10-position mixer table comprises five units of two mixer con-
trols each, which can be inserted and removed similarly to a plug-in
relay (Figs. 5 and 6). This facilitates maintenance, as a pair of mix-
ers can be replaced with a spare with as little effort as a fuse. The
potentiometers themselves have a rotary motion, and are controlled
by a belt of cord, wrapped around a drum on the shaft. The sliding
control knob runs in F-grooved tracks, set back from the slot in the
FIG. 7. Two mixer tables set up for operation by two re-recorders.
panel so that dirt does not fall into the grooves. The mechanism is
sufficiently free from friction that the mixer can be flipped from one
extreme of its range of movement to the other. The knob itself has
a depression for control by one finger, if this should be desirable.
Using this device, about 70 per cent of M-G-M's re-recording is done
by the four individual re-recorders. With the exception of about
Y4 per cent of the product, the remainder is done by teams of two as
shown in Fig. 7. The exceptional reels may require a third man to
operate equalizers.
Sept., 1941] AN IMPROVED MIXER POTENTIOMETER 289
DISCUSSION
MR. CRABTREE: It is still somewhat of a mystery to me as to how mixing is
accomplished. What does the mixer do when he sits down at the mixing panel?
When he gets the various sound-tracks, does he rehearse the picture with all of
them and fiddle around with the keys until he thinks it is all right? Is the mixer
responsible for the result, or does someone else listen in, and does the mixer re-
modify it to suit him? Suppose there is talking, with extraneous noises; does he
first run through the speech record and adjust that, and then the noise record;
or does he adjust the two simultaneously? It would seem that if he has eight
fingers on eight keys he is attempting to integrate eight tracks simultaneously.
MR. HILLIARD: I can explain that as I would explain how I can drive non-
chalantly downtown in Los Angeles through all the congestion of traffic, and yet
pay attention while you are talking to me. Apparently, one does not think about
what is happening, but adjusts himself automatically to the conditions.
As regards the lack of marks on the panel, the re-recording mixer indicates with
a grease pencil how far he wishes to go in a given scene. Sometimes if the scene is
very complicated he will break it down and run only the effects tracks, to get an
idea of what has been given to him. In most cases, he has not previously seen the
original material. In a temporary dubbing for previews a lot of material is given
to him in a very short time, and he has to consult the log that is placed before him
by the cutter, or by whoever is responsible for building up the effects to go along
with the dialog and music. He looks at them and from previous experience knows
roughly where they will be in the reel.
He breaks the material down and tells his operators on what types of machine
to put it, if there is any preference in his mind. He might have higher-quality
reproducers for dialog and music than for effects, so he utilizes the best machines
for the work requiring the greatest precision.
By a cut and try process the mixer works the material up to a point where he
feels he has the situation in hand. Someone may assist in advising him as to the
relative values of the effects to be dubbed into the music, or as to the interpreta-
tion desired in the final released material. In a half hour or so the situation is
sufficiently in hand for making the take. If the take is not satisfactory, the next
day it is done again.
MR. CRABTREE: Does he first rehearse the individual tracks or does he start
rehearsing with the entire six or seven?
MR. HILLIARD : That depends upon the complexity of the scene and upon the
mixer's preference. Some would probably run the dialog track to find out what
is in the sequence, and possibly add some music. Then later the effects would be
brought in. That is one technic ; there is no special way of doing it.
MR. CRABTREE : Having rehearsed it to his satisfaction, can the mixer repro-
duce the effect? He apparently has to remember how loud the various tracks
were.
MR. HILLIARD : Volume indicators are his guides, and it is no effort for him to
remember the scenes. He thinks in terms of the scenes themselves, of the action
that is going on, in terms of a speed of 90 feet a minute. He knows he has to
operate in a split second. You and I go into retrospect on what has happened
and put it together at a later date.
MR. CRABTREE : Does one man do the entire mixing?
290 K. B. LAMBERT [j. s. M. P. E.
MR. HILLIARD: That depends upon the flexibility of the equipment and the
policies of the various studios. In most studios one man handles the simple reels
alone; when more than three or four tracks are involved, there may be two men,
in other cases three.
MR. CRABTREE: What is the greatest number of tracks that one man can
handle?
MR. HILLIARD: That varies with the equipment. In our case one can handle
up to six or eight tracks ; in fact he does so almost every day. It depends to some
extent upon how fast the eight tracks may enter as they are distributed through-
out the reel, or whether eight operations are demanded simultaneously. With
this control he can handle a maximum of eight situations at once.
MR. LAMBERT:* Mixing is a little like playing the organ. Tremendous dex-
terity is demanded. Not only are all ten fingers busy on the keyboards, but they
must operate the stops; and the feet are equally busy playing the pedals and the
swells, etc. It seems beyond comprehension that one mind can simultaneously
control so many different physical operations at the same time. The technic must
be mastered so completely that when the eye sees a note on a sheet of music a
hand or foot moves unconsciously to perform the operation it demands. In the
case of re-recording, the picture on the screen is the sheet of music. On the re-
recording log will be various comments, indicating that at a certain part of a reel
an effect is to be found in a certain sound-track. These are similar to instructions
on the music that a particular passage is to be played on the lower organ manual
with the left hand, and then a few notes on another manual, which has been pre-
set, while playing, to produce a desired effect. That is what occurs when a mixer,
controlling several tracks with one hand, simultaneously changes equalizers, or
throws various circuit keys with the other. The mixer is always guided by the
picture. The action on the screen demands that certain sounds be produced that
will agree well with the scene. The mixer frequently does not analyze these de-
mands consciously, but satisfies many of them with an automatic reaction de-
veloped by experience, his conscious thought being left more free to concentrate
upon some particular effect to be achieved. He will perhaps play a band parade
"automatically," balancing all the street effects and the music to be realistic,
but give very conscious attention to a line of dialog that must be made under-
standable, or a bass drum that must be sounded very loudly.
To be a capable re-recording mixer requires extensive experience and well de-
veloped judgment in :
(a) Dramatic presentation of situations.
(6) The conventionalities by which dramatic situations are developed in mo-
tion pictures.
(c) The technical limitations of presenting dramatic situations in theaters, and
tact in reconciling a producer's wishes with these limitations.
(d) The mechanism, technics, and handling of the re-recording process itself, of
which manual dexterity alone is but a small part.
(e) They must be able to cooperate with each other to produce a uniform prod-
uct.
* Communicated.
Sept., 1941] AN IMPROVED MIXER POTENTIOMETER 291
The best re-recording mixers must have experience that can be gained in no
other way than by doing the work. Our mixers have been from four to ten years
on this type of work, preceded by experience in other forms of motion picture work,
and that again preceded by a technical or theatrical background of some kind.
No matter what his previous experience, a re-recording mixer does not become a
really efficient member of our re-recording organization until he has been at that
specific work for at least six months.
Ordinarily we employ two men if the number of tracks exceeds six. There are
cases where two are required for fewer than this number, and others where one
man can control seven or eight. Each case is handled differently, and the mixer
can save much time and effort by being able to analyze quickly how best to ap-
proach each problem.
REPORT ON THE ACTIVITIES OF THE INTER-SOCIETY
COLOR COUNCIL*
Summary. — A brief discussion of the activities, organization, and functions
of the Inter-Society Color Council is given, followed by abstracts of twenty papers
that have been jointly sponsored by the ISCC and its member bodies. The report
concludes with a plea that members contact the delegates if there are matters that
should be taken up with the Council.
The Society of Motion Picture Engineers has, for the last year and
a half, been a member of the Inter-Society Color Council. A brief
description of this Council may be in order.
There has never been a National Society in the United States de-
voted exclusively to color as a general subject. The growing im-
portance of color in all fields about ten years ago led to the feeling
that such a Society might be successful. After some discussion
among those interested in the project it was decided to follow a sug-
gestion made by the late Irwin G. Priest. This proposal appeared
in the form of a resolution from the Executive Committee of the
Optical Society of America. In brief it proposed that a joint council
be set up, formed of delegates officially appointed and sent to the
council by societies interested in color as one part of their more
general fields.
The Inter-Society Color Council was formed on this basis and
consists of two types of members, the so-called member bodies and
individual members. Member bodies must be national non-profit
societies, and delegates from these member bodies control the policies
of the Council as a whole. Individual members are those who do
not represent national societies but who are individually sufficiently
interested so that they are willing to pay the cost of being placed on
the mailing list of the Council. The group of individual members
is considered as a member body and sends three voting delegates to
the Council meetings.
Each member body is also represented on the Council by three
voting delegates, one of whom is designated as the chairman. The
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 5,
1941. Report prepared by R. M. Evans, Chairman of SMPE delegates to the
ISCC.
292
INTER-SOCIETY COLOR COUNCIL 293
member body may also, if it wishes, appoint up to seven other non-
voting delegates.
The Council at the present time consists of thirteen member bodies
and the group of individual members. The member bodies repre-
sent, respectively, the fields of Textiles; Ceramics; Psychology;
Testing of Materials; Illuminating Engineering; Pharmacy;
Optics; Pulp and Paper; The Textile Color Card; Art; Paint and
Varnish; and, through our own Society, Motion Pictures. The in-
dividual members represent an even more diversified group of in-
terests under their chairman who is an ophthalmologist. The dele-
gates from the SMPE are Ralph M. Evans, Chairman, G. F. Rackett,
F. T. Bowditch, and J. A. Ball.
The purpose of the Council is to act as a joint committee on color
for all the societies and individuals represented. As a joint committee
it is expected to consider problems referred to it and is free to ap-
point sub-committees from among the membership lists of all the
societies involved. In this way the best talent in the whole field of
color can be brought to bear on any problem of sufficient importance.
The separate delegations from each society, on the other hand, are
charged with the responsibility both of forwarding problems and
helpful information to the Council, and of reporting back to their
societies the work of the Council so far as it is of interest to their
fellow members.
The need for such a council and its mode of operation were dis-
cussed at some length- in a very interesting paper presented by Dr.
H. P. Gage at the Spring Meeting of the Society a year ago, and
published in the October, 1940, issue of the JOURNAL.
This paper gives an illustration of how the Council can be of ser-
vice to its member bodies. The U. S. Pharmacopoeia Revision Com-
mittee of the American Pharmaceutical Association requested the
Council to investigate and recommend a list of color names which
would be readily understandable and definite enough to use in the
descriptions of all the material in the U. S. Pharmacopoeia. The
Council undertook this problem with the following results. It ob-
tained for this member body "the advice of the color experts of two
other member bodies (the Optical Society of America and the Ameri-
can Psychological Association) ; it obtained for this member body the
cooperation of the National Bureau of Standards, which had been
previously sought and refused ; and the Council served as an authori-
tative source of information unswayed by commercial considerations
294 INTER-SOCIETY COLOR COUNCIL [J. S. M. P. E.
for deciding which of the various competing systems of material color
standards was best suited to derivation of the color names. In all
this work the allied interests of another member body (The Textile
Color Card Association) were protected by the presence of its dele-
gates at the Council Meetings." The outcome of this work, which in-
volved a problem really too large for one society to handle success-
fully by itself, was a set of definite recommendations which have
been accepted and will be used exclusively in the next edition of the
U. S. Pharmacopoeia. This was published as a research paper of the
U. S. Bureau of Standards, of which the following is an abstract.
METHOD OF DESIGNATING COLORS1
DEANE B. JUDD AND KENNETH L. KELLEY
In 1931 the first Chairman of the Inter-Society Color Council, E. N. Gather-
coal, proposed on behalf of the United States Pharmacopoeial Revision Committee
the problem of devising a system of color designations for drugs and chemicals.
He said, "A means of designating colors hi the United States Pharmacopoeia, in
the National Formulary, and in general pharmaceutical literature is desired ; such
designation to be sufficiently standardized as to be acceptable and usable by sci-
ence, sufficiently broad to be appreciated and used by science, art, and industry,
and sufficiently commonplace to be understood, at least in a general way, by the
whole public." With the assistance of the American Pharmaceutical Association,
and following plans outlined in 1933 by the Inter-Society Color Council, there has
been worked out a solution for this problem, which substantially fulfills the re-
quirements laid down by Dr. Gathercoal.
Contents
(7) History
(77) Scope
(777) Logic of the designations
(7) Surface-color solid
(2} Basic plan of forming the designations
(5) Divisions of the hue circle
(4) Pink, orange, brown, and olive
(5) Some unavoidable disadvantages
(IV) Definition of the color ranges
( V) Hue boundaries for various ranges of Munsell value and chroma
( 77) Color designations for opaque powders
(7) Preparation of sample
(2} Lighting and viewing conditions
(5) Procedure
(4) An example
( VIP) Color designations for whole crude drugs
(1) Comparison with Munsell color standards
Sept., 1941] INTER-SOCIETY COLOR COUNCIL 295
(2} Lighting and viewing conditions
(3} Ways of using the color designations
( VIII) Color designations for any object
(1) For opaque non-metallic materials
(a) With matte surfaces
(b) With glossy surfaces having no regular detailed structure
(c) With glossy surfaces made up of cylindrical elements
(2} For metallic surfaces
(3} For materials which transmit but do not scatter light
(4) For translucent materials
(IX) Summary
(X) References (4)
How to use the color name Charts 1 to 34
The selection of a standardized set of color names was the partic-
ular problem that engaged the early attention of the Council. It is,
however, not the only problem on which the Council is at present en-
gaged. As listed in the last report of the Executive Committee they
are as follows :
(7) Questions concerning the ICI standard observer
(2) Color names
(3) Color for poison label
(4) Designation of theatrical gelatins
(5) Who's who in color
(6) Survey of color terms in use by member bodies
(7) Survey of color specifications in use by member bodies
(8) Survey of color problems of member bodies
(9) Development of a color aptitude test
In addition to its fact-finding activities the Council has been very
active in sponsoring joint meetings with member bodies at Conven-
tions for general educational purposes in color. Several such joint
meetings have been held. At each such meeting there has been a
joint symposium of invited papers on a subject of particular impor-
tance to the member body. These papers have then been published
in the regular journal of the member body and reprints bound to-
gether have been sent to all Council delegates.
In February, 1939, a joint meeting was held with the American
Psychological Society. The topic of the symposium was "Color
Tolerance. ' ' The following papers were presented and later published
in the American Journal of Psychology in July, 1939.2
296 INTER-SOCIETY COLOR COUNCIL [J. S. M. P. E.
THE PSYCHOPHYSICS OF COLOR TOLERANCE2
EDWIN G. BORING
Any particular tolerance can and must be measured in terms of the physical
units of the color-stimulus, since the stimulus-scale is an ultimate system of refer-
ence. One hopes, nevertheless, for some general statement of the limits of any
particular kind of tolerance, and it would seem that such a generalization must be
more likely to be expressible in the units of some psychological scale which is defi-
nitely but perhaps not linearly related to the physical scale of the stimulus.
Tolerance is a matter of perception and must therefore be related to discrimina-
tion. There must be as many kinds of tolerance as there are reasons for fixing the
character of colors, and two colors that are not noticeably different vpould neces-
sarily lie within the range of all tolerances, if they are really not discriminable.
That sentence is, however, much too simple to express the whole truth. While
many tolerances are supraliminal, certainly there are others which are subliminal.
The difference limen is the average "jnd" (just noticeable difference), that is to
say, it is the difference which is as often perceived as not. A difference that is
perceived only forty per cent of the time is subliminal, yet it is perceived almost,
though not quite, as often as not. Complete intolerance, a requirement that two
colors should never be perceived as different, would specify a difference that is far
below the limen.
There are three psychological scales, any one of which might be used for mea-
suring tolerance. (1) The first is the DL difference limen. It can be chosen as the
unit, and a tolerance specified as a permissible number of DL. (2} A more stable
measure is (standard deviation) of the psychometric function. (3) The third
measure is the sense-distance. Since these three measures are, unfortunately,
almost the private property of the psychologists, their explanation is undertaken
to show theu: natures and disadvantages. The thesis is, that for the measurement
of tolerances, the DL has the most disadvantages, and the sense-distance the least.
Unfortunately it is the DL that has been used most often by psychologists, whereas
the sense-distance has hardly been used at all.
THE RATIO METHOD IN THE REVIEW OF THE MUNSELL COLORS3
SIDNEY M. NEWHALL
(7) The ratio method consists hi the estimation by direct impression of the
ratio of supraliminal sense magnitudes. The one magnitude or interval is taken
as standard, and the ratio of the other to it is then estimated.
(2} The spacing problem is to detect and correct errors of allocation in surface
color space of the 400 regular Munsell colors. The readjustments are being made
for the samples as mounted on white, on gray, and on black (cardboard) grounds.
(5) The application of the ratio method to the spacing problem proves to be a
complicated, meticulous, and lengthy process of choosing vectorial equivalents for
sensory magnitudes. The principal attributes are evaluated separately under a
flexible instruction.
(4} Observers are beset by the real and technical difficulties of the first chroma
step, attributive abstraction, and separation of standard and sample. They do
not use the ratio method in pure form, but resort to the methods of ranking and
single stimuli in the effort to achieve results; and results are achieved.
Sept., 1941] INTER-SOCIETY COLOR COUNCIL 297
(5) Preliminary results on the respacing of saturation tend to confirm the use-
fulness of the method and to suggest that many minor adjustments may prove
desirable. Present data indicate that adjustments also will be necessary if the
colors are to be corrected for gross variations in background reflectance.
COLOR TOLERANCES AS AFFECTED BY CHANGES IN COMPOSITION
AND INTENSITY OF ILLUMINATION AND REFLECTANCE OF
BACKGROUND4
HARRY KELSON
The hue, lightness, and saturation of any object depend upon a complex of con-
ditions, chief of which are spectral reflectance of sample and background, spectral
energy distribution of illuminant, and state of the visual mechanism as determined
by its properties as a receptor and mode of functioning. Since color tolerance con-
cerns the extent to which colors match, or if they do not match, in what respects
they differ, any factors which affect the color of standard and variables have im-
portance for the problem of tolerance. The problem as to how illuminant, back-
ground, contrast, adaptation, and so-called constancy operate in literally "color-
ing" objects seems to be nearing solution after recent work with strongly chromatic
illuminants and backgrounds of high, medium, and low reflectance. The prin-
ciples governing color changes which objects undergo in strongly chromatic il-
luminations throw light also on phenomena encountered in ordinary viewing
situations. Hence consideration of the facts discovered under "abnormal" condi-
tions of viewing is of value in bringing into sharp relief the principles operating
in all visual functioning.
REPRESENTATION OF COLOR TOLERANCE ON THE CHROMATICITY
DIAGRAM5
DAVID L. MacADAM
Actual practice in the establishment of color tolerances indicates that visual
examination of a representative group of samples, and agreement between the
manufacturers and representative users of the colored materials is more satisfac-
tory to all parties concerned than any theoretical deduction of tolerances from ab-
stract experiments. Tolerances established by such agreement can be represented
just as clearly on the ICI chromaticity diagram as on any other chromaticity dia-
gram. The ICI chromaticity diagram is recommended for such use because it has
been standardized internationally and used extensively for many years, and be-
cause its use will not encourage any false simplifications of the color tolerance
problem.
SPECIFICATION OF COLOR TOLERANCES AT THE NATIONAL BUREAU
OF STANDARDS6
DEANE B. JUDD
The various parts of a color specification to be administered by working stand-
ards have required different methods for color-tolerance specification. Choice of
method is also affected by the article whose color is specified and by the instru-
298 INTER-SOCIETY COLOR COUNCIL [J. S. M. P. E.
ments used. Three chief methods are discussed: (2) the standard ICI coordi-
nate system; (2) material standard and tolerance sample; and (3) the NBS unit >
of surface-color difference.
The ICI system applies aptly to color specifications not requiring account of the |
perceptibility of differences; it yields a precise and reproducible specification of
color tolerance in fundamental terms. The use of a material standard and
tolerance sample requires a sense-distance judgment, and within the limitations of
precision of that judgment is perfectly adapted to insure tolerances of uniform per-
ceptibility. The NBS unit of surface-color difference is intended to combine the
precision andreproducibility of a fundamental system with the aptness of the sense-
distance judgment by means of a tolerance sample. The formula for. color differ-
ence defining this unit makes use of sensibility data obtained over a period of years
by psychophysical methods, and, in effect, interpolates between the various series
of stimuli for which color-difference sensibility is known. The NBS unit of sur-
face-color difference suffers from two disadvantages: first, the psychophysicai
data on which it is based are so small a proportion of the total required for reliable
evaluation of the complete surface-color solid that attempts to use the unit still
reveal ways in which the interpolations yielded by the color-difference formula may
be significantly improved ; and, secondly, the unit varies with observing conditions
so that the color-difference formula defining it is necessarily rather complicated.
The NBS unit is already successful enough to justify its use in some standardiza-
tion work, but our research continues to be directed toward its improvement. It
should be emphasized, therefore, that changes in definition of the unit are to be
expected, but it is believed that a practical unit of surface-color difference will
result from our study, and that the definition of the unit will be no more compli-
cated than the tentative form discussed here.
INDUSTRIAL COLOR TOLERANCES7
ISAY A. BALINKIN
The results of visual tests indicate that it is possible to establish a numerical
scale for visual perception of magnitude in relation to color differences. A con-
siderable amount of additional data will be required to provide the information
for various spectral regions.
Although the degree of agreement between visual estimations of color differences
and those computed from physical measurements are not satisfactory, their ap-
plication to problems of consumers' acceptance and tolerance control proved of
practical industrial value. The use of Hunter's reflectometer for these measure-
ments gave fairly reproducible results within ±0.1 judd under most careful opera-
tional technic. Further improvements are needed, however, to achieve a better
reproducibility of this instrument for routine industrial use under plant operating
conditions. The reduction of color measurements to differences in terms of visual
perception by the use of Judd's equation is still a very tedious and lengthy process
requiring at least ten minutes for a pair of samples. The possibility of construct-
ing a nomograph for such computations is now being investigated.
In February, 1940, at a joint meeting with the Technical Associa-
tion of the Pulp and Paper Industry a symposium on Spectropho-
Sept., 1941] INTER-SOCIETY COLOR COUNCIL 299
tometry in the Pulp and Paper Industries was sponsored. The papers
were reprinted from Technical Association Papers, 23 (June, 1940),
pp. 473-525; and were published in the TAPPI Section of the Paper
Trade Journal, 111, No. IQetseq. (Sept. -Oct., 1940).
SURVEY OF SPECTROPHOTOMETERS8
KASSON S. GIBSON
Recent progress in spectrophotometry in the visible spectrum has been largely
along photoelectric lines. Most of the new instruments, however, have been de-
signed for transmission measurements only, and are of little direct interest to the
pulp and paper industry. Reference is made to various previous surveys of spec-
trophotometry, and the present paper merely brings the subject up to date, with
particular emphasis on reflection measurements.
Visual spectrophotometric measurements have proved inadequate for the ac-
curate colorimetric specification of white and near-white materials. The colors
of such materials are importantly affected by small changes or differences of spec-
tral reflection, differences that are of the order of magnitude of the uncertainties
inherent in visual spectrophotometry. The photoelectric cell, on the other hand,
is ideally adapted to the determination of small differences, and in apparatus de-
signed on sound electrical and optical principles is capable of yielding spectro-
photometric measurements that are both precise and accurate.
All reflection spectrophotometers measure the apparent reflectance of a material
relative to the apparent reflectance of a working standard, under certain fixed con-
ditions of illumination and observation. The concept of apparent reflectance, the
importance of considering the most desirable conditions of illumination and view-
ing, and the question of primary and secondary standards of apparent reflectance,
are accourdingly briefly considered.
A SURVEY OF ABRIDGED SPECTROPHOTOMETERS9
J. A. VAN DEN AKKER
A survey has been made of abridged spectrophotometers, and a description and
a discussion of the uses of these instruments are given.
TAPPI SURVEY OF COLOR INSTRUMENTS USED IN THE PULP AND
PAPER INDUSTRIES10
The active interest of the pulp and paper industries in the use of instruments for
the measurement and control of color is demonstrated by a survey of a number of
mills in this country and Canada by the Technical Association of the Pulp and
Paper Industry. Of the 43 inquiries mailed by TAPPI headquarters, 38 replies
were received. The summarized answers are given to the questions comprising
the inquiry. They indicate that properly designed instruments for the control
and measurement of the color of pulp and paper are finding increased application
300 INTER-SOCIETY COLOR COUNCIL [J. S. M. P. E.
by the industry. The fact that nearly 90 per cent of the replies showed their
interest in the use of a more satisfactory instrument, if it could be found, demon-
strates the trend of technical control and measurement in the manufacture of
pulp and paper.
By control testing is understood the routine measurement of pulp, paper, and
fillers, such as the determination of brightness. Color measurement usually in-
volves a more thorough measurement of the color of the product for research,
grading, reference standard, or other purposes, such as the determination of the
spectral reflectance of the material.
THE MEASUREMENT OF COLOR OF PULPS11 "
R. S. HATCH
The problem of evaluating the color of pulps is one which involves the exact
physical measurement of the color rather than the expression of color value in
psychological terms. The best instrument and the instrument to which all physi-
cal measurements of color should be referred is an automatic recording spectro-
photometer. Under certain conditions, where pulp is being produced from a
single species, an abridged spectrophotometer may be used for general research
purposes, and a properly designed brightness meter may be used for manufactur-
ing control. Brightness meters should be so constructed that commercial samples
of the pulp itself can be directly measured without resorting to the making of spe-
cial color sheets from the sample in question.
CARD INDEX AND OTHER METHODS USED IN CONTROLLING THE
COLOR OF BEATER FURNISHES12
R. N. GRIESHEIMER
Instrumental methods of controlling the color of beater furnishes are contrasted
with the older method of visual control. Although the advantages of the instru-
mental methods are indisputable, neither method can be wholly successful in
providing for color control of the finished sheet of paper unless account is taken
of certain machine variables that affect sheet color by influencing dye retention,
etc. Important variables are the pH and freenes (refining treatment) of the to-
wire stock.
Instrumental methods are subdivided into empirical methods, and methods in-
volving the absolute calculation of certain optical constants of the raw materials
and finished sheet from either the Kubelka and Munk theory, or the theory of
trichromatic coefficients. The latter methods are principally of academic interest
and have not met wide-spread use in the industry. Two empirical methods are
outlined, both depending on the ability of a technical man to associate various
dyestuffs with the shapes of their spectrophotometric curves, and the displace-
ments of these curves with changes in dye concentrations.
A particular instrumental method called the card index system is discussed in
considerable detail. A program for indexing variations in reflectance at certain
wavelengths (or variations in trichromatic coefficients) with weight of dye produc-
ing these changes for a given furnish is set up. It is then indicated how these data ,
Sept., 1941] INTER-SOCIETY COLOR COUNCIL 301
considered with other data taken on the effect of important machine variables,
can be used for predicting the color of the finished sheet. A double integrating
sphere reflection meter and a time-saving wet pump sampling procedure are
briefly described as tools found useful in applying the card index system.
In conclusion, general aspects of the problem are discussed including considera-
tions of start-up losses due to off -color, color tolerances, and the selecting of
method and instruments to fit a particular mill's requirements.
USE OF COLOR MEASURING INSTRUMENTS IN THE MANUFACTURE
OF UNCOATED PAPER13
MYRL N. DAVIS
This discussion is limited to the use of spectrophotometers and colorimeters in
the design and control of "white," uncoated papers. Experience has shown that
a working estimate of the apparent "whiteness" of paper can be obtained by mea-
surement of reflectance with light of 458 mmu wavelength. Instrumentation is
available and widely used for making such single measurements on paper samples
in controlling the uniformity of paper being manufactured. Hue of the paper is
still controlled visually in all but a few paper mills.
It is possible, by use of the Guerivic, Kubelka-Munk, and Smith theories of the
optical properties of diffusely reflecting materials, to determine scattering and ab-
sorption coefficients for the pulp and pigment, of which paper is manufactured,
and by use of these coefficients to predict the reflectance and opacity of papers
made from any proportions of these raw materials. Examples are given of such
predictions for machine-made papers, the optical coefficients having been obtained
from simple handmade papers.
The effect of supercalendering on opacity and color is discussed. Exact quanti-
tative estimates of the effect of supercalendering are not yet possible.
SPECTROPHOTOMETRY ON COATED PAPERS14
WILLIAM J. FOOTE
By means of spectral reflectance curves, dyestuffs may be identified either in
coated or uncoated papers and rough quantitative estimations made. In addi-
tion, spectral reflectance curves may be used to specify the color of standard
grades, to act as a guide in color matching in general, and particularly in the case of
matching the color of base sheet and coating of tinted whites and to indicate toler-
ances.
A more precise method of specifying standard colors and tolerances consists of
using chromaticity diagrams based on the specification of color by the method
proposed by the International Commission on Color (ICI) in 1931.
By the use of one additional measurement (either R0 or R0.ts at any chosen wave-
length) and calculations involving the Kubelka and Munk relationship, valuable
information is obtained on the fundamental optical coefficients (scattering and ab-
sorption) of coatings made up with various types of pigments. This permits a
study of the effects of different raw materials and the successive steps in the proc-
ess of manufacture on color, gloss, and opacity of the finished product.
302 INTER-SOCIETY COLOR COUNCIL [J. S. M. P. E.
The relation of the physical measurements, brightness and visual efficiency ( Y),
with visual "brightness" and "whiteness" and the possibility of there being an
optimum color for white printing papers is briefly discussed.
SYSTEMATIC COLOR DESIGNATIONS FOR PAPER15
DEANE B. JUDD
The construction at the National Bureau of Standards of a system of color
designations for drugs and chemicals in accord with recommendations by the
Inter-Society Color Council raises the question whether a similar system, or the
same system, would be worth while for description of paper color. ^The ISCC-
NBS color designations are combinations of the simplest and most widely used
color names: red, orange, brown, yellow, olive, green, blue, purple, pink, black,
gray, and white; with such common and easily understood modifiers as light,
medium, dark, weak, moderate, strong, vivid, pale, and deep. There are slightly
more than 300 such designations, each one applying to a defined range of color,
and, taken together, covering all colors.
As an aid to the initiation of cooperative study embracing all branches of the
paper industry, the ISCC-NBS method of designating colors is described in detail.
These designations have been found for about one-fifth of the colors given in the
Blue-Book Manual issued in 1936 by the Trading Committee of the Groundwood
Paper Manufacturers Association, and they have also been found for samples of
bond paper supplied by 4 companies and known by 15 traditional paper-color
names. These results serve to illustrate the descriptive qaulity of the ISCC-NBS
color designations, and they also indicate how closely the traditional color names
are followed for bond paper, and how wide the color departures may be between
bond-paper terms and groundwood-paper terms.
THE PSYCHOLOGY OF COLOR16
I. H. GODLOVE AND E. R. LAUGHLIN
The psychological or subjective aspect of color problems must always be con-
sidered along with the physical or objective phases, sometimes in a subordinate or
supplementary role, but often in a dominant one. The growing use of color and
the role of psychological facts in the scientific and judicious use of color is dis-
cussed.
After citing some color idioms which illustrate the pervasiveness of color as-
sociations, as red and orange with warmth, the relation of the psychology of color
to its physics and physiology is shown. The difficulty of explaining many color
phenomena by means of the "pure physics" of color is pointed out; and in this
connection recent work of the psychologists on the phenomena of "color con-
stancy" is stressed. What is meant by the "true color" of an object is described,
and alternative explanations are given. In this connection work on the process
of adaptation of the eye to changes in the intensity and the spectral character of
the illumination is cited. The legibility of colors on various backgrounds is dis-
cussed on the basis of this work and in relation to our knowledge of the fact of
chromatic aberration in the eye. Rules for the application of the known data to
Sept., 1941] INTER-SOCIETY COLOR COUNCIL 303
the selection of colors for stock or backgrounds, rulings, initial lettering, letter-
heads, decorations, and the best relations of these to each other are given.
The extant data on the preferences of men and women for single colors and for
color combinations; on the attention-attracting values of colors; on the subject
of color harmony (including the utility of contrasts, complementaries, and color
sequences) ; on the appropriateness of colors and combinations which may be con-
sidered in relation to their use in advertising, in packaging, and in commodities.
In September, 1940, at a joint meeting with the Illuminating En-
gineering Society a general symposium on color was sponsored. The
four papers were published in Illuminating Engineering.
THE BASES OF COLOR VISION17
LEGRAND H. HARDY
A brief discussion of the physical, anatomical, physiological, and psychological
bases upon which color vision is dependent. Light, its nature, origin, location in
the electromagnetic spectrum, components, and sources are briefly discussed. The
gross anatomy and physiological data concerning the photoreceptors and some
considerations regarding the action of light upon these receptors are given in
summary. A final short note on some psychological factors involved in color
vision is presented.
COLOR DETERMINATION18
PARRY MOON
A general discussion is given of methods for the measurement of color stimuli.
These measurements may be divided into two classes: measurements of the
spectral-distribution curve,irom which are computed the trichromatic coefficients;
and direct measurements by means of colorimeters and (in certain special cases)
color- temperature meters.
The spectroradiometric method is of fundamental importance in every branch
of illuminating engineering, since the complete spectroradiometric curve gives in-
formation on color, luminous output, and heating effect. Perhaps the most im-
portant problem of illuminating engineering today is the development of a spec-
troradiometer for accurate and rapid absolute measurements. Such an instru-
ment has not yet been produced, though satisfactory comparison instruments
(spectrophotometers) are available.
If the spectroradiometric curve is known, the color stimulus can be computed and
can be expressed in terms of:
The tristimulus values X, Y, Z.
The trichromatic coefficients x, y.
Dominant wavelength and purity, \<i and p.
Color temperature Tc (only applicable if unknown can be approximately
matched by a Planckian distribution).
Color can also be measured directly by means of various colorimeters. Results
obtained with these instruments are generally less accurate than those obtained
304 INTER-SOCIETY COLOR COUNCIL [J. S. M. P. E.
by the spectrophotometric method, and the data are of much less generality.
Nevertheless, colorimeters and color-temperature meters may be of distinct value
in industry where the routine comparison of large numbers of very similar lamps
or materials is encountered.
COLOR SYSTEMS AND THEIR INTER-RELATION19
DEANE B. JUDD
By color system is meant a system of specifying color; that is, a system in
which the color to be specified is matched with that produced by one member of
a system of objects or lights. The specification consists of giving an identification
of the member of the system producing the match.
Lights are combined to produce color systems in one of two ways: either three
lights are added together to produce a tristimulus system, or a light of variable
quality is added to one of fixed quality, the "monochromatic-plus-white" method.
The tristimulus system recommended in 1931 by the International Commission
on Illumination (ICI) is discussed and the method of transforming specifications
from this standard system to other tristimulus systems is given. Seven other
tristimulus systems used for color specifications are defined in terms of the ICI
system; some of these were proposed because of simplicity in the computation of
a color specification from the spectrophotometric curve, some to provide uniform
chromaticity-scales, and others to quantify theories of color vision. Chromaticity
is specified on the tristimulus system by trichromatic coefficients, two of which,
plotted on coordinate axes, produce a Maxwell triangle; on the "monochromatic-
plus-white" system, chromaticity is specified by an angle and a radius. Four
methods of specifying the angle are described, and six methods of specifying the
length.
Sets of material color standards may be made up of transparent media viewed
by transmitted light or of opaque surfaces viewed by reflected or scattered light.
The system of arrangement of standards made up of transparent media is that of
subtractive combination; two such systems are described. Color systems based
on collections of pigmented or dyed surfaces are described. These range from the
Munsell color system, intended to conform as closely as possible to the surface-
color solid, to color dictionaries whose system of arrangement is intended merely
as an aid to discovery of the surface producing the match.
A description is also given of the ISCC-NBS method of designating colors to-
gether with the uses to which it is being put.
THE ILLUMINANT IN COLOR MATCHING AND DISCRIMINATION20
DOROTHY NICKERSON
A study of the part played by the illuminant in color discrimination may be
divided into two broad sections. In one the chief concern is to find an illuminant
under 'which color differences will surely be evident. The single illuminant most
satisfactory for this purpose will depend upon the reflectance curve of the samples
to be examined. In the other, the choice is limited to an illuminant under which
an observer may see the colors with which he is concerned in the same relation to
Sept., 1941] INTER-SOCIETY COLOR COUNCIL 305
each other as he would if they were observed under an illuminant to which he has
become previously accustomed, the most usual example being the selection of an
artificial daylight in substitution for natural daylight. Results of studies made
in the color-measurements laboratory of Agricultural Marketing Service regard-
ing this latter choice are presented in charts and table form. They include stud-
ies of 18 illuminants, actual and theoretical, several pairs of samples expected to
show large color differences under a change in illuminant, and 30 samples of cot-
ton, the product with which this laboratory is chiefly concerned. The final re-
sults are summarized in a table which gives a relative rating of illuminants as
substitutes for each other.
The Council also publishes a mimeographed News Letter to dele-
gates. These News Letters give information regarding the opera-
tion of the council, the advance in the fields of color, dates of impor-
tant meetings, and, finally, a rather extensive bibliography of maga-
zine articles of interest to the member bodies. It is intended that
notes from this News Letter may be reprinted in the journals of the
member bodies.
In 1939 the Council issued a comparative list of color terms in
which was given the definitions of a large number of color terms as
supplied by several member bodies. At the time this booklet was
compiled, the Society of Motion Picture Engineers was not repre-
sented. The delegates have proposed a set of terms and their defini-
tions, derived from our existing Glossary of Color Names. This has
been submitted to the Council for inclusion in their list. It is ex-
pected that the Council's final list will cover all words of definite
meaning relating to color, with definitions as given by the users in
all fields.
A test for color aptitude has been under consideration for some
time by a very active sub-committee of the Council. This work is
beginning to give results and it is expected that in the not-too-dis-
tant future a test will be available which it is hoped will test directly
the inherent ability of a person to detect small color differences.
The usefulness of such a test is apparent.
The Society has not, to date, taken advantage of the opportunity
offered by our membership, to submit problems for consideration.
No such problems are at present known to your delegates, and if any
member has suggestions to offer, the chairman or the other delegates
would greatly appreciate hearing about them. The problem, of
course, must be of broad enough nature to interest experts outside
our own field since all the Council activities are done by voluntary
cooperation of members of other societies.
306 INTER-SOCIETY COLOR COUNCIL
It is also to be hoped that at some' time in the near future a joint i|
meeting with the Council can be arranged for one of our Conventions. v
Suggestions are wanted as to the most desirable subject matter for
invited papers for such a meeting.
REFERENCES
1 /. of Research, Nat. Bur. Stand., 23 (Sept., 1939), RP 1239.
2 Amer. J. Psychology, LII (July, 1939), pp. 384-394.
3 Ibid., pp. 395-405.
4 Ibid., pp. 406-412.
6 Ibid., pp. 412-418.
• Ibid:, pp. 418-428.
7 Ibid., pp. 428-448.
* Tech. Assoc. Pulp and Paper Industry (TAPPI), 23 (June, 1940); Paper Trade
J., HI (Sept.-Oct., 1940), pp. 475-479.
9 Ibid., pp. 480-489.
10 Ibid., pp. 489-490.
11 Ibid., pp. 491-493.
12 Ibid., pp. 494-499.
13 Ibid., pp. 500-505.
14 Ibid., pp. 506-512.
15 Ibid., pp. 512-518.
16 Ibid., pp. 518-525.
17 Ilium. Eng., XXXVI (March, 1941), pp. 295-312.
18 Ibid., pp. 313-335.
19 Ibid., pp. 336-372.
20 Ibid., pp. 373-399.
AIR-CONDITIONING SAFETY DEVICE FOR THEATERS*
E. R. MORIN**
Summary. — A new fire damper release and method of preventing smoke from being
recirculated or pumped into a theater auditorium through the air-conditioning system
in the absence of heat or flame has just been developed by the Motion Picture Division
of the Connecticut State Police and is here described.
The Motion Picture Division of the Connecticut State Police is con-
tinually spending time and money in investigating existing safety de-
vices and adapting them to practical uses for the theaters, as well as
developing new items. It also takes into consideration the necessity
of speeding up the action of such devices to prevent the spreading of
fire and panic. At the same time it is further realized that the ulti-
mate cost of these devices to the theater owner must be kept at a mini-
mum. It is also our aim to localize smoke and flame at its origin so
that there will be no cause for unnecessary alarm.
Like everything else, the theater is being modernized and brought
up to date. One of the noticeable changes being brought about by
this modernization program is the increasing number of air-condi-
tioning equipment installations. This has presented us with a new
problem, since at the present time the only safety devices being in-
stalled are those operating in the event of heat or flame : namely, a
fuse switch by the dampers for the blower motor, and a fuse link hold-
ing the damper.
On March 8, 1936, at 7:40 P. M., the Bridgeport Fire Department
received an alarm that brought their apparatus to the Cameo Fur
Store on State Street, Bridgeport. The Cameo Theater is adjacent
to this store and the dense black smoke caused by the furs burning
came pouring out of the front of the store. It so happened that the
fresh-air intake of the Cameo Theater's air-conditioning system was
located at the front of the theater building, thus allowing a large
quantity of smoke to be pumped into the theater auditorium, result-
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May 5,
1941.
** Connecticut State Police, Hartford, Conn.
307
4 The Society is not responsible for statements by authors 4-
308
E. R. MORIN
[J. S. M. P. E.
ing in a slight panic. At this particular time, very few of our theaters
were equipped with air-conditioning systems and there was no known
device to cope with this situation. Since this fire two or three other
similar occurrences have been brought to our attention. These were
caused by burning incinerators or rubbish on adjacent property, and
in one instance a fire in the immediate neighborhood of the theater.
PHOTOELECTe/l UNIT
2WTJ
Meies CONNCCTION THPOUGH
SffTfy sw/rcHft
— TO CO/L OF-
MfitTEK EMERGENCY PfLffY
OUTOMflTIC BfLEfiSE
r-i
JJ
LIGHT JOueCf
110 VOLT SUPPLY
PHOTOSLKTUC UNIT
FIG. 1. Smoke cut-off unit and wiring diagram.
We have been constantly on the lookout for some method or device
to guard against this panic hazard. During our search it was sug-
gested that we use an existing smoke alarm or detector that rang a
bell and flashed lights. We are very much opposed to alarming the
patrons by the use of bells or buzzers, or by using anything which
would depend wholly upon the human element for shutting off the
air-conditioning equipment. Therefore, this did not appear to be the
answer to our problem, as it would result in a lapse of tune and cause
smoke to be pumped into the theater.
Sept., 1941] AlR-CONDITIONING FOR THEATERS 309
We then asked a manufacturer to submit a smoke cut-off device for
experimental purposes, suggesting that the alarm portion of this de-
vice be replaced by an added relay switch. On receipt of this device,
consisting of a photoelectric cell with an exciter lamp and two relay
switches, we conducted numerous experiments and found this to be a
very practical method of shutting off the blower when smoke was in
the vicinity. However, the minute the smoke cleared, the blower
automatically started up again. We felt this to be an objectionable
feature since if there were sufficient smoke in the vicinity to cause the
blower to be shut down, it warranted investigation before starting
the blower up again. We then incorporated a momentary contact
switch located in the vicinity of the smoke cut-off device so that when
the system was shut down by smoke it would require going to that
location in order to start the system, and if desired, a signal light
could be installed in such a location as to attract the attention of one
of the employees. The experiments proved this to be a worthy and
inexpensive addition. The complete device was presented to the
theater owners and is now being installed in theaters in Connecticut
as a smoke cut-off device and not as a smoke alarm as originally de-
signed.
This solved only half our problem. As in the past, it has been our
custom to approach these difficulties step by step, so we then gave
consideration to the remainder of the problem which was the closing
of the fire damper by the smoke detector.
The only automatic damper we could locate that could be released
with the smoke detector was one of the motorized type. However,
this was not equipped with a fuse link so that the damper might close
in the event of failure of the electric current, and there would also be a
lapse of time from the moment the current was applied until the
damper was completely closed. We felt that this action should be
speeded up to be momentary, and that the damper should be equipped
with a fuse link as an added protection. This motorized damper was
also quite costly to install in existing systems inasmuch as it was
necessary to remove the present dampers and redesign the duct in
that location to accommodate the new one, or else have two dampers.
This excessive expense would, of course, be objectionable to the
theater owners.
We then went in search of some simple and inexpensive device that
would not require any alteration of the present damper or method of
fastening the fuse link and would be instantaneous. We could not
310
E. R. MORIN
[J. S. M. P. E.
find any manufacturer who was interested in developing such a de-
vice. We, therefore, took it upon ourselves to construct such a re-
(O
(D)
0
Cfc3
O
o p o
If"
L J
o
o
SECT/ON
£>-£>
(withp/afe 'X'rtmorfef)
FIG. 2. Magnetic safety release.
Small catch to fit up into
hook plate opening
Short nub on rotating
member to cause elec-
trical contact between
K and L
Projecting pin
Flush hook plate with trip
hook attached
Spring dust closure at-
tached to hook H
Shoulder on hook plate to
align box N
(GO Shoulder on hook plate
with slots J and / to
allow space for pin C
and hub B
(H} Trip hook rotating when
A is released
(/) Slot in hook plate
U) Slot in hook plate
(K) Spring contact as shown
(L) Spring contact as shown
(Jlf) Fiber and metal contact
actuater
(N) Mechanism box
(0) Slot for pin C
(P) Cut-out for M
lease whereby only the catch protruded into the duct, replacing the
eye-bolt now holding a chain or fuse link. We experimented with
several types, such as using a magnet from a holding- type switch;
Sept., 1941] AlR-CONDITIONING FOR THEATERS 311
an electrical furnace with a thermostat cut-off for melting an added
fuse link; a latch held with a fuse wire, which is the short-circuit
method; and several others. The one which has proved most prac-
ticable has as its basic principle an electrical door-lock operating on 12
volts a-c or d-c and releasing the large, heavy dampers with the least
amount of electrical energy. This had one serious objection. A
loud buzzing sound was transmitted through the system that would
naturally tend to alarm the patrons. By incorporating a specially
designed switch using contact springs from a telephone jack-switch
which opens the circuit as soon as the catch is released, this difficulty
was eliminated.
FIG. 3. Photograph of safety release, which is held me-
chanically, released electrically, and re-set manually.
Then we felt that there was still one more thing that should be done.
Some of the large dampers made a considerable noise when released.
We found that a l/2-inch asbestos wicking with a piece of asbestos
cloth around it clamped into a metal frame would act as a cushion
and gasket and practically eliminate the noise of the damper closing.
Since the completion of this electrical release, which can be operated
from a remote station or stations, we have found it to be adaptable to
other uses, such as the releasing of fire doors and asbestos curtains.
In Connecticut, all asbestos curtains are equipped with a safety cord
running from one side of the proscenium arch to the other with a fuse
link in the center of the cord. On the operating side of the curtain is
312 E. R. MORIN
a ball with a half-hitch. On the other side is a screw-eye in the I
floor, and the cord is tied to this screw-eye. On the wall close by is
hung a suitable knife and a sign which reads, "Cut This Cord in Case
of Fire." This new device could replace the screw-eye in the floor by
fastening a ring in the end of the cord and hooking it to the catch of
the device. A switch could be installed in the switchboard where a
man is on duty, and another switch could be installed just outside the
stage door, thus making it possible to release the curtain off stage as
well as on. By so doing, the stagehand's safety would no the jeopard-
ized in trying to cut the cord to release the curtain and it would not
be necessary to wait until such time as the flame had melted the fuse
link, which might cause a delay in closing the curtain. In reference
to fire doors, this could be manually operated, or operated from a
sprinkler alarm system or fire alarm system.
NEW MOTION PICTURE APPARATUS
During the Conventions of the Society, symposiums on new motion picture appara-
tus are held in which various manufacturers of equipment describe and demonstrate
their new products and developments. Some of this equipment is described in the
following pages; the remainder will be published in subsequent issues of the Journal.
FIVE NEW MODELS OF 16-MM SOUND KODASCOPE*
W. E. MERRIMAN AND H. C. WELLMAN**
A new line of Eastman 16-mm sound projectors, identified by the model desig-
nations, F, FB, FB-25, FS-10, and FB-40, has been recently introduced to the
public (Fig. 1). All these projectors are designed to be the ultimate in simplicity
and rugged dependability, with the in-built precision so necessary to reproduce
faithfully the finest existing 16-mm records.
There has been no compromise in the quality of the sound-reproducing systems
or film-handling mechanism of these projectors from the lowest-priced to the most
expensive. The same high accuracy of sprockets, aperture plates, film-gate,
sound-drum, and film-guides is common to all models.
On all models the points at which film contact, and subsequent wear, take place,
the surfaces are the same high quality. Buff chrome and stainless steel are used
exclusively along the film's path through the projectors.
The basic picture projection mechanism used in the five models was developed
several years ago and has since been subjected to continual refinement until we
now find it capable of many hundreds of hours of good service. Technical ad-
vances in methods of hardening and toughening the film contact surfaces has
added many hours to the life of the projectors. Precision cam grinders as well
as sprocket and gear generators now produce mechanism parts with dimensional
tolerances which were considered unattainable only a year or two ago. Tolerances
of 0.0001 to 0.0005 inch are common among the sprockets, shafts, and pull-down
mechanism parts.
The sound-head for these projectors is of simple, though effective design. The
short, easily threaded, film path through the sound-head may be seen in Fig. 2.
An easily threaded, well defined path for the film through the picture and sound-
head provides positive synchronism of picture and sound.
The film need not be threaded through the sound-head when it is desired to pro-
ject silent pictures. Fig. 3 shows the short film path for silent projection.
*Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May 5,
1941.
** Eastman Kodak Co., Rochester, N. Y.
313
OThe Society is not responsible for statements by authors O-
314
NEW MOTION PICTURE APPARATUS [j. s. M. p. E.
Simplicity of threading of the film on the, projectors has been considered very
carefully. It is believed that the new Sound Kodascope models offer something
new in their straightforward threading.
The first three projectors mentioned previously are designed to operate on a
d-c or a-c supply voltage of 100-125 volts. Universal operation of the amplifiers
in these models is accomplished through the use of a conventional ballast resistance
unit, suitably connected to allow series operation of the tube heaters from the
supply lines. The use of a ballast device in the heater supply circuit provides
FIG. 1,
Operating position for models FB, FB-25, and FB-40
Sound Kodascopes.
essentially constant voltage on the various tube heaters in the operating range of
100-125 volts, a-c or d-c. Standard radio receiver type tubes of the metal shell
or GT type with glass shells are used on all models.
A unique feature of the universal models is the method used in supplying plate
or anode voltage for the amplifier tubes. High tube efficiency and output are
obtained through the use of a combination single-unit motor-generator. By
means of this device, power is provided to drive the projector mechanism, and it
also provides, from the generator, a high d-c potential for the anode of the photo-
cell and amplifier tubes. Governor control of the motor-generator unit assures
constant output voltage and mechanism speed so essentail for high-quality sound
and picture projection.
Sept., 1941]
NEW MOTION PICTURE APPARATUS
315
The last two projectors, i. e., the FS-10 and FB-40, are designed to operate on
50-60-cycle, 100-125-volt supply lines. Universal motors are used to drive these
projectors and conventional a-c transformer-rectifier power supplies are used to
supply the high d-c potential necessary for photocell and tube anodes.
Many features, such as ease of threading provided by "latchback" gates,
resilient mounting of amplifier and motor units, providing freedom from micro-
phonics; self -lubricating bearings on mechanisms and motors, assuring a mini-
FIG. 2. Projector threaded for projection of sound-film.
mum of attention and maximum life; double-claw pull-down; aperture plate
framing; and provision for the use of crystal microphone or phonograph pick-up
are common to all models. Also, all projector controls are grouped in the same
plane on the operator's side of the projector.
In order to reduce the mechanism noise to a minimum, models FB, FB-25, and
FB-40 have been equipped with a "blimp," or noise-reducing cover.
Provision for the reproduction of either reversal or dupe prints has been made
on three models, i. e., the FS-10, the FB-25, and the FB-40. In order to accom-
modate both types of film, it is necessary to shift the focus of the scanning beam
from one surface of the film to the other. A carefully machined cam, guides, and
lever system have been assembled so that the movement of the lever (Fig. 4)
316 NEW MOTION PICTURE APPARATUS [J. S. M. p. E.
easily shifts the focus of the scanning beam from one side of the film to the other.
The scanning system is of the slitless or so-called apertureless type on all models
and is characterized by greater freedom from microphonics. A standard 4-volt,
0.75-ampere, prefocus-base exciter lamp is used on all models.
Cost, accessibility, and freedom from microphonics dictated that the photocell
be mounted on the main amplifier chassis for all models. A unique method of
light transfer from the film to the photocell is used; it consists of a slightly bent
glass rod approximately 51/z niches long, silvered its entire length. Light, modu-
lated by the film, falls on the end of the rod and is then transmitted through the
rod to the photocell located on the other side of the sound-head. This method of
light transfer permits all photocell wiring to be of minimum length. ^The conse-
FIG. 3. Projector threaded for projection of silent film.
quent reduction of hum and stray modulation is well known to all designers and
engineers responsible for sound-on-film photocell pick-up circuits.
Fig. 5 shows the entire mechanism removed from the "blimp" case or housing.
It is apparent in Fig. 5 that all mechanism parts, including the flywheel, ampli-
fier, and motor-generator, are made accessible when the "blimp" housing is
removed.
On all models, a governor of the electrical, vibrating-reed type maintains con-
stant sound speed of 24 frames per second ; in addition to this, a rheostat is pro-
vided to enable the user to obtain any desired speed below 24 frames per second.
All models except the FS-10 have a thread-lite conveniently located and con-
trolled automatically by the main control switch. Turning off the projection
lamp turns on the thread-lite.
Model FS-10 is a single-case unit in which the speaker case acts as a carrying
case for the projector when not in use. Also, the back section of the speaker case
Sept., 1941] NEW MOTION PICTURE APPARATUS 317
is provided with folding legs and acts as a platform for the projector when a table
is not available.
Fast rewind for all sizes of reels up to and including the 1600-ft is provided on
all models through the use of a clutch, rewind lever, and main drive motor. The
rewind mechanism on all models has been so designed that film damage can not
occur. This feature has been accomplished by placing the rewind clutch lever in
such a location that with film threaded onto the projector it is impossible to actu-
ate the rewind lever. Also, accessory reel arms may be obtained for 2000-ft reels
if desired.
FIG. 4. Sound-head showing focusing optics and focusing lever.
Another feature common to all projectors is the use of a specially designed, oil-
damped, film-driven flywheel. Uniform speed of the film at the scanning point
is therefore assured.
Models F, FB, and FS-10 provide ten watts of undistorted power output ample
for most home and classroom service. Models FB-25 (25-watt output) and FB-40
(40-watt output) have been designed to cover large audiences. The use of twin
speakers with these projectors provides good sound coverage for such assemblies.
Twelve-inch permanent magnet speakers with 4.6-pound magnets are used in the
twin-speaker assembly.
Model FB-40 is provided with two jacks for inputs from phonograph and micro-
phone plus an exciter-lamp dimmer and separate volume controls for phonograph
318
NEW MOTION PICTURE APPARATUS [J. S. M. p. E.
and microphone. Complete mixing of film, phonograph, and microphone is, J
therefore, possible with the input channels and controls provided.
There are six lenses available for each of the five Sound Kodascopes. They are
the 1-inch, //2.5; the PA-inch, //2.5; the 2-inch lenses, //1. 6 and //2.5; the 3-
inch, //2.0 ; and the 4-inch, f/2.5 lenses.
FIG. 5. Mechanism removed from "blimp."
Projection lamps of standard, medium prefocus base, construction are used, and
range in wattage from 300 up to and including the 750-watt lamp recommended
for large screens.
The five new projectors described fulfill a wide range of requirements in a field
which demands equipment that must be reliable and economical. The design
anticipates their use by many operators who have little or no experience.
HIGH-FIDELITY HEADPHONES *
L. J. ANDERSON**
Although the headphone is by no means of recent origin, the high-fidelity head-
phones and the general analysis of the problem were probably first presented as
late as 1932. l Since that time, considerable work has been done, particularly
with regard to improvement in response, sensitivity, and mechanical design,
though the method of analysis remains unchanged.
The desired characteristic of a high-fidelity headphone is to produce a constant
distortionless sound pressure in the ear when constant voltage is applied to the
unit. The usual method of accomplishing this is to couple to a simple moving
coil and diaphragm system a network as indicated in Fig. 1. Proper choice of
constants will produce phone units that will deliver sound of surprisingly good
fidelity. If the ear could be considered a small compliance, then the problem of
ff* R*
T * '
FIG. 1. Cross-section of phone unit and equivalent circuit.
obtaining a satisfactory response would be considerably simplified, since it would
be necessary merely to move the diaphragm with constant amplitude throughout
the desired frequency range. The ear, however, presents a more complex picture,
which is approximately simulated by the simple circuit shown in Fig. 1. Here
the compliance CE represents the volume of the ear, and the values RL and ML
result from the inevitable leakage between the ear-cap of the receiver and the
ear. Actually, there are more elements to the circuit, and especially so at the
higher frequencies, where the dimensions of the ear become appreciable in terms
of the wavelength of the sound in question.
The problem of providing adequate low-frequency response may be attacked
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received June 12,
1941.
** RCA Manufacturing Co., Indianapolis, Ind.
319
<>The Society is not responsible for statements by author s<>
320
NEW MOTION PICTURE APPARATUS [J. S. M. P. E.
from two points: the seal between the unit .land the ear may be improved by using,
specially designed ear-caps, or the acoustic circuit may be proportioned so that-
the low-frequency response will be relatively flat, in spite of the leakage. In the
case of the phones in question, it was found desirable to resort to a combination
of the two methods. Various ear-caps were checked by placing a dummy receiver
case and the ear-cap in question on the ear and blowing smoke into the case. It
was thus possible to observe the extent and points of leakage around the ear. By
this means, it was found that a soft sponge-rubber-faced cap, about 23/4 inches in
J_
FIG. 2. Curve (/)
(II}
Response of final unit on artificial ear.
Response of an experimental unit operating
into a sealed cavity.
(Ill) Response of experimental unit operating
into artificial ear.
loooo ioooo
FIG. 3. (/) Threshold curve obtained with phones.
(II) Normal threshold curve
diameter, gave a good seal consistent with a moderate amount of pressure. In
this connection, it is well to note that the phones will be far from high fidelity if
Sept., 1941] NEW MOTION PICTURE APPARATUS 321
they are not worn so that they fit snugly on the ears. Incidentally, the ear was
well plugged before this smoke experiment, in order to prevent the entry of smoke
into the ear. The effect of the leakage path ML and RL will obviously become
more and more effective in reducing the response as the frequency of the signal
decreases. In order to compensate for this effect, the velocity of the diaphragm
below 300 cycles must be made very nearly inversely proportional to the fre-
quency.2 This is accomplished by introducing into the circuit the path Mz and
R2, in which Mi is very large and R2 very small. As the frequency decreases, the
impedance of this path decreases rapidly and allows the diaphragm velocity to
increase as desired. Between 300 and 500 cycles, the velocity should be inde-
pendent of frequency. The principal reason for attempting to improve the low-
frequency response by providing a better seal to the ear was to reduce the neces-
sary amplitude of the diaphragm as much as possible, and so avoid distortion due
to non-linearity of the edge compliance. A number of headphones with poor seal-
ing appeared to have quite good low-frequency response, which proved on analysis
to be mostly harmonic.
FIG. 4. Headphone unassembled.
In the actual design of the unit, the choice of circuit constants is to a great
extent limited by the physical size the phones may have. The unit values of
MD and CD are chosen so that their resonance occurs at about 800 cycles. The
compliance C2 back of the diaphragm is then made small enough to produce a
resonance peak at the highest frequency desired. C3 and Mz are then selected to
resonate in combination with Z at approximately the same frequency, thereby
producing a double peak at the high-frequency end. The value of RI is then ad-
justed to give the desired ratio between high and low-frequency response. The
path through M2Ri is predominantly inductive, as previously noted, and serves
to short-circuit M^ at low frequencies, thereby boosting the low-frequency
response.
322 NEW MOTION PICTURE APPARATUS [J. S. M. p. E.
In practice, it is possible to set up the electrical equivalent of the circuit and
make the adjustments in terms of electrical units, and thus save a great deal of
time. Most of the circuit elements come out a feasible size, and, if not, a propor-
tional circuit may easily be set up.
The mechanical considerations of the design are comparable in importance to
the electroacoustic performance. In addition to high sensitivity, low distortion,
and good frequency response, the phones must be comfortable and readily ser-
viced. Comfort for the wearer involves several factors, among the most important
of which are weight, pressure on the ears, and material in the ear-cap. It was
found that weights in excess of 1.25 pounds complete were undesirable when the
phones had to be worn for long periods. Service requirements dictate that the
cords may be changed without disturbing the units, and that the phone motors
may be exchanged without tools.
FIG. 5. Complete headphone set.
Since the final criterion of a good phone unit is the opinion of the listener, the
measurement of the performance of the phone units is of special interest. The
initial measurements were made using an artificial ear,3' 4 but after making a few
listening tests, it became apparent that the leakage path afforded was too large.
This was probably due to the fact that the leak in question was designed to repre-
sent the leakage normally occurring between the hard cap of a telephone receiver
and the ear. The leakage between the ear and the soft rubber cap proved to be
considerably less. This was clearly evident in the fact that more comparable
results between listening tests and measurements were secured by the simple ex-
pedient of completely closing the leak in the artificial ear. The results of these
Sept., 1941] NEW MOTION PICTURE APPARATUS 323
measurements are shown in Fig. 2.. Here Curve /// shows the response of a pre-
liminary model operating into the artificial ear, and Curve II the response ob-
tained with the leak completely closed. The actual performance of the phone
unit no doubt lies between these two extremes. Two other methods, both subjec-
tive in nature, were used for further checks. The first of these might be called the
threshold curve of the listener, and curve / was determined by the following
method : a constant voltage was applied to the phone circuit through an attenua-
tor, and the attenuation between the supply voltage and the phones increased
until the listener could no longer hear the signal. The values of attenuation were
then plotted against frequency, as shown. Had the two curves coincided, the
indication would have been that the preliminary phones were flat in response;
however, variations from this ideal are to be noted, especially at the higher fre-
quencies. As a final check, a direct listening test was made between the phones
and a high-fidelity speaker channel, with results that fairly well substantiated the
frequency response variations indicated by the threshold method. The extent to
which it was adjudged desirable to alter the frequency response in the final unit in
order to obtain the flat response required for high-fidelity performance under
actual listening conditions is indicated by a comparison between curves / and
///of Fig. 2.
REFERENCES
1 WENTE, E. C., AND THURAS, A. L. : "Moving-Coil Telephone Receivers and
Microphones," J. Acoust. Soc. Amer., Ill (July, 1931), p. 44.
2 OLSON, H. F.: "Elements of Acoustical Engineering," D. Van Nostrand Co.
(New York), p. 232(1940).
3 Ibid., p. 270.
4 INGLIS, A. H., GRAY, G. H. G., AND JENKINS, R. T.: "A Voice and Ear for
Telephone Measurements," Bell Syst. Tech. J., II (April, 1932), p. 293.
FIFTIETH SEMI-ANNUAL CONVENTION
OF THE
SOCIETY OF MOTION PICTURE ENGINEERS
HOTEL PENNSYLVANIA, NEW YORK, N. Y.
OCTOBER 20TH-23RD, INCLUSIVE
OFFICERS AND COMMITTEES IN CHARGE
Program and Facilities
E. HUSE, President
E. A. WILLIFORD, Past-President
H. GRIFFIN, Executive Vice-President
W. C. KUNZMANN, Convention Vice-P resident
A. C. DOWNES, Editorial Vice-P resident
R. O. STROCK, Chairman, Local Arrangements
S. HARRIS, Chairman, Papers Committee
J. HABER, Chairman, Publicity Committee
J. FRANK, JR., Chairman, Membership Committee
H. F. HEIDEGGER, Chairman, Convention Projection Committee
Reception and Local Arrangements
P. J. LARSEN
F. E. CAHILL, JR.
H. RUBIN
E. I. SPONABLE
P. C. GOLDMARK
W. H. OFFENHAUSER, JR.
A. S. DICKINSON
W. E. GREEN
R. O. WALKER
R. O. STROCK, Chairman
T. E. SHEA
J. A. HAMMOND
O. F. NEU
V. B. SEASE
H. E. WHITE
L. W. DAVEE
L. A. BONN
J. H. SPRAY
J. J. FINN
A. N. GOLDSMITH
J. A. MAURER
L. B. ISAAC
E. W. KELLOGG
M. HOBART
J. A. NORLING
H. B. CUTHBERTSON
J. H, KURLANDER
C. F. HORSTMAN
E. R. GEIB
P. SLEEMAN
E. S. SEELEY
C. Ross
P. D. RIES
324
Registration and Information
W. C. KUNZMANN, Chairman
J. FRANK, JR.
Hotel and Transportation
G. FRIEDL, JR., Chairman
R. B. AUSTRIAN
R. F. MITCHELL
P. A. McGuiRE
M. W. PALMER
F. HOHMEISTER
H. MCLEAN
F. C. SCHMID
F. M. HALL
J. A. SCHEICK
FALL CONVENTION
325
H. A. GILBERT
G. A. CHAMBERS
Publicity Committee
J. HABER, Chairman
P. SLEEMAN
S. HARRIS
C. R. KEITH
W. R. GREENE
H. MCLEAN
D. E. HYNDMAN
L. A. BONN
E. G. HINES
A. S. DICKINSON
Banquet
O. F. NEU, Chairman
R. O. STROCK
J. C. BURNETT
J. A. SPRAY
J. A. NORLING
W. H. OFFENHAUSER, JR. M. HOBART
P. J. LARSEN
E. C. WENTE
A. GOODMAN
M. R. BOYER
J. A. HAMMOND
MRS. D. E. HYNDMAN
MRS. E. I. SPONABLE
MRS. E. S. SEELEY
MRS. A. S. DICKINSON
Ladies' Reception Committee
MRS. R. O. STROCK, Hostess
MRS. O. F. NEU, Hostess
MRS. H. GRIFFIN MRS. E. A. WILLIFORD
MRS. P. J. LARSEN MRS. J. FRANK, JR.
MRS. J. A. HAMMOND MRS. H. E. WHITE
MRS. G. FRIEDL, JR.
MRS. F. C. SCHMID
F. H. RICHARDSON
L. B. ISAAC
A. L. RAVEN
G. E. EDWARDS
J. K. ELDERKIN
Convention Projection
H. F. HEIDEGGER, Chairman
T. H. CARPENTER
P. D. RIES
J. J. HOPKINS
W. W. HENNESSY
L. W. DAVEE
J. J. SEFING
H. RUBIN
F. E. CAHILL, JR.
C. F. HORSTMAN
R. O. WALKER
Officers and Members of New York Projectionists Local No. 306
Hotel Reservations and Rates
Reservations. — Early in September, room-reservation cards will be mailed to
members of the Society. These cards should be returned as promptly as possible
in order to be assured of satisfactory accommodations. Reservations are subject
to cancellation if it is later found impossible to attend the Convention.
Hotel Rates. — Special per diem rates have been guaranteed by the Hotel Penn-
sylvania to SMPE delegates and their guests. These rates, European plan, will
be as follows:
Room for one person
Room for two persons, double bed
Room for two persons, twin beds
Parlor suites: living room, bedroom, and bath for
one or two persons
$3. 50 to $8.00
$5. 00 to $8.00
$6. 00 to $10. 00
$12.00, $14.00, and
$15.00
326 FALL CONVENTION [j. s. M. P. E.
Parking. — Parking accommodations will, be available to those motoring to the
Convention at the Hotel fireproof garage, at the rate of $1.25 for 24 hours, and
$1.00 for 12 hours, including pick-up and delivery at the door of the Hotel.
Convention Registration. — The registration desk will be located on the 18th
floor of the Hotel at the entrance of the Salle Moderne where the technical sessions
will be held. All members and guests attending the Convention are expected to
register and receive their badges and identification cards required for admission
to all the sessions of the Convention, as well as to several de luxe motion picture
theaters in the vicinity of the Hotel.
Technical Sessions
The technical sessions of the Convention will be held in the Salle Moderne on
the 18th floor of the Hotel Pennsylvania. The Papers Committee plans to have
a very attractive program of papers and presentations, the details of which will
be published in a later issue of the JOURNAL.
Fiftieth Semi-Annual Banquet and Informal Get-Together Luncheon
The usual Informal Get-Together Luncheon of the Convention will be held in
the Roof Garden of the Hotel on Monday, October 20th.
On Wednesday evening, October 22nd, will be held the Silver Anniversary
Jubilee and Fiftieth Semi-Annual Banquet at the Hotel Pennsylvania. The
annual presentations of the SMPE Progress Medal and the SMPE Journal
Award will be made and officers-elect for 1942 will be introduced. The proceed-
ings will conclude with entertainment and dancing.
Entertainment
Motion Pictures. — At the time of registering, passes will be issued to the dele-
gates of the Convention admitting them to several de luxe motion picture theaters
in the vicinity of the Hotel. The names of the theaters will be announced later.
Golf. — Golfing privileges at country clubs in the New York area may be ar-
ranged at the Convention headquarters. In the Lobby of the Hotel Pennsylvania
will be a General Information Desk where information may be obtained regarding
transportation to various points of interest.
Miscellaneous. — Many entertainment attractions are available in New York to
the out-of-town visitor, information concerning which may be obtained at the
General Information Desk in the Lobby of the Hotel. Other details of the enter-
tainment program of the Convention will be announced in a later issue of the
JOURNAL.
Ladies' Program
A specially attractive program for the ladies attending the Convention is be-
ing arranged by Mrs. O. F. Neu and Mrs. R. O. Strock, Hostesses, and the Ladies'
Committee. A suite will be provided in the Hotel where the ladies will register
and meet for the various events upon their program. Further details will be pub-
lished in a succeeding issue of the JOURNAL.
Sept., 1941] FALL CONVENTION 327
PROGRAM
Monday, October 20th
9:00 a. m. Hotel Roof; Registration.
10:00 a. m. Salle Moderne; Technical session.
12:30 p. m. Roof Garden; Informal Get-Together Luncheon for members, their
families, and guests. Brief addresses by prominent members of
the industry.
2:00 p. m. Salle Moderne; Technical session.
8:00 p. m. Salle Moderne; Technical session.
Tuesday, October 21st
9:00 a. m. Hotel Roof; Registration.
9:30 a. m. Salle Moderne; Technical session.
2:00 p. m. Salle Moderne; Technical session.
Open evening.
Wednesday, October 22nd
9: 00 a.m. Hotel Roof; Registration.
9:30 a. m. Salle Moderne; Technical and Business session.
Open afternoon.
8:30 p. m. Fiftieth Semi- Annual Banquet and Dance.
Introduction of officers-elect for 1942.
Presentation of the SMPE Progress Medal.
Presentation of the SMPE Journal Award.
Entertainment and dancing.
Thursday, October 23rd
10:00 a. m. Salle Modems;- Technical session.
2: 00 p.m. Salle Moderne; Technical and business session.
Adjournment
W. C. KUNZMANN,
Convention Vice- President
SOCIETY ANNOUNCEMENTS
1941 FALL CONVENTION
NEW YORK, N. Y.
OCTOBER 20TH-23RD, INCLUSIVE
The 1941 Fall Convention will be held at New York, N. Y., with headquarters
at the Hotel Pennsylvania.
Members are urged to make every effort to attend the Convention, as a very
interesting program of papers and presentations is being arranged.
Details of the Convention will be found elsewhere in this issue of the JOURNAL.
ADMISSIONS COMMITTEE
At a recent meeting of the Admissions Committee, the following applicants
for membership were admitted into the Society in the Associate Grade:
ABBOTT, H.
1311 South Wabash Ave.,
Chicago, 111.
ALBERSHEIM, W. J.
Electrical Research Products, Inc.
20 Vandam St.,
New York, N. Y.
ANDERS, Gus
Droll Theater Supply Co.,
351 E. Ohio St.,
Chicago, 111.
BAILEY, E. L.
Box 81 5N,
1421 Arch St.,
Philadelphia, Pa.
BINGHAM, E. H.
5 East 39th St., South,
Salt Lake City, Utah
BURTON, C. C.
Paramount Pictures, Inc.,
1501 Broadway,
New York, N. Y.
GATES, J. R.
629 N. 15th,
Lincoln, Neb.
328
GAW, E. D.
26 Drake Court,
Omaha, Neb.
GELLERUP, D. W.
Milwaukee Journal,
333 West State St.,
Milwaukee, Wis.
GOLDMAN, A. I.
Box 51,
Assinippi, Mass.
JOHNSON, E. O.
533 East 32nd St.,
Indianapolis, Ind.
MAUTHNER, E. I.
4649 Beacon St.,
Chicago, 111.
MCLEAN, J. F.
Miller Broadcasting System, Inc.
113 West 57th St.,
New York, N. Y.
SACHTLEBEN, L. T.
RCA Manufacturing Co., Inc.,
501 N. LaSalle St.,
Indianapolis, Ind.
SOCIETY ANNOUNCEMENTS
329
S\VEN, MlNG-CHING
Dept. of Educational Cinematog-
raphy,
University of Nanking,
Chengtu, China
THOMAS, P. E.
501 West Mills St.,
Creston, la.
WAGLE, M. M.
17 Mathew Road,
Bombay 4, India
WYSOTZKY, M. Z.
Bolshoi
Gnezdnikovsky Pereulok dom 10
kvartira 724,
Moscow, U.S.S.R.
In addition, the following applicants have been admitted to the Active Grade :
BARNETT, H.
30 Jefferson St.,
Garden City, L. I., N. Y.
LEBEL, C. J.
370 Riverside Drive,
New York, N. Y.
Society of Motion Picture Engineers
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JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
Volume XXXVII October, 1941
CONTENTS
Page
The Stereophonic Sound-Film System — General Theory
H. FLETCHER 331
Mechanical and Optical Equipment for the Stereophonic Sound-
Film System
E. C. WENTE, R. BIDDULPH, L. A. ELMER, AND A. B. ANDERSON 353
The Stereophonic Sound-Film System — Pre- and Post-Equali-
zation of Compandor Systems J. C. STEINBERG 366
Electrical Equipment for the Stereophonic Sound-Film System
W. B. SNOW AND A. R. SOFFEL 380
A Light- Valve for the Stereophonic Sound-Film System
E. C. WENTE AND R. BIDDULPH 397
Internally Damped Rollers. . .E. C. WENTE AND A. H. MULLER 406
A Non-Cinching Film Rewind Machine. L. A. ELMER 418
Current Literature 427
1941 Fall Convention, Hotel Pennsylvania, New York, N. Y.,
October 20th-23rd, Inclusive
General Arrangements 429
Abstracts of Papers 433
Society Announcements 442
JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
SYLVAN HARRIS, EDITOR
BOARD OF EDITORS
ARTHUR C. DOWNES, Chairman
JOHN I. CRABTREE ALFRED N. GOLDSMITH EDWARD W. KELLOGG
CLYDE R. KEITH ALAN M. GUNDELFINGER CARLETON R. SAWYER
ARTHUR C. HARDY
Subscription to non-members, $8.00 per annum; to members, $5.00 per annum,
included in their annual membership dues; single copies, $1.00. A discount
on subscription or single copies of 15 per cent is allowed to accredited agencies.
Order from the Society of Motion Picture Engineers, Inc., 20th and Northampton
Sts., Easton, Pa., or Hotel Pennsylvania, New York, N. Y.
Published monthly at Easton, Pa., by the Society of Motion Picture Engineers.
Publication Office, 20th & Northampton Sts., Easton, Pa.
General and Editorial Office, Hotel Pennsylvania, New York, N. Y.
West Coast Office, Suite 928, Equitable Bldg., Hollywood, Calif.
Entered as second class matter January 15, 1930, at the Post Office at Easton,
Pa., under the Act of March 3, 1879. Copyrighted, 1941, by the Society of
Motion Picture Engineers, Inc.
OFFICERS OF THE SOCIETY
** President: EMERY HUSE, 6706 Santa Monica Blvd., Hollywood, Calif.
** Past-President: E. ALLAN WILLIFORD, 30 E. 42nd St., New York, N. Y.
** Executive Vice- President: HERBERT GRIFFIN, 90 Gold St., New York, N. Y.
* Engineering V ice-President: DONALD E. HYNDMAN, 350 Madison Ave., New
York, N. Y.
** Editorial Vice-President: ARTHUR C. DOWNES, Box 6087, Cleveland, Ohio.
* Financial Vice-President: ARTHUR S. DICKINSON, 28 W. 44th St., New York
N. Y.
** Convention Vice-President: WILLIAM C. KUNZMANN, Box 6087, Cleveland, Ohio
^Secretary: PAUL J. LARSEN, 44 Beverly Rd., Summit, N. J.
"Treasurer: GEORGE FRIEDL, JR., 90 Gold St., New York, N. Y.
GOVERNORS
**MAX C. BATSEL, 501 N. LaSalle St., Indianapolis, Ind.
* JOSEPH A. DUBRAY, 1801 Larchmont Ave., Chicago, 111.
*JOHN G. FRAYNE, 6601 Romaine St., Hollywood, Calif.
*ALFRED N. GOLDSMITH, 580 Fifth Ave., New York, N. Y.
*ARTHUR C. HARDY, Massachusetts Institute of Technology, Cambridge,
Mass.
**LOREN L. RYDER, 5451 Marathon St., Hollywood, Calif.
*TIMOTHY E. SHEA, 195 Broadway, New York, N. Y.
*REEVE O. STROCK, 35-11 35th St., Astoria, L. I., N. Y.
Term expires December 31, 1941.
**Term expires December 31, 1942.
THE STEREOPHONIC SOUND-FILM SYSTEM— GENERAL
THEORY*
HARVEY FLETCHER**
Summary. — The general requirements are discussed for an ideal recording-re-
producing system as determined by the characteristics of hearing of a typical group of
persons listening in a typical concert hall or theater. Quantitative values are set
down as ideal objectives. Although microphones, loud speakers, and amplifiers which
had been developed for the stereophonic transmission system were available for meet-
ing these objectives, no recording medium was known which would record the wide
dynamic range of intensity levels which the objectives indicated was necessary. How-
ever, this wide intensity range objective was met by using a compandor in the electrical
system. A general discussion is given of the reasons for choosing the particular
compandor used, for using variable-area rather than variable-density on the recorded
film, for using three instead of a greater or lesser number of channels. A general
description of the stereophonic sound-film system is given, including the enhancement
feature. This feature makes it possible to re-record from the original recording, at
the same time making any desirable changes in the dynamic range or frequency re-
sponse in each of the three channels.
In 1934 a series of papers1 on auditory perspective was presented
before the A. I. E. E. describing a transmission system which was later
called a stereophonic transmission system. It consisted essentially of
three complete channels working together, each comprising a micro-
phone, a high-gain amplifier, a predistorting and corrective network,
a transmission line, an amplifier, a restoring and corrective network,
a variable distorting network and attenuator, a power amplifier, and
a loud speaker, as shown in Fig. 1. It was shown that by means of
this system symphonic music and other sounds could be picked up in
a hall in Philadelphia, transmitted to a hall in Washington, D. C.,
and there reproduced without the introduction of apparent distortion
or noise.
This system not only made possible the production of a facsimile
of the original music, but it also had what is called an enhancement
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May 27,
1941.
**Bell Telephone Laboratories, New York, N. Y.
331
OThe Society is not responsible for statements by authors O
332
H. FLETCHER
[J. S. M. P. E.
feature. The part of the equipment used for this enhancement is
labeled "control box" in Fig. 1 . By means of these controls the direc-
tor of the orchestra, while listening to the reproduced music, raised
and lowered at will the intensity level of each channel by means of
dials attached to the attenuators. By means of appropriate switches,
he could increase or decrease the level of the bass, or make the music
less or more shrill by throwing in or out networks having a sloping
frequency loss characteristic. These networks were designed to pro-
duce changes in the frequency response characteristics of the system,
as indicated in Fig. 2. There were also auxiliary control circuits
411
CONTROL BOX
•• CONTROL CIRCUITS FOR STARTING ORCHESTRA, GOVERNING TEMPO ETC. ••
FIG. 1. Philadelphia- Washington stereophonic- trans-
mission system.
used for giving signals to the orchestra, governing the tempo, and
giving instructions to the leader of the orchestra.
In the development of the stereophonic sound-film system (SSFS)
nearly all the features of the stereophonic transmission system de-
scribed above were retained. The loud speakers, power amplifiers,
and microphones are the same and the attenuating and equalizing
networks are similar. The fundamental requirements upon which the
design of both the transmission system and the recording-reproducing
system are based are the same and will be reviewed here.
It is well known that when an orchestra plays, vibrations con-
tinually changing in form and intensity are set up in the air of the
hall where the recording is made. An ideal system is one which will
make a record of these vibrations and at any desired later time re-
Oct., 1941]
GENERAL THEORY
333
produce them so as to produce at every position in the hall the same
time sequences of wave motion as were produced during the recording.
To accomplish this for a sound source which is spread out, for ex-
ample, the sound coming into a concert hall from the orchestra on the
stage, requires more than one channel.
Suppose there were interposed between the orchestra and the audi-
ence a sound-transparent curtain on which were mounted small micro-
phones for picking up the sound going through the curtain ; and sup-
pose to each microphone there is connected an ideal recording sys-
tem. Records made with such an ideal system would have stored on
them a complete history of the sound changes at every position on
100 20O 60O IOOO 2000 5000 10000 200OO
FREQUENCY IN CYCLES PER SECOND
FIG. 2. Variable network characteristics.
the curtain. Now if small ideal loud speakers were placed at the
positions occupied by the microphones and connected to ideal ma-
chines reproducing the records, then a sheet of sound would be re-
produced having all the characteristics of the original sound. Theo-
retically, there should be an infinite number of such recorder-repro-
ducer sets. Practically, however, only a few such channels are needed.
On a large stage it has been found that three channels are sufficient
to give a good illusion of the sounds coming from all parts of the
stage. We developed a three-channel system not only because it
gave better representation of movements on a large stage, but also
because of the possibility of using the center channel for solo work
while still retaining the stereophonic features of the orchestra on the
two side channels. If one wished as much flexibility up and down as
334
H. FLETCHER
U. S. M. P. E.
the present system gives sidewise, then the channels would have to be
increased from three to nine.
It is important to recognize the difference between this stereo-
phonic system and a binaural system. The latter requires only two
channels for a perfect reproduction but requires that head receivers
be held tightly against the ears, while the former can use loud speakers
but for perfect reproduction requires an infinite number of channels.
If we design the system to handle any kind of sounds that the ear can
hear and tolerate, then the limits of frequency and intensity are set
by the hearing characteristics of a typical group of listeners. It was
this ambitious objective that was set for the SSFS.
too 1000
FREQUENCY IN CYCLES PER SECOND
FIG. 3. Threshold hearing level curves for quiet and
noisy rooms.
During the past two years a survey of the hearing capabilities of
persons in a typical population has been made by the Bell Telephone
Laboratories. This was done in connection with the exhibits at the
World's Fairs at San Francisco and New York City, sponsored by the
Bell Telephone companies. At these exhibits records of the hearing
of more than one-half million persons were analyzed. The record
expressed the hearing acuity as a relative hearing loss or gain with
respect to an arbitrary reference. Measurements at the Laboratories
on this reference have made it possible to express these data on an
absolute scale. The results were published in a paper entitled "Re-
sults of the World's Fair Hearing Tests," by Steinberg, Montgomery,
and Gardner.2 Fig. 3 has been constructed from data taken from
Oct., 1941]
GENERAL THEORY
335
this paper. The lower curve labeled 95 indicates that 95 out of 100
persons in a typical group can not hear pure tones whose frequency
and intensity levels lie below this curve. The top curve indicates
that 5 out of 100 can not hear these tones until they exceed the in-
tensity levels indicated by this curve. The middle curve indicates
the levels where one-half the group can hear and the other half can
not hear. The dashed portions of the curves indicate regions where
no measurements have been made. Feeling and hurting levels lie
somewhere above 120 db as indicated by the field of dots at the top
of the chart. Our experience with reproduced music has taught us
that it is undesirable and probably unsafe to reproduce sounds for a
general audience that have greater intensity levels than 120 db.
500 1000
FREQUENCY IN CYCLES PER SECOND
FIG. 4. Masking and spectrum levels for average room
noise.
If the listener is in a quiet place, these curves set the limits for the
ideal transmission system. This ideal of no noise is seldom if ever
realized by listeners. Measurements of room noise have been made
by the Bell Telephone Laboratories and from these measurements the
average noise spectrum can be deduced. In a paper by Seacord3 it
was found that 43 db was the average sound level in residences not
having radios playing. The standard deviation of levels in different
residences from this figure was 5.5 db. The distribution about this
average value indicated that about one-half the residences have noise
levels between 39 and 47 db, and 90 per cent are in the range between
33 and 52 db. *
Hoth4 found that the form of the noise spectrum was about the
same for all types of rooms. This is shown for a room noise having a
total level of 43 db in Fig. 4, lower curve. In a paper entitled "Re-
336 H. FLETCHER [j. s. M. P. E.
lation between Loudness and Masking"5 it was shown that the mask-
ing level could be obtained directly from the spectrum level. Using
this relation the curve shown in Fig. 4 labeled Masking Level was
obtained. This curve then gives the level of pure tones which can
just be perceived in the presence of average room noise by a reference
observer. This masking curve is shown in Fig. 3 as a cross-hatched
band. It shows the range of the masking levels for about 90 per cent
of the residences in a typical group. The dotted curve gives the
average. These figures refer to residences. Measurement of noise
levels by Mueller6 in quiet motion picture theaters gave an average
level of 25 db, but with an audience present the average was 42 db,
which is just 1 db below the average room noise given above. Con-
sequently, the curve given in Fig. 4 will apply to this case. It will
probably generally apply also to concert halls, although in this case
for the very quiet listening periods the lower part of this hatched
portion more nearly represents the noise conditions. It is interesting
to note that the threshold hearing levels vary about 20 db due to
noise conditions in residences, theaters, and concert halls, whereas
it varies nearly twice this amount due to differences in acuity of
hearing by different persons considering only the middle 90 per cent
of the cases.
If these deductions are correct, then it is seen that the lowest levels
that can be heard by the average person in a group are determined by
the hearing mechanism when the frequencies are below 200 or above
6000 cycles, but by room noise when they are between 200 and 6000
cycles. For example, the fundamental of a 60-cycle hum should be
kept below a 57-db level, whereas any components of this hum around
1000 cycles should be kept below a 25-db intensity level. It is seen
that for the 5 per cent of the rooms which are quietest the limit is
set entirely by the hearing acuity curve. This condition is nearly
reached in some of the quietest periods in the very best concert halls.
From Fig. 3 one can also set the frequency limits if all sounds that
can be heard by the average listener are to be recorded. This range
is from 20 cycles per second to 15,000 cycles per second for the highest
possible levels, and for any lower levels this frequency range is
smaller, as indicated.
Fig. 3 also gives the maximum levels such an ideal system might
be called upon to transmit. This maximum level is taken as 120
db and the same for all frequencies. To reproduce this high level
in a large concert hall requires about 1/9 kw of sound power. In a
Oct., 1941]
GENERAL THEORY
337
small room, however, this level can be attained with about 3 watts
of sound power.
A summary of the limitations set by average hearing is shown in
Fig. 5. The lower curve may be anywhere in the hatched area,
depending upon the noise conditions of the room. This figure then
sets the design objectives for frequency and intensity determined
by the characteristics of average hearing and average noise conditions
in listening rooms.
As stated above, most of the features of the stereophonic trans-
mission system were carried over to the SSFS. The transmission
120
100 1000
FREQUENCY IN CYCLES PER SECOND
FIG. 5. Hearing limits for pure tones of a typical listener
in a room having typical residential room noise.
line of the former is replaced by a long period delay or storage system
in the latter. The amplified microphone current, instead of flowing
into a transmission line, is translated into a record in a form which
can at a later date be retranslated into a facsimile of the original re-
cording current. While problems of noise, non-linearity, and attenua-
tion distortion are met both in the transmission and storage systems,
their source and character are such that solutions of quite a different
nature are demanded.
Since current in three signal channels must be recorded simul-
taneously and subsequently reproduced in synchronism, a linear type
of phonograph carrier presents itself as probably the most suitable
recording medium. In this class, the photographic sound-film is
farthest advanced in its development and was therefore adopted for
the stereophonic system.
338 H. FLETCHER [j. s. M. P. E.
Two types of sound-film records are now in wide commercial use,
variable-area and variable-density, each having advantages which we
shall not discuss in detail. We shall merely point out the chief rea-
sons for adopting the variable-area method in this particular project.
With a sharply defined boundary between the light and dark areas,
and with clean handling of film, the variable-area record has initially
the advantage of greater volume range, which is soon lost in successive
play ings. The rate of loss in volume range can be kept down by
careful film handling. In the design of the stereophonic equipment,
special thought was given to obviating conditions that result in sur-
face abrasions of the film, which was here made easier by the fact
that the film does not have to pass through a picture projector. A
second advantage possessed by the variable-area record is that the
level of the photoelectric cell current in reproduction is about 6 db
higher, which is of some importance in dealing with signals of low in-
tensity over a broad band system. One of the principal disadvantages
of the variable-area method is that improper film processing will
result in the introduction of high intermodulation components, even
at low signal levels, whereas in variable-density recordings, while
bad processing will result in a signal distortion, the distortion falls
off rapidly with the signal level. Lately better methods of measuring
and controlling this distortion have been developed, so that, where
the choice of positive print stock is unhampered by picture considera-
tions, the non-linear distortion of the variable-area record can be
held to a low value. Special requirements that the variable-area
method demands in the reproducing system, such as the uniformity of
the illumination per unit length in the scanning line of light, it was
felt, could be taken care of by extra precautions in the design and
construction of the apparatus.
The range of maximum signal to noise in a film record 80 mils
wide is of the order of 50 db. It is more or less dependent upon
circumstance and exact definition of the term. Measurements made
with a high-speed automatic level recorder showed a volume range
for a large symphony orchestra of 78 db. The range of signal in-
tensity level that is to be recorded and reproduced when recording
music is then of the order of 80 db. For purposes of discussion, we
shall use simply these rounded figures of 50 and 80 db. The 80-db
signal must, therefore, in some manner be compressed at least 30 db
and if dynamic range is to be preserved, it must be expanded the
corresponding amount in the reproduction. In the sound versus light
Oct., 1941]
GENERAL THEORY
339
DECIBELS
- w W £ o
O O o O O C
A
LIGHT MODULATION^,
x^
X
**«
'-.
X
X
x
X
k^
AMPLIFIER
X
x
iff
*^«
••<
0 10 20 30 40 50 60 70 80
SOUND LEVEL IN DECIBELS
(a)
B
X
X
PHOTO .-ELECTRIC
CELL CURRENT^
x
x^
x
x
„-
,*• *"
^X
X
^~
^AM
NC
PLIFIER GAIN
AND
ISE LEVEL
2^
X^
0 10 20 30 40 50 60 70 80
AMPLIFIER OUTPUT IN DECIBELS
(b)
FIG. 6. Amplifier gain and noise relationships with compandor operating
linearly and continuously over the whole sound level range, (a) recording;
(6) reproducing.
DECIBELS
— ru U) £. LJ
0 0 0 0 0 C
A
LIGHT MODULATION v
X
^b
^
^
^
S*
Vjj,
/
>» i
^-AMPLIFIER
^^ GAIN-
/
"K
10 20 30 40 50 60 70
SOUND LEVEL IN DECIBELS
B
^x
x^
PHOTO-ELECTRIC
CELL CURRENT>^
X
^
x^
^^
**
/
^
„"
z
^ * '
,.^5^ AMPLIFIER GAIN
AND NOISE LEVEL
80
10 20 30 40 50 60 70
AMPLIFIER OUTPUT IN DECIBELS
(a)
FIG. 7. Amplifier gain-and noise relationships with compandor operating
linearly and continuously over the upper 60-db level of the sound signal, (a)
recording; (&) reproducing.
40
30
A
/
b
--
7
- —
"\AMPLIFIER
\ GAIN
10
0
/ LIGHT
/MODULATION
\
\
</
\
\
10 20 30 40 50 60 70
SOUND LEVEL IN DECIBELS
80
B
/
b
./PHOTO-ELECTRIC
/^-CELL CURRENT
/
/
/
/
AMPLIFIER GAIN
AND NOISE /
LEVEL *~/
/
/
/
0 10 20 30 40 50 60 70 80
AMPLIFIER OUTPUT IN DECIBELS
FIG. 8. Amplifier gain and noise relationships with compandor settings fixed
for the lower 50-db range of sound signal level.
340 H. FLETCHER [J. S. M. P. E.
modulation coordinate diagram, two points, labeled a and b, respec-
tively, in Fig. 6 (a) are fixed, if the capabilities of the film are to be
used to full advantage. One of these points is defined by the maxi-
mum sound level and complete light modulation, and the other, by a
sound level 80 db and a modulation level 50 db lower. In between,
the relationship may be of any desired kind so long as it can be repre-
sented by a single-valued line connecting these two points, and pro-
vided that for every change in the gain of the recording amplifier an
equal amount of attenuation is subsequently introduced in the re-
producing amplifier at a corresponding point of the record, provided
further that there is no signal distortion between these two parts of
the circuit.
The departure of the slope of the line connecting a and b from unity
gives the rate of change of amplification of the recording amplifier
with sound level. If, for instance, the characteristic of a recording
system is represented by a straight line connecting a and b, then for
each db increase in sound level, there is a decrease in amplification of
3/8 db. The gain in this case therefore changes continually with the
signal level. If now a record so made is reproduced with an ampli-
fier the gain of which increases 3/g of a db for every 5/8 db increase in
the photoelectric cell current, or, which is equivalent, for every db
increase in amplifier output, the dynamics of the original sound will
be recovered in the output current of the reproducing amplifier.
These conditions are graphically represented in Fig. 6. Fig. 6 (a)
shows the recording and Fig. 6(&) the reproducing conditions. In
Fig. 6 (a) the solid line gives the relation between sound level and the
modulation of the recording light. The dashed line gives the rela-
tion between sound level and amplifier gain. In Fig. Q(b) the solid
line represents the relation between amplifier output and photoelectric
cell current, while the dashed line gives the relation between amplifier
gain and amplifier output level. Similar designations hold for Figs. 7
and 8, which are to be discussed presently. The amplifier gain curve
in Fig. 6(6) also represents the level of the film noise that accompanies
the output signal. If the gain of the amplifier is set so that at the
minimum signal (80 db below maximum) the noise is at or above the
threshold under the particular listening conditions, then the signal
will be accompanied by "hush-hush," a term used to designate the
audible rising and falling of noise with the signal. If, for minimum
signal, the signal and noise are both at threshold, then as the signal
is raised 8 db the noise will be only 5 db below the signal, even though
Oct., 1941] GENERAL THEORY 341
at maximum level the noise is 50 db below the signal. One further
disadvantage of this type of compression and expansion is that dis-
tortion which may be introduced by the process will enter throughout
the whole signal level range.
In the case illustrated in Fig. 7 (a) the compressor system is so
arranged that the signal is recorded in a normal way over the lower
20-db range after which there is a reduction of 1/2 db in the ampli-
fication for every db gain in input signal. The corresponding signal
versus noise relationships in reproduction when the dynamics of the
signal are restored are shown in Fig. 7(b). There will be no hush-
hush until the signal reaches a level of 20 db above the minimum.
After that, for every db rise in signal, there is l/z-db rise in noise level.
In comparing this system with that represented by Fig. 6, we note
from 7(b) that when the output signal has a level of 40 db, the noise
level is 10 db, whereas in the system of Fig. 6, the corresponding
noise level is 15 db. At higher levels this difference in the two sys-
tems becomes less until at the highest level, the noise is down 50 db
in either case, the volume range of the film.
Fig. 7 is of particular interest, as the noise relationships shown in
b are virtually those which would obtain for a 30-db noise-reduction
system of the usual form. An inherent weakness in the ordinary
noise-reduction scheme is that only for the maximum signal is the
full track width utilized. For example, in a system with 10 db of
noise reduction, signals more than 20 db below the maximum are
reproduced from a record which is one-tenth as wide as a record oc-
cupying the full width of the track and which consequently has 10
db less volume range. It is evident that as the characteristic of the
compandor (compressor-expandor) is changed so that the upper
part of the line ab becomes more nearly horizontal, the hush-hush
becomes less.
The ideal type of system is the one represented by Fig. 8. In this
system no change is made in amplifier gain over the lower 50-db
range of sound level, i. e., not until the sound-track is fully modulated,
after which the recording amplifier gain is reduced by 1 db for each
db increase in signal level. In other words, the recording level re-
mains constant. In reproducing, there is then no possibility of hush-
hush, and the quality of the signal can not in any way be impaired
by the action of the automatic gain control system over the lower
50-db range. For the most part sound sequences of the kind ordi-
narily recorded will therefore be entirely free from hush-hush and
342
H. FLETCHER
LF. S. M. P. E.
from distortion in excess of that introduced by a normal system.
The masking of one sound by another increases rapidly with the
level of the masking sound. As all noise-reduction schemes depend
for their success upon the masking of the noise by the signal, it is of
great advantage to restrict noise reduction to the higher signal level
FIG. 9. Record-
ing-reproducing
system with com-
pression and ex-
pansion contro lied
directly by the sig-
nal.
FIG. 10. Record-
ing-reproducing
system with com-
pression and expan-
sion controlled di-
rectly by the signal
and time-delay
network in signal
channel.
range as is done to the greatest extent possible in the system of Fig. 8.
The objective in the design of the stereophonic system was to approach
the ideal of Fig. 8 as closely as possible.
The operations involved in a compression-expansion, or a so-
called compandor system, can be carried out in one or the other of
two general ways. The compression and expansion operations may
Oct., 1941]
GENERAL THEORY
343
be controlled directly by the signal, or they can be placed under the
control of a separate channel which may be called a pilot channel.
In Fig. 9 is shown a recording-reproducing system in which the
signal itself is used to control the amplifier gains in recording as well
as in reproducing. In this particular arrangement, if there were no
distortion in the recording and reproducing processes, it would be
possible to reproduce the sound
without distortion, since both
amplifiers are controlled virtually
by the same dynamically distorted
signal. If the control system is
so constructed that the gain ad-
justments follow the signal very
rapidly, the operation and fre-
quency-range requirements for the
recording-reproducing system be-
come severe. If the adjustments
follow the signal slowly, then
noticeable clipping may result, i. e.,
when the signal tends to rise
suddenly the recording system
may be overloaded before the
proper gain adjustment has been
effected.
This difficulty can 1be overcome
in the system of Fig. 10. In
this arrangement, the gain of
the recording amplifier is con-
trolled by the microphone in-
stead of the recording ampli-
fier output current. By the in-
sertion of a time delay at the part of the circuit indicated, it is
possible to have even a slowly operating control circuit reduce the
gain to the proper value before a sudden increase in signal level can
make itself manifest at the input of the recording amplifier. Clipping
is thus avoided. It will be noticed that in this system, in contradis-
tinction to that represented in Fig. 9, the recording and reproducing
amplifiers are not controlled by current of the same dynamic char-
acteristics, so that there is a greater likelihood of introducing wave-
form distortion in the current finally delivered to the loud speaker.
FIG. 11. Recording-reproducing
system with compression and ex-
pansion controlled through a pilot
channel.
344 H. FLETCHER Q. s. M. P. E.
Since the sound in a room reaches, its maximum value in a rela-
tively short time after its inception, but decays from the maximum
to threshold rather slowly, it has been customary in all noise-reduction
systems to have the controls operate rapidly when the signal level
increases and slowly when the signal decreases. This arrangement
greatly reduces the operating requirements of the recording-repro-
ducing apparatus, particularly as regards phase distortion at low
frequencies.
Neither of the above systems can operate in the way that is repre-
sented by Fig. 8 — set up as the ideal — for in the level region where
gain adjustment must be made the signal current coming from the
reproducing machine is of constant level and so does not contain the
requisite information for the gain adjustments. These systems oper-
ate best under the condition shown in Fig. 6, i. e., where the slope
of the curve relating light modulation and current is large.
In the system shown in Fig. 1 1 a pilot track, for carrying informa-
tion about the required gain adjustment in the reproducing amplifier,
is made along with the signal record. This system is not subject to
the limitations mentioned in the last paragraph.
Current from an oscillator is recorded on a separate track. The
level of this current is modulated by the signal coming from the micro-
phone. This modulated current controls the gain of the recording
as well as that of the reproducing amplifier — in the latter case, of
course, after having been recorded and reproduced. A given change
in the current-level of the pilot channel should produce an equal but
opposite change in gain in the two amplifiers. By a proper design of
the modulator, the system can theoretically be made to operate with
any desired relationship of sound level and recording current, in-
cluding that of Fig. 8. No matter what this adjustment may have
been for any particular record, it can always be reproduced with the
proper dynamics without special adjustment of the modulator con-
trolling the gain of the reproducing amplifier. The introduction of a
time delay in the signal behind the control current presents no basic
difficulties in this system. In subsequent re-recording, the dynamics
may be altered at will by a manual adjustment of the pilot current.
The stereophonic system operates with a pilot track in essentially
the manner indicated.
We should like to point out that in any of the above-described sys-
tems, if the gain-control circuits are properly balanced, the control
current manifests itself in the signal channel only in the desired modu-
Oct., 1941]
GENERAL THEORY
345
lation of the signal. When the signal current is zero, variations in
the control current are theoretically not detectable in the signal
channel. Except for greater ease in the elimination of distortions
introduced by the photographic process, there is here not the ad-
vantage in the push-pull track that there is in an ordinary noise-
reduction system, where the light-modulator biasing current is re-
V
RECORDER
1 1 J 1
1 f
~^— -— — - ~"~ ~"~
a
o
1
o
o
I i
o
0
\\
Jll
FIG. 12. Schematic diagram of stereophonic recording
circuits.
corded directly on the sound-track. The extra complications of a
push-pull recording system were therefore avoided.
Some further gain in noise reduction could be effected if, in addi-
tion, use were made of the ordinary noise-reduction method having
preferably very slow operating speeds, and used only when there are
longer intervals of quiet passages. However, this arrangement can
have an advantage only where the limit of automatic adjustment of
the reproducing amplifier has been reached. As the major difficulties
in a compandor system are not in respect to noise at low signal levels,
346
H. FLETCHER
[J. S. M. P. E.
but rather to hush-hush effects, the extra complication of the addition
of this kind of noise reduction did not seem warranted.
The general features of the recording part of the three-channel
stereophonic sound-film system are shown in Fig. 12. It will be seen
that there are three signal channels and three control channels. The
three frequencies come from the generator and pass through the
modulators and amplifiers, and are combined in the combining net-
work and recorded as the pilot track on the film. If there is no signal
in the signal channels, these three frequencies are recorded with equal
amplitudes. Their phases are controlled at the generator G so that
their combined amplitude is a minimum. A more detailed schematic
drawing of one channel is shown in Fig. 13. As the signal current
FIG. 13. Schematic diagram of one channel of the recording cir-
cuit.
appears in the signal channel a small part of it is tapped off and sent
through the amplifier 2 and the rectifier / into the modulator. The
rectifier is designed so that a constant direct current leaves it and
enters the modulator. As long as the signal current remains below a
critical value, this critical level is controlled by the bias current in
the rectifier. Above this level the direct current fed into the modu-
lator is proportional to the signal current. The critical value can be
easily changed by changing the gain of the amplifier 2 preceding the
rectifier.
The modulator is designed so that the amplitude of a single fre-
quency leaving it and going to amplifier 3 is proportional to the
direct current entering it from the rectifier. So it is seen that the
amplitude on the pilot track is constant until the critical level in the
Oct., 1941]
GENERAL THEORY
347
signal channel is reached. Above this level the amplitude of the pilot
track is proportional to the rms amplitude in the signal. At the
same time that this modulated frequency is recording on the pilot
track it is also sent back through the filter and the rectifier //, to the
compressor. The compressor is designed so that the change in loss
introduced by it into its signal channel is equal to the change in level
FIG. 14.
A A A
Schematic diagram of the stereophonic repro-
ducing circuits.
of the rectified current coming into it. Consequently, the signal
levels beyond the compressor never exceed the critical value and re-
main constant for all levels coming into the compressor above the
critical one. In other words, as the signal level rises and falls on the
input side of the compressor, the loss and gain in the compressor are
I automatically regulated by just the right amount to keep the output
level constant, and the amount of this regulation is recorded on the
348
H. FLETCHER
[J. S. M. P. E.
pilot track. This corresponds to thexase discussed above and shown
in Fig. 8.
The general features of the reproducing circuits are shown in Fig. 14.
Three signals coming from the three tracks enter the upper three
channels as indicated. The combined three modulated frequencies
from the pilot track enter the lower channel where they are amplified
the proper amount and separated by three filters. Each frequency is
sent through a linear rectifier to the appropriate expandor. A more
detailed diagram of one channel is shown in Fig. 15. The rectifier
must be linear through a much wider range than the corresponding
one in the recording system. By "linear" is meant that the rectified
direct current be proportional to the alternating input current. The
FIG. 15. Schematic diagram of one channel of the re-
producing circuit.
reason for this wider range is to take care of the enhancement feature,
which will be explained later. Each expandor is designed so that its
gain is directly equal to the change in level of the rectified current
entering the expandor. In other words, if the rectified current is
increased tenfold, the signal current leaving the expandor is also in-
creased tenfold. Consequently, it will be seen that whenever a loss
is introduced during recording there will be an equal gain introduced
during reproducing so that the signal will be restored to its normal
relative values. For the same reason that was given for the recti-
fier, the expandor must be linear for a much wider range than thfe
compressor. Since the signal before recording must be compressed
30 db, the objective in the design of the compressor was to obtain a
linear relationship over this range.
Oct., 1941]
GENERAL THEORY
349
It was considered desirable to include in the design of the system
the enhancement feature so that the music could be re-recorded and
additional interpretations added by the director of the orchestra as
desired. In order to give the director as much freedom as possible,
the maximum level should be limited only by the ear, that is, at 120-
db intensity level. It has been found that the maximum peak in-
FIG. 16.
Control box for enhancement and quality
control.
tensity level for a large orchestra sometimes reaches 110 db. For
this reason the system was designed so that the maximum levels
of the original orchestra could be raised 10 db if desired. The control
box is designed so that level changes can be made over a range of 30
db, from 10 db higher to 20 db lower than the normal signal levels.
The 10-db increase and 5 of the 20-db decrease were produced by
changing the level of the pilot channel. The other 15-db decrease
350 H. FLETCHER [j. s. M. P. E.
was introduced directly in the signal, channel. If a signal near the
film noise level is decreased 5 db by the pilot channel and the maxi-
mum signal level is increased 10 db, the enhanced music will have an
intensity level range of 95 db. The maximum intensity of an orches-
tra occurs at frequencies between 400 and 800 cycles. So it is seen
from Fig. 5 that this range is all that the ear can hear and tolerate.
The networks which change the frequency response of the system
were introduced directly into the signal channel before the recorder.
Except for changes made by these networks, the re-recorded signal
track was the same as the recorded signal track. It will be seen then
from these figures that the expandor must operate at levels 5 db lower
and 10 db higher than for the compressor, or through a range of 45
db.
In Fig. 16 is shown a picture of the control box which the director
uses for this enhancement feature. It is evident that this enhance-
ment can be done at the first recording, rather than on re-recording.
This procedure would save any inherent distortions due to the re-
recording process. In this case, however, the director must have an
associate, either operating the control box or directing the orchestra.
It has been pointed out that although the film noise is at the same
level as the orchestra noise at its ''silent" period in the unexpanded
music coming from the film, this is not true when the music is ex-
panded to normal or enhanced, because in this case the film noise is
also raised, being at most 50 db below the signal. In general, for
orchestral music such noise is masked. However, for intense low-
pitched tones the film noise, being principally in the high frequencies,
is audible and may be heard as a hissing sound varying in intensity as
the tone increases and decreases in level. To provide as large a
margin as possible against this, a predistorting network is introduced
into the system which makes it possible to record the higher fre-
quencies at greater amplitudes on the track than normal. This is
possible without increasing the track width because these higher fre-
quencies come from the orchestra at considerably lower intensity
levels. When the signal is reproduced, a restoring network cuts down
the intensity of these high frequencies by the same amounts that they
were raised during recording, and at the same time lowers the film
noise in these high-frequency regions.
The frequency characteristic of the pre-equalizer used in the SSFS
is shown in Fig. 17. The philosophy back of choosing this particular
characteristic is rather involved and will be dealt with in the paper by
Oct., 1941]
GENERAL THEORY
351
Dr. Steinberg. * It is sufficient to say here that this effective reduc-
tion of noise is possible only because music has a different sound
spectrum from that due to noise. In recording orchestral music, this
gives an effective reduction of noise of about 10 db. In other words,
for such recorded music using these pre-equalizers and restorers, the
film noise is effectively 60 db below the signal. This additional re-
duction in noise also provides a margin in case the director, who is
doing the enhancing, raises the level of the soft passages in the re-
recording process.
It has already been mentioned that, because of the time required
for the compressing system to operate, a signal of suddenly increased
intensity may overload the light modulator initially and thus undergo
0 10
z
200 500 1000 2000
FREQUENCY IN CYCLES PER SECOND
FIG. 17. Transmission-frequency characteristic of predistorting
network.
"clipping," the audible effect of which is a low-frequency thud. It
was also suggested that this difficulty could be overcome by the intro-
duction of a delay in the signal channel. The required amount of
delay depends upon the speed of operation of the compressor. Un-
fortunately, delay equipment had not been incorporated in the cir-
cuits that were used in making the orchestral records which will be
demonstrated at the end of this paper. The effect of the introduc-
tion of delay for various impulsive sounds was subsequently studied
by using two microphones — one for the signal, and the other for the
control circuit. The delay time was easily varied by altering the
relative distances of the two microphones from the source. It was
found that the method eliminates the "clipping" thud and otherwise
operates entirely satisfactorily.
* Page 366, this issue of the JOURNAL.
352 H. FLETCHER
The object in the development of the recording and reproducing
machines was to provide a means of putting into and taking out of
storage the compressed signal wave. The frequency range to be
covered was set at 20 to 14,000 cps without the introduction of any
audible amount of distortion.
The various mechanical and optical features of the recording-
reproducing equipment are described in some of the other related
papers.* It will here be sufficient to say that the four records to be
demonstrated are made on one standard 35-mm strip of film by four
special light-valves, one for each track, and that in reproducing, a
separate optical system and a separate photoelectric cell are used with
each track. Except for the optical features, the same machines are
used for reproducing that were used in the making of the records.
REFERENCES
1 A symposium of six papers on "Auditory Perpsective," Electrical Engineering,
LIII (Jan., 1934), p. 9; Bell Syst. Tech. J., XIII (April, 1934), p. 239.
2 STEINBERG, J. C., MONTGOMERY, H. C., AND GARDNER, M. B.: "Results of
the World's Fair Hearing Tests," /. Acoust. Soc. Amer., XH (Oct., 1940), p. 291;
Bell Syst. Tech. J., XIX (Oct., 1940), p. 533.
3 SEACORD, D. F. : "Room Noise at Subscribers' Telephone Locations,"
/. Acoust. Soc. Amer., Xn (July, 1940), p. 183.
4 HOTH, DANIEL F. : "Room Noise Spectra at Subscribers' Telephone Loca-
tions," /. Acoust. Soc. Amer., XII (April, 1941), p. 499.
5 FLETCHER, HARVEY, AND MUNSON, W. A. : "Relation between Loudness
and Masking," /. Acoust. Soc. Amer., IX (July, 1937), p. 9.
6 MUELLER, W. A.: "Audience Noise as a Limitation to the Permissible
Volume Range of Dialog in Sound Motion Pictures," /. Soc. Mot. Pict. Eng.,
XXXV (July, 1940), p. 48.
* Published in this issue of the JOURNAL.
MECHANICAL AND OPTICAL EQUIPMENT FOR THE
STEREOPHONIC SOUND-FILM SYSTEM*
E. C. WENTE, R. BIDDULPH, L. A. ELMER, AND A. B. ANDERSON**
Summary. — The same mechanism is employed for propelling the film in both
recording and reproducing. To permit recording the longer orchestral selections
without interruption, the machines are designed to handle film in 2000-ft lengths.
Special features of the film-propulsion system for obtaining great uniformity of speed
at the translation points are described. The three signal-currents and one control-
channel current are recorded by means of light-valves of identical construction. All
four tracks are exposed while the film is passing over a free-running supporting roller,
mounted on the same shaft with a new type of internally damped roller. In repro-
duction, each track is exposed through an objective of high aperture to light from an
incandescent source. After passing through the film, the light from each track is
carried by a glass rod to a photoelectric cell.
The primary function of the mechanical part of the recording ma-
chine of the stereophonic system is to move film from one magazine
to another and intermediately pass it at a uniform speed over a sup-
port that will hold the film in accurate focus at four translation points
simultaneously. The reproducing machine must perform essentially
the same function with film reels substituted for the magazines.
Aside from the optical systems, the same machines have, therefore,
been used both for recording and reproducing. In order that long
orchestral selections may be recorded without interruption, the ma-
chines must be capable of handling film successfully in 2000-ft
lengths.
The optical system for each of the channels in the recorder must
transmit light from the modulator in sufficient quantity for exposing
the film adequately in a sharply defined image. The width of the
image in the direction of film travel should be as small as the resolv-
ing power of the photographic emulsion warrants. When the record-
ing is of the variable-area type, the ends of the image must also be
sharply defined so that the boundary line between the light and dark
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 1,
1941.
** Bell Telephone Laboratories, New York, N. Y.
353
OThe Society is not responsible for statements by authors •&
354 WENTE, BIDDULPH, ELMER, AND ANDERSON [J. S. M. P. E.
areas in the record will be clear cut. ^A border of high contrast will
permit the fullest modulation of the sound-track area and keep the
noise from this part of the record to the lowest value.
The reproducing optical system for each channel must form an
image on the film slightly longer than the width of the track, and, in
the direction of film travel, the image must be narrow or have a
narrow intense region, but does not need to be so sharply defined as
the image in the recording system, since the scanning losses, if they
are not too large, can here be equalized in the electrical circuit.
FILM PROPELLING MECHANISM
The apparatus as set up for recording is shown diagrammatically
in Fig. 1. The passage of film is from left to right and is controlled
FIG. 1. Diagrammatic view of recording machine.
by the two 24-tooth sprockets Si and S2, which are driven through
spiral gears from a transverse shaft directly connected to the motor.
Between the two sprockets, the film passes over a series of rollers
having various functions. They are all mounted on ball bearings.
Rollers 1,2,11, and 12 are pad rollers for the sprockets; 3 and 10 are
guide rollers which lead the film around a shield placed over the photo-
electric cells when the machine is used as a reproducer; 3 and 5 are
flanged rollers limiting the weave of the film; 7 is the main sound or
scanning roller, which is depended upon to keep the film in focus at
all four tracks and moving at a uniform speed; 6 is a steel pressure
Oct., 1941] MECHANICAL AND OPTICAL EQUIPMENT 355
roller which presses the film against the sound roller with sufficient
force to prevent film slippage. This roller bears only on a narrow
portion of the film along its center line. It is mounted in a fork
which is pivoted at p\ so that the roller can seek a position in which it
will not exert sidewise thrust on the film. The axis of this pivot is
rigidly connected to the lever arm a\t which in turn is pivoted on the
horizontal axis £2. The pressure roller tension is controlled by ad-
justment of the spring Si connected to the lower end of the lever arm
a\. The purpose of 8 is to give the film a relatively large angle of
wrap around the sound roller, thus providing good seating for the
film at the translation points and greater insurance against slippage.
A damping roller is rigidly mounted on the sound roller shaft.
While the power needed to drive the sound roller under steady-
state conditions is small, the driving torque must be increased con-
siderably when the roller is being brought up to speed from rest be-
cause of the relatively high moment of inertia of the damping roller.
During this time, the roller 6 must be pressed against the film with
increased force if film slippage is to be prevented. The linkage,
shown by the dashed lines in the figure, is provided for this purpose.
The lever / of this linkage is pivoted at p%. A loose-fitting rod r con-
nects the arm a\ to the lower arm of the lever /. The upper end of
the lever / carries the supporting shaft of the flanged roller 9. The
spring $2 forces this shaft against a stop so that, in normal operation,
the axis oi roller 9 is virtually fixed. When the sound roller is being
accelerated, the tension-of the film between it and the leading sprocket
will increase, the upper arm of the lever / will be pulled down from
its stop, the slack in the bearings of the rod r will be taken up, and
pressure of roller 6 on the film will be increased until the sound roller
is up to speed, when the film tension will become normal and the
shaft of roller 9 will again be pulled back against the stop. With this
arrangement, the sound roller is brought up to speed in about four
seconds.
Flanged reels may be used in reproducing, but in recording the
film must be wound in a light-tight magazine. It is practically im-
possible to avoid a certain amount of rubbing of the film against the
side walls by the time most of the film has passed into the magazine.
Consequently, a greater torque must be applied to the take-up shaft
when the magazine is nearly full than would be necessary for a flanged
reel holding the same amount of film. If the take-up shaft were
driven by a slipping clutch, the torque would be the same for the
356
WENTE, BIDDULPH, ELMER, AND ANDERSON [J. S. M. P. E.
empty as for the full magazine. If the clutch were set so that it
would turn the shaft of the full magazine without risk of failure, the
film tension would be so high at the beginning as to expose the film to
serious injury. This condition is greatly aggravated in going to 2000
instead of the usual 1000-ft lengths of film. In place of the friction
clutch, a spring belt type of drive was, therefore, substituted. This
drive was so designed that the troque delivered by it increases with
the diameter of the film spiral up to a certain point, and from there
on remains constant. The belts and pulleys are so proportioned
that the spring material never becomes strained beyond the endur-
CD
-
CD
CD
Q
a
CD
CD
CD
CD
CD
D
CD
a
CD
CD
^
o
0.080"
«
-*-
-0.325"-
»•
•*-
-0.367"-
-fr
•-0.325"-*
1
n tAn
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n 1A<V'
FIG. 2.
Location of sound-tracks on
35-mm film strip.
ance limit. Danger of breakage is, therefore, small. To avoid
serious trouble if there should be such breakage, two belts are used
in parallel. The belts run in F-grooved fiber pulleys. The driving
pulley is driven from the leading sprocket shaft through a pair of
spur gears.
The light-valves L V, shown above the sound roller, are held in ex-
act position by brackets provided with suitable guide surfaces. They
are clamped down with spring clips so that they can be readily re-
placed. All valves are assembled and adjusted on one fixture so as
to render them completely interchangeable. No further adjust-
Oct., 1941] MECHANICAL AND OPTICAL EQUIPMENT
357
ments need be made after they have been attached to the brackets.
They are so located along the axis of the sound roller that the four
sound-tracks are generated at the positions on the film shown in
Fig. 2. The outer tracks are sufficiently far from the sprocket-holes
to avoid sprocket-hole flutter resulting from processing variations
round the sprocket-holes.
Fig. 3 is a skeleton side view of the frame. Above to the left is the
>und roller with the damping roller to the right, carried by the same
Ft. This damping roller consists of a casing having an annular
FIG. 3. Skeleton side view of film-
propelling mechanism.
channel carrying a liquid which is coupled to the casing by a porous
partition. This roller is described in more detail in a separate paper. l
Below is shown the sprocket shaft, carrying the leading sprocket at
the left. Toward the right, the spiral gears through which the shaft
is driven by the motor may be seen. The gear in the middle is one
that meshes with another gear attached to the driving pulley of the
spring belt drive for the take-up reel shaft.
A 3000-cycle record was made with one of these machines and the
frequency modulation in this record measured for us by Electrical
Research Products, Inc., on one of their flutter meters. While the
358 WENTE, BIDDULPH, ELMER, AND ANDERSON [J. S. M. P. E.
stereophonic records were made on acetate base, this particular rec-
ord was made on a nitrate base to reduce the modulation caused by
uneven film shrinkage. Measurements were made on a number of
sections taken at random along the record. The results of these
measurements are given in Table I.
TABLE I
Frequency Observed Minimum Perceptible
of Flutter Speed Variations Speed Variations
(Cps) (Per Cent) (Per Cent)
96 0.02-0.085 0.05_
9 0.055-0.09 0.0045
1.2 0-0.038 0.0055
Drift 0-0.018
The first column gives the frequencies of flutter, and the second,
corresponding magnitudes in per cent. The upper and lower figures
given in the second column for each frequency region represent the
extreme values that were found in the entire set of measurements.
Zero percentage, of course, means no more than that the flutter
meter was not sensitive enough to indicate the flutter actually pres-
ent. The value given for drift was said to be not in excess of that
which normal unevenness of shrinkage could produce. The source of
the flutter at the various frequencies is easily accounted for except
that at 9 cycles. It was later reported that at least some, if not all,
of the indicated flutter at this frequency was assignable to a fault in
the particular meter used in these measurements. The maximum
values of flutter are in excess of those which the ear can detect under
the most favorable listening and frequency conditions on an A-B com-
parison test. These values, taken from an unpublished memoran-
dum by W. A. MacNair, are given in the third column of the table.
While these values are considerably lower than the higher figures of
column 2, it is known that, for the complex tones ordinarily recorded
when reproduced in a moderately dead room, the values of column 3
can be exceeded without detection.
RECORDING OPTICAL SYSTEM
Fig. 4 shows the optical system used with each one of the four light-
valves on the recording machine. The light- valve itself is described
in detail in another paper.2 A and B are two views in sections pass-
ing through the axis of the valve, the former at right angles and the
latter parallel to the axis of the sound roller. The filament of an ex-
Oct., 1941] MECHANICAL AND OPTICAL EQUIPMENT
359
citer lamp is brought to a focus by means of a single-element con-
denser on the ribbons of the light-valve. In the bottom pole-piece
of the light-valve is mounted a commercial ten-power apochromatic
microscope objective having a numerical aperture of 0.3. With
this, the inner edges of the two light- valve ribbons are imaged on the
film, at a magnification of 10:1. L2 is a cylindrical lens which forms
a reduced image of the edges of a fixed slit 5 in the film plane. The
slit and the cylindrical lens together define the height of the image
FIG. 4. Recording optical system.
in the direction of film travel, while the microscope objective and the
slit formed by the light- valve ribbons define the length of the image.
The lens L% consists of a round circular rod provided with a stop
limiting the image angle to about 20 degrees. The clearance be-
tween this lens and the film surface is about x/32 inch. The cylindrical
lenses for all four channels are mounted in a single block, which can
be removed from the machine as a unit. This block has embossed
ridges on its undersurface, running parallel to the film travel, which
prevent accidental contact between the film and the lens surfaces.
In this arrangement, no trouble has been experienced from particles
360
WENTE, BIDDULPH, ELMER, AND ANDERSON [J. s. M. P. E.
becoming attached to the glass surfaces of sufficient size to impair .
image quality.
The slit 6" acts as a stop on the system, as seen in view A . In this
plane, therefore, the aperture, and consequently the image aberration j
of the condensing lens, is small. Hence, in the narrow direction, the j
image of the lamp coil is sharply defined at the valve ribbons. In •
the plane of Fig. 4(J5), the optical system operates at the full aperture !
of the microscope objective, and the single-piece condenser has,
r0.7 -0.6 -0.5 -0.4 -O.3 -0.2 -O.I 0 O.I O-2 O.3 0.4 0.5
DISTANCE FROM MIDDLE OF IMAGE IN MILS
FIG. 5. Light distribution in scanning image of micro-
densitometer.
therefore, a relatively large amount of spherical aberration. Here,
this is no disadvantage since a lamp may be chosen having a coil
length such that the image formed by the lens has a central uniformly
bright portion long enough for amply covering the operating area at
the light- valve ribbons. At the same time, the aberration prevents
sharp focusing of the individual turns of the lamp coil, which would
result in a recording image of uneven brightness along its length.
An exact determination of the frequency characteristic of a sound
record free from effects introduced by the optics of the measuring
Oct., 1941] MECHANICAL AND OPTICAL EQUIPMENT
361
system presents some difficulty. Such a characteristic could be ob-
t tained theoretically by scanning the record with a line of light of in-
; finitesimal width and observing the transmitted light. While this
i procedure is impracticable, a pretty good estimate of what the results
! of such measurements would be can be gained from scanning data ob-
I tained with a narrow, but finite, line image in which the distribution
of the light is known. Such measurements were carried out on rec-
ords made at a number of discrete frequencies in the range from 1000
to 16,000 cycles with the light-valve modulation maintained at 50
per cent. The light distribution in the scanning image used in these
measurements is shown in Fig. 5. The nominal width of the image
0 6
z
0 2 4 6 8 10 12 14 16
FREQUENCY IN KILOCYCLES PER SECOND
FIG. 6. Relative modulation in negative sound record.
was between 0.25 and 0.3 mil. This curve was obtained from meas-
urements on the variation of the light passing over a knife-edge, set
and held accurately parallel while it was moved in measured steps
across the image. The fact that the curve is unsymmetrical is prob-
ably due to some imperfections in alignment. The frequency records
were scanned very slowly with this image, while the transmitted
light was measured with a microdensitometer. The amplitudes of
the microdensitometer records for the various frequencies were
measured. The value so obtained, when translated into decibel
losses, are plotted as A of Fig. 6. Next, the scanning vs. frequency
characteristics of an image having the light distribution shown in
Fig. 5, when used to scan sine-wave records of constant amplitude,
362 WENTE, BIDDULPH, ELMER, AND ANDERSON [J. S. M. p. E.
were computed. The values so obtained are shown by the dashed
curve B in Fig. 6. The ordinates of this curve were then subtracted
from the corresponding ordinates of curve A. From these values,
C was plotted. This curve shows a loss of 6 db at 16,000 cycles, but
it must be considered in the light of the method whereby it was ob-
tained. It does not represent the true state of affairs accurately.
The correction for the image was made on the assumption that sine-
wave records were scanned — which is here not the case, certainly not
at the higher frequencies. Prints of these sound records, ideally
made with proper exposure and^processing,
should show less high-frequency loss than the
negatives which were here explored. Practi-
cally, the high-frequency losses are likely to be
higher as the result of failure in meeting the
ideal conditions. While thus there remains
some doubt regarding the frequency charac-
teristic of the records as made for reproduction
in the stereophonic system, it is likely that
curve C does give at least the order of magni-
tude of the part of the total overall transition
loss of the recording-reproducing system that
is attributable to the photographic processes
involved in making the records.
THE REPRODUCING OPTICAL SYSTEM
The objective in the planning of the repro-
ducing optical system was to get a large quan-
FIG. 7. Reproducing tjty of n^fa from an exciter lamp into a reason-
optical system. J ° .
ably small scanning line, so that a high signal
current might be generated in the photoelectric cell. The optical
arrangement had to be such that the light transmitted by the film
could be brought conveniently from each record to a photoelectric cell.
Fig. 7 shows the arrangement of the optical system used for each
one of the four channels. The lamp coil is parallel to the axis of the
sound roller. M is a commercial ten-power achromatic microscope
objective with a numerical aperture of 0.3. It forms an image on the
film of the illuminated slit S. This slit is adjusted to give an image
normally equal to 0.5 X 85 mils. Actually, because of lens aberra-
tions, the width is greater, but no measurements have been made on
the light distribution within the image. The slit S has the length of
Oct., 1941] MECHANICAL AND OPTICAL EQUIPMENT
363
about 0.85 inch. The condensing system between the lamp and this
slit must provide uniform illumination of this slit and it must fill
completely the microscope objective if the advantage of the large
aperture of the latter is not to be sacrificed. The condenser used is
a one-piece combination prism, lens, and reflector shown at P. All
its surfaces are either plane or cylindrical: 1 is a plane surface ap-
proximately normal to the center line of the light beam; 2 is a cylin-
drical surface with its center line of curvature in a plane that is per-
pendicular to the roller axis and passes through the center line of the
FIG. 8. Diagrammatic view of reproducing machine.
light beam; 3 is also a cylindrical surface with its center line of cur-
vature parallel to the roller axis and lying between the center line of
the objective and the lamp. The prism surface 2 reflects the light
internally through the slit S and brings it to a focus in the direction
of the roller axis at the objective M. Light from the different coils
of the lamp is, therefore, completely diffused along the length of the
slit and the slit is otherwise illuminated with great uniformity along
its length. The surface 3 forms an image of the lamp coil on the slit
S in the transverse direction. The illumination angle is small —
only one-tenth of that which obtains at the image on the film. Spheri-
cal aberration is, therefore, not a serious problem.
364
WENTE, BIDDULPH, ELMER, AND ANDERSON [J. S. M. P. E.
After the light has traversed the fijm, it enters a glass rod through
which the light is deflected around a roller shaft to the photoelectric
cell. The plane surface through which light enters the rod is ap-
proximately perpendicular to the light-beam. After it has passed
this surface, the light is reflected internally by the cylindrical sur-
face 4. The curvature and orientation of this surface are such that
the light is reflected along the axis of the rod with minimum spread.
The surface 6 is cut at a slight angle to the rod so as to refract the
beam to the middle of the photoelectric cell cathode. This rod serves
not only to lead the light from the film to the photoelectric cell, but
it also acts as a diffuser so that light coming from various parts of
the line image on the record is spread over approximately the same
surface area of the cathode. The introduction of wave-form dis-
25
15
10
\
40 60 100
20O 400 600 10OO 2OOO 4000 6000 10000 200OO
FREQUENCY IN CYCLES PER SECOND
FIG. 9. Insertion loss of recording-reproducing system.
tortion by any non-uniformity of sensitivity of the cathode surface
is thus avoided.
The arrangement of the various optical parts on the machine when
set up for reproducing is shown in Fig. 8, which shows also the gear
and pulley arrangement of the spring belt drive. The condensing
prism and lamp for each channel are mounted in one casing, which is
fastened to a bracket that holds a light-value in recording.
No measurements have been made on the scanning efficiency as a
function of frequency of this optical system. The variation of the
insertion loss of the recording-reproducing system is shown in Fig. 9.
In this loss are included those due to scanning in recording and repro-
ducing, processing and printing, and photoelectric cell and photo-
electric cell amplifier losses.
Oct., 1941] MECHANICAL AND OPTICAL EQUIPMENT 365
In conclusion, we wish to point out that all records and prints used
in the measurements here reported, as well as those used in the dem-
onstration of orchestral music, were made with white light and the
surfaces of none of the lenses were coated for reduction in reflec-
tivity.
REFERENCES
1 WENTE, E. C., AND MULLER, A. H.: "Internally Damped Rollers," J. Soc.
Mot. PicL Eng., XXXVII, (Oct., 1941), p. 406.
2 WENTE, E. C., AND BIDDULPH, R.: "Light-Value for the Stereophonic
Sound-Film System," /. Soc. Mot. Pict. Eng., XXXVII, (Oct., 1941), p. 397.
THE STEREOPHONIC SOUND-FILM SYSTEM— PRE- AND
POST-EQUALIZATION OF COMPANDOR SYSTEMS*
JOHN C. STEINBERG**
Summary. — In order best to fit the volume range of the program material into
the volume range available in sound-film, it is generally advantageous to pre-equalize
the program material before recording, and to compensate for the equalization by
means of a complementary post-equalizer on reproduction. The type and amount
of pre-equalization depends upon the properties of hearing and on the characteristics
of the program material and the film noise. This paper discusses the relations
between these quantities for systems using compandors, -where the film noise varies
up and down in level as the compandor gains vary. Ideally, different types of pre-
equalization are needed for different types of program material, and a compromise
must be made if a single type is to be used. The considerations leading to the choice
of the pre-equalization used in the stereophonic recording and reproducing system
are discussed.
The purpose of introducing pre- and post-equalizers and com-
pandors into a recording and reproducing system is to bring about a
better fit than would be obtained otherwise, of the intensity range of
the program material into the intensity range afforded by the system.
The form that such elements take and the good that is accomplished
by their use depend upon the type of program material, the properties
of hearing, and the particular characteristics of the system. It is the
purpose of this paper to discuss the ways in which these factors enter
into the problem, which are general in character, and then to describe
the pre- and post-equalization used in the stereophonic sound-film
system (SSFS).
It will serve our present purpose to take as our program material
the sounds produced in a concert hall by a large symphony orchestra,
and to take as our system one involving sound-film recording and re-
production. The intensity range afforded by the sound-film is deter-
mined by the difference between the intensity levels which cause ob-
jectionable overloading in recording, and those intensity levels which
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 1,
1941.
** Bell Telephone Laboratories, New York, N. Y.
366
<> The Society is not responsible for statements by authors <>
EQUALIZATION OF COMPANDOR SYSTEMS 367
are just audible in the background of film noise. The maximum
orchestral intensity levels can be recorded so as to just avoid over-
loading, and then, on reproduction, they can be amplified to their
original maximum values. Under these conditions, the intensity
range of the reproduced sounds is limited by the reproduced film
noise, and the amount that the film noise must be attenuated in order
that it be just inaudible in the presence of the background of audience
noise is a measure of the amount of the original intensity range that
has been lost in reproduction. The reduction in the reproduced film
noise that is afforded by the use of equalizers and compandors gives
a measure of the increase in the intensity range of the recording and
reproducing system due to the use of such elements.
From measurements reported by Sivian, Dunn, and White,1 in-
formation is available on the maximum intensity levels produced by
a large orchestra, and by individual instruments. The peak ampli-
tudes occurring in alternate Vs-second intervals were measured for
the whole spectrum, and for various frequency bands throughout the
spectrum. In the case of the orchestra, the measurements were made
at a point near the conductor's stand, on four different musical selec-
tions. The results may be expressed in the form of the instantaneous
peak intensity levels (referred to hereafter as peak intensity levels)
which are exceeded in given percentages of the intervals. The total
or whole spectrum peak intensity levels that were exceeded in only
one per cent of the intervals for the different selections are as follows :
109, 1 12, 108, and 1 1 1 db from 10~16 watts per sq-cm. A study of the
data indicated that for a given selection the one per cent values were
rarely exceeded by more than 2 db. It seems reasonable, therefore,
to take 115 db as representative of the maximum peak intensity levels
for an orchestra. Measurements made on the Philadelphia Orchestra
at the Academy of Music by somewhat different methods indicate
similar values.
Fig. 1 shows the peak intensity per cycle levels which are exceeded
in one per cent of the intervals, for a whole spectrum peak intensity
level of 115 db. The peak intensities per cycle were obtained by
dividing the peak intensities measured in the different frequency
bands by the band width in cycles.
For good recording and processing conditions, the total level of
film noise in a 10,000-cycle band may be taken as 53 db below the
peak sine wave level at which overloading occurs in recording. The
noise intensity may be considered as distributed uniformly through-
368
J. C. STEINBERG
LJ. S. M. P. E.
out the frequency band. Hence, the intensity per cycle level of the
noise is 93 db below the peak sinusoidal intensity level at which over-
loading takes place.
Measurements have been made in which the maximum peak in-
tensity levels of the orchestra were compared with sine-wave peak
intensity levels when both were at the overload point. The results
indicated that the peak intensity levels of the two classes of signals
were nearly the same when overloading occurred, with perhaps a
80
70
60
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MAXIMUM PEAK LEVELS (ORCHESTRA)
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FILM NOISE LE
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0 5OO 1000 5000 1OO
FREQUENCY IN CYCLES PER SECOND
FIG. 1. Spectrograms of audience and film noise and
orchestral sounds.
tendency for the maximum peaks of the orchestra to be slightly higher
than the sine-wave peaks. If these levels be taken as equal, which is
the conservative procedure for our present purpose, the film noise
will be reproduced at an intensity per cycle level of 22 db above
10~16 watts per sq-cm, when the system is set to record and reproduce
peak intensity levels of 115 db.
Measurements made with the sound level meter on the audience
noise at the Philadelphia Academy of Music, and also at Constitution
Hall in Washington, when filled with audiences, indicate minimum
sound levels of 33 db, when measured with meters employing the so-
called 40 db weighting. From auxiliary measurements, a spectrum
Oct., 1941] EQUALIZATION OF COMPANDOR SYSTEMS 369
of audience noise corresponding to a sound level of 33 db was ob-
tained, and is shown in Fig. 1 along with the spectrum of the repro-
duced film noise.
The three curves of Fig. 1 represent quantities which must be con-
sidered in attempts to increase the intensity range of the recording
and reproducing system. In order to provide the original intensity
range, the system must be capable of reproducing the maximum
orchestral levels, and the reproduced film noise must be masked by
the audience noise, or by the sounds of the orchestra. The amount
that the film noise must be reduced in order that it may be masked
by the audience noise is best shown by the curves of Fig. 2.
In this figure, the film noise and audience noise are plotted in terms
of the intensity level per critical frequency band. In sensing a ran-
dom type of noise, such as film noise, the ear tends to integrate, over
a small frequency interval, the intensity carried by each cycle of the
noise.2 This interval, called the critical frequency band, depends
upon frequency, and has the property that a pure tone having the
mid-band frequency and an intensity level equal to that of the noise
in the critical band, will be just audible in the presence of the noise.
The lower curve in Fig. 2 shows the intensity levels of pure tones
at the threshold of hearing for people having very good hearing and
listening in a quiet place. The curves for film and audience noise
show the threshold intensity levels for pure tones when heard in the
presence of the respective noises. The audience noise and pure tone
threshold curves set theJower limit of the original intensity range in
the concert hall, and, to preserve this limit in reproduction, the film
noise levels must be reduced, at least to the levels indicated by these
two curves, the audience noise curve setting the limit for those fre-
quencies at which it lies above the threshold curve. The required
reduction is shown by the solid curve of Fig. 3.
As previously noted, the pure tone threshold curve of Fig. 2 indi-
cates the threshold values for people having very good hearing. The
results of the World's Fair hearing tests3 indicate that less than five
per cent of the people are able to hear such tones. If threshold levels
which can be heard by at least 50 per cent of the people are used in
obtaining the required film noise reduction, the dashed curve in Fig. 3
results.
The figure shows that a film noise reduction of some 42 db at fre-
quencies near 7000 cycles is needed if the recording and reproducing
system is to provide an intensity range equal to the original range in
370
J. C. STEINBERG
U. S. M. P. E.
the concert hall. It should be noted that this amount of reduction is
in the nature of a maximum. It is trie reduction required for the re-
produced film noise to be inaudible to a listener having very good hear-
ing when located at a point near the conductor's stand in a concert hall
PURE TONE THRESHOLD LEVELS\j
-20
100
500 1000 5000 10000
FREQUENCY IN CYCLES PER SECOND
FIG. 2. Levels of film and audience noise in critical
bands.
similar to the original hall, and when the recording and reproducing
system is set to reproduce the maximum peaks of the orchestra with-
out noticeable overloading, using, in the process sound-film which has
a peak signal to noise ratio of 53 db. For a listener located in the
seating area of the concert hall, the reproduced film noise might be 5
to 10 db below audibility under these conditions.
REQUIRED FILM NOISE
REDUCTION IN DECIBELS
- IM u fe o
^0 0 0 0 0 C
!
"-s,
^
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0 500 1000 5000 100
FREQUENCY IN CYCLES PER SECOND
FIG. 3. Required film noise reduction.
For a given film technic, there are three ways in which the film
noise may be reduced: (1) by the use of pre- and post-equalizers, (2)
by the use of compandors, and (3} by the use of appropriate combina-
tions of 1 and 2.
Pre- and post-equalization makes use of the fact that the maximum
Oct., 1941] EQUALIZATION OF COMPANDOR SYSTEMS
371
peak intensity levels for the higher frequencies are generally smaller
than the peak levels for the lower frequencies, as may be seen from
Fig. 1. In reducing the noise by this method, the high frequencies
are recorded at greater than normal levels, relative to the low fre-
quencies, by using a pre-equalizer, and then on reproduction they are
reduced back to their normal levels by means of a complementary
post-equalizer. The penalty that is paid for reducing noise by this
method is in the reduced capacity of the system for recording and re-
producing high-frequency peak intensity levels. Although this does
not limit the usefulness of the system for orchestral sounds, it may
present difficulties for other types of sounds.
In order to show the amount of noise reduction that may be accom-
plished by the use of pre-equalizers, two different types will be con-
sidered, which will be designated as pre-equalizers No. 1 and No. 2.
GAIN
PRE-
TOR EQUALIZING ORCHES
EQUALIZER NO. 1
EQUALIZER NO. 2
TRAL PEAKS
PRE-
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FREQUENCY IN CYCLES PER SECOND
FIG. 4. Pre-equalizer characteristics.
With pre-equalizer No. J, the insertion gain is such as to produce at
the recorder, equal maximum peak levels for the different frequencies
in orchestral sounds. The solid curve in Fig. 4 shows the gain that
is required to equalize the orchestral peak levels indicated in Fig. 1,
and the dashed curve shows the gain characteristic of pre-equalizer
No. 1, which approximately meets this objective. The dotted curve
shows the gain characteristic of pre-equalizer No. 2, in which the
increase in gain is confined principally to the frequency regions above
3000 cycles.
The use of such pre-equalizers produces an increase at the recorder,
in the maximum whole spectrum peak levels of the orchestra. The
increase may be obtained by integrating, over the frequency range,
the peak intensity per cycle spectrum of the orchestra before and
after pre-equalization, and taking the difference between the inte-
grated values. The appropriate spectra may be obtained from the
peak intensity per cycle level curve of Fig. 1 and the equalizer gain
372 J. C. STEINBERG fj. S. M. P. E.
characteristics of Fig. 4. The calculations indicate that the whole
spectrum peak levels for pre-equalizef No. 1 are 11.5 db higher than
the corresponding levels for the unequalized orchestra. The corre-
sponding figure for pre-equalizer No. 2 is 3.0 db.
If the same peak levels in different parts of the frequency range were
equally effective in producing noticeable overloading, the whole spec-
trum peak levels should be a criterion of overloading. Hence, in
order to avoid overloading, it would be necessary to reduce the gain
ahead of the recorder by 11.5 and 3.0 db, respectively, when pre-
equalizers Nos. 1 and 2 are used.
It is believed, however, that high-frequency peaks are not as effec-
tive as low-frequency peaks in producing noticeable overloading be-
cause of their relatively shorter durations, and also because of the
greater tendency of the modulation products of high-frequency peaks
to fall outside the audible band. In sensing sounds, the ear inte-
grates peak amplitudes over a time interval of some Vs to y4 of a
second. To determine the auditory characteristics of peak levels, it
would be more appropriate to deal with the intensity levels inte-
grated over Vs-second intervals than with the peak intensity levels
occurring in such intervals. These two quantities differ by a peak
factor which is larger for the high-frequency peaks than for the low-
frequency ones. Unfortunately, quantitative measurements of such
peak factors for orchestral sounds are not available, and it has been
necessary to estimate, from rather fragmentary data, a set of weight
factors for indicating the relative overloading effects of peak levels
at different frequencies. The estimated values which are shown in
Table I, should be subtracted from the peak intensity per cycle level
curve of Fig. 1 . The curve thus obtained is used for calculating the
whole spectrum peak levels in the manner indicated above.
TABLE I
Factors for Weighting the Peak Levels of Orchestral Sounds
Frequency 100 200 500 850 1200 1800 3000 5000 10,000
Weight factor db 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
The calculations employing the weight factors indicate that, in
order to avoid overloading, the gain ahead of the recorder must be
reduced by 8.5 db for pre-equalizer No. 1, and by 1.5 db for pre-
equalizer No. 2. It is believed that these values are more nearly
correct than the corresponding values of 11,5 and 3.0 db calculated
Oct., 1941]
EQUALIZATION OF COMPANDOR SYSTEMS
373
from the unweighted peak levels, and they will be used in the subse-
quent discussion.
When the sounds are reproduced, the post-equalizer introduces a
loss equal to the gain introduced by the pre-equalizer, and thus re-
duces the reproduced film noise levels. Also, since the gain ahead of
the recorder was reduced in order to avoid overloading, the gain follow-
ing the reproducer must be increased by a corresponding amount in
order to reproduce the sounds at their original levels. This increases
the reproduced film noise level, and the net change in level is given by
the difference between the post-equalizer loss and the overloading
gain adjustment.
The effects achieved by the use of the equalizers may be seen from
the curves of Fig. 5. The solid curve, which was replotted from Fig.
FILM NOISE
IN DECIBELS
8fc o
o c
WITHOUT
WITH PRE
. WITH PRE
PR
i-EQUALIZER
OUALIZER NO. 1
3UALIZER NO. 2
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REQUIRED
REDUCTION
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FREQUENCY IN CYCLES PER SECOND
FIG. 5. Required reduction for equalized film noise.
3, shows the film noise reduction that is required when a pre- and
post-equalizer is not used, in order that the original intensity range
in the concert hall be reproduced. The dashed curve shows the re-
duction that is required when pre-equalizer No. 1 is used. This pre-
equalizer represents the maximum amount of pre-equalization that
may be used advantageously. Any larger amount will increase the
low-frequency noise levels without further affecting the high-fre-
quency levels. If a smaller amount of pre-equalization were used,
low-frequency noise levels would be diminished, but the intermediate
and high-frequency levels would be increased as indicated by the
dotted curve for pre-equalizer No. 2. In either case, the high-fre-
quency noise levels have been decreased by some 10 db only, and a
further reduction of some 30 db is needed.
A further reduction in the reproduced film noise may be achieved
by the use of compandors. Such devices compress the signals before
374 J. C. STEINBERG [j. s. M. P. E.
recording, and then expand them to their original values on reproduc-
tion. In the case of a 30-db compressor, for example, this is done by
providing a gain increase of 30 db ahead of the recorder. During
silent periods, this gain remains unchanged at the value of 30 db.
As the signals increase in level, the gain decreases in accordance with
the increase in signal level and becomes zero for the maximum signal
levels. On reproduction, a 30-db loss is provided by the expander
during silent periods. As the signal levels increase, the loss dimin-
ishes in a manner which is complementary to the gain changes of the
compressor and becomes zero for the maximum reproduced signal
levels. Hence, during silent periods the reproduced film noise levels
are 30 db below the values they would have if the compandor were not
used. They increase and approach the latter values as the signal levels
increase and approach their maxima. If during this period the film
noise were masked by the signals, and if during the silent periods it
were masked by the audience noise, then the recording and reproduc-
ing system would be capable of reproducing the original intensity
range in the concert hall.
It will be recalled (Fig. 3) that a film noise reduction of 42 db is
needed in order that it be masked by the audience noise. Hence, a
42-db compandor would be required if the film noise were to be in-
audible during silent periods. Such a compandor is practicable and
could be used, but generally it would be advantageous to achieve
part of the reduction by the use of pre- and post-equalizers. The re-
duction achieved by their use is effective not only during silent inter-
vals but during the intervals that the noise levels change in accord-
ance with the signal levels. By the proper choice of equalizer char-
acteristics, the noise may be more effectively masked by the signals
than would be the case if the noise reduction were accomplished by
the compandor alone. Also, in the case of the SSFS it was desired
to combine a 15-db enhancement feature with the expandor, so that
the reproduced signal levels could be increased by 10 or diminished
by 5 db. This would require a 57-db expandor, which becomes some-
what impracticable. It was decided, therefore, to use an equalizer
in combination with a 30-db compressor and a 45-db expandor.
The pre- and post-equalizer No. 1, which has been described, was
chosen for this purpose. It provides a film noise reduction of 10 db
(Fig. 5) and the combination affords a reduction of 40 db during silent
intervals. The film noise levels thus obtained are shown on Fig. 6
in relation to the audience noise levels and pure tone threshold levels.
Oct., 1941] EQUALIZATION OF COMPANDOR SYSTEMS
375
The reduction is sufficient to reduce the film noise below audibility
during silent periods, and it remains now to investigate how effec-
tively the film noise is masked by the signals during other than silent
periods.
It may be noted in passing that equalizers may be combined with
the compandors in such a way that the gains in different parts of the
frequency range depend upon the signal levels in those parts. Such
variable-equalizer compandors would probably be the most ideal way
of fitting the program material into the intensity range afforded by
sound-film, particularly for widely different program materials.
Their use, however, presents a number of practical problems which
seemed to outweigh their advantages for the present application.
500 1000 5000 10000
FREQUENCY IN CYCLES PER SECOND
FIG. 6. Reproduced film noise during silent periods.
Many types of compandors have been used, involving different
relationships between the change of compressor gain and signal level.
In the type used in the SSFS, the compressor gain increases 1 db for
each 1-db decrease of signal level in the range of signal levels begin-
ning a few db below the maximum levels and extending to levels 30
db lower. For still lower signal levels, the compressor gain remains
fixed at the 30-db value. The loss provided by the expandor changes
in a similar fashion on reproduction. These changes are accomplished
by means of a pilot or control current which is modulated by the
signal levels. The pilot current operates the compressor on record-
ing, and is also recorded. On reproduction, the reproduced pilot
current operates the expandor. The considerations leading to the
choice of this type of compandor for the SSFS are discussed in the
paper by Dr. Fletcher.* One of the principal reasons for the
* In this issue of the JOURNAL.
376
J. C. STEINBERG
LT. S. M. p. E.
choice was that this type provides the greatest possible margin be-
tween signal levels and noise levels during other than silent periods.
The performance of the compandor is shown by the curves of Fig. 7.
The upper solid curve shows how the output level of the compressor
(i. e.t recorder input level) increases as the signal levels increase, then
flattens off and finally increases again until the compressor (and also
the recorder) overload level of 115 db is reached. The lower solid
curve shows the reduction in the level of the reproduced film noise
due to the complementary action of the expandor. The input signal
level to the compressor is shown as equal to the acoustic peak in-
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in
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PEAK INPUT SIGNAL LEVEL TO COMPRESSOR
FIG. 7. Noise reduction afforded by compandor.
tensity level input to the system provided a pre-equalizer is not used.
To obtain the acoustic level when a pre-equalizer is inserted ahead of
the compressor, the signal level shown in Fig. 7 must be corrected by
the gain changes caused by its insertion.
The control current operating the compressor must be set such that
the output level will flatten off before overloading occurs. If the
compressor acts infinitely fast, and if the compressed signals have no
more tendency to overload than the uncompressed ones, the level for
flattening off should be 115 db. In practice, it was found necessary
to flatten off the compressor output at a level of 106 db, as shown by
the solid curve of Fig. 7. When the signal levels reaching the com-
Oct., 1941] EQUALIZATION OF COMPANDOR SYSTEMS 377
pressor were delayed some two or three milliseconds with respect to
those modulating the control current, so as to give the compressor
time to act, it was found that the flattening off level could be raised 5
db. As shown by the dotted curves of Fig. 7, this reduces the re-
produced film noise an additional 5 db during the period when the
compressor acts. The records for the most part, however, were made
in accordance with the solid curve of Fig. 7, as the technic for intro-
ducing delay was not well worked out at the time of recording.
The combination of equalizers and compandor affords sufficient
film noise reduction that it will be inaudible during all silent periods
and during all periods when the full orchestra is playing. The periods
when the noise is most likely to be audible are when either low or high-
frequency tones only are being produced by single instruments or
small groups of similar instruments, as happens rather frequently in
symphonic programs. To obtain a conservative estimate of the
audibility of the noise under such conditions, the levels of the repro-
duced film noise have been determined when the system is called
upon to record and reproduce either a 200 or a 4000-cycle pure tone.
From the levels of the reproduced film noise, it was possible to deter-
mine, from the pure tone masking curves, the levels of the noise above
threshold in the presence of the reproduced pure tones.
The curves in Fig. 8(A) show the levels above threshold of the re-
produced film noise as heard in the presence of the reproduced pure
tones of 4000 and 200 cycles, for the combination of equalizer No. 1
and the 30-db compandor. The tone levels are shown by the #-axis,
and represent either the input or the reproduced tone levels, since
they are equal. The frequency range of the noise that is audible in
the presence of the 200-cycle tone extends from 5000 to 10,000 cycles.
The range that is audible in the presence of the 4000-cycle tone ex-
tends from 100 to 3000 cycles. This noise, as it rises and falls with
fluctuating tone levels, produces the so-called "hush-hush," which
may be either low or high-frequency in character. As suggested by
the dotted curves of Fig. 7, the hush-hush may be reduced 5 db below
the values shown, by delaying the signals two or three milliseconds at
the compressor input.
Fig. 8(B) shows similar curves for the equalizer No. 2 when com-
bined with the 30-db compandor. They suggest that equalizer No. 2
would have been a better choice than No. 1, as less low-frequency
hush-hush is produced, i. e., the hush-hush of the noise when heard in
the presence of the high-frequency masking tone.
378
J. C. STEINBERG
[J. S. M. P. E.
In an earlier paragraph it was noted that the use of a compandor
alone, to accomplish the noise reduction required for silent periods^
would not generally result in the most effective masking of the noise
by the signal. This is illustrated by the curves of Fig. 8(C), which
obtain for a 40-db compandor without an equalizer. Although such
a compandor reduces the film noise to inaudibility during silent
periods, the high-frequency hush-hush is considerably greater than
obtained with an appropriate equalizer.
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100 120 60 80
PEAK INTENSITY LEVEL OF TONE
100
FIG. 8. Hush-hush effect from compandor action.
In the paper by Fletcher, it was pointed out that a self-acting
compandor would not give as good a discrimination against
noise as one of the type described here, involving the use of a pilot
channel. The curves of Fig. 8(D) show the noise levels for a 30-db
2:1 self-acting compandor when combined with equalizer No. 2. In
this type of compandor, the compressor gain changes through a 30-db
range at the rate of 1 db per 2-db change in signal level. Although
the two types are equally satisfactory during silent periods, the self-
acting type results in hush-hush over a wider range of signal levels
than does the type used in the SSFS.
The foregoing discussion has been predicated on such a gain setting
of the SSFS as to afford unity reproduction, i. e., reproduced intensity
levels equal to the original levels. The enhancement feature pro-
vides for a gain increase of 10 db above unity. Since the gain in-
Oct., 1941] EQUALIZATION OF COMPANDOR SYSTEMS 379
crease takes place on reproduction, it will cause a corresponding in-
crease in the level of the reproduced film noise. Reference to Fig. 6
will show that the film noise during silent intervals is just about at the
threshold, so that any increase in gain during these periods will result
in audible noise. Gain increases made while the full orchestra is
playing will not result in audible noise, as the full orchestra provides
sufficient masking. When single instruments play at levels below
the operating level of the compressor, i. e., 60 to 70 db, a gain increase
raises the film noise from the level which it has during silent intervals.
Fig. 6 shows that at this level, the film noise below 3000 cycles is
some 7 db below the audience noise. Hence, if the instruments are
producing high-frequency tones, the gain may be increased 7 db with-
out the noise becoming audible. When low tones are produced,
however, a gain increase would produce audible high-frequency noise,
as low-frequency tones at these levels would not mask the high-
frequency noise. When single instruments play at levels high enough
to operate the compressor, the increase in noise levels caused by a gain
increase correspond, in the worst cases, to those shown in Fig. 8(a),
as it is unlikely that any single instrument producing a relatively pure
tone would ever produce a level within 10 db of the overload level of
the compressor. Many instruments cover a wide enough frequency
range to mask the film noise for any gain increase up to 10 db. Hence,
the system makes some, although not complete, provision for in-
audible noise levels during periods when the reproduced sounds are
enhanced by gain increases.
REFERENCES
1 SIVIAN, L. J., DUNN, H. K., AND WHITE, S. D. : "Absolute Amplitudes and
Spectra of Certain Musical Instruments and Orchestras," /. Acoust. Soc. Amer.,
II (Jan., 1931), p. 330.
2 FLETCHER, H.: "Auditory Patterns," Rev. Modern Phys. (Jan., 1940),
Fig. 16.
3 STEINBERG, J. C., MONTGOMERY, H. C., AND GARDNER, M. B.: "Results
of World's Fair Hearing Tests," Bell Syst. Tech. J., 19 (Oct., 1940), p. 533 (Fig. 8).
ELECTRICAL EQUIPMENT FOR THE STEREOPHONIC
SOUND-FILM SYSTEM*
W. B. SNOW AND A. R. SOFFEL**
Summary. — An electrical system is described which permits the use tf sound-film
with its limited signal-to-noise ratio, as a recording medium for wide-range stereo-
phonic reproduction of symphonic music. Noise reduction is accomplished both by
pre-eqtyalization, rising to 18 db above 8000 cycles, and by automatic signal compres-
sion and expansion of 30 db.
To secure maximum suppression of noise and freedom from distortion, a pilot-
operated, flat-top compandor system was selected. In each channel low-level signals
are recorded on a separate track with constant gain 30 db above normal, which places
them above the film noise. Higher-level signals cause automatic gain reductions and
are recorded at substantially full modulation. These signals vary the intensity of a
pilot tone, which in turn controls the compressor gain. There is a pilot frequency for
each of the three channels, and the three are combined and recorded together on the
fourth film track. During reproduction they are separated by filters, and operate ex-
pandors which restore the signals to their original forms but reduce the noise to in-
audible levels.
The compressor and expandor gains are made proportional to pilot level in db, and
the expandor range over which this relation holds is 45 db. Therefore a 15-db variation
in average pilot level during reproduction causes a corresponding average level change
but no distortion. This is used to allow expansion of the original signal intensity
range during recording or re-recording by simple gain controls in the pilot circuits.
The paper describes the apparatus and circuits developed to accomplish these re-
sults, and discusses the frequency, load, distortion, noise, and dynamic characteristics
of both constant and variable-gain elements. Also included are considerations of
microphone and loud speaker arrangement and equalization to secure high fidelity of
reproduction.
The general requirements for performance of the stereophonic
sound-film system (SSFS) and the mode of operation of its principal
parts have been described in other papers of this series, L 2> 3 and we
shall attempt a minimum of repetition. This paper will deal with
the actual apparatus used in making and playing the records and the
characteristics of the parts and the system. Therefore, only a brief
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May 26,
1941.
** Bell Telephone Laboratories, New York, N. Y.
380
-OThe Society is not responsible for statements by authors-^
ELECTRICAL EQUIPMENT
381
review of the material from the other papers as it affects the electrical
system will be given.
In order that the system may reproduce all sounds without deteriora-
tion of quality, the aim in its design has been to reproduce the entire
range of frequencies and intensities to which a person of unimpaired
hearing is sensitive under conditions existing in quiet auditoriums.
This leads to the requirement that the complete system have an
overall frequency response characteristic that is essentially uniform
from 40 to 15,000 cycles per second. The acoustic output of the
system should allow maximum sine- wave intensity levels of +120,*
yet any noise generated within the system must be low enough so as
not to be heard at any time. For the quietest auditoriums and
auditors with acute hearing this means that the 60-cycle component
of power hum which is often the most prominent must be at least 75 db
below maximum signal, and the 120-cycle component should be at
least 90 db below. Even if only average conditions are expected it is
well to be strict on hum limits because standing- wave patterns can
build up the steady hum in local spots where random noise of the same
projected intensity would be inaudible. Thermal noise, usually
generated in the microphone and first amplifier, will be about 95 db
below the maximum signal with the more efficient of the microphones
in use at present. In the critical bands around 4000 cycles this is
somewhat above threshold in quiet rooms for acute hearing. The
system should not add appreciably to this noise.
These requirements are determined by the characteristics of hearing
rather than those of particular types of sounds or program. The
acoustic intensity level ranges discussed are for single frequency
signals, therefore, and should not be confused with the volume ranges
of program material measured by volume indicator.
The electrical equipment, it will be recalled, is used in conjunction
with a sound-film recorder and reproducer capable of handling three
signal channels and one control channel. The ratio, in this recorder
and reproducer, of maximum rms sine-wave to total band of film noise
as measured by volume indicator is about 50 db. In order to meet
the requirements set forth above, pre-equalization rising to 18 db
above 8000 cycles and automatic compression and expansion of signals
by 30 db are used to reduce the effects of this film noise. The effec-
tive level of the film noise is reduced approximately 10 db by pre-
— —
* Acoustic levels will be expressed in db from 10" 16 watts per cm2.
382
W. B. SNOW AND A. R. SOFFEL
tf. S. M. P. E.
equalization and 30 db by compression for zero signal and it is thus
about equal to the thermal noise during quiet periods. Provision is
made for a manual manipulation of the expandor gain which allows
sounds to be reproduced up to 10 db above and down to 5 db below
unity ratio. These gain changes can be introduced either during
recording or re-recording. In general, the frequency range and hum
requirements . are satisfied. Three-channel stereophonic reproduc-
tion, with quality comparable to the best direct transmission and
reproduction, was accomplished with the use of this electrical system.
Fig. 1 is a block diagram of the recording system and Fig; 2 of the
FIG. 1. Block diagram of the recording circuit.
reproducing system. Of course, in the actual case there are three
identical signal channels but to conserve space only one is shown
completely. The three pilot tones are handled together in a fourth
channel the details of which are given. The system consists of three
main parts: the acoustic and fixed gain amplifying and equalizing
portion; the compandor and recording and reproducing circuits;
and the pilot system. We shall describe them in this order.
ACOUSTIC AND AMPLIFYING SYSTEMS
To supply the large amplification necessary before the compressor
and following the expandor the original amplifiers of the 1934 demon-
strations were used with some rearrangement.4 This includes
Oct., 1941]
ELECTRICAL EQUIPMENT
383
amplifiers A i to A 3 of Fig. 1 and A12 to Au of Fig. 2. The pair of
power amplifiers Au on each channel is rated at +52* maximum
rms sine-wave, which is sufficient to produce the desired maximum
acoustic intensity level +120. The total hum output is set by these
amplifiers at 87 db below maximum, a close approach to the ideal.
At higher frequencies the thermal noise of the microphone resistance
sets the limit. All these amplifiers have satisfactory gain-frequency
characteristics over the 40 to 15,000-cycle range desired.
The microphones M used for the records made in Philadelphia
were the same ones used in the 1934 demonstration5 which combine
FIG. 2. Block diagram of the reproducing and monitoring circuit.
high efficiency with a smooth characteristic to 15,000 cycles. The
cardioid 642A microphone has since become available and has been
used in Hollywood with excellent results. While the microphone
characteristic was relatively smooth, it was not uniform and it was
therefore necessary to equalize for it. In the direct transmission of
1934 the microphone equalization was included with the loud speaker
equalization. However, when a record was to be made of the sound
it was thought better to employ the separate microphone equalizer
designated ME. Precisely, the equalizer is a constant-resistance net-
work the loss-frequency characteristics of which is inverse to the field
* Throughout the paper electrical levels are expressed in db from 1 milliwatt.
384 W. B. SNOW AND A. R. SOFFEL [J. S. M. P. E.
calibration at normal incidence of the microphone. This calibration
was chosen since the microphones were used close to the sound sources
and received most of their sound directly and at approximately nor-
mal incidence. This procedure yields records which are a faithful
copy of the sound and consequently can be reproduced on any flat
loud speaker system.
Pre-equalization is thoroughly discussed in the other papers1-3 of
the series. The pre- and post-equaliers they describe are repre-
sented in our drawings as Pr. E and Po. E. PI is an attenuator which
sets the recording gain.
The loud speaker equalizer designated LSE is also a constant-resis-
tance network, with a loss-frequency characteristic inverse to the
response characteristic of the loud speaker. These loud speakers are
similar to those used in 1934. 5 The low-frequency units are identical,
but for frequencies above 300 cycles standard W. E. 594A units are
used with a special coupling to the old 4X4 cell horns. This gives
the advantages of the commercial unit combined with the 60 X 120-
degree coverage of these horns. The horns are mounted behind the
low-frequency ones far enough to reduce the delay between upper and
lower frequency ranges to about 5 milliseconds.
The characteristic of the loud speaker was measured in a unique
manner.* Two systems were set up, one consisting of an oscillator,
an interrupter key, an attenuator, and amplifier, and the loud
speaker. The other consisted of a microphone, an amplifier, and the
film recorder used in the regular system. The loud speaker was
placed at the front edge of the stage of the Academy of Music in
Philadelphia and the microphone was hung out in the auditorium 18
feet from the loud speaker with its diaphragm parallel to the speaker
mouth. Then, at a number of single frequencies, tones giving roughly
equal outputs were abruptly started and stopped on the loud speaker.
The output of the microphone was recorded on sound-film and the
sound-tracks thus obtained were examined and measured under a
microscope. On each spurt of tone there was recorded an initial few
cycles of constant amplitude representing the direct wave from the
loud speaker followed by transient waves and finally by a steady wave
as determined by the acoustics of the hall. It was the first section
that was measured and used for the calibration of the loud speaker.
Since the electrical equipment, microphone, and recorder were of
* Suggested by Mr. E. C. Wente.
Oct., 1941]
ELECTRICAL EQUIPMENT
385
known frequency characteristics the actual values of loud speaker out-
put could be determined. This calibration was equivalent to a free
space calibration since the effects of reverberation were eliminated.
500 1000 5000 10000 20000
FREQUENCY IN CYCLES PER SECOND
FIG. 3. Acoustic reproduction characteristic. The curve gives
the ratio, expressed in db, of direct sound output of the loud speaker
to field pressure at the microphone.
The compandor system is designed to have a uniform frequency
characteristic. Consequently the equalizers described above deter-
mine the acoustic characteristic of the system. This is considered to
be uniform when the direct sound output of the loud speaker is equal
to the direct sound at normal incidence at the position of the micro -
N
-OPERATING RANGE
70 65 60
55 50 45 40 35 30 25 20 15 10
RELATIVE PILOT LEVEL IN DECIBELS
FIG. 4. Compressor schematic and characteristic. The curve
shows the relation of the gain of compressor and amplifier to the
level of pilot tone at the rectifier input.
phone. It will be remembered that the microphones are placed dose
to the sound source. The argument is that by this means the sound
is projected into the listening hall as if the original sound source were
there and is acted upon normally from an acoustic standpoint. As
little as possible of the acoustic effect of the originating auditorium is
386 W. B. SNOW AND A. R. SOFFEL [J. S. M. P. E.
transmitted. For reproduction in a large auditorium this is felt to be
the correct method of equalization and pick-up, although it might
not be satisfactory for reproduction under considerably different
conditions. The overall air-to-air response of the system as calcu-
lated from the microphone, loud speaker, and electrical system charac-
teristics, appears as Fig. 3.
The 40-cycle high-pass filters were employed to protect the loud
speakers from injury caused by very low-frequency transients which
sometimes arise in the system from power-line disturbances or in-
judicious switching. At frequencies below the cut-off or the low-
frequency horn (40 cycles) there is little acoustic load on the dia-
phragm and a small voltage may cause a large displacement and
consequent injury.
COMPANDOR-RECORDING-REPRODUCING SYSTEM
The compressors and expandors are of the general types that have
been described by Bennett and Doba.6 The average characteristic of
the three compressors labeled C in Fig. 1 is shown by Fig. 4 together
with a simplified schematic drawing of the circuit. Control current
is fed in longitudinally in a balanced bridge formed by the four
varistor elements. The amplifier which follows the varistors and is a
part of the compressor is operated at a maximum 1000-cycle output
level of + 14 and a hum level of — 57, giving a signal-to-hum ratio of
71 db. Since the output signal of the compressor is recorded on film
with a signal-to-noise ratio of only 50 db, this seems more than ample.
However, the film noise covers a wide band, whereas the hum consists
of fixed frequencies which will be masked only by the critical bands
of the film noise. Our experience has been that a 65 to 70-db range is
required for hum in the compressed portion of the circuit.
Upon leaving the compressor, the signal enters the recording
amplifier A±, which is a balanced feedback type amplifier with a very
sharp upper limit of voltage output. This limiting action is em-
ployed to prevent clashing of the strings of the light-valves, an
especially important point when a compandor is used. The arrow in
this and other amplifier boxes indicates that the gain is adjustable.
The output of A 4 is split into three paths by the two impedance-
adjusting networks NI and NZ. One branch is labeled monitor and
connects to the expandor input potentiometer PIO (Fig. 2) when the
full reproducing system is used for monitoring in recording or re-
recording. The other branch consists of a light-valve equalizer VE
Oct., 1941]
ELECTRICAL EQUIPMENT
387
and provision for operating the light-valves of two recording machines
simultaneously with individual level control by the attenuators P2A
and P2B.
The valve equalizer is a network to correct in advance for the fre-
quency characteristic of the light- valve as measured in terms of de-
flection of the valve strings. This characteristic was measured by
observing valve string deflection with a microscope when known
voltages and frequencies were applied to the input of A±. The over-
-65 -60 -55 -50 -u45 -40 -35 -30 -25 -20 -15 -10 -5
RELATIVE PILOT LEVEL IN DECIBELS
FIG. 5. Expander schematic and characteristic. The curve
shows the gain of the expander and amplifier as a function of the
level of pilot tone at the rectifier input.
all result then is an amplitude on the sound-track proportional to the
original sound amplitude except as it is purposely modified by the pre-
equalization and compression.
The photoelectric cell output is coupled to the amplifier A 10 (Fig. 2)
by a transformer mounted in the reproducing machine and a low-
impedance shielded line. The reproducing equalizer RE corrects for
the overall optical, film, photoelectric cell, and coupling losses of the
recording-reproducing chain. To obtain the characteristic, a film
negative was made of a number of single frequencies recorded at con-
stant amplitude and the output of Aw was observed when a positive
print of this negative was reproduced. The equalizer was built to
388 W. B. SNOW AND A. R. SOFFEL [j. S. M. P. E.
have the inverse of this characteristic and since AH has a flat fre-
quency characteristic, the signal delivered to the expandor Ex is the
same as the signal leaving the compressor. Attenuator PIO is used
to adjust the expandor input to a standard level. The amplifiers
between compressor and expandor are very uniform in frequency
characteristic and have a maximum signal-to-hum ratio of 70 db.
The expandor for this system must perform a double duty. It
must automatically restore the signal to its uncompressed character-
istics and in addition produce the relatively slow gain changes for
enhancement of the signals. It must, therefore, be capable of oper-
ating over a wider range of pilot levels than the compressor. The char-
acteristic of the expandor and a simplified schematic circuit are
shown in Fig. 5, where in the normal range of pilot levels for reproduc-
ing original records and the extended range employed for full enhance-
ment are indicated.
The signal channels are operated at levels which insure that the non-
linear distortion on the highest peaks is at least 35 db below the funda-
mental.
PILOT SYSTEM
The pilot system provides the operating mechanism for compressor
and expandor and offers the possibility of enhancement. Fig. 1
shows the parts used for recording. The three pilot tones are gener-
ated by a special magnetic generator driven by a constant-speed
motor. This device delivers 1260 cycles and its third and fifth har-
monics, 3780 and 6300 cycles, with provision for adjusting the phase
relations between them. The output is between — 12 and — 17 with-
out amplification and the harmonic distortion is less than one per cent.
The three tone outputs are connected through volume controls P3
to the three modulator inputs. The modulators1 are similar to the
expandors previously described and operate under signal control to
vary the pilot tone levels. This signal control is effected by the modu-
lator rectifiers MR which are bridged across the compressor input and
supply rectified current to the modulators. The range through which
the modulators vary the pilot tone can be adjusted and also that part
of the signal amplitude range causing the change can be selected by
adjusting the amplification of the rectifier.
The signal-controlled modulator outputs are combined in a network
7V3 that prevents interaction between them. This combined output
is at a very low level and must be amplified considerably before it can
Oct., 1941] ELECTRICAL EQUIPMENT 389
be used to operate the light- valve or compressor rectifiers. Since the
lowest frequency is 1260 cycles, an 850-cycle high-pass filter is con-
nected between A$ and A* which greatly diminishes the shielding re-
quired against power hum. Amplifier AQ is a limiting amplifier
similar to A^ in which the limiting feature is used to protect the light-
valve strings from clash. The output of this amplifier is arranged to
drive two recording machines simultaneously and the reproducing
pilot circuit for monitoring, as in the signal channels. In addition
there is a third branch of N* which passes through the three volume
controls P± and feeds three band-pass filters that separate the pilot
tones for operation of the compressor rectifiers. Pilot tone leaving
each filter operates the appropriate compressor rectifier which in turn
operates the compressor according to the characteristic of Fig. 4.
In the pilot channel reproducing circuit (Fig. 2) the photocell out-
put of the reproducing machine is amplified by AI$ and is then de-
livered to a circuit similar to that feeding the compressor rectifiers.
Any irregularities in frequency characteristic can be corrected by ad-
justments for each pilot frequency at P\^. When a recording is being
made the photoelectric cell circuit can be patched out and network
NU substituted which connects the recording pilot output direct to the
reproducing pilot input for monitoring. It is important to note in
either case that compressor and expander are operated by as nearly as
possible identical circuits so that no matter how the pilot tones vary,
the gain changes of compressor and expander will remain satisfac-
torily inverse.
In general the gain changes should take place as rapidly as possible
when signals increase so that the compressor output will remain below
the overload point of the limiting amplifier. However, once the com-
pressor has reduced its gain to accommodate the wave it should not
follow individual cycles. Although the pilot-operated expander
tends to restore the wave this would require an impractically perfect
pilot system. This system operates with about a six-to-one ratio
between attack and decay timing and will repeat a 50 -cycle tone with
excellent fidelity.
It was, of course, impossible to make the compressor gain changes
instantaneous. Some wave-fronts are steep enough so that part of
the beginning of the wave is not reduced sufficiently by the compressor
and is chopped by the limiting amplifier A^ with resultant objection-
able distortion when compression begins at the limiting input of the
amplifier. It was found by listening tests on symphonic music that
the characteristic of Fig. 6 gave satisfactory recording. The desired
390
W. B. SNOW AND A. R. SOFFEL
[J. S. M. P. E.
30 db of compression is retained, but the sensitivity of the pilot con-
trol is adjusted so that compression begins at a lower signal intensity
and the "flat- top" portion of the characteristic is not recorded at full
modulation. This must be done because the curve shown is for
steady-state inputs. When a program is being recorded the com-
pressor output continually for short intervals exceeds the values
given by the curve of Fig. 6, and the "margin" between full modula-
tion and the flat top was set to render peak-chopping unobjectionable
during the listening tests.
It is important to note that the form of this curve is determined
wholly by the pilot-level-to-signal characteristic of the pilot control.
-60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0
COMPRESSOR INPUT LEVEL IN DECIBELS
FIG. 6. Recording Characteristic. This curve shows the rela-
tionship of signal output of the recording amplifier to signal input
to compressor and modulator rectifier, for a single-frequency signal.
The expander and compressor merely respond linearly to pilot level
changes. Therefore both the shape and amount of compression may
be varied by adjustments at the time of recording and might even be
changed for each type of program recorded.
PHYSICAL LAYOUT
All the signal equipment except the power amplifiers4 is mounted
on four tall cabinet racks, shown at the left of Fig. 7. The experi-
mental nature of the installation is apparent. Corresponding units
of all channels were mounted together to keep level differences at a
minimum. It also facilitates checking levels and making substitu-
tions and on the whole is preferable in an experimental system such as
this. Another useful feature from the experimental angle is the
Oct., 1941] ELECTRICAL EQUIPMENT 391
provision of input, output, and bridging jacks with "normal- through"
contacts to make the regular set-up continuous without patch cords.
The rapid rearrangement and checking of the circuit which this made
possible proved very useful.
In all amplifiers where the noise requirements are severe external
supplies for both plate and heater power are used. These are
mounted on racks which are situated a few feet away from the signal
FIG. 7. Recording and reproducing channels. The three racks
at the left carried the program equipment, while the two smaller
cabinets on the right contained the pilot apparatus. The medium-
size rack, center, contained auxiliary equipment used for some
special tests but not part of the regular channels.
channels, thereby reducing greatly the difficulties of shielding the latter
from hum pick-up. Several of the amplifiers of narrower range have
small self-contained power supplies. They are mounted only on the
two racks housing equipment handling compressed signals, and are
well shielded.
The apparatus associated only with the pilot circuits is installed in
two short cabinets appearing at the right of Fig. 7.
All the foregoing racks must be fairly close together to facilitate
operation. The other essential parts of the system may be located at
the most convenient points. In the installation for our Hollywood
392 W. B. SNOW AND A. R. SOFFEL [J. s. M. P. E.
demonstration they were all installed on the rehearsal stage of the
Pantages Theater, and may be seen in tear view in Fig. 8. At the left
are rectifiers for the loud speaker fields supplying 22 amperes at 32
volts. To their right are rectifiers for the exciting lamps of the ma-
chines which draw a maximum of 20 amperes at 16 volts per machine.
In the second row, left, are the power amplifiers, and at the right the
plate and heater power supplies for the signal channels.
FIG. 8. Power equipment. In the foreground are rectifiers to
supply low voltage direct current to loud speaker fields and exciting
lamps in the machines. The next row consists of power amplifiers
(left) and plate and heater-supply equipment in the three cabinets
at the right.
If all the apparatus is at hand the following can be done : record-
ing, recording with full-range monitoring, reproducing, or re-recording
with or without enhancement plus monitoring.
OPERATION
Placement of microphones and loud speakers for stereophonic re-
production is still largely a matter of trial. For our Philadelphia
Orchestra records the microphones were suspended 10 feet above the
stage and 5 feet inside the front row of performers. The orchestra
width was about 40 feet and our outside microphones were 28 feet
apart. At the Carnegie Hall demonstration the side loud speakers
Oct., 1941] ELECTRICAL EQUIPMENT 393
were 40 feet apart and they and the center loud speaker were 11 feet
behind the main curtain.
When the circuit is to be set up for recording an orchestra with
monitoring, patches are made as indicated in Figs. 1 and 2. Each
channel is then lined up with a single-frequency tone applied at the
input of A i of voltage corresponding to the maximum expected from
the microphone. Adjustments are made so that the compressor out-
put just drives the recording amplifier to its limit. However, the ex-
pandor gain is set at 10 db below maximum to allow for enhancement.
Then program is picked up and PH is adjusted to give unity ratio re-
production on the basis of listening test or measurements. The final
setting of PI is then made such that audible overloading does not occur
on the highest program peaks, and PH can be readjusted accordingly.
The circuit is now set to record at proper level any sound less intense
than an orchestra without further changes in gain. Of course with
practice this setting can be determined by measurement.
In order to reproduce records it is desirable to have previously
made a test record with the circuit conditions described for recording.
This record is played over the reproducing circuit. The gain of A 15
and the setting of volume controls Pi2 are adjusted so that the proper
pilot levels are obtained at the expandor rectifier input on each pilot
tone. Finally the signal levels into the expandor are adjusted to be
at the allowable maximum and PH is set as for monitoring. Then any
record recorded with setting given above will be reproduced at the
same level at which it "was heard in the monitoring circuit. Any
desired alteration in listening level provided it does not exceed +10
db or — 5 db may be made very conveniently by changing the gain of
v4i5 which changes all channels at once.
The use of a pilot-operated compandor makes it possible to increase
the volume range of a re-recorded selection, or to enhance it, introduc-
ing only a slight increase in film noise. This process was described in
detail in another paper.1 Since only 30 db of the expandor gain
variation are needed for automatic control the 15 db remaining is
available for manual manipulation. Our amplifier capacity is suffi-
cient for 10-db increase in orchestral sound so that the 15-db range is
used as +10 and -5. It was to allow for this range that the ex-
pandor adjustments were 10 db below maximum in the recording
monitoring line-up. If greater reductions in level are required they
can safely be made by lowering the re-recorded signal level because
the noise is inaudible with expandor gain at 5 db below normal
394
W. B. SNOW AND A. R. SOFFEL
[J. S. M. P. E.
minimum. Quality-control networks may also be switched into the
signal channels.
Parts of the recording set-up are combined with the reproducing
circuit to make up this arrangement. The signal from each photo-
electric cell is amplified and then sent through a quality control and an
attenuator of 15-db continuous range which is part of the enhance-
ment volume control.1 Then they are equalized, amplified, and re-
recorded. The pilot track is picked up, amplified, and separated into
i^55^j
FIG. 9. Recording channels of new stereophonic system.
its components by the filters used with the compressors during re-
cording. After separation each tone passes through an attenuator of
15-db continuous range. The tones are recombined, amplified, and
re-recorded. Monitoring is accomplished in the same manner as
when recording. The attenuators for each channel in the signal path
and pilot are controlled by the same lever and arranged so that in the
normal position there are 10 db of loss in the pilot channel and none in
the signal. The lever can be moved up to zero loss in both channels
and down to 15 db in the pilot circuit with no loss in the signal channel
Oct., 1941] ELECTRICAL EQUIPMENT 395
and then still farther down to hold 15 db in the pilot and add up to 15
db to the signal loss. The enhancement volume control levers and
the quality-control keys are mounted in a control cabinet and are so
arranged that they can be conveniently manipulated by an operator
while he is listening to the music and can hear the effects of any
changes he makes. Of course these changes are all recorded at the
same time on the re-recorded negative.
CONCLUSION
During the past summer another complete stereophonic sound-film
system was built for Electrical Research Products, Inc., by the
Laboratories. Throughout this design commercial equipment was
used except in the pilot and compandor systems where standard
articles were not available. The new system always equalled and at
many points exceeded the performance of the experimental one
described in this paper. Fig. 9 shows the improved appearance of
the recording system and the standard height cabinet rack used
throughout.
The authors have had the full cooperation of many members of the
Laboratories' staff. In particular they wish to thank Mr. K. D.
Swartzel who developed the expander circuit and Mr. R. W. Bunten-
bach for his aid in equalizing and operating the system.
REFERENCES
1 FLETCHER, H.,: "The Stereophonic Sound-Film System— General Theory"
(this issue of the JOURNAL, p. 331).
2 WENTE, E. C., BIDDULPH, R., ELMER, L. A., AND ANDERSON, A. B.: "Me-
chanical and Optical Equipment for the Stereophonic Sound-Film System" (this
issue of the JOURNAL, p. 353).
3 STEINBERG, J. C.: "The Stereophonic Sound-Film System — Pre- and Post-
Equalization of Compandor Systems" (this issue of the JOURNAL, p. 366).
4 SCRIVEN, E. O. : "Auditory Perspective — Amplifiers," Electrical Engineering,
LHI (April, 1934), p. 278.
6 WENTE, E. C., AND THURAS, A. L.: "Auditory Perspective — Microphones
and Loud Speakers," Electrical Engineering, LIU (Jan., 1934), p. 17.
6 BENNETT, W. R., AND DOBA, S. : "Vario-Losser Circuits," Electrical Engineer-
ing, LX, Trans. No. 17 (Jan., 1941), p. 17.
DISCUSSION
MR. KELLOGG : Was a special copper oxide rectifier used with the compressor?
MR. SOFFEL : The copper oxide rectifiers were made especially for the job in the
Bell Telephone Laboratories and possess characteristics that give the compressors
and expanders a wide linear gain change without harmful effects caused by recti-
fier shunt capacity.
396 W. B. SNOW AND A. R. SOFFEL
MR. CRABTREE : In listening to the records from which the background noise
was not eliminated and attempting to eliminate the background noise mentally, I
got the impression that the sound quality was better than in the record that had
been subjected to equalizaton; that is, in going through the mill of equalization
and compression, some quality has been lost. This demonstration would seem to
stress the necessity of eliminating background noise by the use of even still finer
grained emulsions in recording.
MR. SOFFEL: If it were possible to get a film with a signal-to-noise ratio 40 db
better, the quality would be slightly superior on sounds of very impulsive nature,
but for ordinary sounds there would be little difference.
MR. MAURER: Is any information available as to the overall harmonic dis-
tortion of the system?
MR. SOFFEL: Each unit in the sytsem was designed to have a total harmonic
distortion of a pure tone of 400 cycles, 35 db below the fundamental at all levels.
The distortion of the complete system is probably slightly greater.
MR. KELLOGG : Are there certain frequency ranges of film noise that are mpre
disturbing than others? There is also the question as to the effect of a narrow
band of noise frequencies vs. a wide band.
MR. STEINBERG: Although studies reported by Professor Donald Laird of
Syracuse University tend to indicate that high-frequency noise would be more
disturbing than low-frequency noise when the two are equally loud, more study
will be needed with particular reference to film noise as a background to program
signals before a final answer can be given to that question.
A LIGHT-VALVE FOR THE STEREOPHONIC SOUND-FILM
SYSTEM*
E. C. WENTE AND R. BIDDULPH**
Summary. — This paper describes a light-valve incorporating large electromag-
netic damping and operating directly through the ribbon resonance region. Resonance-
region response, 5 db above that at low frequencies, is equalized by a suitable equalizer
to provide uniform ribbon displacement per unit driving voltage over the band 30-
14,000 cycles with very nearly constant phase-shift per cycle. Problems of structure
and size have furnished a mechanical design with several interesting features, among
which are mechanical robustness, protection against dirt and moisture, built-in rib-
bon and optical adjustments, and an optical system integral with the valve structure.
This unit has proved a rugged, stable, light-modulator especially free from inter-
modulation products.
The light-valve for recording sound-on-film consists essentially of
a pair of coplanar ribbons, supported in a transverse magnetic field
forming a light-slit which varies in size in accordance with the cur-
rent flowing through the ribbons. Its design involves the solution
of a combination of mechanical, electrical, magnetic, and optical
problems. Because of the limitations in the physical properties of
materials, the vibrating elements must be very small — a circumstance
which demands close tolerances and great stability in the mechanical
construction. However, the light- valve is well adapted to fulfill the
general requirements for a light-modulator in a high-quality sound-
recording system. It is simple in principle, and can be built in a
stable, convenient, and rugged form.
THEORY OF OPERATION
In order to obtain an analytical expression for the operating char-
acteristics of the light-valve in terms of its physical constants, we
shall assume that the ribbons are connected to an alternating source
of emf having an internal resistance equal to Ri. Except at very
high or very low audio-frequencies, this condition corresponds with
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received March
18,1941.
** Bell Telephone Laboratories, New York, N. Y.
397
<>The Society is not responsible for statements by author sO
398 E. C. WENTE AND R. BIDDULPH [J. S. M. P. E.
that under which the valve is used in practice. In order to simplify
the discussion, let us assume that the ribbons have exactly the same
physical constants, so that the motion of the two ribbons is the same
for a given current flow. We can then treat the two ribbons as a
unit. Our discussion will, therefore, proceed as if we were dealing
with a valve having a single ribbon. For reasons that will become
evident, the ribbon is preferably provided with a resistance shunt.
If this resistance is Rs, if the impedance of the valve is Zv, and if the
emf of the source is E'ejwt, the circuit diagram for the system is
analytically equivalent to that shown in Fig. 1, where E is equal to
ErR5/(Ri + R5) and R is equal to RiRs/(Rt + RJ.
Only the central portion of the ribbon is used in controlling the
light transmitted by the valve ; we shall therefore restrict our inter-
A/W
R
Eejwt,
FIG. 1. Equivalent electrical circuit of recording ampli-
fier and light- valve.
est to the motion of this region. If the displacement is designated by
x, the problem of finding the response frequency characteristic then
consists in the determination of |#/E|. The ribbon is ordinarily
tuned to a frequency which is near the upper part of the operating
frequency range. When current flows through the ribbon, it will,
therefore, be displaced into a form which is close to that of one lobe
of a sine-wave for all frequencies of practical interest. Assuming this
form of displacement we can readily determine the equivalent simple
harmonic system in which the constants are lumped and in which
the motion is the same as that of the middle of the ribbon. The ef-
fective mass of the ribbon in this equivalent system is equal to that
mass which, when moved at the velocity of the middle of the ribbon,
has the same kinetic energy as the ribbon itself. The effective stiff-
ness is equal to the effective mass multiplied by w02 where o>0 is the
angular frequency at resonance. The effective force is equal to that
force which, when multiplied by the displacement of the center of
Oct., 1941]
THE LIGHT- VALVE
399
the ribbon, will represent as much work as that which is actually done
in displacing the ribbon. If m, s, and F are the effective mass, stiff-
ness, and force, respectively, for the sinusoidal form of ribbon dis-
placement, we get, on applying the definitions just given,
where p is the density of the ribbon material; a is the cross-sectional
area, and / the length of the ribbon; B is the flux-density, which is
assumed to be uniform along the full length of the ribbon; and i
u 2
0.3
0.5
0.6
O.I 0.2_
FIG. 2. Effect of damping upon resonance response.
0.4
A/WO
is the current in amperes. We find also that the potential developed
between the ends of the ribbon when it vibrates is equal to
2BI
x 10~8 = k x 10-8 volts
We can now set up the following dynamical equation
k
mx + m co02 x = T *
If Rv is the resistance of the ribbon when its motion is fully con
strained, we have the following relationship for the circuit of Fig. 1
E = (R +
k x
(*)
400 E. C WENTE AND R. BIDDULPH
If E = Eejwi, we get from these two equations
kE
x =
W(R
"
S
[J. S. M. P. E.
(5)
This is the equation of motion for a simple vibrating system having
the angular resonance frequency co0, a mass m, and a mechanical
resistance 10 ~*k*/ (R + R,,), when subjected to a driving force equal
FIG. 3. Magnetic circuit of light- valve.
to kE/W(R -f- RV). The damping constant A of the system is
equal to lO~9k2/2(R + R^m. Substituting this value in equation 3,
and setting 77 = co/co0, we obtain the desired solution :
IQ(R +
- 01
The greatest uniformity of response is obtained when co0 is made
greater than the highest angular frequency to be impressed. This
procedure is impracticable for a broad-band system, coo is increased
either by shortening the ribbon or increasing its tension. For
Oct., 1941] THE LIGHT- VALVE 401
optical reasons, the maximum displacement of the ribbon must
not be too small and so the ribbon must not be too short. The
fatigue limit of ribbon materials restricts the working tension. Be-
cause of these limitations, it becomes practically necessary to keep co0
within the operating frequency range.
In this case, however, it is impossible to adjust the constants of a
simple vibrating system, such as the ribbon presents, so that the
response will be uniform. Equalization must be introduced into
FIG. 4. Photograph of ribbon supporting structure.
*
the electrical circuit if the system as a whole is to have a uniform
response. Networks may be designed to equalize for almost any
characteristic, but the tolerance limits both in the construction and
for stability become prohibitively severe for a sharply tuned system.
The equalization problems become increasingly simpler as the height
of the resonance peak is reduced.
Unless a unique kind of circuit closely coupled to the ribbon is
used for the equalization, such networks can not eliminate so-called
"valve clash." When the ribbons of a valve are over-modulated
and not well damped, they will oscillate every time they strike each
402 E. C. WENTE AND R. BIDDULPH U. S. M. P. E.
other. These oscillations are free oscillations at the resonance fre-
quency, which were not in the original signal. The damping of
these oscillations is practically unaffected by equalization in some
other part of the circuit. The effect of valve clash can be reduced
only by an increase in the damping of the valve. For this reason,
as well as because of the greater ease of equalization, the ribbon
should be as well damped as is practically possible.
From equation 4 we see that the maximum response occurs when
co2 = co02 — 2 A2. The ratio of peak response to the response at oo = 0
is, therefore,
This ratio thus depends solely upon A/co0. The relationship is shown
graphically in Fig. 2. The relative height of the resonance peak
thus can be decreased to any desired value down to unity by in-
creasing A/COQ. We should therefore try to make A/co0 as large as is
practically possible. The limit to which co0 can be reduced is set by
the fact that for frequencies very far above resonance, the displace-
ment of the ribbon no longer is sinusoidal, but takes on the form of a
higher mode of motion. This change will result in irregularities in
the response characteristic which are not easily controlled. Stabil-
ity of ribbon adjustment also sets a lower limit in the ribbon tension.
Having reduced co0 to the lowest practical value, we must seek an
increase in A. The damped resistance Rj, of the ribbon is equal to
al /a, where a is the resistivity of the ribbon material. Let
R, /(R +RJ) = a. We then get
According to equation 6, the best ribbon material is one having the
lowest value of <rp. This criterion, together with the fact that a
comparatively high value of co0 must be maintained, indicates that
duralumin is probably the most satisfactory ribbon material available.
For greatest reduction in the relative height of the peak value of re-
sponse, a should be made as large as possible, but the advantage
in going in this direction is limited, since the maximum value of a is
unity and a recording amplifier of increasingly greater power capacity
is required. A varies as the square of B and the power required to
Oct., 1941]
THE LIGHT- VALVE
403
operate the valve decreases as B is increased. For these reasons,
much is gained in making the flux-density as great as is practically
possible.
CONSTRUCTION AND PERFORMANCE
The magnetic circuit of the valve used in making four-channel
stereophonic records is shown in Fig. 3. Two conical pole-pieces,
spaced by a surrounding supporting cylinder of permanent-magnetic
material, form the air-gap in which the ribbons move. The flux-
density produced in this air-gap is 32,000 gauss.
40 60 100
200 400 600 1000 2000 40OO 60OO 10000 20000
FREQUENCY IN CYCLES PER SECOND
FIG. 5. Response vs. frequency characteristic.
Mounted securely on one of the pole-pieces is a ring structure
carrying clamping blocks for the ribbons, as shown in the photograph
of Fig. 4. These blocks are adjustable and the ribbons can be read-
ily placed in their correct position relative to the pole-piece structure
and tuned to the required frequency. The adjustment provisions
include means for moving the ribbons in the axial direction in order
that they may be made accurately coplanar. When all adjustments
of position and tension have been made, the clamps are fastened
solidly to their supporting base. Ribbons are spaced 0.004 inch
apart, and each is tuned to a resonance frequency of 8400 cps. In
order that all contact surface resistances may be kept to a small
and stable value, all ribbon clamps and the base upon which they
mount are gold-plated. A shunting resistance across each ribbon is
also fastened directly to the ribbon clamps. While these shunt
circuits increase the power requirements for operating the valve,
404 E. C. WENTE AND R. BIDDULPH []. S. M. P. E.
they add to the damping constant and decrease the dependence of
the frequency response characteristic upon the output impedance^
of the recording amplifier.
The response-frequency characteristic of the completed valve
when connected directly to the recording amplifier without any fur-
ther equalization is given in Fig. 5. This response has a broad maxi-
mum in the neighborhood of 8000 cps and is easily equalized with
electrical networks.
FIG. 6. Photograph of assembled valve.
The light-source is focused directly in the ribbon plane by a con-
densing lens mounted in one pole-piece of the valve, and a micro-
scope objective focusing the ribbons directly on the film is mounted
in the other pole-piece. These lenses, as well as the ribbons of all
valves, are adjusted in a common fixture. The completed valves
after adjustment in this fixture are nearly enough alike, optically
and mechanically, so that they can be mounted on the fixed brackets
of the recording machine without the need of further adjustment
for position or focus of the recording image on the film. Condensing
and objective lenses in the pole-pieces serve also to close the valve
structure against dirt and dust, and the interior may be hermetically
Oct., 1941] THE LIGHT- VALVE 405
sealed if so desired. A photograph of one of the valves is shown in
Fig. 6.
A lot of ten such valves has been used for a considerable
amount of experimental recording over an extended period and in
various parts of the country. Operated from a feedback amplifier
acting both as a driver and a voltage limiter, no difficulty has been
experienced with broken ribbons, and no readjustment of either rib-
bon tuning or ribbon spacing was required during this time. This
valve has proved to be rugged in all experimental work where reason-
able precautions were observed and is free of the instability frequently
characterizing compact vibrating structures.
The writers are indebted to Mr. L. A. Elmer for a number of im-
portant suggestions in connection with the design of this valve.
DISCUSSION
MR. KELLOGG: What is the magnification in the horizontal plane?
MR. BIDDULPH: The objective in this valve operates at a lateral magnification
of 10 X in the horizontal plane to form an unmodulated track 0.040 inch wide on
the film.
MR. STEPHENS: Is the valve equally well adapted for variable-area and vari-
able-density recording?
MR. BIDDULPH: Variable-area recording methods were employed in our
stereophonic work; hence the light-valve was designed to operate in conjunc-
tion with an optical system producing a satisfactory variable-area trace. To
employ this valve for variable-density work would require some modifications of
both valve geometry and optics.
INTERNALLY DAMPED ROLLERS*
E. C. WENTE AND A. H. MULLER**
Summary. — Special damping rollers, capable of damping oscillations of rotating
shafts without adding a steady load, were first devised by Prof. H. A . Rowland. These
rollers had either an annular channel along the periphery filled with a liquid, or a
wheel mounted loosely on a shaft coaxially fixed in an outer shell, the interspace being
filled with a liquid. The theory of the action of such rotters in reducing fluctuations
in the speed of rotation caused by disturbances from either the load or the driving side
is developed and the results are illustrated by graphs. A new form of rotter is de-
scribed in which liquid filling an annular channel within the shell of the roller is
coupled to the shell by a mechanical resistance.
A flywheel elastically connected to the shaft of a machine will
hunt, i. e., execute resonance oscillations as the result of any depar-
tures from steady-state conditions, unless the effective torque load
on the flywheel increases at an adequate rate with the speed of rota-
tion. In most machines, a small amount of hunting is not harmful.
In some cases, however, means must be provided for keeping the os-
cillations below undesired amplitudes. Special devices for prevent-
ing hunting are, for instance, commonly applied to synchronous
motors, in which the rotor has enough moment of inertia to behave
as a flywheel. These devices are usually of a kind that do not in any
way impede the steady rotation, i. e., they absorb no power when the
speed is constant, an important characteristic here and in some other
applications.
Prof. H. A. Rowland early made use of the viscosity of liquids for
damping rotating shafts in a way that did not produce an increase
in the steady load. His method was disclosed in U. S. Patents Nos.
691,667 and 713,497, relating to a printing telegraph — both of the
year 1902. Its particular purpose there was to reduce hunting in
synchronous motors. "Viscous damper" was the name given to the
device by Rowland.
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received March
15, 1941.
** Bell Telephone Laboratories, New York, N. Y.
406
•t>The Society is not responsible for statements by authors^
Oct., 1941]
INTERNALLY DAMPED ROLLERS
407
One form of this viscous damper is shown in Fig. 1, taken directly
from the patent specification. This consists of a shell, or casing,
having a hollow annular channel enclosing a liquid. When the shell
is revolving, the liquid is distributed by centrifugal action along the
outer boundary of the channel. When the speed is uniform, the
liquid and shell move together as a unit. If the speed of the shell
changes, the liquid is gradually brought back into step by viscous
shear in the liquid. This shear involves dissipation of energy and
consequent damping of oscillations of the rotating shell. The
amount of damping that can be obtained in this way depends upon
a number of factors, among which the viscosity and density of the
liquid are of first importance.
FIG. 1. Liquid form of Rowland "viscous damper."
Almost any value of resistance per unit length of channel may be
obtained by a proper choice of liquid and cross-sectional area. The
damping of an oscillating system, however, depends, not upon the
resistance alone, but upon the ratio of resistance to mass, and, in
this type of damper, anything that will increase the resistance will
also increase the mass reactance by an equal or greater amount.
The amount of damping that can be applied by this method is there-
fore restricted. Within limits, the resistance is proportional to the
square-root of the product of density and viscosity of the liquid.
Liquids of high viscosity generally have relatively low density, so
that, when such liquids are used, the damper will be relatively bulky
since a certain amount of inertia in the liquid is required against which
the mass of the casing may react. Rowland, therefore, while men-
408
E. C. WENTE AND A. H. MULLER
Lf. S. M. P. E.
tioning oil, preferred to use mercury in his damper, as this has much
greater density than any other normally liquid substance. Mercury
dampers of this general type have been used in certain forms of print-
ing telegraph for many years.
Another form of damping wheel was disclosed by Rowland, also
in U. S. Patent No. 713,497, and is here reproduced in Fig. 2. In
this form, a wheel is mounted freely on an axle, fixed within a shell,
coaxially with the main shaft. Liquid is placed in the shell which,
by centrifugal action, fills the peripheral clearance spaces between
the wheel and the shell when the shell rotates. Rowland^ comparing
this form of damper with that of Fig. 1, states, "The loose body has
the effect of making a light liquid, such as oil, act as a heavier liquid."
71
FIG. 2. Wheel form of Rowland "viscous damper."
This structure has a further advantage in that the dissipation of
energy for a given rate of annular displacement between the inner
and outer members is determined entirely by the viscosity of the
liquid, the clearances, and the effective shearing area, all of which
can be independently controlled.
The use of a viscous damper of the general form shown in Fig. 2 in
sound-film apparatus was first described by E. D. Cook.1 Cook de-
veloped the theory of operation of the device and gave the mechanical
circuit diagram for it when connected to a film-driven scanning roller
shaft. He showed that the system could not be made aperiodic un-
less the moment of inertia of the inner wheel was about eight times
as great as that of the shell.
The chief disadvantages of the solid over the liquid mass within
the shell of the roller are: the inner wheel must be supported on a
bearing which may not always be free, particularly when the oscilla-
Oct., 1941]
INTERNALLY DAMPED ROLLERS
409
tions are small; since, in operation, the shell and wheel may have any
angular position relative to each other, they must be balanced inde-
pendently and the geometrical form of the wheel and the inner sur-
face of the shell must be such that the liquid as distributed in the in-
terspace during rotation is also maintained in balance. These dif-
ficulties are avoided in dampers having a homogeneous liquid only as
the movable substance within the shell. There can then be no ques-
tion of static frictional forces, and, if a completely assembled damper
has once been dynamically balanced, it will not be thrown out of
balance by any angular shift of the liquid.
FIG. 3. Damped roller used in stereophonic film system.
The difficulty inherent in the device of Rowland, as shown in Fig.
1, namely, limitation in the control of the coupling resistance between
the liquid and shell, is overcome in the structure shown in Fig. 3.
Here, as in Fig. 1, there is a shell with a closed annular channel carry-
ing a liquid. The coupling between the liquid and the shell is, how-
ever, not determined by the frictional drag between the channel walls
and the liquid, but is controlled by a porous partition P placed trans-
versely across the channel so that the liquid can not move circum-
ferentially without forcing liquid through the pores of the partition.
This partition will act as a pure resistance* to the motion of the
* Throughout this paper, the term "impedance" is to be understood as "me-
chanical impedance," i. e., the ratio of force to velocity. When expressed in
complex form, the term "resistance" is to be understood as the real part and
"reactance" as the imaginary part of this ratio.
410 E. C. WENTE AND A. H. MULLER [J. S. M. P. E.
liquid relative to the casing, if certain conditions are fulfilled. One
of these is that the ratio of reactance to resistance for each pore be^
small at the resonance frequency, where energy dissipation is to be
effected. For example, if the pores are small and circular in section,
the resistance of each hole has a value equal to 8fj.l/r2 and a mass re-
actance of 4cop//3 per unit area,2 where ju is the viscosity and p the
density of the liquid, r the radius and / the length of the hole, and o>
is the angular frequency, r should, therefore, be small compared
with (6/-i/wp)1/2. If this condition is satisfied, the resistance of the
whole partition will be equal to al A 2/a, where A is equar to the sec-
tional area of the channel, a is the total area of all the pores in the
partition, and a is the resistance of the pores per unit area per unit
length. The coupling resistance between the shell and the liquid
can, therefore, be given almost any desired value for any liquid by a
proper choice of size and number of pores.
i
''^- • i
4 i
i i
FIG. 4. Mechanical circuit diagram of damped
roller system.
The mechanical circuit diagram for a system in which a roller of
this kind is driven through an elastically yielding coupler is shown in
Fig. 4. For the particular case where the roller is driven by a re-
silient film running over a scanning drum rigidly connected to the
shell of the roller, C is the compliance of the portion of film connect-
ing the driving sprocket and the scanning drum. M is the mass of
the liquid, m the mass of the shell and other parts that may be rigidly
connected to it, and R the resistance of the partition — each divided
by the square of the ratio of its own radius of gyration to the radius
of the scanning drum. This circuit is of the same form as that given
by Cook for the film-driven roller having a solid internal member.
In this system, there are two principal types of disturbances which
can produce variations in the speed of rotation of the scanning drum
to which the shell of the roller is assumed to be rigidly connected.
These may be designated as driving-side disturbances and load-side
disturbances. The former are variations in the speed of film travel
Oct., 1941] INTERNALLY DAMPED ROLLERS 411
at the driving sprocket and are represented in the circuit diagram by
V. The latter are such disturbances as variations in the friction of
the sound roller bearings and in the film compliance C. Their com-
bined effect is equivalent to that of an alternating force acting at the
periphery of the scanning drum and can be represented by the force
F at the point indicated in the diagram. The variation in the speed
of the roller resulting from either type of disturbance is represented
in the diagram by v. An analysis of the circuit should, therefore,
give expressions for v/V and v/F as a function of frequency and the
physical constants of the system. Our interest is, however, restricted
to the absolute value of these ratios.
We readily derive the relation
jr ?">«> M /"* 9/ i i >r\ To i I >ro o/ -i n xi\ o \ /
If now we set
Q -
Equation 1 reduces to
1 ~l~ Q x fn\
(1 - x)2 + Q2 x (1 - kx)2
When x is small, this expression approaches unity, and when x is
large it approaches l/k2x2. Intermediately, it passes through a
maximum, the magnitude and frequency of which depend upon Q
and k. In the region of this maximum and at all lower values of x,
it is greater than 1 . This means that, in this range, the system does
not attenuate, but actually amplifies the speed flutter that may be
present at the driving sprocket. It is an advantage, therefore, to
make the resonance frequency co0 as low as possible and to keep the
peak value of this amplification to a low value, so far as this can be
accomplished without impairing the effectiveness of the filtering ac-
tion at other frequencies. In order to find the lowest peak that it
is possible to obtain for a given value of k by adjustment of Q, we set
= 0 and
= 0
d<22
and solve the two simultaneous equations so obtained. When this
is done, we find
412
E. C. WENTE AND A. H. MULLER
[J. S. M. P. E.
Qz =
(4)
When these values are substituted in equation 2, we obtain, for the
height of this minimum peak
,\,
vl
In deriving corresponding expressions for the load-side disturbances,
let us set F/v equal to z, and (M -\- m) coo equal to ZQ, the impedance
that would obtain for the load-side disturbances at the angular fre-
quency w0 if the liquid and the casing were rigidly interlocked and
the compliance C were very large. 20/2 is then the ratio of the velo-
city of the shell to its velocity at the frequency co0, if the same force
were a'cting on the combined liquid and shell masses alone. From
the circuit diagram, we see that
- kx)
(6}
This function also passes through a maximum. In order to find the
lowest value that the peak can have for a given value of k, we pro-
ceed as before and set
2 = 0 and A
dx
0
Solving the two equations so obtained, we have
and
1 + k 3 + k
2 3k + 1
Substituting these values in equation 6, we get for the minimum
possible peak value at a given value of k
_2(l + *)
(1 -
Equations 3, 4, 7, and 8 show that if k, the ratio of the mass of the
shell to the total mass of the roller, is kept constant, and Q is adjusted
so that the peak value of flutter is a minimum, the peak frequency
will be the same whether the adjustment is made for load-side or for
Oct., 1941]
INTERNALLY DAMPED ROLLERS
413
driving-side disturbances, but that the values of Q required to reduce
these two peak values to a minimum are different. These values of
Q differ more and more as k is decreased. This statement gives one
U W
•*
•81
cn a
El
3s
good reason why k should not be made too low. However, we can
not draw the general conclusion that a sound-film machine, equipped
with a roller having a small value of k, is necessarily going to exhibit
a relatively large amount of flutter in the resonance region for one or
414
E. C. WENTE AND A. H. MULLER
[J. S. M. P. E.
the other of the two types of disturbances. This would depend
upon a number of other factors — primarily the frequency of resonance r
and the magnitude of the disturbances. It can safely be said that if
all other factors remain the same, an increase in the liquid mass
always leads to an improved flutter vs. frequency characteristic.
Aside from cost, the disadvantages of such an increase are that the
Oct., 1941] INTERNALLY DAMPED ROLLERS 415
load on the bearing carrying the roller will be greater and that either
a greater starting torque or a longer starting time will be required.
An increase in bearing load means an increase in the load-side dis-
turbances and a decrease in the compliance C of the driving film
link. Both effects operate to give an increase in flutter. In an anal-
ysis of the effect of k on performance, it is more rational to proceed
under the assumption that the total mass M -+- m of the roller re-
mains fixed. Then the bearing load, the angular resonance fre-
quency co0, and the starting conditions will all remain the same. In
the discussion immediately to follow, this condition will be assumed.
From equations 2 and 6, we see that when x is large
1
and
J_
kzx
We therefore conclude that for greatest protection against high-
frequency flutter, k should be as large as possible. On the other
hand, equations 5 and 9 show that where the main interest lies in
<eeping the flutter small in the resonance region, k should have the
owest possible value.
In order to present a better picture of these various relations for a
vider frequency range, a series of plotted curves are reproduced in
?igs. 5 to 8. In these figures, Qc is set equal to [(1 + k)/2k] 1/2, the
ritical value of Q which will reduce the peak value of the flutter re-
ulting from driving-side disturbances to a minimum.
On comparing the curves for k = l/2 with those for which k = 1/s,
ve see that both for the load-side and for the driving-side distur-
ances, there is a marked improvement in the peak flutter in going
o the smaller value of k. Unless co0 can be made so low in frequency
lat no flutter is easily noticeable in this frequency region, it will
enerally be advisable to go to the smaller ratio — even though the
ttenuation at the higher frequencies is only half as great. In going
still lower values of k, a high price has to be paid in high-frequency
ttenuation for a little gain in low-frequency performance. The
reatest possible gain in the resonance region would be a reduction
the peak value of the flutter by a factor of 2. But this gain could
ot be obtained for both types of disturbances since the peak fre-
uency separation is increased as k is decreased. Referring to the
gures, we see that as a rule Q is advantageously adjusted to a value
>wer than Qc. For instance, if Q is made equal to Qc/\/2, when k is
jual to 2, the peak flutter produced by the driving-side disturbances
416
E. C. WENTE AND A. H. MULLER [j. s. M. p. E.
is raised about 10 per cent, but the peak occurs at a lower frequency,
where a given amount of flutter is less audible, and, at slightly higher
frequencies, the attenuation is greater. For example, at co = 2coo,
it is 30 per cent greater. Fig. 6(5) shows that, as regards flutter
caused by load-side disturbances, the larger value of R gives im-
proved performance in every respect.
FIG. 9. Experimental performance curves
of | damped roller used in stereophonic film
system.
The curves show that relatively little is gained in respect to the
height of the peaks in going to values of k less than 1/s, and much is
lost in attenuation at the higher frequencies, particularly for the
load-side disturbances. A general statement about an optimal value
of k with reference to flutter attenuation can not be made, as in
sound-film apparatus this would depend upon the relative audibility
of flutter in the various frequency regions, the resonance frequency
of the system, and the magnitudes and frequencies of the distur-
bances on both the driving and the load sides.
Oct., 1941] INTERNALLY DAMPED ROLLERS 417
Fig. 9 shows the characteristics of a roller of one of the machines
used in making the stereophonic orchestra records.* The roller was
hung by a wire with the wire and roller coaxial, to form a torsional
pendulum. Means were provided for twisting the top of the wire
sinusoidally at a controllable frequency. One curve shows the am-
plitude of oscillation of the roller so supported when the amplitude
of the "disturbance" at the point of support was held constant and
the frequency varied. This curve was taken at room temperature.
The other curve was obtained in the same way, but at a temperature
of 56° C. The effect of raising the temperature is a decrease in the
viscosity of the liquid and a consequent lowering of the coupling re-
sistance. From the coordinates of the point of intersection of these
two curves, k and Q may be determined by expressions 3 and 4.
These measurements showed that the effective masses of the liquid
and shell were in the ratio of 1.59, whereas the roller was designed to
have a value of 2 for this ratio. When subsequently better account
was taken of the amount of liquid that effectively adheres to the walls
of the shell at the resonance frequency, excellent agreement was
found between the computed and empirical values.
REFERENCES
1 COOK, E. D.: "The Technical Aspects of the High-Fidelity Reproducer,"
/. Soc. Mot. Pict. Eng., XXV (Oct., 1935), p. 289.
2 CRANDALL, I. B.: "Theory of Vibrating Systems and Sound," D. Van Nos-
trand Co. (New York) p,235._
* The writers are indebted to Mr. L. A. Elmer for these measurements.
A NON-CINCHING FILM REWIND MACHINE*
L. A. ELMER**
Summary. — Cinching, or the sliding between layers of film within a reel, produces
scratches and surface abrasions which increase the film noise level. Cinching is]
more likely to occur in rewinding than anywhere else in the normal usage of sound-film.
At the beginning of rewinding, when the supply reel is full and the take-up reel is'i
empty, a small amount of torque is nseded for rotating the take-up reel. Under this\
condition the film will be wound rather loosely. When the supply reel is nearly\
empty, relatively high film tension is required to produce a given torque on the supply]
reel. The torque to be applied to the take-up reel will then be high, on account on
both the high film tension and the large radius arm of the film spiral on the reel. Thisl
high torque is almost certain to cause cinching in the loosely wound bottom portion 0/1
the reel. The conditions to be satisfied if cinching is to be avoided are analyzed. A jj
power-driven rewind is described which meets these requirements. The film tension is\
controlled by the weight of the film on the supply reel at all times during the rewind.-!
It is generally known that a sound-film record becomes more noisy ;
with successive play ings. Some of this increased noise is the result !
of dust and grease which accumulate on the film while it passes
through the projector. More of it is caused by scratches and abra-
sions on the film. In modern projectors and sound-heads and other
film-handling machinery, that area of the film which carries the
sound record never comes into contact with anything which could
mar its surface except the adjacent layers of film on the supply andji
take-up reels. The sound record areas of film, therefore, should '.
suffer very little damage during projection if sliding of the layers of f-
film within the reels is prevented. In every operation involving the ;
handling of stereophonic films, care was taken that this sliding or,
cinching did not occur.
A rewind machine was needed that could handle a 2000-f t reel of
film rapidly and protect the film from cinching and exposure to even
small amounts of dust. A commercial automatic rewind machine j
was found where the film was wound directly from the upper supply j
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received March
14, 1941.
** Bell Telephone Laboratories, New York, N. Y.
418
"£ The Society is not responsible for statements by authors &
FILM REWIND MACHINE
419
reel to the lower take-up reel in a closed cabinet. The supply reel,
however, supplied a constant torque and this would cause the film
to be wound on the take-up reel at the start with a comparatively
low tension and at the end with about three times the tension. Such a
condition is conducive to serious cinching, especially if the film on the
supply reel spindle is on a 2-inch diameter core instead of the usual 5-
inch diameter reel core. To overcome this, a device was designed
that provided a supply reel torque con-
trolled by the amount of film on that reel.
A diagrammatic sketch of the machine
is shown in Fig. 1. If the film is wound
upon the take-up reel with such a tension
as to give a constant torque to the take-up
reel, it obviously can not cinch on this reel
in the rewind process. It should be wound
with a certain minimum tension at the
outside of the take-up reel so it can be
handled or threaded in a machine with-
out danger of cinching. If this minimum
tension was 250 grams for a 15-inch reel
with a 5-inch diameter core, the maximum
:ension at the start of the reel would be
50 grams. With the machine of Fig. 1,
his tension must be supplied by the feed
eel, and this would cause cinching to
ccur on the supply reel at the start of
he rewinding, unless the supply reel had
)een wound very tightly. Such a process would not be a convenient
me.
At times it was necessary to transfer the film from a 2-inch core
o a reel with a 5-inch diameter hub or vice versa. To fit these many
onditions, it was decided to make the initial and final winding
ensions the same, so that a supply reel would have the same initial
mil as it had final pull when it was in the take-up position. These
ensions were set at about 1/2 pound.
The supply reel is mounted on a shaft that is carried in the end
)f a lever pivoted on ball bearings. A friction drum, keyed to the
haft and supply reel, rotates with a loose fit in a ring lined with a
)rake material fixed in position and held against rotation. At the
ther end of the lever a means is provided for attaching weights
FIG. 1. Diagram of re-
wind machine and forces
involved (Case 1).
420
L. A. ELMER
[J. S. M. P. Ei
which partially counterbalance the weight of the film, reel or ph
friction drum. etc. The friction drag of the supply reel is caused
the weight of the film, reel, lever, and film tension pressing the
against the brake shoe. The characteristics will be determine
for such a rewinder when the film is fed (1) downward from the supph
reel, (2) upward, and (3) horizontally. In the last two cases, th<
film would be fed over suitable rollers not shown in Fig. 1.
BRAKE DRUM
FIG. 2.
Diagram of preferred arrangement of
rewind machine (Case 2).
The following notation will be used :
L = the desired load on the brake shoe due to the weight of the film, reel
lever, and pull of the film (latter may be positive, negative, or zero)
n — coefficient of friction between drum and brake shoe.
P = tension in the film between reels at any instant.
Po = pull of the film when the supply reel is nearly empty.
Pi = pull of the film when the supply reel is nearly full.
W = weight of the empty reel (or plates and core), drum, and lever reaction
Wo = the portion of W required to give the desired film tension.
w = weight of the film on the supply reel at any instant.
wi = weight of 2000 feet of film = 8.44 pounds.
= radius of friction drum.
= radius to outside of supply reel film at any instant.
= radius to outside of take-up reel film at any instant.
r
R
Rt
radius of supply reel core.
Ri = radius to outside layer of supply reel film when full.
Li/ L* = lever ratio (reel arm/weight arm).
M — weight to be hung on lever to give proper film tension.
Case 1. Film Pulled Downward from Supply Reel. — By taking
loments about the reel axis pLr = PR and since L = Wo + w + P,
)ct, 1941]
Ro
FILM REWIND MACHINE
421
R-pr
W|(R0-/ur)
MINIMUM P
IN POUNDS
0.3/57
R IN
NCHES
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
RADIUS OF FILM ROLL IN INCHES
FIG. 3. Chart of film tension for Case 1, downward pull.
ie moments equation becomes nr(Wo + w -f- P) = PR, which re-
uces to
R - fir
Since it is desired to have P the same for a full or empty reel
422 L. A. ELMER [J. S. M. P. E.
Solving for the necessary pressure on the brake shoe
;
(5)
#1 - Ro
The weight to be hung on the other end of the lever is
M = (W - Wofe (4)
W0:
RI-RO
§0.54
£0.50
5
MINIMUM P
IN POUNDS
218
R IN
INCHES
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
RADIUS OF FILM ROLL IN INCHES
FIG. 4. Chart of film tension for Case 2, upward pull.
Case 2. Film Pulled Upward from the Supply Reel. — In this case
in the denominator of equation 1 has a positive sign and
w)
R +
(5)
and
Oct., 1941]
FILM REWIND MACHINE
423
and the weight to be used as a counterbalance will be found by equa-
tion 4 with the new value of WQ used (see Fig. 2).
Case 3. Film Pulled Horizontally from the Supply Reel. — In this
case L = WQ + w and as before pLr = PR so
(7)
R
Mr
R
W, R0
7.5 7.0 65 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
md
ind
FIG. 5. Chart of film tension for Case 3, horizontal pull.
P =
or Case 3 with a horizontal pull, W0 may be calculated directly
rom the known factors in equation 9. Placing this in equation 8, r
424 L. A. ELMER Q. S. M. P. E
may be obtained from the value of P that was chosen as desirable
This value of r will give a film tension, P, between the values fron
corresponding curves of Figs. 3 and 4. If P is to be l/z pound, ;'
is !3/8 inches, assuming a coefficient of friction of 0.2.
For Cases 1 and 2, the solution is slightly more complicated as
size of the drum radius, r, must be assumed. If the r found for G
is used for Cases 1 and 2, the film tension will be very nearly eqi
to that given by equation S and will be the same for the start of ;
reel as for the end of a reel. The weight of the film on the suppl)
reel is at any instant.
1 Ri2 - R0
Placing this in 1
R - »r
A similar formula is obtained for Case 2:
-p _
R
If it is desired to find the maximum departure of the film tensior
from a constant value, equation 11 may be differentiated with respeci
to R and setting
R will be a minimum at
R = »r + vV'2 + £1*0
by substituting the value of WQ given in equation 3. For example
r may be chosen as 1.375 inches and let ju = 0.2, R\ = 7.15 inches anc
RQ = 2.5 inches. Then P is a minimum at R = 4.186 inches and P =
0.433 Ib at this radius. At the start and end of the reel, P = 0.5 Ib
If the film is pulled upward from the supply reel, the pull wiT
reach a minimum in a similar manner. The pull is a minimum at
R = -
Using the same dimensions as before, P is a minimum for Case t
at R = 4.264 inches. W0 is found from equation 6 to be 5.035 Ibs
t.. 1941]
FILM REWIND MACHINE
425
,nd the pull P = 0.441 Ib at this minimum point. At the begin-
ing and end of the rewinding P is again 0.5 Ib.
I Figs. 3 and 4 show curves for the film pull plotted against radius
ro outside turn on the supply reel for Cases 1 and 2, respectively,
or various drum radii. Fig. 5 is a similar chart for Case 3, the condi-
ion where the film is pulled horizontally from the supply reel. The
ilm pull for the three directions has been plotted on a combined
•hart, Fig. 6, for comparison.
? 0.50
7.0 6.5 60 5.5 5.0 4.5 4.0 3.5 3.0 2.5
RADIUS OF FILM ROLL IN INCHES
FIG. 6. Comparison of film tensions for Cases 1, 2, and 3.
The weight M to be hung on the other end of the lever depends
ipon the size and weight of reel or plates and core to be used, as
hese determine the W in equation 4 and the RI and RQ in equations 3,
>', and 9.
Case 2 has an advantage over the other designs in that the fric-
ion can never become so great as to break the film. A brake-band
s shown in Fig. 2 is preferable to a brake shoe, as it can be made to
ive a more constant drag. The rocking lever to which the belt loop
s fastened operates a contact switch S if the supply reel is rotating
n either direction. If the supply reel is stationary, a pair of springs,
ot shown, return the lever to its mid-position, opening the contact
426 L. A. ELMER
switch. The key shown in parallel with switch 5 is a jogging switch
for starting the rewind.
DISCUSSION
MR. CRABTREE: Does this machine insure the winding of a tight roll? Even
though the roll is wound at a constant tension, unless it is tight, cinches and
scratches will result from subsequent handling.
MR. ELMER: The machine insures the winding of a tight roll with the proper
weight hanging on the lever arm. Cinches and scratches will result from handling
unless the roll is wound under sufficient minimum tension. The machine is
designed to allow sufficient tension at a substantially constant and predeter-
mined value.
CURRENT LITERATURE OF INTEREST TO THE MOTION PICTURE
ENGINEER
The editors present for convenient reference a list of articles dealing with subjects
cognate to motion picture engineering published in a number of selected journals.
Photo static copies may be obtained from the Library of Congress, Washington, D. C.,
or from the New York Public Library, New York, N. Y. Micro copies of articles
in magazines that are available may be obtained from the Bibliofilm Service, Depart-
ment of Agriculture, Washington, D. C., at prevailing rates.
American Cinematographer
22 (August, 1941), No. 8
Let's Design Pictures for the Camera (pp. 366-367, 394) G. WILES
A Versatile New Lighting-Control Switchboard (pp. 368,
396)
Canada's War Movies (pp. 370, 396-397)
Filming Underwater Movies from the "Hole" in the Water
(pp. 371, 397-398)
Electronics
14 (August, 1941), No. 8
Photographic Analysis of Television Images (pp. 24-29)
Institute of Radio Engineers, Proceedings
29 (July, 1941), No. 7
The Synthetic Production and Control of Acoustic Phe-
nomena by a Magnetic Recording System (pp. 365-371)
International Projectionist
16 (May, 1941), No. 5
First Commercial Television Theater in America Is
Rialto, New York City (pp. 7-9)
Screen Brightness, Theater Design, Power Survey on
SMPE Agenda (pp. 11-15)
A Report of the Theater Engineering Committee of the
SMPE
A Unique Film Scanner for Testing Television Transmis-
sion Images (pp. 18-19)
16 (June, 1941), No. 6
Lubricants and Their Applications (pp. 7-8, 10)
"Increased Range" System Promised to Revolutionize
Photography (pp. 11-12)
The Intermittent Carbon Arc (pp. 13-18)
H. NYE AND
M. MORAN
C. W. HERBERT
L. KNECHTEL
D. G. FINK
S. K. WOLF
L. CHADBOURNE
W. A. KNOOP
L. CHADBOURNE
W. KAEMPFFERT
F. T. BOWDITCH,
R. B. DULL, AND
H. G. MACPHER-
SON
427
428 CURRENT LITERATURE
RCA Theater-Television Technical Data (pp. 19-21)
Motion Picture Herald (Better Theaters Section)
144 (August 23, 1941), No. 8
Determining the Picture Size and Screen Light Required
(pp. 24, 26-27)
Ventilating Projection Rooms and Arc Lamps (pp. 32-35,
37)
I. G. MALOFF AND
W. A. TOLSON
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FIFTIETH SEMI-ANNUAL CONVENTION
OF THE
SOCIETY OF MOTION PICTURE ENGINEERS
HOTEL PENNSYLVANIA, NEW YORK. N. Y.
OCTOBER 20TH-23RD, INCLUSIVE
OFFICERS AND COMMITTEES IN CHARGE
Program and Facilities
E. HUSE, President
E. A. WILLIFORD, Past-President
H. GRIFFIN, Executive Vice-President
W. C. KUNZMANN, Convention Vice-President
A. C. DOWNES, Editorial Vice-President
R. O. STROCK, Chairman, Local Arrangements
S. HARRIS, Chairman, Papers Committee
J. HABER, Chairman, Publicity Committee
J. FRANK, JR., Chairman, Membership Committee
H. F. HEIDEGGER, Chairman, Convention Projection Committee
Reception and Local Arrangements
P, J. LARSEN
F. E. CAHILL, JR.
H. RUBIN
E. I. SPONABLE
P. C. GOLDMARK
W. H. OFFENHAUSER, JR.
A. S. DICKINSON
W. E. GREEN
R. O. WALKER
R. O. STROCK, Chairman
T. E. SHEA
J. A. HAMMOND
O. F. NEU
V. B. SEASE
H. E. WHITE
L. W. DAVEE
L. A. BONN
J. H. SPRAY
J. J. FINN
A. N. GOLDSMITH
J. A. MAURER
L. B. ISAAC
E. W. KELLOGG
M. HOBART
J. A. NORLING
H. B. CUTHBERTSON
J. H. KURLANDBR
C. F. HORSTMAN
E. R. GEIB
P. SLEEMAN
E. S. SEELEY
C. Ross
P. D. RIES
Registration and Information
W. C. KUNZMANN, Chairman
J. FRANK, JR.
Hotel and Transportation
G. FRIEDL, JR., Chairman
R. B. AUSTRIAN
R. F. MITCHELL
P. A. McGuiRB
M. W. PALMER
F. HOHMEISTER
H. MCLEAN
F. C. SCHMID
F. M. HALL
J. A. SCHEICK
429
FALL CONVENTION
[J. S. M. P. E.
H. A. GILBERT
G. A. CHAMBERS
D. E. HYNDMAN
L. A. BONN
E. G. HINES
A. S. DICKINSON
Publicity Committee
]. HABER, Cfiairman
P. SLEEMAN
S. HARRIS
C. R. KEITH
Banquet
O. F. NEU, Chairman
R. O. STROCK
J. C. BURNETT
J. A. SPRAY
J. A. NORLING
W. H. OFFENHAUSER, JR. M. HOBART
W. R. GREENE
H. MCLEAN
P. J. LARSEN
E. C. WENTE
A. GOODMAN
M. R. BOYER
J. A. HAMMOND
MRS. D. E. HYNDMAN
MRS. E. I. SPONABLE
MRS. E. S. SEELEY
MRS. A. S. DICKINSON
Ladies' Reception Committee
MRS. R. O. STROCK, Hostess
MRS. O. F. NEU, Hostess
MRS. H. GRIFFIN
MRS. P. J. LARSEN
MRS. J. A. HAMMOND
MRS. G. FRIEDL, JR.
MRS. E. A. WILLIFORD
MRS. J. FRANK, JR.
MRS. H. E. WHITE
MRS. F. C. SCHMID
Convention Projection
H. F. HEIDEGGER, Chairman
T. H. CARPENTER
P. D. RIES
J. J. HOPKINS
W. W. HENNESSY
L. W. DAVEE
F. H. RICHARDSON
L. B. ISAAC
A. L. RAVEN
G. E. EDWARDS
J. K. ELDERKIN
Officers and Members of New York Projectionists Local No. 306
J. J. SEFING
H. RUBIN
F. E. CAHILL, JR.
C. F. HORSTMAN
R. O. WALKER
Hotel Reservations and Rates
Reservations. — Early in September, room-reservation cards will be mailed to
members of the Society. These cards should be returned as promptly as possible
in order to be assured of satisfactory accommodations. Reservations are subject
to cancellation if it is later found impossible to attend the Convention.
Hotel Rates. — Special per diem rates have been guaranteed by the Hotel Penn-
sylvania to SMPE delegates and their guests. These rates, European plan, will
be as follows:
Room for one person
Room for two persons, double bed
Room for two persons, twin beds
Parlor suites: living room, bedroom, and bath for
one or two persons
$3. 50 to $8.00
$5. 00 to $8.00
$6. 00 to $10. 00
$12.00, $14.00, and
$15.00
Oct., 1941] FALL CONVENTION 431
Parking.— Parking accommodations will be available to those motoring to the
Convention at the Hotel fireproof garage, at the rate of $1.25 for 24 hours, and
$1.00 for 12 hours, including pick-up and delivery at the door of the Hotel.
Convention Registration. — The registration desk will be located on the 18th
floor of the Hotel at the entrance of the Satte Moderne where the technical sessions
will be held. All members and guests attending the Convention are expected to
register and receive their badges and identification cards required for admission
to all the sessions of the Convention, as well as to several de luxe motion picture
theaters in the vicinity of the Hotel.
Technical Sessions
The technical sessions of the Convention will be held in the Salle Moderne on
the 18th floor of the Hotel Pennsylvania. The Papers Committee plans to have
a very attractive program of papers and presentations, the details of which will
be published in a later issue of the JOURNAL.
Fiftieth Semi- Annual Banquet and Informal Get-Together Luncheon
The usual Informal Get-Together Luncheon of the Convention will be held in
the Roof Garden of the Hotel on Monday, October 20th.
On Wednesday evening, October 22nd, will be held the Silver Anniversary
Jubilee and Fiftieth Semi-Annual Banquet at the Hotel Pennsylvania. The
annual presentations of the SMPE Progress Medal and the SMPE Journal
Award will be made and officers-elect for 1942 will be introduced. The proceed-
ings will conclude with entertainment and dancing.
En tertainmen t
Motion Pictures. — At the time of registering, passes will be issued to the dele-
gates of the Convention admitting them to several de luxe motion picture theaters
in the vicinity of the Hotel. The names of the theaters will be announced later.
Golf. — Golfing privileges at country clubs in the New York area may be ar-
ranged at the Convention headquarters. In the Lobby of the Hotel Pennsylvania
will be a General Information Desk where information may be obtained regarding
transportation to various points of interest.
Miscellaneous. — Many entertainment attractions are available in New York to
the out-of-town visitor, information concerning which may be obtained at the
General Information Desk in the Lobby of the Hotel. Other details of the enter-
tainment program of the Convention will be announced in a later issue of the
JOURNAL.
Ladies' Program
A specially attractive program for the ladies attending the Convention is be-
ing arranged by Mrs. O. F. Neu and Mrs. R. O. Strock, Hostesses, and the Ladies'
Committee. A suite will be provided in the Hotel where the ladies will register
and meet for the various events upon their program. Further details will be pub-
lished in a succeeding issue of the JOURNAL.
432 FALL CONVENTION
PROGRAM
Monday, October 20th
9:00 a. m. Hotel Roof; Registration.
10:00 a. m. Salle Moderne; Technical session.
12:30 p. m. Roof Garden; Informal Get-Together Luncheon for members, their
families, and guests. Brief addresses by prominent members of
the industry.
2: 00 p.m. Salle Moderne; Technical session.
8:00 p. m. Salle Moderne; Technical session.
Tuesday, October 21st
9: 00 a.m. Hotel Roof; Registration.
9:30 a. m. Salle Moderne; Technical session.
2: 00 p.m. Salle Moderne; Technical session.
Open evening.
Wednesday, October 22nd
9:00 a. m. Hotel Roof; Registration.
9:30 a. m. Salle Moderne; Technical and Business session.
Open afternoon.
8:30 p. m. Fiftieth Semi-Annual Banquet and Dance.
Introduction of officers-elect for 1942.
Presentation of the SMPE Progress Medal.
Presentation of the SMPE Journal Award.
Entertainment and dancing.
Thursday, October 23rd
10:00 a. m. Salle Moderne; Technical session.
2: 00 p.m. Salle Moderne; Technical and business session.
Adjournment
W. C. KUNZMANN,
Convention Vice- President
ABSTRACTS OF PAPERS
FOR THE
FIFTIETH SEMI-ANNUAL CONVENTION
HOTEL PENNSYLVANIA
NEW YORK, N. Y.
OCTOBER 20-23, 1941
The Papers Committee submits for the consideration of the membership the follow-
ing abstracts of papers to be presented at the Fall Convention. It is hoped that the
publication of these abstracts will encourage attendance at the meeting and facilitate
discussion. The papers presented at Conventions constitute the bulk of the material
published in the Journal. The abstracts may therefore be used as convenient refer-
ence until the papers are published.
A. C. DOWNES, Editorial Vice-President
S. HARRIS, Chairman, Papers Committee
G. A. CHAMBERS, Chairman, West Coast Papers Committee
F. T. BOWDITCH C. R. KEITH W. H. OFFENHAUSER
F. L. EICH E. W. KELLOGG R. R. SCOVILLE
R. E. FARNHAM P. J. LARSEN S. P. SOLOW
J. L. FORREST G. E. MATTHEWS W. V. WOLFE
Dynamic Screen — a Speculation; ROBERT W. RUSSELL, Training Film Produc-
tion Laboratory, Ft. Monmouth, N. J.
Within its present limits, various phases of the motion picture have been
brought close to technical exhaustion and artistic satisfaction. Competition
with color television and other forms of entertainment require that motion pic-
tures come forth with another "sudden impact of novelty" similar to its other
great discoveries: screen personalities, story, montage, sound, color. One great
frontier remains for film-makers and engineers : the selective delimitation of the
screen. The familiar rectangular screen shape forces the motion picture to ac-
complish everything within a rigid opening like a window. Feeble attempts have
been made to vary this arbitrary shape, usually by trying to substitute other
arbitrary shapes: the "Grandeur" wide-film, the square frame, the circular
"iris-in," camera matte shapes. Unprogressive justification for the present
rectangle is in static painters' composition, in commercial standardization, and
in a false claim of relationship to the "Golden Section" rectangle. It is possible
to speculate on a new type of motion picture production using the unlimited,
unframed "Dynamic Screen," permitting another "sudden impact of novelty" to
meet the increasing competition of similar medium of entertainment. Great new
frontiers of cinematic effect are opened up by making the screen area the entire
433
434 ABSTRACTS OF CONVENTION PAPERS [J. S. M. P. E.
proscenium wall, by employing a projector lens that will throw the 35-mm frame
to cover this whole wall as a potential, and by selectively limiting the projected
image to smaller pictures within this potential, using peculiarly appropriate or
eccentric delimitations in an overall montage of boundaries. Such a production
can be imagined, described, and even accomplished with present-day equipment.
Mobile Television Equipment; R. L. CAMPBELL, R. E. KESSLER, R. E. RUTH-
ERFORD, AND K. V. LANDSBERG, Allen B. Du Mont Laboratories, Passaic, N. J.
While portability is a necessary requirement for outside pick-up equipment,
several advantages result when portability is carried into the studio. To equip
a studio of adequate size with fixed equipment for operation of several cameras
involves considerable time and expenditure. However, with portable studio
equipment, the entire equipment installation can be located to suit studio needs,
as well as moved to different studios or outside locations.
The dolly type of equipment is described in some detail, and systems for pro-
gram control are discussed. Some of the design features discussed are port-
ability and flexible synchronizing equipment; electronic view finders; oscillo-
scope monitors; and other operating facilities.
Production and Release Applications of Fine-Grain Films for Variable-Density
Sound Recovery; C. R. DAILY, Paramount Pictures, Inc., Hollywood, Calif.
Fine-grain film materials have supplanted the normal positive type emul-
sions for all variable-density sound-recording and printing operations. The
sound-quality improvement realized by the reduction in noise and distortion
is now available for all sound operations, including release prints. The paper
describes a number of problems encountered and solved in the commercial ap-
plication of such films for sound recording, including factors affecting the choice of
negative and print materials, noise, distortion, sensitometric characteristics,
recorder lamp supplies, and noise problems on stages.
Laboratory Modification and Procedure in Connection with Fine-Grain Release
Printing; J. R. WILKINSON AND F. L. EICH, Paramount Pictures, Inc., Holly-
wood, Calif.
While fine-grain emulsions have been in general use for specialty purposes for
three years or more, their use as a medium for release prints is comparatively
recent. This paper discusses the necessary modifications required in a release
print laboratory to produce satisfactory fine-grain release prints. The discussion
covers the light-source, power supply, light-testing, and printing equipment
Observations noted while processing the first thirty million feet of release prints
are made relative to the behavior and characteristics of the film.
A Note on the Processing of Eastman 1302 Fine-Grain Release Positive in
Hollywood; V. C. SHANER, Eastman Kodak Co., Hollywood, Calif.
A brief historical resume is given of a series of fine-grain films that have been
put upon the market during the past four years. This series of fine-grain
films culminated with the acceptance of Eastman 1302 fine-grain release positive
at one Hollywood laboratory to the exclusion of regular positive of the 1301 type
Oct., 1941] ABSTRACTS OF CONVENTION PAPERS 435
for release printing. Experimental data are presented to show the comparative
sensitometric characteristics of fine-grain positive 1302 and regular positive 1301
at various pH values and potassium bromide concentrations typical of Hollywood
positive developers. A basic positive developer formula derived from chemical
analyses of every release positive developer in Hollywood was used in the experi-
mental work. Some practical facts are discussed, based upon the experiences ob-
tained from the initial use of the fine-grain film in Hollywood.
A Frequency-Modulated Control Track for Movietone Prints; J. G. FRAYNE
AND F. P. HERRNFELD, Electric Research Products, Inc., Hollywood, Calif.
A 5-mil frequency-modulated track located between sound and picture areas
is proposed to control reproduction in the theater from one or more sound-tracks.
A variation of approximately one octave in the control frequency provides a 30-db
change in volume range which may be used in part for volume expansion of loud
sounds or as noise reduction for weak sounds. The control-track frequency is
varied manually and recorded simultaneously with the sound-track in the dub-
bing operation, the gain of the monitoring channel being varied in accordance
with the control frequency to produce automatically the enhanced volume range
desired from the release print. The track is recorded in line with the standard
sound-track, and does not require separate printing or reproducing apertures.
It is scanned by a separate photosensitive surface, the output being converted
from frequency to voltage variations by a frequency-discriminating network
identical to that used in the monitoring channel. The output from the network,
applied to the grid of a variable-gain amplifier in the sound channel, controls
automatically the volume of the reproduced sound in accordance with that ob-
served in the dubbing operation.
The Design and Use of Film Noise-Reduction Systems; R. R. SCOVILLE
AND W. L. BELL, Electrical Research Products, Inc., Hollywood, Calif.
The factors underlying the design and use of biased recording systems are
described. In order to minimize noise and "shutter bump" special precautions
in filtering must be taken. Suitable values for "attack" and "release" times are
dependent upon the type of recording, margin settings, and reproducing condi-
tions. Comparison of variable-density and variable-area requirements is made.
Methods used in designing the rectifiers, filters, and other circuit details are given
and the application to a new equipment known as the RA-1124 noise-reduction
unit is shown.
Streamlining a Sound Plant; L. L. RYDER, Paramount Pictures, Inc., Holly-
wood. Calif.
This paper discusses the trend in modern sound-recording equipments. It
reviews the objectives and requirements that are now existing in regard to studio
recording as contrasted to previous recording systems. Several new develop-
ments in the art of sound recording are discussed and from this group are selected
a complementary series of improvements which together are streamlined into a
new recording plant.
436 ABSTRACTS OF CONVENTION PAPERS [j. s. M. p. E.
A Precision Direct-Reading Densitometer;, M. H. SWEET, Agfa Ansco Corp.
Binghamton, N. Y.
The history of physical densitometers is briefly discussed. In spite of develop-
ments in modern electronic circuits, simple photoelectric instruments suitable
for routine sensitometry are not yet in common use. The present densitometer
is designed to fill this need.
The minimum requirements for a satisfactory instrument are outlined. Photo-
graphic density as such, and density standardizations are discussed.
The densitometer density of the present instrument as related to that of other
types is demonstrated. The optical aspects, including the geometry and spec-
tral qualities of the system, are explained, and the problem of calibration dis-
cussed. Emphasis is placed upon the practical agreement of different optical
systems suitably calibrated, and specific examples are shown.
The circuit arrangements of previous photoelectric densitometers are outlined.
The theory and practical development of the present electrical circuit are de-
scribed, and the effects of the novel features are shown. An accurate linear den-
sity scale is obtained in a single-stage d-c amplifier, and the sensitivity is suf-
ficient to permit the use of a rugged output meter. A density range of 0 to 3.0 is
covered, and the characteristics of the output meter are given.
The technics used in prior densitometers in attempting to secure a linear den-
sity scale and adequate scale length for good legibility are discussed, and the tech-
nic used in the present instrument is compared with them. The performance
characteristics of the electrical circuit make it suitable for application to record-
ing instruments.
The routine operation is described and the permanence of calibration is shown.
Data are given on the warm-up period and drift, and on the influence of varying
line voltage. Operation is entirely by alternating current. Practical perform-
ance considerations such as convenience in reading, eye fatigue, etc., are reviewed,
and figures showing the comparative speed of operation and reading accuracy are
given.
A Review of the Question of 16-Mm Emulsion Position; WM. H. OFFEN-
HAUSER, JR., Precision Film Laboratories, New York, N. Y.
When a 16-mm sound-film is properly threaded in a 16-mm projector, the
emulsion of the film may face the screen (which position is called the "standard"
position), or it may face the projector light-source (the "non-standard" emulsion
position). The well designed 16-mm sound projector of today should be capable
of projecting either "standard" or "non-standard" prints.
In the case of 35-mm film, the standard position for the emulsion of a print is
opposite that for 16-mm; in 35-mm, the emulsion faces the light-source of a
projector. The anomaly of the 16-mm emulsion position arose from the fact
that a large number of the earliest 16-mm commercial sound-films were made by
optical reduction from 35-mm negatives. Since the "standard" was established,
however, numerous developments have occurred in direct 16-mm production
which now practically compel the recognition of so-called "non-standard" prints
as a factor of fast-growing importance in our rapidly growing 16-mm industry.
The expression "non-standard" emulsion position no longer carries the stigma
ordinarily associated with other things that are called non-standard.
Oct., 1941]
ABSTRACTS OF CONVENTION PAPERS
437
Motion picture films may be printed either by contact (the emulsion of the film
to be copied is in physical contact with the raw film upon which the copy is to be
made) or by optical printing (the emulsions of the two films are not in physical
contact; some form of lens system is interposed between the film to be copied
and the raw film upon which the copy is to be made). By far, the largest per-
centage of picture film printed today is printed by contact methods. It does
not seem likely that 16-mm picture film will be printed optically in the near future
for a number of reasons, not the least of which is the lack of available lenses due to
the defense program.
The use of Kodachrome duplicates has been growing very rapidly and since
contact printing of Kodachrome originals will continue to be used for some time,
the "non-standard" emulsion position will continue to be a rapidly growing factor
in 16-mm sound-projection that can not be ignored.
Some Equipment Problems of the Direct 16-Mm Producer; L. THOMPSON,
The Calvin Company, Kansas City, Mo.
The production of industrial films by the direct 16-mm method is now de-
finitely out of the experimental stage.
As more industrial work is done by this method there is an increasing demand
for more and better 16-mm equipment suitable for professional use. Such equip-
ment can be developed successfully only after the professional user has found by
actual experience what he needs and wants.
A number of 16-mm professionals were asked for suggestions as to what is needed .
These suggestions, combined with the author's own ideas gained over a period of
10 years in the professional 16-mm field, form the basis of this paper. Some of
the ideas presented could be acted upon immediately; some of them can not be
put into practice until the demand for 16-mm service becomes even greater.
A Constant-Torque Friction Clutch for Film Take-Up; WILLIAM HOTINE,
The Rotovex Corp., EastTNewark, N. J.
From the standpoint of film protection, a take-up mechanism should be reli-
able, wear should not appreciably alter its characteristics, and it should maintain
the film tension between safe limits. These objects are attained by driving the
take-up spindle through a constant-torque clutch of novel construction and de-
sign. A new type of friction-clutch is described, which, when adjusted initially
to deliver a given safe torque to the take-up spindle, maintains this torque at a
constant value which can not be exceeded. The clutch construction is simple and
rugged, and wear of the friction element does not appreciably affect the operation.
Due to the fact that the torque at the take-up spindle is maintained at a constant
value, a safe value of film tension is not exceeded. An analysis of the forces and
mechanical constants of the clutch mechanism is given, deriving an equation of
these in terms of torque delivered.
Recent Developments in Projection Machine Design; E. L. BOECKING AND
L. W. DAVEE, Century Projector Corp., New York, N. Y.
This paper discusses the design features of a new projector to meet the ever-
increasing demands for accuracy and simplicity required by modern projection
in the theater. Basic, fundamental, scientific functions of motion picture mecha-
438 ABSTRACTS OF CONVENTION PAPERS [J. S. M. p. E.
nism design are discussed relative to perfection of film motion, clearer definition,
light transmission, and picture steadiness.
As in the design of any scientific mechanical device, the stability and inherent
durability must first begin with perfection in the basic design and it must be built
upon a foundation of engineering knowledge proved by practical operating experi-
ence. In order that these design features may be appreciated it will be the pur-
pose to show how every step of the engineering design, every part of the mecha-
nism, and every motion were carefully planned so that mechanical perfection could
be achieved.
The design and operation of the gear-train are discussed with respect to its
simplicity, mechanical accuracy, and long life; the design and operation of the
bearings are reviewed in the light of recent developments relating to permanent
operation with minimum servicing; and the intermittent movement operation is
analyzed in relation to more stable operation and steadier picture reproduction.
The film-gate and film-trap design, providing more uniform film travel at less
film tensions, is described as well as methods of obtaining perfect placement of
the film plane with respect to the optical axis. Finally, the theoretical design
features of single- and double-shutter operation are outlined and the actual operat-
ing results expected and realized discussed.
Economic and Technical Analysis of Arc Lamp and Screen Light Character-
istics; H. D. BEHR, New York, N. Y.
Many exhibitors do not understand what is meant by the relative inefficiency
of power for ultimate consumption at the arc in comparison to power actually
delivered at arc. Deficiencies in various parts of the projection plant are de-
scribed and a value is placed upon losses to emphasize the need for constant atten-
tion to details.
Tables are presented showing the excessive carbon and current costs that result
when arcs are operated at higher currents due to defects in equipment. Em-
phasis is placed upon the fact that too many arcs operate at or near the upper
limits for which they were designed and too little leeway is left for extra current to
increase light for dull prints or color-prints.
Some ideas are given as to what to look for in competitive arc equipments.
Various procedures are described for minimizing current and carbon waste due to
poor reflector mirrors.
Suggestions of projectionists have too long been ignored by managements.
The latter should take a little time from their booking and other problems to as-
certain that poor screen light is costly and definitely contributes to drops in at-
tendance.
The IR System: An Optical Method for Increasing Depth of Field; ALFRED
N. GOLDSMITH, Consulting Engineer, New York, N. Y.
This paper is submitted as a report of progress made in the development of the
increased range (IR) system. In it is described the solution of a long-standing
problem in the field of optics, namely: the attainment of greater depth of field
than is attainable by any previous method of utilizing a lens system for image
formation.
Oct., 1941] ABSTRACTS OF CONVENTION PAPERS 439
The solution is particularly applicable in the fields of photography and tele-
vision under conditions of controllable lighting of the external objects to be de-
picted. In this paper there will also be included methods for demonstrating the
correctness and effectiveness in practice of the increased-range system which, as
stated, has been invented to meet the need for increased depth of field, as well as
indications of certain of the directions in which the practical evolution of the IR
System may reasonably be expected to proceed under studio conditions.
Adventures of a Film Library; RICHARD GRIFFITH, Museum of Modern Art
Film Library, New York, N. Y.
Collecting and circulating important films of the past is not as dusty an occupa-
tion as it sounds, as the director and curator of the Museum of Modern Art Film
Library discovered when this institution was founded in 1935 for the purpose of
instituting a considered study of the motion picture as art, industry, and social
influence. Even the mechanical acts of collecting and preserving film have in-
volved the human factor : people feel strongly about works that they themselves
have created, criticized, or merely seen, and the collection of films both in this
country and in Europe has been fraught with emotional, financial, and even politi-
cal complications, while the number of illustrious personalities who have in one
way or another become involved in the Film Library's work is prime evidence of
the ability of even the most ancient fragments of celluloid to retain a contemporary
as well as an archaeological interest.
Circulation of the Film Library's motion picture programs has also proved
illuminating in its revelation of the attitude taken toward the film medium by all
varieties of persons. The Film Library's purpose has from the first been to pro-
vide students with the opportunity to form a critical attitude by examining im-
portant films at first hand. But experts as well as laymen so warmly enjoy
movies that many were at first reluctant to "spoil" their pleasure in films by ex-
amining them critically. As more and more historic films have been restored to
the screen, however, there has gradually grown up throughout the United States a
new appreciation which has learned not only to marvel at the rapid development
of this new medium but also to discern its enormous and largely untapped poten-
tialities.
A New Electrostatic Air-Cleaner and Its Application to the Motion Picture
Industry; HENRY GITTERMAN, Westinghouse Electric & Manufacturing Co., New
York, N. Y.
The principle of electrostatic precipitation is not new. To the best of our
knowledge, it was first enunciated in 1824. It was used in England in the late
80's of the last century. In this country the Cottrell process has been in use for
approximately 30 years with great success. However, it was not until 1932 that
Dr. G. W. Penney of the Westinghouse Research Laboratories produced an elec-
trostatic precipitator that could be used in connection with atmospheric air
breathed by human beings. This advance was due to the fact that Dr Penney's
apparatus did not produce ozone in any appreciable amounts. Much lower volt-
ages and currents have been possible through the use of this system. Instead of
440
ABSTRACTS OF CONVENTION PAPERS
imposing huge voltages and currents upon a single system, in which ionization and
precipitation took place in the same chamber, the new system consists of two
parts. The first is made up of cylindrical rods alternating with small tungsten
wires on which a potential of 12,000 volts at very low current is imposed. This
creates an electrostatic field that charges all solid particles passing through it.
The air-stream carrying these charged particles then passes through plates alter-
nately charged negative and positive. The charged particles are precipitated
against the oppositely charged plates. The efficiency of the system is such that
guarantees can be made to remove 90 per cent of all solid particles in the air-
stream down to and including Vio of one micron in size. Ordinary air filters
range in efficiency from 12 to about 40 per cent by particle count.
Of particular interest to motion picture engineers is the fact that three of the
leading film-producing manufacturers in this country have adopted this system for
air-cleaning in their plants. Several of the more prominent exhibitors are con-
sidering using it in some of their theaters. It is possible to maintain a much
cleaner condition in the theaters themselves and thereby produce economy in re-
decorating the interiors. Furthermore, great savings are possible in the amount
of outside air needed in air-conditioning systems, which will enable engineers to
specify lower capacity cooling units without sacrificing any cooling effect whatso-
ever.
A number of installations are discussed describing the various aspects of par-
ticular interest to motion picture engineers.
Color Television; PETER C. GOLDMARK, Columbia Broadcasting System, Inc.,
New York, N. Y.
The paper will be introduced with a brief history of color television and the
reasons leading up to the CBS color television System. A general theory for
color television, including color, flicker, and electrical characteristics, will be given.
Equipment designed and constructed for color television transmission and recep-
tion will be discussed. Slides illustrating circuits, equipment, and actual color
pictures will be shown.
Synthetic Aged Developers by Analysis; R. M. EVANS, W. T. HANSON, JR.,
AND P. K. GLASOE, Eastman Kodak Co., Rochester, N. Y.
The dropping mercury electrode is applied to the problem of analyzing aged
photographic developers, and new tests are described for elon and hydroquinone.
The question of suitable tests for bromide is discussed and it is shown that the
bromide test must be independent of chloride. Such a test is described.
Using these new technics and others it is demonstrated that it is possible and
practicable by chemical analysis alone to match exactly the photographic char-
acteristics of an aged MQ developer. The only elements necessary for such an
analysis are elon, hydroquinone, sulfite, salt concentration, pH, bromide, and
iodide. The precision required for the proper analysis for each constitutent has
been investigated and is reported for three developer-film combinations. In
general the precision required is different for every combination.
Oct. 1941] ABSTRACTS OF CONVENTION PAPERS 441
Iodide Analysis in an MQ Developer; R. M. EVANS, W. T. HANSON, JR., AND
P. K. GLASOE, Eastman Kodak Co., Rochester, N. Y.
A method is described for the analysis of iodide in a developer, involving pre-
cipitation of the halide with silver nitrate and oxidation of the iodide while it is
in the form of solid silver iodide to iodate with chlorine water. The iodate is then
determined polarographically. Quantities of iodide from 2.5 to 10 milligrams of
potassium iodide were analyzed with an accuracy of 2 to 4 per cent. Thiocyanate
in the developer interferes but it can be removed by boiling with strong sulfuric
acid before precipitation.
Using this method of analysis it was shown that an equilibrium amount of
iodide is obtained in a developer. Curves are given showing the attainment of
equilibrium for development of Eastman panchromatic negative motion picture
film in Kodak D-76 and in D-16 developers, and Eastman panchromatic positive
motion picture film in Kodak D-16 developer. The equilibrium value depends
upon the emulsion, the developer, the developed density, and perhaps other
variables which will be investigated more thoroughly.
SOCIETY ANNOUNCEMENTS
FIFTIETH SEMI-ANNUAL CONVENTION
HOTEL PENNSYLVANIA, NEW YORK, N. Y.
OCTOBER 20-23, 1941
The 1941 Fall Meeting will be the Fiftieth Semi- Annual Convention of the
Society commemorating the Silver Anniversary of the Society's founding. Mem-
bers are urged to make every effort to be present, as the various Committees of
the convention are endeavoring to make this convention an outstanding one.
Details will be found in a preceding section of this JOURNAL.
PACIFIC COAST SECTION
A meeting of the Pacific Coast Section was held on Tuesday, September 23rd,
at the Naval Reserve Armory at Los Angeles, Calif. The program of the meeting
opened with the introduction of Capt. I. C. Johnson, Director of Naval Reserves,
followed by two discussions by officers of the Unit. Lt. Comdr. John Ford, U. S.
N. R., spoke on "The Organization of the U. S. Naval Reserve Photographic
Unit," followed by Lt. Comdr. A. J. Bolt on, U. S. N., Retired, who discussed
"Personnel and Equipment Requirements of the Photographic Unit."
Following these presentations a number of demonstration films produced by
the Photographic Unit were projected, and the meeting concluded with a discus-
sion by Mr. Emery Huse, President of the Society, on "Photographic Materials
for Military Purposes."
The meeting was arranged as a joint session of the Pacific Coast Section with
the U. S. Naval Reserve Photographic Unit.
MID-WEST SECTION
The meeting of the Mid-West Section was held at the meeting rooms of the
Western Society of Engineers, Chicago, on September 30th, at which engineers of
the DeVry Corporation described "A New and Improved Theater Sound Pro-
jector." The presentation included an interesting demonstration of the equip-
ment.
AMENDMENTS TO THE BY-LAWS
At the meeting of the Board of Governors on July 24, 1941, several amend-
ments to the By-Laws were proposed and approved for submission to the Society
membership at the next Business Meeting, to be held during the 1941 Fall Con-
vention. These proposed amendments are as follows:
442
SOCIETY ANNOUNCEMENTS 443
PROPOSED AMENDMENTS FOR STUDENT MEMBERSHIP
By-Law I
Membership
Sec. I—It is proposed that the first paragraph of this Section shall be changed
to read as follows :
The membership of the Society shall consist of Honorary members, Fellows, Active
members, Associate members, Sustaining members, and Student members.
It is proposed that a new paragraph d be added to Sec. 1 as follows:
(d) A student member is any person registered as a student, graduate or under-
graduate, in a college, university, or educational institution, pursuing a course of
studies in science or engineering which evidences interest in motion picture tech-
nology. Membership in this grade shall not extend more than one (1 } year beyond the
termination of the student status described above. A student member shall have the
same privileges as Associate members of the Society.
Sec. 2. — It is proposed that a new paragraph e be added to the end of this
Section as .follows:
(e) Applicants for student membership shall give as reference the head of the
Department of the Institution he is attending; this faculty member not necessarily
being a member of the Society.
By-Law VIII
Dues and Indebtedness
Sec. 1. — It is proposed that the first sentence of this Section be changed to
read as follows:
The annual dues shall be Fifteen ($15.00} Dollars for Fellow and Active members,
Seven Dollars and Fifty Gents ($7.50} for Associate members, and Three ($3.00}
Dollars for Student members, payable on or before January 1st of each year
PROPOSED AMENDMENTS FOR FELLOW MEMBERSHIP
By-Law I
Membership
Sec. 3(b). — It is proposed that this Section be changed to read as follows:
Fellow Membership may be granted upon recommendation of the Fellow Award
Committee, when confirmed by a three-fourths majority vote of the Board of Governors.
By-Law IV
Committees
Sec. 4(a}. — It is proposed to add to the list of standing committees appointed
by the president and confirmed by the Board of Governors a new Fellow Member-
ship Award Committee.
444 SOCIETY ANNOUNCEMENTS
PROPOSED AMENDMENT RELATING TO TECHNICAL COMMITTEES
By-Law IV
Committees
Sec. 4(b). — It is proposed that to the list of standing committees appointed by
the engineering vice-president be added a new Committee on Cinematography.
JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
Volume XXXVII November, 1941
CONTENTS
Page
Resume of an Extemporaneous Address by H. HANSON 449
Analysis of Sound-Film Drives
W. J. ALBERSHEIM AND D. MACKENZIE 452
A Suggested Clarification of Carbon Arc Terminology as Ap-
plied to the Motion Picture Industry. . .H. G. MACPHERSON 480
Improved Methods of Controlling Carbon Arc Position
D. J. ZAFFARANO, W. W. LOZIER, AND D. B. JOY 485
Symposium on Projection
Projection Room Equipment Requirements. . . . J. J. SEFING 502
The Projection Room — Its Location and Contents
J. R. PRATER 506
Factors Affecting Sound Quality in Theaters. .A. GOODMAN 510
Progress in Three-Dimensional Pictures J. A. NORLING 516
Solving Acoustic and Noise Problems Encountered in Recording
for Motion Pictures W. L. THAYER 525
Report of the Standards Committee 535
New Motion Picture Apparatus
A New 13.6-Mm High-Intensity Projector Carbon
M. T. JONES, W. W. LOZIER, AND D. B. JOY 539
Current Literature
(The Society is not responsible for statements by authors.)
JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
SYLVAN HARRIS, EDITOR
BOARD OF EDITORS
ARTHUR C. DOWNES, Chairman
JOHN I. CRABTREE ALFRED N. GOLDSMITH EDWARD W. KELLOGG
CLYDE R. KEITH ALAN M. GUNDELFINGER CARLETON R. SAWYER
ARTHUR C. HARDY
Subscription to non-members, $8.00 per annum; to members, $5.00 per annum,
included in their annual membership dues; single copies, $1.00. A discount
on subscription or single copies of 15 per cent is allowed to accredited agencies.
Order from the Society of Motion Picture Engineers, Inc., 20th and Northampton
Sts., Easton, Pa., or Hotel Pennsylvania, New York, N. Y.
Published monthly at Easton, Pa., by the Society of Motion Picture Engineers.
Publication Office, 20th & Northampton Sts., Easton, Pa.
General and Editorial Office, Hotel Pennsylvania, New York, N. Y.
West Coast Office, Suite 928, Equitable Bldg., Hollywood, Calif.
Entered as second class matter January 15, 1930, at the Post Office at Easton,
Pa., under the Act of March 3, 1879. Copyrighted, 1941, by the Society of
Motion Picture Engineers, Inc.
OFFICERS OF THE SOCIETY
** President: EMERY HUSE, 6706 Santa Monica Blvd., Hollywood, Calif.
*" 'Past-President: E. ALLAN WILLIFORD, 30 E. 42nd St., New York, N. Y.
**Executive V ice-President: HERBERT GRIFFIN, 90 Gold St., New York, N. Y.
^Engineering Vice-President: DONALD E. HYNDMAN, 350 Madison Ave., New
York, N. Y.
** Editorial Vice-President: ARTHUR C. DOWNES, Box 6087, Cleveland, Ohio.
* ''Financial Vice-P resident: ARTHUR S. DICKINSON, 28 W. 44th St., New York
N. Y.
** Convention Vice-President: WILLIAM C. KUNZMANN, Box 6087, Cleveland, Ohio
^Secretary: PAUL J. LARSEN, 44 Beverly Rd., Summit, N. J.
*Treasurer: GEORGE FRIEDL, JR., 90 Gold St., New York, N. Y.
GOVERNORS
**MAX C. BATSEL, 501 N. LaSalle St., Indianapolis, Ind.
* JOSEPH A. DUBRAY, 1801 Larchmont Ave., Chicago, 111.
*JOHN G. FRAYNE, 6601 Romaine St., Hollywood, Calif.
*ALFRED N. GOLDSMITH, 580 Fifth Ave., New York, N. Y.
*ARTHUR C. HARDY, Massachusetts Institute of Technology, Cambridge,
Mass.
**LOREN L. RYDER, 5451 Marathon St., Hollywood, Calif.
TIMOTHY E. SHEA, 195 Broadway, New York, N. Y.
*REEVE O. STROCK, 35-11 35th St., Astoria, L. I., N. Y.
*Term expires December 31, 1941.
**Term expires December 31, 1942.
RESUME OF AN EXTEMPORANEOUS ADDRESS BY
HOWARD HANSON*
AT THE JOINT MEETING OF THE SOCIETY OF MOTION PICTURE ENGINEERS
AND THE ACOUSTICAL SOCIETY OF AMERICA AT
ROCHESTER, N. Y., MAY 5, 1941
I feel somewhat concerned in attempting to address this eminent
group of scientists and technicians. I am under the impression that
you may expect me to embark upon a technical discussion of problems
in sound reproduction and an evaluation, from the standpoint of the
musician, of the effectiveness of the solution of those problems. Per-
haps I am even expected to make criticisms and to suggest directions
in which the musician feels that sound reproduction may be improved.
Some years ago in speaking before this same body I had the
temerity to attempt something of the sort. Today I feel quite unable
to carry that discussion further. This reluctance is due to my con-
viction that you as scientists are already far beyond us, the musicians.
You have progressed in the matter of sound reproduction to the point
where the fidelity of the recorded sound to the original is remarkable.
You have embraced in your recording, frequencies, both high and low,
which formerly were lost. You have even solved to a startling degree
the problem of gradations in intensity so that a dynamic range which
gives an adequate representation of an actual orchestral performance
is possible.
In fact in some respects you have gone beyond "natural" sound in
such a way as to raise a question in my mind as to the validity of tak-
ing as our final criterion the direct comparison of recorded sound with
"natural" sound. In certain experiments such as those which we are
having the privilege of witnessing this week Doctor Fletcher and his
colleagues of the Bell Laboratories are convincing us that the term
"enhancement' ' seems quite justified. Scientists and technicians have
for some time held before themselves the ideal of reproducing sound
qualities which could not be distinguished from the original. This
* Director, Eastman School of Music, University of Rochester.
449
450 ADDRESS BY HOWARD HANSON [J. S. M. P. E.
they have accomplished to an amazing degree. It seemed to me as I
listened to the results of Doctor Fletcher's experiments that the time
has come when sound reproduction can itself become creative — that
science may produce tonal beauty of a quality which has no counter-
part in the sounds of the musical instruments and ensembles which
we know today. This seems to me to be a legitimate goal.
But there is something else which is much closer to my heart today.
I have the uncomfortable feeling that you as scientists are too good
for us — that you have given us tools which are beyond our ability to
understand and to use wisely. The terrible condition of the world
today is all too vivid an illustration of what I am saying. Science
gives us the airplane with which we can annihilate space and bring
mankind closer together. We convert the airplane into a bomber and
use it to kill our fellows. The sciences of chemistry and of medicine
give us the blessed means of easing human pain. We divert the same
scientific inventiveness to the production of gases which burn out
men's lungs. Science puts into our hands magnificent tools but we are
so spiritually unprepared for these miracles that we are quite likely
to use them for the purpose of committing physical and spiritual
suicide.
Does the scientist have any responsibility in all of this moral chaos?
Is his task only to produce the tools and to allow us to misuse them as
we will? I do not believe so. It seems to me that those of you who
are creating the mechanisms which the rest of us are to use must be
interested in the use to which they are put. You can not remain aloof
to the manner in which these products of your hands and brain serve
humanity for good or ill.
I have spoken with enthusiasm of the work which you have done
in the field of sound and 1 say again, you have been too good for us.
In the field of radio, for example, you must at times have the feeling
that all of us — composers, performers, scientists, and technicians —
are banded together for the high and noble cause of selling soap.
Now I have nothing against the selling of soap. Certainly from the
amount of time devoted to it we must be the cleanest nation in the
world. The women of America must have the softest of hands and
the whitest of teeth or our efforts have been in vain. All this is
doubtless important but it was certainly not for this that you have
labored.
In the field of the motion picture too often the same condition
obtains. Magnificent invention is used to serve a cause which is too
Nov., 1941] ADDRESS BY HOWARD HANSON 451
frequently unworthy. The creative brain of the scientist dreams a
vision and labors to realize it only to find that his invention has been
used for meretricious ends.
Certainly today we must pause and consider whither we are pro-
gressing. Today as seldom before in history the world needs spiritual
awakening. It needs the quickening and sensitizing spirit of beauty.
It demands the re-birth of man's soul. Of what good is it if all of our
science, all of our material production, leads only to poverty of the
mind and the heart? Of what merit is it if through the invention of
man's mind we save our bodies but lose our souls?
ANALYSIS OF SOUND-FILM DRIVES*
W. J. ALBERSHEIM AND D. MACKENZIE**
Summary. — In order to avoid audible flutter, the velocity of sound-films past
the scanning light-beam must not vary more than about 0.1 per cent. Such precision
can not be obtained solely by constant speed motors and high-quality gears. Me-
chanical filters must suppress the "drive side" disturbances originating in motor,
gear -train, and sprocket-teeth, and the "load side" disturbances due to variations in
the film and in the friction load.
Early designs filtered only the drive-shaft rotation and steadied the film by recording
on a large sprocket drum. Filtered sprockets in combination with fixed reproducer
gates were not adaptable to modern requirements and were superseded by film-driven
damped impedance drums (rotary stabilizers').
The recommended design avoids the troublesome inner flywheel bearings by a
liquid stabilizer and overcomes the uncertain filtering properties of film compliance
by means of elastic driving sprockets.
It has been stated that the main function of a film-driving mecha-
nism is to pull the film out of the upper magazine into the lower
magazine. This is true with the provision that the motion of the
film must be uniform as it passes through the focal line of a beam of
light. If the film speed varies, the frequency of every sound recorded
on the film will vary in proportion, causing flutter. How small varia-
tions of speed or frequency can become noticeable is seen from Fig. 1
which shows the frequency variations of pure oscillator tones which
were barely audible to trained observers in a live auditorium.1 Some
of the listeners were able to notice a rhythmical speed variation off
0.005 per cent in a 3000-cycle tone at the rate of 1.5 cycles per second.
What the ear hears under such conditions are not the frequency
variations directly, because if the same tones are heard through head-
phones the frequency variations must be increased nearly one hun-
dred-fold to become audible. What the observers actually noticed
was an amplitude pulsation due to shifting of a standing-wave pat-
* Presented at the 1940 Fall Meeting at Hollywood, Calif. ; received June 12,
1941.
** Electrical Research Products, Inc., New York, N. Y.
t Above and below mean frequency is understood throughout the paper.
452
ANALYSIS OF SOUND-FILM DRIVES
453
tern between parallel walls. Fortunately, under practical condi-
tions, one does not have to listen to oscillator tones nor in empty
straight walled halls, so that practical flutter tolerances are consider-
ably higher, as indicated by Fig. 2. Curve 1 of this figure shows the
flutter limits for the sound heard in the theater which we set for our
own guidance as early as 1935. It amounted to 0.25 per cent at
flutter rates above 25 cps, to 0.15 per cent at flutter rates between 25
and 1 cps and increased with the inverse square-root of the frequency
for frequency "drift" below 1 cps.
10
100
FIG. l.
MODULATION RATE CPS. — —
Minimum perceptible frequency flutter (oscillator
tones in live room).
The sound-film reproduction in the theater is the product of proc-
esses which involve generally at least three passages through film-
driving mechanisms: the original recording process, the printing
operation, and the reproduction. The number of cumulative speed
distortions may be increased by re-recording operations. Assuming
that the irregularities of the film motion are superimposed at random,
one must take the total speed deviation as the root-sum-square of all
contributory deviations. The irregularities of each step must there-
fore be held so far below the tolerance limit that they add up to a
satisfactory total.
454
W. J. ALBERSHEIM AND D. MACKENZIE [J. S. M. P. E.
Economy requires us to impose the most lenient tolerances upon
the apparatus used in the greatest quantity — that is, the theater re-
producers— and to hold the recording and re-recording machines to
closer limits. Accordingly, we allowed for the reproducer a high-
frequency limit of 0.20 per cent, a low-frequency limit of 0.12 per
cent, and drift limits increasing from 0.12 per cent as shown on Curve
2. This left only a small margin for recording and re-recording
flutter which were fixed at 0.10 per cent for high frequencies and 0.05
per cent for low frequencies (Curve 3). In 1938 the Research Council
1 ]
I
CURVE NO.
1 OVERALL SOUND
2 REPRODUCING p- ERPI -1935
3 RECRDG. &, RE-RECRDG. J
0
4 TOTAL REPRO. FLUTTER - A.M.PA.S. , 1938
o^
UJ
^<L1
o .25
^^^^v^
25
\ ^*v
.^
J-
_i 15
^
o\
f
15
or
^^
***
y--
. "Zj. I k
.1
!^x
s^
V-2
|U
u 05
H
i
Vj^
J
.06
V-3
_i
".02
.02
.5 1 1.5 10 50
FLUTTER RATE— - CYCLES PER SECOND
FIG. 2. Practical flutter tolerances.
100
of the Academy of Motion Picture Arts & Sciences recommended
that the total reproducer flutter measured on our flutter bridge should
not exceed 0.15 per cent (Curve 4). This does not specify the fre-
quency distribution. The bridge in its position for the measurement
of "total flutter" has a flat output characteristic for flutter rates
above 2 cps and a somewhat lower sensitivity for slow drift; it meas-
ures the power sum of individual flutter frequencies (rms addition).
The Research Council requirement would therefore be satisfied by a
0.12 per cent 96-cycle flutter combined with a 0.08 per cent 3-cycle
flutter and a 0.08 per cent 1/4-cycle drift.
Even these practical tolerances are by no means easily held. As
the U. S. Circuit Court of Appeals stated in a well known decision :
Nov., 1941]
ANALYSIS OF SOUND-FILM DRIVES
455
The rapidly moving, flimsy, curling film must be uniform in its movements
and so controlled that the position and motion of each fine line at the beam of
light must be accurate within the thousandth of an inch.
Such precision requires high constancy of the motor speed, but it
can not be attained by this means alone as will be seen from Fig. 3,
which illustrates the flutter sources in unfiltered film drives. These
are:
(1} Drive-Side Disturbances (irregularities in the force which pulls the film
past the reference point).
(a) The motor supplies not only the steady (d-c) torque but it is subject to
periodic power main surges and to vibrations at the high frequencies of the alter-
nating current with its harmonics and at its own speed of revolution.
DRIVE SIDE DISTURBANCES
DRIVING & HOLD-
BACK SPROCKETS
FILM
FREQUENCIES \ IRREGULAR FILM
\ AND SPROCKET
GEAR TRAIN OF SOUND\HOLE ""ENSIONS
& PICTURE DRIVE \
TOOTH. SHAFT i \
INTERMITTENT
FREQUENCIES
_MQTOR.
FOWER FLUCTUATIONS
AND HIGH FREQUENC
FLUTTER
LOAD SIDE DISTURBANCES
FIG. 3. Flutter sources in unfiltered film drives.
(b) The gear-trains needed for speed reduction and synchronization of picture
and sound introduce the frequencies of their shafts and gear teeth — and their
sums and differences. The most pronounced tooth frequency is that of the
driving-sprocket teeth, 96 cps. Another disturbance impressed upon the gear-
train of theater reproducers is the 24-cps intermittent frequency. This frequency
may also be introduced by the variation of free loop length between the scanning
point and the picture hold-back sprocket.
(c) The irregularities of the film intervene between the driving sprocket
and the scanning light-beam.
(2) Load-Side Disturbances (irregularities in the mechanical impedance op-
posing film motion past the reference point).
(a) If scanned in a gate, the film is subject to irregular gate friction; if scanned
on a rotating drum, to irregular bearing friction and to unbalance of rotating
parts.
The requirement of passing without reduction the d-c motion and
attenuating the unwanted a-c components can be met only by a type
456
W. J. ALBERSHEIM AND D. MACKENZIE [J. S. M. P. E.
of structure which is the mechanical equivalent of a device well known
in electrical transmission theory as a low-pass filter. A low-pass
filter chain consists of series inductances L and of shunt capacities
C. Taken by themselves these would resonate at a frequency
Fr = l/(2-jr\/l/LC). Arrayed in a properly terminated filter they
pass freely all frequencies below, and attenuate those above 2 Fr.
The attenuation, increasing rapidly at first, asymptotically approaches
FRICTION
BETWEEN
FILM AND
/ DRUM
REFERENCE / FILM
VELOCITY / COMPLIANCE
SPROCKET
TEETH
VELOCITY
JvW-
FIG. 4. Filtered sprocket drum.
a straight line through zero attenuation at Fr which slopes at the
rate of 12 db per octave per filter section. This means that in each
section the high-frequency amplitudes are reduced in proportion to
the square of the frequency. In a mechanical filter, inertia takes the
place of inductance, and compliance that of capacity. Since our
ears remain sensitive to flutter rates even slower than one per second
and since low-pass filters lose effect near their resonance frequency,
one must either adjust the resonance to less than l/* cps by heavy
but very pliant filter structures, which tend to be unstable, or one
Nov., 1941] ANALYSIS OF SOUND-FILM DRIVES 457
must make the power source very constant at low frequencies.
Neither of these objectives is easily attained and the elimination of
audible film flutter has been a very gradual and difficult accomplish-
ment as will be realized from the following short survey of some
typical past and present film-driving mechanisms.
In recording, one of the oldest successful drives was the "filtered
sprocket drum" (Fig. 4). This reduces the flutter components of
motor and gear-train by a mechanical low-pass filter. In the pre-
ferred form of this filter, the motion of the drive shaft is coupled to
the heavy flywheel by an arm which engages two spiral springs and
two oil-filled sylphon bellows connected by a small aperture. Rela-
tive rotation between the drive shaft and the flywheel tensions the
springs and forces oil through the friction aperture. The electrical
analogy of this mechanism is shown in heavy lines on the left part of
Fig. 4. Vi is the sum of d-c and a-c angular velocity components
impressed upon the flywheel shaft by the gear- train. Due to its
great rigidity the gear- train approximates a constant- velocity gen-
erator, as symbolized in the analogy by an infinite impedance to
ground, Zim combined with an infinite torque T = V\Z\m. In the
simplest form of the analogy, a condenser represents the torsional
compliance C\ of the springs, a resistance the oil friction R, and an in-
ductance the moment of inertia J, of the flywheel. The analysis of
such a combination is given in the appendix. One sees that due to
the absence of a heavy load the filter is not terminated and therefore
resonant. The resonance peak is damped by a resistance in series
with the condenser which cuts the high-frequency attenuation from
12 db per octave to 6 db per octave. (A more detailed analysis
shows that the bellows themselves contain a small compliance which
in the electrical analogy is shown in dotted lines as a second con-
denser in parallel with the resistance. This bellows compliance
brings the high-frequency attenuation back to 12 db per octave and
modifies the response characteristic in a manner discussed in the ap-
pendix.)
An advantage of the filtered flywheel drive is the absence of load-
side trouble. It requires, however, careful matching of the sprocket-
tooth pitch to the film length, and accommodates only the shrinkage
range of reasonably fresh recording film stock. The residual
sprocket-tooth impacts are reduced by providing simultaneous con-
tact of the film with several teeth, a construction which requires a
large sprocket drum, low angular speed and a large flywheel. In
458
W. J. ALBERSHEIM AND D. MACKENZIE [J. S. M. P. E.
theater reproducers, the shrinkage difference between new and old
films is uncontrollable, and a large sprocket drum can not be accom-
modated within the standard 15-inch distance between picture and
sound scanning point. Consequently, the filtered sprocket drum has
been used mainly in studio type recording and re-recording machines.
The first commercial reproducers in this country attempted to
overcome drive-side disturbances by a filtered flywheel similar to
that of Fig. 4. It was rigidly coupled to a small sprocket which
pulled the film through a closely adjacent gate. Figs. 5 (a) and 5(&)
show two stages in the evolution of the gate. The straight gate of
Fig. 5 (a) was designed to insure that the film was held in the focal
FIG. 5(o)
FIG. 5(6)
FIG. 5(a). Reproducer straight sound gate with filtered sprocket.
(6) . Reproducer curved sound gate with filtered sprocket.
plane of the scanning-beam. Its sound quality was satisfactory at
the time but the 96-cycle flutter exceeded the narrow tolerance limit
later demanded and shown in Fig. 2. This flutter, originating at the
mesh of sprocket-teeth and film, could not be reduced by the flywheel
filter. The absence of flexibility between sprocket and gate im-
pressed all irregularities directly upon the gate, where nothing but
the solid friction of the gate shoes opposed them.
The difficulties encountered in using solid friction as a damping
means are explained by Fig. 6. The upper graph (A) shows the
forces of viscous and solid friction as a function of velocity. While
viscous friction force is proportional to velocity, indicated by a
straight line through the origin, solid friction is high at rest, rapidly
falling off to a minimum with increasing velocity in either direction
Nov., 1941]
ANALYSIS OF SOUND-FILM DRIVES
459
and then slowly rising again. The lower graph (B) shows the fric-
tional resistances derived from the above-illustrated forces. The
viscous resistance is a constant, equal to the slope of the force-veloc-
ity line : R = p/V. The solid resistance depends on the magnitude
and the starting velocity of the oscillatory motion. For periodic os-
cillations around zero velocity, the effective damping resistance is
very high at small amplitudes and falls off with amplitude to a mini-
mum, and then slowly increases again as shown by the upper (p/v)
SOLID FRICTION
(SMALL OSCILLATIONS) dP/dv
-7-6-5-4-3-2-1 1 234 567
FIG. 6. Characteristics of viscous and solid friction.
curve. For small vibrations superimposed on a d-c velocity, the re-
sistance equals the gradient of the resistance forces and follows the
lower (dp/dv) curve. At points remote from zero d-c velocity, it has
a small value which in a certain range becomes negative, so that im-
properly used solid friction, instead of damping, may amplify ex-
ternal disturbances and even generate free vibrations like those of a
violin string under the steady pull of the bow.
A considerable improvement was brought about by the curved
gate shown in Fig. 5(6). By reducing the film tension and introduc-
ing a short length of relatively loose film loop, the drive when well
460 W. J. ALBERSHEIM AND D. MACKENZIE [J. S. M. P. E.
adjusted was capable of reducing the film flutter to about l/4 of 1 per
cent.
It was found, however, that the best reproducing quality could be
obtained by abandoning gates altogether and scanning the sound-
track on a smooth impedance roller. The general nature of a typical
impedance-roller drive is shown in Fig. 7. A sprocket wheel, usually
unfiltered, pulls a loose film loop over a smooth drum rigidly coupled
to a flywheel. The film loop supplies the shunt compliance, the fly-
wheel the series inertia of a mechanical low-pass filter which, in this
new location, attenuates sprocket-hole disturbances as well as motor
and gear flutter. In order to act as a filter, this combination must
be terminated by a proper load impedance. If left unterminated.
it is a purely reactive structure which tends to oscillate at its reso-
FIG. 7. Damped impedance drum.
nance frequency, and therefore may increase rather than attenuate the
film speed variations. The reactive elements must be damped in
some fashion by resistive components. Experience has shown that
such damping can be successfully incorporated, making the damped
impedance drum the most economical and practicable sound-film
drive for both recording and reproducing purposes.
It should be kept in mind that even a smooth recording drum in-
troduces disturbances at sprocket-hole frequencies because the film
bends more sharply in the regions weakened by the sprocket-holes.
This bending stretches the sound-track, causing an increase of fre-
quency in recording and a corresponding decrease in reproduction.
The two effects neutralize each other if the same drum diameter can
be used in recording and reproduction. Since the space available in
theater reproducers limits the drum size to about 2 inches, and this
diameter is too small for recording sprockets, the impedance drum is
Nov., 1941] ANALYSIS OF SOUND-FILM DRIVES 461
the only universally adaptable drive mechanism. Recording ma-
chines equipped with large sprockets are most useful if the films re-
corded on them are reproduced on mechanisms also using large scan-
ning drums either with or without sprocket- teeth.
In providing damping for the impedance drums, the most obvious
procedure is to add a straight resistance termination to the reactive
filter elements. Fig. 8 shows schematically the mechanical arrange-
ment of a resistance-terminated filter and its electrical equivalent.
FIG. 8. Resistance-terminated filter.
Fig. 9 shows the response characteristics of such filter sections, as
calculated in the Appendix. As a reminder, Curve 1 in this figure
shows the ideal low-pass filter characteristic. The nearest approach
for one resistance-terminated section is obtained with a torsional
frictional resistance Rc = 2 J/C, as shown by Curve 2. This is a
rather high resistance which opposes the steady rotation of the film
drive as well as its speed fluctuations, causing unnecessary load on
bearings and motor. If one reduces the resistance as shown in Curve
3, the structure becomes resonant and amplifies some frequencies;
if one desires to attenuate frequencies below the resonance frequency
462
W. J. ALBERSHEIM AND D. MACKENZIE [J. s. M. p. E.
as shown in Curve 4, the required damping resistance increases
greatly, making the demands upon the drive even more impracticable.
A logical next step is to compensate for the resistance load of the
film by supplying an auxiliary driving torque through the damping
resistance. This leads to the resistance-coupled auxiliary drive
schematically shown in Fig. 10, with its electrical equivalent. This
type of drive has been successfully used. One recording mechanism
now in the field uses eddy currents generated by electrodynamic in-
duction to produce a friction drag between a copper drum and electro-
.1 .2 .512
RELATIVE FREQUENCY F/Fr -
FIG. 9. Characteristics of resistance-terminated niters.
(7) Ideal L. P. filter section. (2) Peakless damping R =
Rc = V2L/C. (3) Underdamped R = OARC. (4) Over-
damped R = 2.5RC.
magnets mounted on a flywheel as shown on Fig. 10. A successful
16-mm reproducer uses the viscosity of an oil film to produce the drag
between film-drum and auxiliary drive. As previously shown in
Fig. 9, the resistance drive can be damped down to a peakless char-
acteristic if the resistance is sufficiently high; but only at the price
of a tight coupling between the film drive and the auxiliary drive.
Naturally, the auxiliary drive is subject to speed variations of its
own and one must therefore consider the response characteristics of
resistance-coupled auxiliary drives to disturbances originating on the
auxiliary drive or "load" side as well as on the film or "drive" side.
In a recording mechanism previously described in the JOURNAL,2 the
Nov., 1941]
ANALYSIS OF SOUND-FILM DRIVES
463
auxiliary drive moved with a 15 per cent higher angular velocity
than the recording drum and supplied just about enough torque to
neutralize the small friction in the ball bearings of the recording drum.
This means that the effective coupling friction was only about seven
times as large as the bearing friction, and in view of the fairly large
moment of inertia of the magnetic flywheel, the film-side transmission
characteristic showed a decided resonance peak.
Such a condition is illustrated in Fig. 11, which shows also the re-
sponse to disturbances originating at the load side; the latter being
computed in the Appendix under the favorable assumption that the
FIG. 10. Resistance-coupled auxiliary drive.
auxiliary drive is free from shunt compliance. The film-side response
has a 5-db peak near the resonance frequency, and at about the same
frequency the load-side admittance has a maximum shown as 0-db
attenuation to indicate that disturbances originating in the magnetic
drive are freely passed on to the film. If the coupling resistance is
further reduced, the peak of the film-side response becomes higher;
that of the load-side response remains equally high but it becomes
sharper so that a narrower range of drive-side disturbances affects
the film motion. The load-side response peak means that at low
frequencies near resonance the auxiliary drive must be of an excel-
lence approaching that of the film drive ; the broadness of the reso-
nance demands that even at considerably higher frequencies up to
about 6 cps, the magnetic drive and the coupling resistance must be
464
W. J. ALBERSHEIM AND D. MACKENZIE [j. s. M. P. E.
well balanced magnetically, mechanically, and electrically. These
structural requirements are severe and lead to expensive cons true-
tion. Before adopting this type of drive, it is therefore advisable to
investigate whether its performance can not be matched or bettered
by a simpler mechanism.
Such a simpler device does exist; it is known as the "stabilized
flywheel." The main mechanical elements of a stabilized flywheel
film drive are schematically shown in Fig. 12, with the equivalent
electrical filter. This structure contains all the elements of the resist-
FIG. 11. Resistance-coupled auxiliary drive; attenuation
of film and auxiliary drive disturbances.
ance-terminated low-pass filter but it eliminates the d-c resistance
drag by shunting the damping resistance with a large inductance.
Mechanically this means that the resistance operates not between
the film-drum and a stationary friction pad as in Fig. 8, but between
an outer flywheel shell and an inner flywheel mass which itself is free
to rotate. The resistance may be supplied by the viscosity of an oil
film in the small clearance between the flywheel and the shell. The
greater the flywheel inertia compared to the inertia of the shell, the
more the device resembles a resistance-terminated filter, and the lower
becomes the resonance peak which is inherent in this design. The
characteristics of stabilized flywheels are derived in the Appendix
Nov., 1941] ANALYSIS OF SOUND-FILM DRIVES 465
and illustrated by Fig. 13 for a stabilized flywheel in which the inner
flywheel inertia is only two and one-half times that of the shell, in-
cluding the scanning drum and other moving parts. The drive-
side impedance shows a peak of about 5 db and asymptotically ap-
proaches a straight 12-db-per-octave slope similar to the auxiliary
drive characteristic shown in Fig. 11. In addition to this drive-side
response, Fig. 13 also shows a load-side impedance characteristic.
This takes into acount the eccentricities of flywheel load and the ir-
regularities of bearing friction which may be transferred back to the
light-scanning drum. In this respect, too, the stabilized flywheel
WHEE
L
RESIS- FOCUSED INERTIA ANCE 0. C . GEAR
TANCE LIGHT OF OF 8. SPROCKET
BEAM SHELL FILM VELOCITIES
t, V% r*-r^ \ /C\
>
l)u
'
^o 1
. M V
F* c=
" F-
2E
FIG. 12. Stabilized flywheel drive.
drive is similar to the resistance-coupled auxiliary drive structure.
Several successful film reproducers make use of this type of drive
which incidentally is by no means a new development. It was in-
vented by H. A. Rowland in 1899 for the damping of synchronous
telegraph motors.3
The damping properties of a stabilized flywheel are demonstrated
by a simple experiment used some time ago in a court room ; the fly-
wheel is mounted on a shaft with two light cylindrical rollers and
allowed to roll freely on two rails which are slightly higher at the
ends than in the middle. In one flywheel the shell is locked to the
inner flywheel by two small screws. When placed on the end of the
rails it rolls back and forth for several minutes before it comes to rest.
466
W. J. ALBERSHEIM AND D. MACKENZIE [j. s. M. P. E.
Another wheel is identical with the first but the inner flywheel is per-
mitted to rotate freely against the friction of the oil film. Whe
placed on the rails, the wheel comes nearly completely to rest after
one or two excursions. We say nearly because close observers may
notice that sometimes the wheel continues to teeter back and forth
at very small amplitudes as if the inner flywheel had become frozen
to its shaft.
This actually can happen not only in this experiment but also under
operating conditions. Remember that once the reproducer has come
DRIVES
u£ -*-
^ —
\
/
/
/
x\
^x
\ \
g 10
1
5 "o
4
.&,
-/
5
\
^ 20
£
1 ™
S"
*/
\
\
\ 3°
\
\
.2
10
.512 5
RELATIVE FREQUENCY F/Fr - -
FIG. 13. Stabilized flywheel; response to drive and load-
side disturbances.
up to speed, the average rotational velocity of the flywheel equals
that of the shell although there occur small cyclical speed differences
between them. At least twice during each such cyclical variation,
the relative velocity passes through zero and the friction coefficient
of the inner flywheel bearing increases from rolling friction to the
considerably greater static friction. The flywheel becomes locked
to its shell and the mechanism becomes temporarily undamped until
it exerts sufficient acceleration or deceleration torque upon the fly-
wheel to break the friction lock. This effect, which is symbolized in
the electrical analogy by a spark-gap or breakdown tube Rb in series
with the damping resistance R, is the more pronounced the less high-
frequency flutter components are there to maintain a rapid vibration
Nov., 1941] ANALYSIS OF SOUND-FILM DRIVES 467
j between shell and flywheel. It causes hunting or drifting at the low
([ resonance frequency of the locked flywheel.
This has been the main objection to the stabilized flywheel drive
! until it was recently overcome by a structure called the "liquid fly-
j wheel," used in the recording and reproducing machine designed by
the Bell Telephone Laboratories for the stereophonic sound system
I which has been demonstrated in New York, Hollywood, and Roch-
ester.
The new design replaces the solid flywheel by a heavy liquid of low
viscosity which has to force its way through narrow channels within
I the flywheel shell. It thus eliminates the objectionable bearings
j and, incidentally, the very small clearance between flywheel and shell
j which contributed to the expense of the previous design. If the di-
! mensions of liquid flywheel shell and friction channels are so chosen
that the viscous resistance is concentrated at the channels and only
slightly increased by friction on the surface of the shell walls, and if
care is taken to avoid turbulence in the flow of the liquid, the new
structure becomes equivalent to the solid stabilized flywheel pre-
viously discussed and its drive-side response characteristic identical
with that of Fig. 13. The main source of load-side disturbance, how-
I ever, is eliminated by the avoidance of inner flywheel bearings. The
variations in the low resistance of the outer shaft precision ball bear-
ings are superimposed on a high d-c velocity which according to Fig.
I 6(5) reduces the solid friction resistance. One may therefore greatly
discount the influence of the load-side characteristic of Fig. 13.
The above design considerations were concerned with inertia and
j resistance components of the filter structures. Compliance, the
I third essential filter element, is supplied by the film itself in most
j damped impedance drives. In other words, the very flimsiness and
i the curling propensity of the film which the Court had stressed as the
main obstacle to smooth film propulsion, is utilized to buff the irreg-
I ular shocks of the gear drive. The more one reduces the d-c tension
of the film by auxiliary drive torque or by precision ball bearings, the
looser and more compliant becomes the film loop between sprocket
drive and drum. While in highly resistive structures it resembles a
straight line, the low tension film bends into a loop shaped like a U
or preferably like an S. The compliance of such film loops has been
determined by experiments4 and by analysis (see Appendix). Its
worst characteristic is that it is highly variable. It is approximately
proportional to the 1.5 power of the film bending stiffness and in-
468
W. J. ALBERSHEIM AND D. MACKENZIE [J. S. M. p. E.
versely to the 2.5 power of the loop tension. The film stiffness
and loop shape vary considerably according to weather condition,
film age, and film pressure in the storage cans, and the film tension
naturally depends on the conditions of the scanner bearings which
also are subject to change. It is therefore impossible to determine
and maintain a fixed resonance and cut-off frequency of the mechan-
ical filter, and low-frequency flutter which is well suppressed at the
time of installation may become noticeable after short use and re-
quire servicing. When uniformity and predictability of filtering
FIG. 14.
High-quality drive with liquid flywheel and elastic
sprocket.
performance are required, the objection of the Court to the "flimsy,
curling film" must be sustained. The filtering can (and should) be
made independent of the film loop by supplying an auxiliary com-
pliance based on the more permanent properties of metallic springs.
The most convenient method for the introduction of this compliance
is an elastic sprocket in which a spiral spring is interposed between
the drive shaft and the sprocket rim. Fig. 14 shows schematically
the mechanical arrangement of a high-quality film drive which em-
bodies the liquid stabilized flywheel and an elastic driving sprocket.
The electrical equivalent looks rather complicated. The reason is
that the small inertia of the sprocket rim must be depicted by an
Nov., 1941]
ANALYSIS OF SOUND-FILM DRIVES
469
additional small inductance and that the d-c and gear frequency ve-
locity component Vi is impressed at the sprocket shaft, the 96-cycle
velocity component V* at the sprocket rim. At some relatively high
frequency there is danger of resonance between the compliance of
film loop and sprocket springs and the inertia of the sprocket rim.
This will ordinarily be damped out by the internal viscosity of the
film loop itself and by bearing friction, but in order to play doubly
safe, the elastic sprocket is provided with a little internal damping by
means of a small friction pad.
Summing up, the recommended film drive, which is schematically
shown in Fig. 14, should contain the following elements :
(1) A motor which does not appreciably change speed with line
variations; i. e., an amply powered synchronous motor or a low-slip
induction motor.
TANGENT POINT
INFLECTION DISTANCE, 0
FIG. 15. Film loop compliance.
(2) A smooth recording or scanning drum of standardized diame-
ter (between 1.5 and 2.5 inches).
(3) A mechanical low-pass filter attenuating high frequencies at
least 12 db per octave with a cut-off below l/i cps. The main shunt
compliance of this filter should be independent of film and weather
conditions and preferably consist of metal springs. The series in-
ductance and damping resistance should be provided by a stabilized
liquid flywheel.
A film drive based on these design principles should be able to per-
form consistently with the low flutter amplitudes now obtained only
by frequent maintenance adjustments and reduce film speed varia-
470 W. J. ALBERSHEIM AND D. MACKENZIE [J. S. M. P. E.
tions to a level which is unnoticeable in the reproduction of sound-
film records.
APPENDIX
(1 ) Drive-Side Transmission Characteristic of Filtered Sprocket Drum ( Fig .4). —
Assuming that the motional impedance of the motor and gears is very high com-
pared to that of the filters, one may regard the drive shaft as a constant-velocity
source. If one calls the angular velocity of the drive shaft v\, that of the drum vz,
then the transmission factor is
R +
F=v-2 = _ _ , with co = 2ff (1)
The absolute value of F is
with
n = Ci/co2 (frequency factor) (5)
and
r = R2Ci/J (damping factor) (4)
The transmission factor for d-c is
(5)
At high frequencies it approaches
| Fa, 1 = L = * (6}
n /co
which is inversely proportional to frequency, corresponding to an attenuation of
6 db per octave. The transmission factor has a peak at the frequency
(7)
The peak transmission is
1 FP I = rl - 2(1 + r - VFT20
The peak is always greater than one and depends only on the damping factor r.
Nov., 1941] ANALYSIS OF SOUND-FILM DRIVES 471
(2} Drive-Side Transmission as Modified by Bellows Compliance. — By the
addition of C2 (dotted in Fig. 4) the transmission factor becomes
C,)
1 - /do;2 + £;*(Ci + C2 - J
Substituting the following symbols :
= n (10)
= k (11)
/C,
(12)
one finds
For a given coupling factor &, the transmission is a function of the frequency
and damping factors, n and r. The influence of r is limited to the second term
of (13) and is eliminated for
2 2d + 2C2
+ 2C2
At this frequency the transmission has a gain determined solely by the coupling
factor:
| Fp | may be called the peak factor because it becomes the absolute response
maximum if one makes dF/dn equal to zero at np. This is achieved by the
"optimum" choice of the resistance factor.
'•* - ^ - 1 + 55- (16}
or
»
:
The d-c transmission is
Fo = 1
The high-frequency transmission approaches
which is inversely proportional to the square of the frequency with an attenua-
tion of 12 db per octave.
472 W. J. ALBERSHEIM AND D. MACKENZIE [j. s. M. P. E.
(3) Drive-Side Transmission Characteristic of Resistance-Terminated Filter
(Fig. 8}. — The transmission factor is
RCj*
Using the symbols (3) and (4), the absolute value is
\F\ = [(1 - n)2 + rn]-o.6 (21)
The d-c transmission is
^o = 1 (22)
The high-frequency transmission approaches
(23)
JCu*
that is, 12 db per octave.
The shape of the characteristic depends on r.
is the critical value.
For
This condition is shown as Curve 3 in Fig. 9.
For
(24)
r< rc (25)
the transmission has a peak
1 4J"2
Fp = r - 0.25r2 = 4R*CJ - R*CZ
which it reaches at the frequency
r > rc (28)
This curve is flat at low frequencies and droops smoothly near resonance fre-
quency as shown in Curve 2, Fig. 9.
For
r > rc (30)
the transmission begins to droop at low frequencies in accordance with Curve 4.
Fig. 9.
Nov., 1941] ANALYSIS OF SOUND-FILM DRIVES 473
(4) Resistance- Coupled Auxiliary Drive (Fig. 10}. — (a) Transmission of film-
side disturbances: Assuming that the compliance Cz of the auxiliary gear drive
is negligible, the transmission becomes identical with the "drive-side" character-
istic of the resistance-terminated filter (Fig. 9, Curve 3).
(b) Transmission of disturbances from auxiliary drive ("load-side" distur-
bances) : For negligible C% the transmission factor is
~ RCju + 1 -
Using the symbols (3) and (4) the absolute value of transmission becomes
r/n _ /,«\
The peak value of transmission occurs at
np = 1 (33)
and equals
Fp = 1 (34}
At low frequencies F approaches
| Fo | = RCu (35)
which increases 6 db per octave.
At high frequencies F approaches
| Fm | = £ (36)
Leo
which decreases 6 db per octave.
The two forms of (32) show that when plotted on a logarithmic frequency
scale the curve is symmetrical with regard to n = 1.
(5) Stabilized Flywheel (Fig. 12).— (a) Transmission of drive-side distur-
bances :
(57)
1 -I- CJOJ I Jsjw T T . - — £ }
\ JmJO) + R/
Introducing the symbols
(39)
(40)
r = ^r (41)
one finds
474 W. J. ALBERSHEIM AND D. MACKENZIE [J. s. M. P. E.
| | = (n - 1)* + «2 (1 - *) + r (4*)
The d-c transmission is
F0 = 1 (43}
At high frequencies it approaches
j-
kn
The attenuation increases 12 db per octave.
At the frequency
2JS + 2Jm
(45)
1 + k 2J + Jm
the second term on the right side of (39) vanishes, leaving FP independent of r
and equal to
Fp = -1— - = 1 + 2 £ (46)
This value is the peak of the whole response characteristic if one adjusts r to
make the value of (42) an extreme for n = np. One finds for the "optimum
drive-side" resistance rd the equation
2(np - 1) - V i^A* = 0 (47)
wp + r d
and
2fe 27,
fd = =
7? , _ 2JsJm2
'
(6) Admittance to load-side disturbances: The impedance is
v 1 T . Rjmjco 1 , ,
Z = -f^- + J&* + PIT' = "BY^7" ^
Cju R + /mja; rCjw
At low frequencies the admittance approaches
G0 = IT = Cju (51)
£o
increasing 6 db per octave. At high frequencies the admittance approaches
Go, = yi-f (52)
JgJU
decreasing 6 db per octave. Introducing the symbol
' -J? (53)
Nov., 1941] ANALYSIS OF SOUND-FILM DRIVES 475
one finds
This is independent of r at the common point frequency np defined by (45).
The "common" value of admittance is
Gc = _ j\z = V%C(Jm + 2Js)/Jm (55)
It is the "peak admittance" if one adjusts r to make the value of (55) an extreme
for n = np. One finds for this "optimum load-side" resistance rL the equation:
, 1 _ «p(l - W)
an
Wp tip ~\~
and
(56)
2 1 + 3fe _ 2/» + 2J8 Jm + 47,
" 1 + * ' 3 + k " Jm + 2J, ' 3Jm + 47.
.
+ 4/.
^?L is always larger than J?d but for most practical designs the difference is small
enough to allow a compromise which only slightly increases the peak values per
(46) and (55).
(6) Compliance of Film Loop (Fig. 15).—
If E is the modulus of elasticity of the film,
J the moment of inertia of its cross section,
P the longitudinal loop tension,
Q the curvature,
s the length of film loop and
one finds the "Loop equation"
ff-£ <«»
which has the general solution
0 =-smh-+-icosh (61)
If the fijm is wound around two drums or sprockets in opposite directions, forming
an S, the loop contains an inflection point which we choose as origin of co-
ordinates. If one calls R the drum radius and <f> the angle between film and ab-
scissa,
R sinh s/B
476 W. J. ALBERSHEIM AND D. MACKENZIE [J. S. M. P. E.
"For flat loops one may use the approximations
<*>, <C 1 (63)
y" = O
x ^ s
y' = <*>;
„ = sinh x/B , }
R sinh /
then
where
and
D = inflection length per Fig. 15
Based on (64) it can be shown that the increase of film length due to looping is
AS = J^-2[6a2/ - 12a + 2 (a - coth /)3 + 3//sinh 2/ + 3 coth /] (65)
in which
a = coth/ - / + V/2 - 2/coth/ + 2 ((56)
For large/ (long flat loops) one can expand (65) into a power series in I//:
AS = [1 — 1.5// + . . .] = — + . . . (67)
one finds
C = dAS/dP = dAS/dB (68)
± _[B^ B* IT ^-|
[_4R2 2R2D 2EJ]
(69)
REFERENCES
1 SHEA, T. E., MACNAIR, W. A., AND SUBRIZI, V. : "Flutter in Sound Records,"
/. Soc. Mot. Pict. Eng., XXV (Nov., 1935), p. 403.
2 DREW, R. O., AND KELLOGG, E. W.: "Filtering Factors of the Magnetic
Drive," J. Soc. Mot. Pict. Eng., XXXV (Aug., 1940), p. 138.
3 ROWLAND, H. A.: U. S. Pat. No. 691,667 (1899).
4 COOK, E. D.: "The Technical Aspects of the High-Fidelity Reproducer,"
/. Soc. Mot. Pict. Eng., XXV (Oct., 1935), p. 289.
Nov., 1941] ANALYSIS OF SOUND-FILM DRIVES 477
DISCUSSION
MR. KELLOGG : * This paper is a valuable contribution to the theory of mechani-
cal filters and their application to sound-film drive, and the expedient to which
the authors have resorted to eliminate the last vestige of solid friction in a sta-
bilized flywheel is of much interest. There are several matters, however, concern-
ing which I feel that something further should be said in justification of the types
of constructions that my associates have adopted for our recording and reproduc-
ing machines.
The damping coefficient for the magnetic drive was calculated on the basis of
its supplying only enough forward torque to overcome bearing friction. Since
in all magnetic drive applications, the drum is overdriven and the film is pulling
back on the drum, the magnets are supplying considerably more torque than
that which just balances bearing friction. In view of this, and the fact that ball
bearings have not been employed, but sleeve bearings with small clearance, the
magnetic coupling, and therefore the damping is much greater than the assump-
tion made in the paper would indicate. Sleeve bearings have been employed for
the reason that they have been found to give a smoother action than any ball
bearings, despite the low average friction of the latter.
I have, in several publications, recognized non-uniformity in magnet rotation
as a conceivable source of disturbance in drum motion. In prolonged experience
with this system and in numerous tests, we have found this to be a negligible fac-
tor, and the reason. I believe, may be explained in terms of Fig. 11. This figure
shows that the full amplitude of speed variation of the magnets can be trans-
mitted to the drum provided that the magnet speed irregularity is of exactly the
resonance frequency established by the inertia of the flywheel and the stiffness of
the film loop. This zero attenuation at a single frequency is simply one aspect
of the fact that a resonant system with zero resistance has zero impedance. Fig.
13 shows the same characteristic. There is no possibility, it will be noted, that
the magnetic coupling will cause a greater amplitude of drum disturbance than
that which is in the magnet-drive itself, and actually it would be less because of
some damping in bearings. At all frequencies except that at which the elastic
and inertia reactances cancel, there is substantial attenuation. It is character-
istic of a system of gears that there are no disturbances of lower fundamental
frequency than the rotation frequency of the slowest gear in the train. (No
difference frequency or beat effects are produced.) There is thus no occasion
for any irregularity in magnet speed or for lower frequency than the rotation of
the magnets themselves, or about 31/* revolutions per second. As compared
with this, the natural frequency of the drum and flywheel is of the order of 1/t
cycle per second. According to Fig. 11, there would be something like 27 db at-
tenuation of the lowest frequency magnet disturbance. Components of higher
frequency would be attenuated still more.
As compared with an internally damped flywheel, the auxiliary drive makes it
practicable to employ a much greater directly connected mass on the drum shaft
and still provide adequate damping. The weight on the drum bearings, however,
is not materially increased. The greater mass, with corresponding damping,
* Communicated.
478 W. J. ALBERSHEIM AND D. MACKENZIE [j. s. M. P. E.
means a higher mechanical impedance to resist "load-side" disturbances, due, for
example, to bearing irregularities.
The 5-shaped loop is mentioned as preferable to a ^/-shaped film loop. This
does not agree with our experience, although the S-shaped loop can serve very
well in a filtering system, and may often be chosen for the sake of simplicity in
design.
Turning next to the oil-damped wheel, which has sometimes been called a
"rotary stabilizer" and sometimes a "kinetic scanner," the friction in the ball
bearing within the stabilizer should not be included as a disturbing force. When
the drum is up to speed, there is no continuous rotation on this bearing. It is
acknowledged that bearing friction, if it locks the inner flywheel to the drum shaft,
can prevent damping. Oscillations, however, must be extremely small in order
not to cause some relative motion, and when there is any relative motion there is
damping. Some tests and demonstrations have been given that indicate a loss
of damping for very small oscillations, but these tests were made without, the
wheel turning over, and not with the oscillation superimposed on continuous ro-
tation, as would be the condition in actual operation. With continuous rota-
tion, the direction of gravity on the bearing is always changing and goes through
several revolutions in a single period oi the natural oscillation. With this chang-
ing gravitational force, the probability of locking so as to prevent all relative mo-
tion is reduced to almost zero.
Since in any system with flywheel damping without an auxiliary drive, the film
must accelerate the entire rotating system, it is not feasible to employ as heavy
elements as with an auxiliary drive, nor is it practicable to get as high a directly
connected moment of inertia. In view of this, it is difficult for me to see how the
very low natural frequency mentioned near the end of the paper (l/± cycle per
second) can be obtained without an extremely flexible spring. I should expect
that a spring with sufficient flexibility to accomplish this would be wound up
through a large angle by the frictional torque, and since friction varies with tem-
perature and other factors, there would be considerable departures from syn-
chronism.
For the benefit of those who may wish to refer to earlier publications relating
to sound-film drives, the following list is submitted.
KELLOGG, E. W.: "A New Recorder for Variable-Area Recording," J. Soc. Mot.
Pict. Eng., XV (Nov., 1930), p. 653.
COOK, E. D.: "The Technical Aspects of the High-Fidelity Sound-Head," /.
Soc. Mot. Pict. En?., XXV (Oct., 1935), p. 289.
KELLOGG, E. W. : U. S. Pat. Re. 19,270.
HANNA,C.R.: U. S. Pat. No. 2,003,048.
KELLOGG, E. W., AND DREW, R. O.: "Filtering Factors of the Magnetic Drive,"
/. Soc. Mot. Pict. Eng., XXXV (Aug., 1940), pp. 138-164.
KELLOGG, E. W.: "A Review of the Quest for Constant Speed," J. Soc. Mot.
Pict. Eng., XXVIII (April, 1937), p. 337.
MR. ALBERSHEIM:* In our survey of resistance-coupled auxiliary drives, such
*Communicated .
Nov ., 1941] ANALYSIS OF SOUND-FILM DRIVES 479
as the magnetic drive, we made the most favorable assumptions. These include
a loose film loop. If in Mr. Kellogg's design the pull of the film loop increases
the required magnet torque considerably, its stiffness must be greater, and its
filtering properties less than we estimated. However, from the paper, "Filtering
Factors of the Magnetic Drive" by Messrs. Kellogg and Drew, it appears that
the average torque needed to overcome the film loop pull is only about one inch-
ounce.
We can not agree that in a system of gears there are no disturbances of lower
fundamental frequency than that of the slowest gear in the train. A disturbance
caused by the meshing of two high teeth has a period determined by the smallest
common multiple of the numbers of teeth in all the gears. Assuming, for in-
stance, that a 20-tooth gear drives a 25-tooth gear, the same teeth will not mesh
until 100 teeth have passed the line of contact, i. e., until after five revolutions of
the smaller gear and four revolutions of the larger gear. Unless special precau-
tions are taken it is therefore always possible that the auxiliary drive may gener-
ate disturbances in resonance with the flywheel-loop system. With regard to
the shape of the film loop, Mr. Kellogg appears to prefer the U loop. In an actual
drive, these distinctions are not always sharp. Fig. 5 on p. 146 of the paper by
Kellogg and Drew, for instance, shows S bends on each side of the main U loop,
so that the filter combines properties of both loop shapes. Such structural de-
tails are, of course, a matter of design choice, and we have already stated in the
text of the paper that auxiliary overdrives have been successful. Our own pref-
erence for damped flywheels was based on economy of design.
The locking of inner ball bearings in damped flywheels or "kinetic scanners,"
which the liquid flywheel avoids, does not occur radially between balls and shaft
but laterally between balls that are wedged together. This locking action has
been verified, even during rotation of the flywheel, by quantitative tests of the
Bell Telephone Laboratories. Experience has shown that introduction of the
liquid flywheel reduced the flutter content, compared to previous designs, and
eliminated the occasional jumps of flutter amplitude which we attribute to locking.
As in this case, practical success is the ultimate test in the design of elastic
flywheel sprockets. A recently built re-recording machine embodying this fea-
ture obtains the low natural frequency recommended in the paper and has been
found to have consistently low flutter, to require less maintenance than previous
designs, and to be free from any observable asynchronism.
A SUGGESTED CLARIFICATION OF CARBON ARC
TERMINOLOGY AS APPLIED TO THE MOTION
PICTURE INDUSTRY*
H. G. MACPHERSON**
Summary. — This paper presents definitions of the three general types of carbon
arcs used in the motion picture industry, the distinction between them being based upon
the origin and the character of the radiation in each case. In the low-intensity arc, the
principal light-source is incandescent solid carbon at or near its sublimation tempera-
ture; in the flame arc, the entire arc stream, made luminescent by the addition of flame
materials, is used as the light-source; while the high-intensity arc is one in which, in
addition to the light from the incandescent carbon, there is a significant amount of light
originating in the gaseous region immediately in front of the carbon. With these con-
cepts as a basis, the theory of light generation in each case is presented with the object
of further clarifying the distinction between the three types of carbon arcs.
The carbon arcs used in the motion picture industry are of three
general types — the low-intensity arc, the flame arc, and the high-in-
tensity arc. The low and high-intensity arcs have been used in
both motion picture photography and in projection, although the
former is now obsolete in photography and is steadily being replaced
by the more efficient high-intensity type in the projection field as
well. The most important use of the flame arc in the motion picture
industry is in photography, where it provides a broad beam of suit-
able color quality for general set illumination. The system of no-
menclature that has grown up with the industry is more descriptive of
certain types of lamp than of the character of the arc. Names such
as "mirror arc," "Hi-Lo," "Simplified High-Intensity," "M. P.
Studio," "Baby Spot," and "Sun- Arc" are in common usage, but
some of these terms are not descriptive of either the arc itself, the
mechanism, the optics, or the service. It is the purpose of this paper
to define the arc itself, irrespective of the other factors just men-
tioned, so that a given trim may be readily classified as to whether
it is a low-intensity, a flame, or a high-intensity arc.
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May 1,
1941.
** National Carbon Company, Cleveland, Ohio.
480
CARBON ARC TERMINOLOGY 481
As a basis for classification, the physical nature of the light-source
[offers the most logical distinction. Therefore the definitions have
been phrased from this standpoint, followed in each case by descrip-
tive material in their support.
The Low-Intensity Carbon Arc. — The low-intensity carbon arc is
one in which the principal light-source is incandescent solid carbon
at or near its sublimation temperature.
In the vast majority of cases, this arc is operated on direct current,
although a few carbons are still sold for alternating-current service.
The direct-current arc uses neutral cored positive electrodes and either
solid or cored negative electrodes. A neutral cored carbon contains
a core consisting predominantly of carbon, less dense than the sur-
rounding shell, and incorporating a small percentage of an arc-sup-
porting material such as a potassium salt, which does not contribute
significantly to the light. "White Flame A.C." carbons are used in
the alternating-current, low-intensity arc. The core of these car-
bons contains flame-supporting material the function of which is to
steady the arc, quiet the hum, and whiten the light. In the direct-
current arc, the crater face of the positive electrode is used as the
light-source for projection, since it operates at a much higher tem-
perature than the negative electrode and so provides about 90 per
cent of the total light from the arc. The bright spot on the end of
this positive carbon has a rather sharply delineated boundary which
is called the anode spot or the positive crater. This crater marks the
region within which mos± of the electric current passes between the
anode and the arc stream.
The surface of the crater is heated to its high temperature as the re-
sult of the absorption of energy from electrons discharged there, and
the absorption of energy from the gaseous region known as the anode
layer directly in front of the anode. The arc gas in the major part
of the arc stream is very hot, having a temperature of 6000 °C or
more, and is therefore highly ionized. In its highly ionized condition,
it can carry the current with a fairly low voltage drop per unit length,
amounting to about 20 volts per centimeter. In the anode layer,
however, the gas is cooled by the proximity of the anode to such an
extent that its degree of ionization, and therefore its electrical con-
ductivity, is very low. Because of its low electrical conductivity
and because of space-charge effects, a high voltage drop must be con-
centrated in the region of this anode layer in order to force electrons
through it and thus conduct the arc current. This voltage is called
482 H. G. MACPHERSON [J. s. M. P. E.I
the anode drop, and is of the order pf magnitude of 35 volts for a low-
intensity arc.
This energy dissipated at the anode heats it to incandescence, the
maximum temperature obtained being limited by the sublimation
temperature of carbon. This limits the maximum brilliancy of the
low-intensity arc to a value of about 175 candles per square-milli-
meter. The area of the anode spot or crater adjusts itself for a given j
current so that the heat input is sufficient to bring the crater to a
value near this sublimation temperature. An increase in current in
the low-intensity arc will, therefore, not increase appreciably the'
maximum brightness, but will increase the area of the crater surface. ;
Compared to a high-intensity arc, the current-density of a low-inten-
sity arc is quite low. For the familiar commercial lamps, the current- j
density in the positive carbon ranges from approximately 50 to 200 !
amperes per square-inch.
It is interesting to observe that carbon is an ideal material for use I
as an electrode in such an arc, because it remains a solid at a higher
temperature than any other substance of suitable electrical and
thermal conductivity, so that a more brilliant light may be produced;
while its property of volatilizing directly from a solid to a gaseous
state permits convenient disposal of the consumed portion without
danger to the associated mechanism.
The Flame Arc. — A flame arc is one in which the entire arc stream,
made luminescent by the addition of flame materials, is used as a
light-source.
The flame arc was a natural development from the low-intensity
arc, obtained by enlarging the core in the electrodes and replacing
part of the carbon there by chemical compounds capable of radiating
efficiently in a highly heated gaseous form. These compounds are
vaporized along with the carbon and diffuse throughout the arc flame,
rendering it luminescent. The high concentration of flame materials
in the core reduces the area and brilliance of the anode spot so that,
at the low current-densities used in flame arcs, the contribution of
the electrode incandescence to the total light becomes unimportant.
The evaporation of flame materials is slow relative to that obtained
in a high-intensity arc, and the resulting concentration of flame ele-
ments in the arc stream is low so that a high brilliance does not re-
sult. Since the whole flame is made luminous, however, the light-
source is one of large area and the radiating efficiency is high.
The radiation emitted by the flame arc consists chiefly of the
jrov., 1941] CARBON ARC TERMINOLOGY 483
Biaracteristic line spectra of the elements in the flame material, and
ih the band spectra of the compounds formed. The rare earth metals
If the cerium group are used as flame materials where, as in most
ases, a white light is desired, while calcium salts are used to give a
ellow light and strontium salts red.
•\ The High-Intensity Carbon Arc. — The high-intensity carbon arc as
Used for projection is one in which, in addition to the light from the
Incandescent crater surface, there is a significant amount of light
riginating in the gaseous region immediately in front of the carbons
IB the result of the combination of a high current-density and an
(tmosphere rich in flame materials.
To produce a direct-current high-intensity arc, the positive carbon
must be cored with chemical compounds similar to those used in flame
Ire electrodes. The current-density, however, is much higher, so
lat the anode spot spreads over the entire tip of the carbon, result-
ig in the rapid evaporation of flame material as well as carbon from
le core. Since the flame material is more easily ionized than car-
Dn, its presence in the anode layer results in a lower anode drop at
ic core area than at the shell of the carbon. This tends to concen-
\ -ate the current at the core surface, resulting in the hollowing out of
|| crater as the current is increased. The rapid evaporation of the
lime material produces a high concentration of this efficiently radi-
ling gas in the crater and immediately in front of it. This gas, of
(purse, radiates in all directions, even back toward the crater surface,
|id consequently tends to serve as a blanket preventing the radiative
Idling of the crater face. The heat liberated at the crater face must
[lien be dissipated entirely through evaporation of more flame ma-
rial and through conduction back along the positive carbon. This,
K course, tends to increase the evaporation of material within the
l|ater and aids in the tendency for crater formation. Thus in a
jlgh-intensity arc there is a close correlation between the crater depth
I id the brilliancy of the arc gas within and immediatey in front of the
l|ater; for a given type of positive carbon, there is a linear relation-
[kp between the crater depth and the excess brightness over that of
| low-intensity arc.
I An increase of current in a high-intensity arc increases the crater
Dea only slightly, but produces a marked increase in brilliancy.
||he maximum brilliancy of the crater obtained in various types of
Irect-current high-intensity arcs used in common commercial lamps
luges from 350 to 1200 candles per square-millimeter with current-
484 H. G. MACPHERSON
densities in the positive carbon ranging from 400 to well over 100C
amperes per square-inch. Experimental carbons have been produced
with brilliancies in excess of 1500 candles per square-millimeter.
The increased brilliancy of a high-intensity over that of a low-in^
tensity arc is produced by radiation from the high concentration o[
flame materials within the confines of the crater. The thermal energ)
supplied by the electrical power input to the arc continually excites
the atoms of the flame materials to higher energy states, and the ex-
cess energy of these atoms is being continually released in the forir
of radiation. The high density of radiation results in the productior
of a strong continuous spectrum in addition to the line spectrum of thf
flame elements. Since radiation in the visual range of wavelengtt
from 4000 to 7000 Angstroms is required in motion picture services
the most efficient compounds to use as flame materials are those pro-
ducing the most radiation in this spectral band. Nothing bettei
than the rare earth metals, of which cerium, lanthanum, neodymium
and praesodymium are typical examples, has ever been found for this
purpose. With complex atoms having many electrons, counties*
opportunities for the energy exchanges that give rise to radiation in
the visual region are provided, so that no one part of the spectruir
is unduly exaggerated, and a white light is naturally produced.
The alternating-current high-intensity arc is also a true high-in-i
tensity arc within the meaning of the definition proposed. The higl:
current-density and the high concentration of flame materials com-
bine to produce light both from the incandescent electrode and froir
the gaseous region immediately adjacent, as they do on direct current.
Summary. — The fundamental distinction between the different
types of arc is based upon the origin and character of the radiation
The chief contributing factors associated with this are compositiot
of carbon, current-density, and brilliancy. The low-intensity arc ii
one in which the principal light-source is incandescent solid carbor
at or near its sublimation temperature. The high-intensity arc if
one in which in addition to the light from the incandescent cratei
surface there is a significant amount of light originating in the gase-
ous region immediately in front of the carbon. In the flame arc the
entire arc stream, made luminescent by the addition of flame ma-
terials, is used as the light-source.
Many members of the Cleveland and Fostoria laboratories, anc
of the Carbon Sales Division of National Carbon Company have
contributed to the material presented here, and the author grate
fully acknowledges their assistance in this connection.
IMPROVED METHODS OF CONTROLLING CARBON ARC
POSITION*
D. J. ZAFFARANO, W. W. LOZIER, AND D. B. JOY**
Summary. — This paper shows, both from previous data and fundamental con-
siderations, the close control of carbon position necessary to obtain constant light on the
projection screen, particularly with reflector-type high-intensity carbon arc lamps.
Review of the characteristics of this type of lamp and optical system reveals that in
order to obtain constant light on the screen it is necessary to avoid variation of carbon
position and changes in arc current due to line-voltage fluctuations. Methods of arc
control employing photoelectric cells and bimetallic thermostats directly responsive to
carbon position have been analyzed with regard to their applicability for this pur-
pose. Some examples of these have been constructed and have demonstrated that
automatic devices of simple construction are capable of maintaining constant the in-
tensity, distribution, and color of the light on the projection screen.
Recent years have seen great advances in the carbon arc light-
sources used for motion picture projection. The "Suprex" type of
arc and the more recent ''One Kilowatt" arcs have brought to both
the medium and small-size theaters much-needed increases in screen
brightness, a more favorable color quality, and improvement in
efficiency. The fundamental factors important to the operation of
these reflector-type high-intensity arc lamps have been described in
several publications in this JOURNAL.1-2'3 One of the important re-
quirements for uniform light is the fact that the arc must be accurately
maintained at the proper distance from the reflector. The purpose of
this paper is to show how automatic devices can be employed with
these lamps to position the arc and deliver a more constant light to
the screen.
The necessity of accurate positioning of the arc is made clear by
examination of the geometry of the optical system of the reflector
type lamp. Fig. 1(^4) shows the essentials of the optical system
commonly employed. The light-source and the film aperture are
placed at the two foci F and F' of the elliptical reflector, which gathers
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May
10, 1941.
** National Carbon Company, Fostoria, Ohio.
485
486
ZAFFARANO, LOZIER, AND JOY
[J. S. M. P. E.
the light from the crater of the positive carbon and directs it to the
film aperture, which in turn is imaged on the screen by the projection
lens. Fig. l(A) shows the path of a ray from one focal point, F, to the
margin of the mirror and to the center of the aperture, F'. It can be
seen that if the crater of the positive carbon is positioned at Q, the
Projector? fas
1 i
I
k
H
I/-
F
P
Rqu fo center of
/ film <7/>erfure
\
FIG. 1C4). Optical system of reflector-type projection lamp, showing
relation of position of positive carbon to the light-ray travelling to the center
of the film aperture. 1(5). Showing movement A within which light-ray
to center of film aperture will originate on light-source of diameter D.
light from the center of the crater is focused at the center of the film
aperture. If the positive carbon is moved ahead to position P, the
ray travelling to the center of the aperture originates from the cooler
portion of the carbon back of the crater which results in a change in
color and intensity of the light at the center of the aperture and
projection screen. Similarly, if the carbon recedes to position R,
;NOV., 1941] CONTROLLING CARBON ARC POSITION 487
the ray travelling to F' originates from the arc stream in front of the
rjcrater, which is blue in color.
Fig. l(B) shows the movement A of the light-source of diameter D
thin which the ray passing to the center of the film aperture F'
iwill originate on the light-source. The two quantities A and D are
related to the angle C by the equation:
tan C = ^
A
U)
n
«3
-It
OF Li<
Of
\
3.7S 540 3.SS 390
DiSTXMSCE IM InCHES
FROM POSITIVE CAHBOM To REFLECTOR.
FIG. 2. Light on projection screen vs. position
of arc: 7-mm positive, 6-mm negative carbons;
45 amperes; 5/i6-mch arc length.
With lamp and carbon combinations in use today, angle C may be as
at as 70 to 75 degrees, for which the tangent is approximately
iree, indicating from equation 1 that the movement A would be
ibout one- third the useful diameter D of the light-source. The use-
rul diameter of the light-source in some examples may be as small as
p. 15 inch in which case equation 1 would indicate a movement A of
488
ZAFFARANO, LOZIER, AND JOY
[J. S. M. P. E.
0.05 inch. Although the basic considerations used in deriving equa-
tion 1 are greatly simplified compared with those that actually exist,
the values calculated for the movement A roughly agree with labora-
tory determinations of the allowable arc movement for satisfactory
screen color, especially with carbons burned at low current-densities,
where the light-source has limited depth. Equation 1 suggests that
CURRENT in AMPS.
FIG. 3. Light on projection screen vs. current:
7-mm positive, 6-mm negative carbons; 5/ie-inch
arc length; positive carbon 3.76 inches from
reflector.
the allowable arc movement can be increased by limiting the collect-
ing angle C to a smaller value and by increasing the diameter of the
light-source. Combinations of these two factors can be chosen so
that there is no decrease in speed or relative aperture of the optical
system and therefore no loss in light on this account. Under these
circumstances greater allowable arc movement is observed. Ex-
amples are the condenser-type high-intensity lamp and some of the
earlier low-intensity reflector arc lamps. However, the smaller
Nov., 1941] CONTROLLING CARBON ARC POSITION
collecting angle results in incomplete utilization of the available cone
of light, and the increase in light-source size necessitates larger
carbons and higher currents to cover the film aperture and main-
tain the same brilliancy and light on the screen. Both these result
in an undesirable reduction of efficiency.
Even within the range of allowable movement of the positive
carbon for satisfactory screen color, there are changes in total screen
light and in the distribution of light over the screen. The relations
between screen light, screen distribution, arc length, current, and
arc position have been previously published2 and are reproduced in
Figs. 2, 3, and 4. Fig. 2 shows the variation in total screen light and
distribution with change in the arc position, at constant arc length
and current, for one of the popular "Suprex"-type reflector lamp
combinations. This clearly indicates that to hold the variation in
screen intensity to a few per cent would require that the arc position
be held within 0.01 to 0.02 inch.
The above discussion shows the necessity for accurate positioning
of the arc. The degree to which this is accomplished with most of the
present lamps depends to a large extent upon a favorable combina-
tion of the stability of the power source, the speed characteristics of
the electrical feeding motor, the uniformity of burning characteristics
of the carbons, and the attentiveness of the projectionist. When it
is realized that high-intensity reflector lamps may consume from 2 to
4 inches of positive carbon during a 20-minute reel, and that a move-
ment of the crater position of 0.01 inch would amount to only x/4 to
l/2 per cent of the total length of carbon consumed, it can be seen
that this degree of control is probably beyond the capabilities of any
control system except one that is directly responsive to the position
of the positive carbon.
Automatic methods of arc control responsive to the position of the
carbons offer practicable means of holding the light on the screen
constant and maintaining optimum burning conditions at all times.
Some of what will be described in this paper is not new. Patents exist
covering various embodiments of controls, and to insure freedom from
infringement in adopting arc controls for specific lamp apparatus, the
active patent art on the subject should be examined. Automatic
devices responsive to carbon position have been employed to a limited
extent with condenser-type projection lamps and to a greater extent
on searchlights. They have not, however, found appreciable usage
as yet on reflector-type projection lamps.
490
ZAFFARANO, LOZIER, AND JOY
[J. S. M. p. E.
Requirements for Constant Screen Light. — Figs. 2, 3, and 4, giving
the fundamental characteristics of high-intensity reflector lamps,
point out essential requirements for constant light on the screen. As
already discussed, Fig. 2 shows that movement of the arc position
greatly changes both the intensity and the distribution of the screen
light. Fig. 3 demonstrates that an increase in arc current increases
the screen light. Fig. 4 shows that the arc length may be varied
AVTI
ASC SCREE
1 UGMT
LOCATIOM
USED
Of PCW1T3
FOR LI8HTP
A\
DM SCREEN
lATIO
A
\
\l
w
OfiKT DlSTfi
iBurion
'. p
'Is
.1 s|
?I5
}
,
E
•
ARC LOlSTH In INCHES
FIG. 4. Light on projection screen vs. arc length:
7-mm positive, 6-mm negative carbons; positive car-
bon 3.76 inches from reflector; constant current, 45
amperes.
considerably without affecting screen light so long as the arc cur-
rent and positive crater position are held constant.
Fixing the positions of both the positive crater and the tip of the
negative carbon with respect to the reflector will result in constant
light on the screen if all other conditions of the arc remain constant.
However, with some types of power supply, line-voltage changes pro-
duce corresponding changes in arc current which, as shown in Fig. 3,
would result in changes in screen light even when the positions of
both carbons are fixed.
Nov., 1941] CONTROLLING CARBON ARC POSITION
491
Where changes in power supply do occur, their effect upon the
screen light can be avoided through the use of a method of arc control
in which the position of the positive crater is fixed and the negative
carbon position is controlled by a current-responsive device that
changes the arc length so as to keep the current constant. This latter
method is particularly effective with the low-voltage power sources
commonly employed with "Suprex" and "One Kilowatt" d-c arcs
with which small changes in arc length result in relatively large
changes in arc current.
Methods for Controlling Arc Position. — One approach to the
problem of controlling the positions of the burning electrodes in the
rv'
ARC
ARC
image \
Lens
Receiwr
reports/ ve fo
arc
/fej*
FIG. 5. Optical system for arc controls, showing image of arc focused on the
receiver.
arc is to use the intense radiation emitted by the arc to actuate sensi-
tive receivers. Such devices include photoelectric cells, which con-
vert radiation directly into electrical energy; or thermocouples,
resistance thermometers, and thermostats, which function indirectly
through conversion of the radiation into heat. A simple method of
using this radiant energy for arc control is to project a side image of
the arc by a fixed lens as shown in Fig. 5. As the arc moves, the
image will also move, and a fixed receiver at the image will be sub-
jected to changes in radiation intensity as a direct result of the
displacement of the burning electrodes.
The relative intensity of radiant energy emitted along the axis
XX ' (Fig. 6) of the arc as detected by a thermopile and galvanometer
492
ZAFFARANO, LOZIER, AND JOY [J. s. M. p. E.
I
J
4s transmitted thru lens
\s transmitted thru lens and
rnmg No. 154 (infrared
insmitting) filter
A A->
\\
/ 1 \\ A
D- /i
Co
22
\
\
\
, tn
_^
x
\
\
\
_L
i=,
\
^ 3^^2^
x\
LSnou-uhife crater gases
Blue -tinted arc j/rea/n -
Yellou-uhite incandescent carbon decreasing in fe/i?per afore
and intensify auay from the elect rode tip
FIG. 6. Image of arc and distribution of intensity of radiant energy across
carbons and arc stream. The ordinates of Curve B would need to be reduced
by a factor of 2.5 to express them in correct proportion relative to Curve A.
is plotted in Curve A of Fig. 6, where the various features can be cor-
related with the portions of the arc from which they originate. The
intensity of emitted radiant energy exhibits maxima at both the posi-
tive and negative electrode tips and decreases rapidly a short distance
away. The intensity at the positive carbon tip is about three times as
great as at the negative tip. Variation of the arc current results
principally in changes in the intensity along the arc stream but does
Nov., 1941]
CONTROLLING CARBON ARC POSITION
493
not destroy the essential features shown in Fig. 6 or result in much
displacement of the positions of the maxima. The visual appearance
of the arc reveals at a glance that the spectral energy distribution of
the radiation originating from the various portions of the arc varies
markedly. This is further borne out by the Curve B in Fig. 6, ob-
tained after passing the radiation through a 5-mm thickness of
Corning No. 254 infrared-transmitting filter which absorbs the visi-
ble light. This shows that the arc stream is rich in visible light while
the incandescent carbons are relatively richer in infrared radiation.
By means of this filter, the radiant energy gradient between the posi-
tive carbon and the arc stream can be made much more abrupt.
?m RCA type 2051
*' fabe
Smegohms
Guardian Electric Co.
Ser/cs 60 relay u/th lODlh ohm cat/
150 ohm poknf/ometer 450 ohms
FIG. 7. 'Circuit of photocell and amplifier.
There are further marked differences in the radiation within the
visible wavelengths originating from the different portions of the arc
though these are not illustrated by the energy measurements of Fig.
6. How the marked change in energy along the carbons and arc
stream can be used to operate arc control devices will be explained.
Arc Control with Photoelectric Cells.— A vacuum-type photoelectric
cell was used as a receiver behind a slit y4 inch wide at an enlarged
arc image, as shown in Fig. 5, and was made to operate a relay through
an electronic amplifier in response to changes in the light-intensity
associated with movement of the arc and its image. With the arc
burning, the amplifier was biased so that the relay was inoperative
when the light from the positive carbon just behind the crater struck
the photocell. The lamp-feeding motor was adjusted to advance the
carbons at a rate slower than their consumption rate, and was con-
494
ZAFFARANO, LOZIER, AND JOY
[J. S. M. p. E.
nected to the photocell-actuated re^ay so that it would run at high
speed when the relay was energized. When the positive carbon burned,
back, the more intense light from the vicinity of the carbon tip struck
the photocell, tripping the relay which allowed the carbon to feed up
until the light at the photocell was reduced to its original level, at
which point the relay again became inoperative and the feed-motor
returned to its normal speed.
The photocell control circuit used is shown in Fig. 7, and employs
a single gas-discharge "trigger" tube. The speed of the feeding motor
is controlled as shown in Fig. 8 by means of a resistor m the motor
field circuit which is short-circuited by the relay when the motor is
running at low speed.
fa/ay
Mofor
fie/c/
Arc
Vo/fao(
Feedmofor
•for carbons
Method of connecting photocell and amplifier to control the speed
of the motor feeding the carbons.
Since a variation in light-intensity exists also at the negative carbon
tip, a double photoelectric control was constructed for controlling
both positive and negative carbons by means of two photocells and
associated amplifiers, giving constant arc length as well as constant
arc position. This control was used in conjunction with a "Suprex"
type of lamp modified to employ separate feed-motors for the posi-
tive and negative carbons. A photocell circuit essentially similar to
that of Figs. 7 and 8 was provided for each of the carbons. A side
image of the arc was focused on the photocells placed outside the lamp-
house. The light emitted from the vicinity of the two carbon tips
was admitted to the respective photocells through a double slit
placed at the arc image in front of the photocells. Performance data
on this combination are given in a later section of this paper.
Nov., 1941]
CONTROLLING CARBON ARC POSITION
495
Arc Controls with Thermostats. — It has been found possible to make
bimetal arc controls which possess sufficient sensitivity and are cap-
able of carrying the current necessary to change the speed of the
lamp-feeding motor, and which therefore do not require amplifying
equipment.
Since the Curve A in Fig. 6 is a plot of the variation in total energy
across an arc image, a curve of the deflection of a blackened bimetal
strip versus the position across the image would be expected to have
a similar shape with the maximum deflection occurring at the posi-
tions of the peaks on the curve. The simplest thermostat would
consist of a single bimetal strip with one end fixed and the other end
Bimetal
VA
Bimetal strips
r- Contacts
Mounting
(a)
Mask uith slit
(b)
Contacts
Bimetal strips
(C)
FIG. 9. (a and &) Singly compensated thermostats; (c) doubly compensated
thermostat.
free to deflect and make or break a circuit to a fixed electrical con-
tact in response to changes of temperature of the bimetal caused by
movements of the arc image. Such a thermostat, however, would be
unable to differentiate between the radiant energy received from the
arc and the heat received from the adjacent surroundings which may
vary during the "warm-up" period of the arc lamp or because of
room temperature changes. Compensation can be made for the
variable heat received from extraneous sources by replacing the
fixed electrical contact point by one mounted upon a "dummy" or
compensating piece of bimetal which is free to respond to the heat
received from the surroundings but which is shielded from the direct
radiation from the arc. This compensating member eliminates the
effect of the surroundings and leaves the relative motion of the contact
points dependent only upon the direct radiation from the arc. Such
496 ZAFFARANO, LOZIER, AND JOY [j. s. M. p. E.
a thermostat is shown in Fig. 9(a)^in which the "dummy" bimetal
strip is placed behind the "active" 'strip and thereby shielded from
the direct radiation of the arc. For purposes of discussion, we have
chosen to call this type of thermostat a "singly compensated" one.
Another example of a singly compensated thermostat is shown in
Fig. 9(6). This differs from the one of Fig. 9 (a) in that the length-
wise direction of the bimetal strip is placed parallel to the axis of the
carbons instead of perpendicular. Another difference is the use of
a mask with a narrow vertical slit to restrict the portion of the arc
image admitted to the bimetal. With the thermostat of Fig. 9 (a),
the orientation and narrow width of the bimetal strips effectively per-
form this function of a slit. In this arrangement of Fig. 9(6), wider
and thinner, and more sensitive pieces of bimetal have been em-
ployed. A lengthwise slot in the center of each strip was used to
avoid "cross-buckling."
Singly Compensated Thermostat. — With the singly compensated
thermostat, the initial setting of the positions of the electrical contacts
determines the amount of radiation the bimetal must receive from
the arc to effect interruption or completion of the electrical circuit.
This type of thermostat can be utilized as follows for control of carbon
position. The thermostat contacts are connected to short-circuit a
resistance in the field circuit of the motor that feeds the carbons,
giving a high speed when the contacts are open and a low speed when
they are closed. The thermostat contacts can be set to close at a
point on the arc image on the falling part of the energy curves of Fig.
6 in the arc stream just in front of the positive carbon. If the carbon
burns back, less energy will be received by the thermostat, which can
be arranged to open its contacts and speed up the motor, feeding the
carbon forward and increasing the energy on the thermostat until the
contacts close and reduce the speed of the motor. The gradient of the
energy vs. position curve along the arc, such as A in Fig. 6, determines
the sensitivity to arc position with which such a thermostat will func-
tion. Therefore, any procedure that increases this gradient, such as
the use of a filter described in connection with Curve B of Fig. 6, will
improve the sensitivity of the types of thermostat shown in Figs.
9 (a) and (b). Changes in the overall level of intensity of radiation
received at the arc image would tend to result in a shift along the arc
image of the point at which the singly compensated thermostat closes
its contacts due to its inherent property of requiring a fixed amount of
radiant energy. The gradient in intensity from carbon to arc stream
Nov., 1941] CONTROLLING CARBON ARC POSITION 497
as shown in Curve B of Fig. 6 is sufficiently abrupt, so that the point
along the arc stream where the contacts of the thermostat close will
not shift appreciably. With energy distribution curves along the arc
such as shown in Fig. 6, there will necessarily be two points, one on
either side of the maximum, at which the thermostat will close. If
one of these is used for arc control as described above, the other will
affect the feed motor in a sense opposite to what is required. The
possibility of the thermostat's being displaced out of its operating
range will be lessened if the two points at which the thermostat closes
are separated as widely as possible on the arc image which, as can be
seen from Fig. 6, means operating the thermostat at as low energy as
practicable. This is more feasible with Curve B than Curve A of Fig. 6
because the gradient, which has been shown above to be important to
sensitivity, can be kept abrupt at low values of energy.
Doubly Compensated Thermostats, — Another form of thermostat is
well adapted to the type of energy distribution along the arc shown in
Fig. 6. An example is shown in Fig. 9(c). Two adjacent strips of
! bimetal are employed, rigidly linked together at one end. With the
thermostat illustrated in Fig. 9(c), mounting is accomplished at one
of the unlinked ends of the strips, leaving the other unlinked adjacent
end free to move and make contact with a fixed point. Both bimetal
strips receive radiation from the arc and bend in the same direction
when heated. They are placed with respect to the arc image so that
one is on either side of the maximum of intensity at the positive car-
bon shown in Fig 6; thus both strips are approximately equally
heated. If the arc image is displaced in either direction, due to move-
ment of the carbon, one strip becomes heated more strongly than the
other, resulting in opening or closing of the contacts, which can be
made to control the feed-motor in the same manner as previously
described. Such a thermostat has two degrees of compensation. In
the first place, as with the singly compensated type described above,
it is compensated for heat received from the surroundings, since this
causes equal deflection of both strips which leaves the separation of
the contacts unchanged. Second, it responds only to displacements
of the position of maximum intensity on the arc image and is un-
affected by general overall increases or decreases of the level of the
curves of Fig. 6, such as would occur with increase or decrease of arc
current. We have called this a "doubly-compensated" thermostat be-
cause of this twofold degree of compensation.
The various examples of singly and doubly compensated thermo-
498 ZAFFARANO, LOZIER, AND JOY [j. s. M. p. E.
stats shown in Fig. 9 have been constructed and tested. A simple
bracket mounting the lens and thermostat was fastened to one window
of a "Suprex"-type lamp. The lens was supported about four inches
from the arc, giving an image on the thermostat just outside the
window. The thermostats were constructed of W. M. Chace Co.'s
Type 2400 bimetal,4 heat treated by the manufacturer to a tempera-
ture of 700 °F. The thickness of the bimetal was 0.010 inch except
for the thermostat of Fig. 9(6) which was 0.005 inch thick. The
material was used in the form of strips about 1 inch long and 0.1 to
0.2 inch wide. Platinum-faced contact points obtained from the
H. A. Wilson Co. were employed.4
PERFORMANCE OF ARC CONTROLS
Double-Photocell System. — To evaluate the performance of the
double-photocell control previously described, "Suprex" trims were
burned for 20 minutes each and the positions of both carbons were
read on an enlarged side image. Readings were begun two minutes
after the arc was struck, and no adjustments were made on the lamp,
photocells, or amplifiers during the test. The observations on the
accuracy with which the carbon positions were maintained have been
reduced to the statistical basis shown in Table I, giving the per cent
of the total time that the carbons were held at various distances from
the original arc position.
TABLE I
Tests of Accuracy of Carbon Position Control with Double Photocell
Per Cent of Time Held within Limits Specified
0.000 In. to 0.015 In. Greater than 0.015 In.
Positive carbon 80 20
Negative carbon 90 10
This shows that the double-photocell control was capable of
limiting the variation of carbon position for the most part to less than
0.015 inch, with a few excursions greater than this. Since the photo-
cells are biased so as to respond to departures from light levels de-
termined at the time of the initial adjustment, any change from the
initial conditions that causes light variations, such as line-voltage
fluctuations, results in a change of the positions at which the carbons
are held. This is probably the reason for the few cases in which the
change in position exceeded 0.015 inch.
v., 1941] CONTROLLING CARBON ARC POSITION 499
Performance Tests on Thermostats. — Performance tests were made
)n a "Suprex"-type lamp adapted to accommodate various examples
)f the singly and doubly compensated thermostats described above.
These were used to control the position of the positive carbon.
The feeding of the negative carbon was accomplished through a
separate motor which was controlled by a magnetic relay responsive
,o the arc current. In this manner the negative carbon was advanced
just the amount necessary to maintain the arc current constant.
This combination is designed to eliminate the effect upon screen light
i movement of the arc crater and variations in power supply.
A considerable number of trims of "Suprex" carbons were tested and
)bserved as described in connection with the photocell evaluation.
The data on the accuracy of positioning the positive carbon are shown
n Table II on the same statistical basis as used for Table I.
TABLE n
Tests of Thermostats Shown in Fig. 9 in Conjunction with Constant- Cur rent
Control
Per Cent of Time Positive Carbon Held within
Limits Specified
Type of Thermostat 0.000 In. to 0.015 In. Greater than 0.015 In.
ingly compensated (Fig. 9a)
Without heat filter 78 22
With heat filter 85 15
ingly compensated (Fig. 9&) 96
Doubly compensated (Fig. 9c) 99 1
The data in Table II show that all the thermostats restricted the
Dosition of the positive carbon most of the time to within 0.015 inch
f the correct position. The use of the heat filter described in con-
lection with Fig. 6 improved the accuracy of control obtained with
lie singly compensated thermostat of Fig. 9a. The superior per-
ormance of the singly compensated thermostat of the type shown in
Pig. 9(&) may be due to its differences in construction and method of'
ipplication as discussed in an earlier portion of this paper. The best
Derformance of all in Table II is shown by the doubly compensated
hermostat. This can probably be attributed to its different princi-
ples of operation and inherently greater degree of compensation. It
nust not be assumed that these data in Table II represent the ulti-
mate in performance. Further improvements may bring the per-
brmance of the singly compensated thermostats up to that shown by
the doubly compensated one.
500
ZAFFARANO, LOZIER, AND JOY
[J. S. M. P. E.
While comparison of Tables I and II indicates that the double-
photocell control was not quite as effective as the thermostat devices,
refinements are, however, possible for the photocell that can improve
the precision of arc control obtainable with it. For example, the use
of the constant-current relay for the negative carbon and one photo-
cell to fix the positive carbon position would no doubt result in more
precise control. Furthermore, just as a filter was used to increase
the gradient in radiation between the positive carbon and arc stream
as shown in Curve B of Fig. 6, suitable filters may be employed to in-
crease the sensitivity of photoelectric cells to movement of the arc
image. It is possible also to devise photoelectric means utilizing one
of the important principles of the doubly compensated thermostat —
namely, the property of responding only to the position of maximum
intensity on the arc image and not to the level of intensity. This can
FIG. 10. Record of light at center of screen over 20 minutes, using doubly
compensated thermostat plus constant-current relay.
be achieved through the use of a special photocell consisting of two
adjacent cathodes placed one on each side of the maximum of in-
tensity in the arc image.
These control devices have been used in connection with com-
mercial reflector-type lamps in the experimental work described above.
Some modification of the mechanism and method of operation of
these lamps is necessary in order to obtain independent control over
both the positive and negative carbons. While the emphasis in this
paper has been chiefly on the application of these methods of arc con-
trol to reflector- type high-intensity lamps, they can be used also with
other types of carbon arc lamps to effect automatic control.
The primary aim of all these arc-control devices is to maintain
constant light on the projection screen. The chart shown in Fig.
10 is a record of the light-intensity at the center of the screen over a
20-minute period without the projector shutter running, using the
doubly-compensated thermostat whose performance is given in
Table II. This thermostat plus the constant-current control was used
Nov., 1941] CONTROLLING CARBON ARC POSITION 501
with a "Suprex"-type lamp burning the 8-mm — 7-mm "Suprex"
trim at 62 amperes. The trace shows that over a 20-minute period,
the average light level remained constant within about two per cent
and the extreme variation from this level was only about four per
cent. This demonstrates that these automatic controls can effectively
maintain constant light on the screen. The employment of such
methods of arc positioning, therefore, makes possible significant ad-
vances in the quality of motion picture projection.
REFERENCES
1 JOY, D. B., AND DOWNES, A. C. : "Direct- Current High-Intensity Arcs with
Non-Rotating Positive Carbons," /. Soc. Mot. Pict. Eng., XXII (Jan., 1934),
No. 1, p. 42.
2 JOY, D. B., AND GEIB, E. R.: "The Non-Rotating High Intensity D-C Arc
for Projection," /. Soc. Mot. Pict. Eng., XXIV (Jan., 1935), No. 1, p. 47.
3 LOZIER, W. W., JOY, D. B., AND SIMON, R. W.: "A New Negative Carbon
for Low-Amperage Trims," 7. Soc. Mot. Pict. Eng., XXXV (Oct., 1940), No. 4,
p. 349.
4 We wish to acknowledge the generosity of the W. M. Chace Co. of Detroit,
Mich., and the H. A. Wilson Co. of Newark, N. J., in furnishing us thermostatic
bimetal and platinum-faced contacts for our experimental work.
SYMPOSIUM ON PROJECTION*
PREPARED FOR THE PROJECTION PRACTICE
COMMITTEE AND PRESENTED AT THE ROCHESTER CONVENTION
Summary. — This symposium on projection comprises three parts: (1} Projec-
tion Room Equipment Requirements; (2} The Projection Room — Its Location and
Contents; and (3) Factors Affecting Sound Quality in Theaters.
PROJECTION ROOM EQUIPMENT REQUIREMENTS
J. J. SEFING
What is installed in a modern projection room is of great impor-
tance to all connected with the motion picture industry. A pro-
jection room may possess all the requirements for a safe and efficient
layout and still remain equipped with obsolete or inadequate appara-
tus. To set a 100 per cent workable standard is quite impossible, but
from every-day practical experience, much knowledge has been gained
that tells us quite accurately just what a piece of equipment will do
and how the equipment can best be applied.
In the old "magic-lantern" days, a projector was bought hap-
hazardly, not as a matter of choice but because of the limitations of
the infant industry. At the present time, there is no legitimate ex-
cuse for not knowing what is best and most efficient in motion pic-
ture equipment, as nearly everyone is "picture conscious" and is clam-
oring for good screen performance. A good projection room layout
has well planned and sufficient working space around the various
pieces of equipment for the convenience of the projectionist, and the
equipment installed therein is adequate for the needs of the par-
ticular theater. However, in many instances, projection rooms in
theaters are not provided with adequate and suitably planned space
for the workers and the equipment, and it is for the designers of such
rooms that reliable information should be available as to the most
practicable methods and procedures.
* Presented at the 1941 Spring Meeting at Rochester, N. Y.
502
SYMPOSIUM ON PROJECTION 503
In purchasing and installing projectors, several important items
should be carefully considered. The pedestal or base should be suf-
ficiently strong and steady to support properly the heavy load of the
lamp house, magazines, and mechanisms in order to produce a steady
picture on the screen. An old-type, obsolete pedestal, even with
makeshift braces, can not be as steady and reliable as a pedestal es-
pecially designed to carry the load of the modern projector and sound
mechanisms. A lamp house should be selected that will be adequate
for the size of the picture and the auditorium. At present, there are
three arc lamps on the market that are widely used, each having its
advantages and specific applications. Theaters having an average-
size picture of 14 X 18 feet should have at least a "1-kw" arc, thus
providing a minimal screen brightness of 9 foot-lamberts; with a
picture size up to 17 X 22 feet the "Suprex"-type arc will produce
minimal brightness; for pictures wider than 24 feet, high-amperage
condenser type arc should be used.
With regard to the upper and lower magazines on the projector,
there is nothing special about them except that the 2000 and 3000-
foot types are quite regularly used. The take-ups at the lower
magazine are of several types: the friction type, the friction even-
tension type, and the fluid drive. There are three methods of
driving — belt, bicycle chain, and silent chain.
The selection of the projector mechanism is of prime importance,
as its operation is very delicate and the parts must be precision-made,
with a high degree of accuracy. The average projector mechanism
must operate about twelve hours a day, over a period of one to two
years, pulling a 35-mm film intermittently at a rate of 90 feet per
minute, or 24 pictures a second and magnifying a frame area of about
1/2 square-inch to about a screen area of 350 square-feet. It can be
seen that the proper selection of the projector mechanism is of great
importance in assuring trouble-free operation and screen results as
fine as it is possible for modern mechanisms to produce.
For picture change-over from one projector to another, a good
type of electrical device should be used. A projectionist can not
make a good change-over when he has to manipulate a home-made
device. Everything in the projection room is timed so precisely
that anything that disrupts or hinders the timing will show itself
quickly on the screen.
Regarding the sound equipment, a choice of several well known
systems can be had today. The amplifier, monitor, volume controls,
504 SYMPOSIUM ON PROJECTION [j. s. M. p. E.
and change-over device should be installed as near to the projection-
ist as is practicable, within the available working area, for convenience
of manipulation. The dials, switches, and pilot-lights should be
so arranged in the sound equipment as to be easily distinguishable.
In planning the projection facilities, three separate rooms should be
provided: one for the two projectors, the spotlight or third pro-
jector, the sound equipment, and, in the larger theaters, the dimmer
bank; a second room, for the rewind equipment, and a third room
for the d-c generating equipment. A separate toilet and wash room
should be provided near the projection room proper; income states
this is compulsory. The walls of the rooms must be fire-proof, with
metal access doors and two main metal doors on opposite sides of the
projection room. Two port-holes should be provided for each pro-
jector, one for projection and the other for viewing the screen. If
a spotlight is included in the equipment, a port somewhat larger
than the projection port should be provided as well as another ob-
servation port of the same size as the projector observation port in the
rewind room. Over these various port-holes, approved metal fire-
shutters must be installed and so arranged by a master trip system
that the shutters will drop and cover the openings in case of fire, auto-
matically, by the melting of fusible links, or manually, by the pro-
jectionist. For exhausting the hot, stale air from the various rooms,
a mechanical blower with a metal duct system and grille-taps into
the rooms should be installed. The blower may be controlled elec-
trically by a snap-switch and by a special switch connected to the
master trip arrangement on the fire-shutter apparatus, which will
automatically turn on the blower in case of a fire. Another blower
with a metal duct system and taps into the arc lamp houses should
be provided for exhausting the heat, gas, and ash of the arc. This
blower should also be mechanically electrically controlled and of suffi-
cient capacity to exhaust the arc lamp house properly and yet not
affect the burning of the arc.
For sound-proofing the projection room or for cutting down noise
transmission to a minimum, a good practice is to use cement plaster
up to a height of 5 feet from the floor, all around the room, and above
this height acoustone D or other approved material of equal acousti-
cal properties. The port-holes in the projection room may be sound-
proofed by glass in a separate track over the shutters or by installing
acoustical baffles inside the openings. If glass is used in the projec-
tion ports, it should be special "optical" glass.
Nov., 1941] SYMPOSIUM ON PROJECTION 505
The projection room floor should be coated with a good grade of
paint to stand the wear and tear and the penetration of oil or, better,
it should be covered with a good grade of "battleship" linoleum.
A popular color scheme is olive-green on the floor and walls, up to a
height of 5 feet all around the room, and buff or gray on the upper
walls and ceiling. The complete fire-shutter apparatus should be
painted a flat green color instead of the former black enamel finish.
The projection room proper should be so planned that the hori-
zontal center-line of the auditorium and screen is midway between the
two projector lenses, which latter should be 5 feet apart. If
the projection room is located an appreciable distance off this screen
and auditorium center-line, due to disadvantageous structural condi-
tions, a definite and noticeable "keystone" will result on the screen.
The edge of the screen image farther from the lens will be longer than
the opposite edge, and so the screen picture will not be rectangular.
The "keystone" effect will likewise occur if the projector lenses are
too high above the center of the screen, necessitating a steep projec-
tion angle. There are two ways to help overcome the "keystone"
effect; one is to use dark, heavy velour masking around the picture
to absorb light falling upon the screen outside the required rectangular
area, and the other is to file a blank aperture plate to the proper di-
mensions required for a rectangular screen picture. This method is
quite critical as the filing must be quite precise.
A safe working area around the projector and other equipment in
the projection room can not be stressed too strongly. At least 30
inches of clear space should be provided at the sides and rear of each
piece of equipment in the projection room. The projection room, re-
wind room, and generator room must be constructed of substantial,
approved, fire-proof materials. In all cases, before proceeding with
the construction, approval of the design should be obtained from the
local, state, or city authorities having jurisdiction. This will avoid
any costly revisions or penalties after the work is done.
The architect, engineer, or even the theater owner can obtain re-
liable, up-to-date information from the Society of Motion Picture
Engineers' specifications on projection room planning, prepared by
the Projection Practice Committee, which provides all the impor-
tant and desirable dimensions.
506 SYMPOSIUM ON PROJECTION [j. s. M. p. E.
THE PROJECTION ROOM— ITS LOCATION AND CONTENTS*
J. R. PRATER
Before selecting the location for the projection room, let us con-
sider the individual factors involved.
(1) Effect on Screen Image. — The primary purpose of the projec-
tion room is to provide a place from which the screen image can be
projected to the best advantage. This requires that :
(a) The projection angle be kept as small as possible, both^aterally
and vertically. The depth of focus of the projection lens is taxed
severely to maintain a sharply defined image over the entire screen
area under working conditions. Add to this the buckling of the
film at the projector aperture, plus any uneven wear of the aperture
tracks and tension shoes, plus the unavoidable lateral projection angle
imposed by the spacing necessary between projectors, and the best
we can hope for in theater practice falls considerably short of the
ideal. Now if we add a vertical projection angle to the already
difficult situation, the screen image definition suffers visibly with
only a very small vertical angle. Long before the maximum ap-
proved limit of 15 degrees is reached, the screen image suffers visibly
from distortion as well as from loss of definition. The added depth
of focus of the longer E.F. lenses may hold the definition within tol-
erable limits, but the distortion is unavoidable.
(&) The projection distance should be such that a projection lens
with an equivalent focus within normal limits will produce the de-
sired size screen image. The lens must have a speed of //2.0 to match
that of modern arc lamp optical systems. Such projection lenses are
available in focal lengths from 2 to 5 inches to fit existing projectors.
Although //2.0 lenses are also available in focal lengths of 6, 7, and 8
inches, they are too large for standard projectors to accommodate.
If longer than 5-inch E.F. lenses are used with standard projectors,
speed must be sacrificed, with resultant loss of light efficiency. On
the other hand, fast lenses of extremely short E.F. have a lesser depth
of focus, and the lateral spacing between projectors at the necessarily
short projection distances imposes an undesirably heavy lateral pro-
jection angle. For example, a 2-inch E.F. lens will form a screen
image 20.5 feet wide at a projection distance of only 50 feet. At
this short projection distance, with the recommended spacing of 60
inches between projectors, the lateral projection angle is approxi-
Nov., 1941] SYMPOSIUM ON PROJECTION 507
mately 2l/2 degrees for each projector where two are used; 5 degrees
for both outside projectors where 3 are used. Lateral angle is
more serious than the same amount of vertical angle, because of the 4
to 3 proportion of the image width to its height. Also, it is impossible
to cancel any of the lateral projection angle by tilting the screen.
This brings our ideal projection distance to that which will give the
desired picture size with an//2.0 projection lens of 5 inch E.F.
(c) The lowest point of the light-beam projected to the screen
must have a clearance of at least 6 feet 4 inches above any seating or
traffic floor area to prevent interference with the picture. The high-
est point of the light-beam must be sufficiently below any ceiling
obstruction to afford a clear view by the projectionist.
(2) Accessibility. — The projection room should be easily accessible
from outside the theater without passing through the seating area
or other public areas. Under no conditions should the projection
room open directly into an audience area without double doors so
arranged as to prevent any patron from seeing into the projection
room at any time.
(3) Fire Hazard. — To reduce the fire and audience panic hazard,
and consequently the insurance rates, the projection room should be
located outside the fire wall of the theater or within a fire wall of its
own.
(4) Heating and Ventilating. — Provision must be made for a non-
combustible vent duct of ample capacity leading to the open air;
also for fresh air in-takenot connected with the main air-conditioning
system. If unit heaters or steam radiators are placed directly in the
projection room they must be covered with wire mesh. A much
more satisfactory plan is to place heating coils or radiators in the
projection room supply ducts.
(5) Plumbing. — Plumbing facilities must be extended to the pro-
jection room location, space being allowed immediately adjoining.
(6) Noise Isolation. — The projection room noise and mechanical
vibration must be kept from the audience area of the theater. While
it is possible to do so by employing massive construction and acous-
tical materials, regardless of the location, the task can be accom-
plished much less expensively if only the front wall of the projection
room is directly exposed to the auditorium.
(7) Additional Space Immediately Adjoining.— It is highly desirable
that motor-generators, rheostats, rectifiers, and other apparatus
necessary to projection, as well as supplies, spare parts, test equip-
508 SYMPOSIUM ON PROJECTION [j. s. M. P. E.
ment, tools used only occasionally, and clothes lockers be located as
conveniently as possible to the projection room without being placed
directly therein.
Considering all these factors, it is obvious that the location of the
projection room will necessarily be a compromise in many respects.
In making the compromise, it is well to remember that the screen
image is what the theater has to sell. The requirements for excellent
projection should come first and foremost, even at an added initial
cost.
The contents of any projection room should be limited 'strictly to
what is necessary for carrying on the performance with safety, de-
pendability, and excellence. The following should be within the
projection room proper :
(1 ) Fire-proof shutters on all ports, with both automatic and manual
controls as described in the SMPE approved plans. (Also NBFU*
Pamphlet 40, Sec. 191c.)
(2) A switch controlling the auditorium lights. Provision must
be made also for turning these lights on from at least one other con-
venient point in the building. (NBFU, Sec. 191;.)
(3) Fire extinguishers of types using water or water solutions, such
as soda and acid, calcium chloride, pump tank, and loaded stream.
(NBFU, Sec. 144.) It seems that there is room for argument on this
point. Water extinguishers are dangerous to use on electrical equip-
ment, besides being themselves a source of extensive damage to such
equipment. Carbon tetrachloride or compressed carbon dioxide
extinguishers would seem much more appropriate for location inside
the projection room. If the water types must be provided, it is the
author's opinion that they should be located just outside the projec-
tion room, and be used only after the projectionist is outside. If
such procedure does not satisfy local fire authorities, however, it can
not, of course, be followed.
An interesting fact is that the NBFU does not recommend the
use of fire extinguishers by the projectionist at all. Section 218 of
NBFU Pamphlet 40 states: "Procedure in Case of Fire. — In the
event of film fire in a projector or elsewhere in a projection or rewind
room, the projectionist should immediately shut down the pro-
jection machine and arc lamps, operate the shutter release at the
nearest point to him, turn on the auditorium lights, leave the pro-
* National Board of Fire Underwriters.
Nov., 1941] SYMPOSIUM ON PROJECTION 509
jection room, and notify the manager of the theater or building."
If such procedure is followed, why should there be any hand ex-
tinguishers at all inside the projection room?
(4) Waste Receptacles. — (a) A suitable container for keeping scrap
film under water, separate from waste paper and other rubbish.
(NBFU, Sec. 183.)
(b) A metal container for hot carbon stubs. This should have a
funnel-shaped cover with an opening only large enough to admit
the largest diameter carbon used.
(c) If we adhere strictly to regulations, there would be no need for a
receptacle for other waste material, because in Sec. 191/ the NBFU
states: "No combustible material of any sort whatever shall be per-
mitted or allowed to be within such enclosure (projection room), ex-
cept the films used in the operation of the machine, and film cement."
Such a condition would indeed be ideal from a fire-hazard standpoint,
if it could be maintained ; but in actual practice there will almost in-
evitably be waste material of various sorts to be disposed of. There
are available on the market cans suitable for such material.
(5) A work-table or bench of metal or other non-combustible ma-
terial, not provided with racks or shelves underneath, which might be
used for keeping film or other materials. (NBFU, Sec. 117.)
(6) Such tools as are necessary for changing carbons and making
minor adjustments or repairs during the performance. These should
be permanently located as conveniently as possible to the place where
they will be most frequently used.
(7) Two good flashlights. One may burn out when needed in a
hurry, or may be in use in an adjoining room. An approved portable
trouble-lamp with metal guard, such as a "Reel-Lite," is good for
long repair jobs, but should never be used around machinery in op-
eration, and should not take the place of the flashlights.
(8) Two or more projectors and arc lamps. Sound equipment
including a double-channel amplifier, all of NBFU approved design
and manufacture.
(9) An enclosed metal cabinet for supplies and spare parts most
likely to be needed during a performance.
(10) All controls necessary to operate the projection equipment,
including associated apparatus not located in the projection room
proper, such as rectifying equipment, ventilating fans, effect lighting,
stage curtains used for motion picture presentation, etc.
510 SYMPOSIUM ON PROJECTION [j. s. M. p. E.
(11) A house phone or other means of communication between pro-
jection room, auditorium, and manager's office.
(12) No film other than that actually in projectors or being
threaded. This requires, of course, that there be an adjoining room
for rewinding, inspection, and storage.
(13) One or more qualified projectionists, who shall not be minors.
(NBFU, Sec. 217.) To operate a projection room with minimum fire
hazard and first-class screen results requires that at least two pro-
jectionists be on duty at all times. Large theaters using more than
two projectors, spotlights, effect machines, etc., in the^ projection
room must have more men in proportion to the additional equip-
ment.
This outline covers only what must be in the projection room
proper. Even for the one-man room, there is need for adjoining space
to accommodate rectifying equipment, shipping cans, a complete stock
of supplies and spare parts, oil cans, tools and test equipment not
ordinarily used during a performance, a work-bench with vise, clothes
lockers, books, records, and any other items necessary to the opera-
tion of the projection room, but which need not and should not be
inside the projection room proper.
FACTORS AFFECTING SOUND QUALITY IN THEATERS
ADOLPH GOODMAN
During the past ten years a great deal of technical progress has been
achieved in recording technic, and in recording and reproducing ap-
paratus, so that today these advances should be reflected in greater
entertainment value of the motion picture. In spite of such improve-
ments there is much to be desired in the final presentation in theaters,
mainly because there is a lack of proper coordination between the
various phases that go to make up the ultimate sound as heard by the
audience.
In this discussion, we shall point out the factors that must be con-
sidered and how they affect each other from the standpoint of the pre-
sentation in the theater. Assuming that the sound-track on the film
is a faithful record of the original sounds, final results that the
theater patrons hear depend upon the following five important,
closely related factors:
Nov., 1941] SYMPOSIUM ON PROJECTION 511
(1) The sound-reproducing system.
(2) The theater acoustic condition.
(5) The screen.
(4) The adjustments of the sound system.
(5) The operation and maintenance of the sound system.
The Sound-Reproducing System. — It is fundamentally important
that the sound-reproducing system be adequate, since it is through
this medium that the audience is expected to hear sounds as the
studio directors and technicians originally conceived them. It is well
known that inadequate sound reproduction can ruin an otherwise ex-
cellent picture, while sound properly reproduced adds greatly to the
entertainment value of the motion picture action.
In the early days, equipments having output power up to 10 or 12
watts were considered satisfactory, while in many instances the power
available was as low as 1 or 2 watts. Modern presentation of sound
motion pictures requires considerably increased power for proper
dramatic effects, and it is not unusual for the larger theaters to use
as much as 150 watts of undistorted power. Even greater power is
needed for showing pictures such as Disney's Fantasia for creation of
effects designed to stimulate the audience.
Realism in sound effects adds tremendously to the appeal of the
screen action. Earthquake and warfare scenes must have sound
accompaniment loud enough to make the audience feel that they
are actual spectators at the scene of action. Thus, the small thea-
ters as well as the large- ones need apparatus having many times the
power considered adequate in the past.
The Society of Motion Picture Engineers and the Academy of
Motion Picture Arts and Sciences have studied the requirements for
adequate theater sound equipment to meet the needs of modern pic-
tures, and the following specifications represent the results of these
studies :
(1) Volume range of 50 to 60 db.
(2} Amplifier capacity in accordance with recommendations of Academy of
Motion Picture Arts and Sciences. (See Research Council Bull, June 19, 1940.)
(5) Frequency response of 50 to at least 8000 cycles, with provision for exten-
sion to 10,000 cycles.
(4) Stage loud speaker system should have a high degree of efficiency, so that
the required amplifier capacity need not be too great. The loud speaker system
should have proper angular distribution so that all frequencies can be properly
distributed throughout the theater.
(5) The sound-head should have a "flutter" content imperceptible to the ear.
512 SYMPOSIUM ON PROJECTION [j. s. M. p. E.
(6) The equipment should be easy to install and operate. Necessary operating
controls should be accessible.
(7) Components of apparatus should be easily accessible for maintenance and
service operations.
(8) Adequate emergency provisions should be incorporated.
(5) Provision should be made for addition of apparatus that may be required
in the future due to advancements in the art.
Theater Acoustics. — Regardless of how well sound is reproduced by
the stage speakers, the theater acoustics greatly influence the final
result. If a theater is properly designed acoustically, it will allow the
sound to arrive at the listeners' ears with naturalness and realism.
If the theater has any acoustic defects, the sound may be so changed
in character that it arrives at the listeners' ears harsh, distorted, and
very unsatisfactory.
In view of the technical progress that has been made in both re-
cording and reproducing apparatus, it is more important than ever
before that careful consideration be given to the acoustic design of the
theater. This is necessary in order to take full advantage of the abil-
ity of modern equipment to give a faithful reproduction of the original
sound.
Some of the more common defects found in auditoriums that are
detrimental to good reproduction are high reverberation-time, echo,
resonance, and extraneous noise from auxiliary equipment, or noises
from sources outside the theater. Many of these can be overcome
or eliminated by proper consideration of such problems in the original
design. Specifically, attention should be given to the shape and size
of the theater, the location and frequency characteristics of absorbent
materials, and the insulation of walls and air-conditioning ducts to
minimize the transmission of noise to the auditorium proper.
Fortunately, the present trend is toward coordination between
acoustic treatment and the other functions of the auditorium such
as lighting, decoration, air conditioning, etc. Thus the theater archi-
tect can carry out a definite decorative scheme and at the same time
incorporate the necessary provisions to make the theater suitable
from an acoustic standpoint.
Screen. — After the sound leaves the loud speaker system it must
pass through the screen before reaching the audience. Just as the
acoustic condition of the theater plays an important part in the final
result, so does the screen influence the sound as heard by the listeners.
One of the improvements made in modern sound equipment is the
Nov., 1941] SYMPOSIUM ON PROJECTION 513
extension of the upper audio-frequency range. A poor screen will not
allow the high-frequency tones to be transmitted with the proper
intensity, resulting in a loss of brilliance of the music and lack of in-
telligibility of speech.
The sound-transmission properties of a screen depend upon sev-
eral factors, the most important of which are the size and number
of perforations per square-inch and the thickness of the screen ma-
terial. If the holes are too small or the material is too thick, then
the screen presents too high an acoustic impedance to permit good
sound transmission.
Even though a screen may be satisfactory when first installed, it
may adversely affect the sound transmission after a period of use.
The perforations will gather dust, and eventually the hole diameters
will be restricted, causing a reduction in high-frequency transmission.
More frequently loss of transmission qualities are due to resurfacing
the screen, in an attempt to improve the light-reflecting qualities.
Any attempt to overcome such adverse conditions of the screen by
recompensating the sound system to accentuate certain frequency
bands results in ragged response and uncomfortable hearing conditions
as far as the audience is concerned.
Adjustments of the Sound System. — While present-day theater
sound apparatus is capable of reproducing with greater fidelity, the
various components must be more carefully installed and adjusted
than has heretofore been necessary. Low-level circuits should be
carefully shielded and grounded to prevent the introduction of ex-
traneous noises into the system. Correct power-transformer taps
; should be used, depending upon the line voltage. Voltages and cur-
j rents in tubes, exciter lamps, and loud speaker fields should be
j checked to be sure they conform to specifications. In addition, the
mechanical apparatus should be carefully inspected, oiled, and ad-
justed before any film is run. After these preliminary adjustments
have been made, then the amplifier system should be set to conform
to the frequency response characteristic set up for that particular sys-
tem. Experience with a large number of installations has shown that
the standard electrical characteristic will prove to be satisfactory in
the vast majority of theaters.
To secure uniform frequency balance, proper distribution of high-
frequency tones, and equalized volume levels in the various parts of
the theater, it is necessary to pay special attention to the installation
and adjustment of the stage loud speaker system. One of the most
514 SYMPOSIUM ON PROJECTION [j. s. M. p. E
satisfactory speaker set-ups is that in which the high frequencies are
reproduced by a cellular type of horn and the low frequencies by sown
type of folded horn, with a suitable cross-over network to separate
the two frequency bands properly. Since frequencies above 300 cy-
cles become directional and beyond 2000 cycles have a beam effect
the positioning of the high-frequency horn is extremely critical in ar-
riving at the best setting for uniform sound distribution. Also, the
high-frequency horn must be properly set with respect to the low-
frequency unit to obtain the correct phase relation between the sounds
emanating from both sources. Usually, this dimension is specified by
the manufacturer, but the actual relative positions are subject to
slight variation in practice and must be checked during the tune-up
process.
At present, the most satisfactory means for adjusting the balance
and distribution in the auditorium is by use of the Academy Research
Council Theater Sound Test-Reel and by careful listening tests in all
parts of the theater. Since the test-reel contains selections of regular ,
release prints from the various major Hollywood studios, once the!
equipment has been adjusted properly, it will reproduce the product
of all studios with uniformly good quality.
Operation and Maintenance of the Sound System. — The preceding dis-
cussion pointed out how the condition of the theater and the equip-
ment affects the sound reproduction. Of equal importance are the op-
eration and maintenance of the sound system. Since the apparatus
consists of delicate mechanical parts and sensitive electrical circuits,
it must be kept in good condition at all times.
An important point in practical operation is the setting of the
sound volume level for the auditorium to allow the audience to hear
comfortably. It must be remembered that the frequency response
of the human ear changes for different sound levels. When the re-
sponse of the sound system is adjusted for proper balance between
high and low frequencies for a certain optimal level in the audi-
torium, the pictures reproduced at this level are natural and pleasing.
However, if the average level is increased or decreased, the sound
quality changes appreciably and the balance is destroyed. Gener-
ally, if the level is set too low, the sound loses "screen presence,"
giving the impression that the actors are far behind the screen. If
the level is too high, certain features of voice reproduction are over-
accentuated and the sound becomes extremely irritating, (e. g., exces-
sively strong sibilants). Projectionists can determine the average
Nov., 1941] SYMPOSIUM ON PROJECTION 515
gain setting for their theaters that will give the most pleasing and
understandable sound. Once this has been determined, there
should be no necessity for "riding" the gain control during the show-
ing of a picture.
Because of the many delicate adjustments that must be main-
tained it is extremely important that the equipment be inspected
periodically. Quite often the quality of the sound will deteriorate
slowly, but not enough to be noticed immediately. Such a condition
can be checked quickly, provided the system is regularly adjusted,
to be sure that it performs in accordance with the standards origi-
nally set for that particular type. Such inspections require the use
of proper tools and test equipment, including electrical meters
specially designed for the purpose, flutter indicator, and special test-
films. Worn parts in the sound-heads should be replaced before
they adversely affect the sound reproduction.
I
PROGRESS IN THREE-DIMENSIONAL PICTURES
J. A. NORLING**
Summary. — Recent years have seen improvements in still and movie stereoscopy
that have given impetus to their commercial exploitation. The developments that have
resulted in their commercial acceptance have been in the nature of refinements rather
than in radically new devices. Experimental work on many such new devices has
received notice in the public press and in technical journals.
Some of the problems encountered in the production of three-
dimensional motion pictures and the methods suggested for exhibit-
ing them have been reviewed in a previous article.1 The present
paper is in reality a supplement to the earlier one, and, in addition,
will deal with some of the problems of projected three-dimensional
still pictures.
The first commercial application of Polaroid to three-dimensional
pictures was in 1939, when a 35-mm black-and-white three-dimen-
sional production was used as a featured attraction at the Chrysler
Corporation's exhibit at the New York World's Fair.
During the year 1940, two other 35-mm three-dimensional films
were made and exhibited. One was a new film entitled New Dimen-
sions, for Chrysler's 1940 New York World's Fair Exhibit and was
produced in Technicolor; the other was a 35-mm black-and-white
film called Thrills for You, which was the major attraction in the
Pennsylvania Railroad's exhibit at the Golden Gate International
Exposition in San Francisco.
About four million persons have viewed these three films, so it is
probably safe to say that real three-dimensional motion pictures
have emerged from the experimental and novelty stage.
The success of three-dimensional motion pictures both with Pola-
roid as a projecting and viewing means as well as the earlier ana-
glyphs2 using red-and-green spectacles, has stimulated great in-
terest in further exploration of the possibilities of projected stereo-
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May 1,
** Loucks & Norling Studios, New York, N. Y.
516
THREE-DIMENSIONAL PICTURES 517
scopic pictures. Still stereograms as well as cine stereograms have
received attention, and a few recent improvements have been made,
particularly in projectors. Most of the projection devices presented
have employed polarized light. The "eclipse" system has been ex-
perimented with for motion pictures, and a still picture projector
utilizing this method was put on the market recently. This method
requires a shutter on the still projector, synchronized with shutters
on individual viewing devices. Another method3 uses prism view-
ing spectacles fitted with a baffle for each eye to block the unwanted
images.
All these methods and devices have interesting possibilities, but at
present the polarized-light system is the only one that provides
simplicity and economy together with a satisfactory quality in the pro-
jected picture.
CAMERA EQUIPMENT
To photograph three-dimensional pictures requires cameras having
twin lenses or some other provision for obtaining pictures from spaced
viewpoints. When two lenses are used, it is recognized that they
must be very closely matched. For practical reasons there must be
some tolerance in matching. Lenses that match each other within
one-half of one per cent in focal length will be satisfactory. It is ad-
visable to keep the two images to the same size within a tolerance of
not more than one-half per cent.
Definition is more important in stereoscopic picture making than
in ordinary photography. Three-dimensional images should be
crisp, clear, and as sharp as possible throughout the whole scene depth.
Lenses should be highly corrected and capable of being operated at
small apertures. Surf ace- treated lenses are particularly advanta-
geous since they are capable of producing images of superior quality.
Matched lenses in sets of various focal lengths are required to ex-
tend the operating range of the camera. However, it is question-
able whether extreme long-focus lenses are ever going to be widely
used, if at all. In my judgment, the useful range of focal lengths is
from the shortest (widest angle) that can be used up to a focal length
of about four times the diagonal of the picture.
The mounting of the lenses is important. The ordinary stereo-
scopic camera has its lenses mounted so that the axes are parallel and
extend perpendicularly from the center of the picture plane. This
is acceptable and good practice for most subjects but it may be de-
518 J. A. NORLING [J. S. M. P. E.
sirable to change the axes so that ttie image centers will converge at
some point in the scene. It is therefore advantageous to have the
lenses mounted so they can be rotated or shifted or both.
Most stereoscopic cameras have the lenses mounted at a fixed in-
terocular distance. In many cases it is desirable to use less than the
normal 2V2-inch spacing, and in some cases it is desirable to extend
the spacing to many times the normal. A versatile stereo camera
will, therefore, have provision for changing the lens interocular.
There are on the market many types of stereoscopic still cameras.
They range in size from those using 35-mm film up to such cameras
as the "StereoGraphic" which makes the pair of pictures on one 5 X
7-inch plate. There are also several attachments employing prisms
or mirrors. These are made to fit on a single-lens camera and pro-
duce two images on the plate or film within the space occupied by the
single image when using the lens without the attachment.
These cameras and attachments are adequate for making stereo-
grams that are to be looked at through a lens or prism-type stereo-
scopic viewer, but are lacking in versatility for the production of
stereograms to be projected on a screen.
To obtain results beyond the capacity of the standard stereo still
camera it is necessary, at present, to have the desired features built
into existing models.
In order to obtain pictures with proper "borders" the operator has
to be able to shift the lenses in relation to the centers of the plates
(or to shift the plates in relation to the optical axes). To obtain the
best three-dimensional effect he has to be able to select a narrow in-
terocular for close-up work and a wide interocular for distant scenes.
In the ordinary stereoscopic camera with parallel lens axes the
"border" is at infinity. Under these conditions there is no actual
stereoscopic "border," or stereoscopic "window" at all. It is gener-
ally conceded that the most pleasing projected stereogram results
when the spectator sees it as if looking through a window — when the
scene seems to exist behind the window or screen frame.
For still-life subjects a single camera may be used, the exposures
being made successively. The camera is mounted on a slide-board
and the interocular may be any selected value from zero to as great
as the capacity of the slide-board. For action shots or exposures of
short duration the two pictures must be made simultaneously.
Apparatus for action shots may be made up of two cameras
mounted on a common base and so arranged that the interocular may
Nov., 1941] THREE-DlMENSIpNAL PICTURES 519
be varied by moving one or both cameras The shutters must be
accurately synchronized and the timing of the shutters closely
matched.
The requirements for making still stereograms apply also to mo-
tion picture stereoscopy. For instance, in scientific films it may be
necessary to photograph a very small object, such as an insect, quite
close to the camera. This demands a very narrow interocular. On
the other hand, some scenic shots are vastly improved by spreading
the lenses apart, thus obtaining a greater three-dimensional effect.
Obviously it is difficult, if not impossible, to build one camera with
such a wide range. Several cameras may be required to cover a wide
variety of subjects.
Since photoplay production does not demand the photography of
minute objects, it seems reasonable to assume that only a limited
interocular range will be needed. A range of interocular from ll/%
inches for close-ups up to 4 inches for long shots should be adequate
for the average photoplay. It is possible to provide this range in
one camera.
The same desirable features regarding convergence of the picture
centers in making stereo movies, because "bordering," that is, es-
tablishing the proper margins at right and left, must be done, and can
be done, only in the camera.
The finder on a stereoscopic motion picture camera is an important
accessory, and its functions differ in some important respects from
standard practice. It is desirable to view the scene in three dimen-
sions and to see both images so that proper alignment for conver-
gence and bordering can readily be effected. Naturally the finder
images must be right side up and not reversed left for right. A bin-
ocular finder of the right kind enables the cameraman and director to
determine by visual means the lens interocular considered best for
any given scene. Of course, general rules must be established for
interocular spacing depending upon distance of principal object and
magnification of the lenses employed, but occasionally it may be de-
sirable to increase the depth of a scene to enhance its dramatic ef-
fectiveness.
No data are included in the present paper on interocular spacing
versus distances and magnifications because there is little agreement
among research men and operators as to recommendations. Every-
body agrees that "excessive" interocular spacing creates distortion.
The controversial point is to define the words "excessive" and "dis-
520 J. A. NORLING [J. S. M. P. E.
tortion" as applied to the problem. Broadly, the whole matter o
interocular spacing and magnification in the taking of the scene
should be influenced by the conditions of projection under which the
picture will be shown. Therefore, it is of great value to know before-
hand what will be the average conditions of screen angles, seating
arrangement, etc.
John T. Rule, of Massachusetts Institute of Technology, has con-
tributed valuable data on the geometry of stereoscopic projection
in a recent paper.4
PROJECTION
The projection of the Polaroid three-dimensional 35-mm motion
pictures that have been mentioned has been done through two syn-
chronized projectors. In one case synchronism was obtained by
electrical interlock; in the other, by mechanical means. Both sys-
tems worked excellently. Since projection of the pictures was on a
"grind" basis, with very short periods between shows, and there were
no breakdowns, it is evident that either method is satisfactory.
Considerable experimental work has been done with 16-mm pro-
jection but no actual use has been made of 16-mm stereograms for
commercial purposes. The indications are that such equipment will
be available sometime this year.
Several types of stereoscopic still projectors have been introduced,
and the three-dimensional projected still picture is coming into wide
use for display and advertising purposes.
At present there are on the market two types of projectors using
Polaroid and one using the "eclipse" system. One of those using the
Polaroid method projects stereograms consisting of pairs of standard
3 X 4-inch lantern-slides; the other is equipped for both 2 X 2-inch
slides and 35-mm slide-films.
All these projectors employ dual optical systems. One type uses
two lamps, and the projector for slide-films uses a special lamp con-
taining two filaments.
These new projection facilities should be of interest to the scientist
as well as the advertiser. The medical profession can utilize them
for many purposes. Gross specimens, operations, and radiographs
may be enlarged in three-dimensional form and may be viewed by
large groups. Engineers can obtain photoelastic records obtained
by polarized light in three-dimensional form to facilitate the study of
stresses and strains in the various planes of the plastic model. Any
number of other interesting possibilities present themselves.
Nov., 1941] THREE-DIMENSIONAL PICTURES 521
The projection of polarized-light stereograms demands a screen
that will not affect the angles of polarization of the projected images.
A metallic surface, preferably aluminum unadulterated by the ad-
mixture of white or gray pigment, is indicated. Several screens now
on the market meet these requirements.
The angles of polarization recommended by the Polaroid Corpora-
tion and adopted as standard practice is a 45-degree slant upward
to the right for the right-eye picture and a 45-degree slant upward to
the left for the left-eye picture. Arranged this way it does not matter
whether the viewers are turned left for right or not. The earlier
vertical-horizontal polarization axes required of the user that he face
the viewers in one selected direction. The new arrangement re-
quires no special instruction to the audience.
THE VECTOGRAPH
No review of progress in three-dimensional photography would be
complete without a mention of E. H. Land's remarkable process for
combining the two disparate images on one film. The vectograph,
as this new type of print has been called, was reported at a meeting
of the Optical Society of America held at Rochester last year, and
those interested in its technical features may refer to that valuable
paper.6 At present the vectograph is available for stills, either as
slides or mounted on aluminum-surfaced paper. When this new
method becomes available for motion picture printing it will simplify
enormously the present projection difficulties. Ordinary projectors,
without any changes at all, will be used for the projection of vecto-
graph films.
OTHER METHODS RECENTLY PROPOSED FOR PRODUCTION OF THREE-
DIMENSIONAL MOVIES
S. J. Ivanov is credited with having developed a new method of
projecting stereoscopic movies.6 From the description of the Ivanov
method it appears that the cost of the special screen required must
be rather high. Projection is from two projectors, but it is claimed
no viewing accessories are required. It is not apparent that the
Ivanov method differs essentially from other systems employing
grids in front of, or behind, the screen. Many variations of the
principle have been proposed during the last thirty years or so.
Another grid device, recently patented by Suzanne Carre,7 is an
interesting variation on the grid principle. The grid is composed
522 J. A. NORLING [J. S. M. P. E.
of thin rods or wires spaced apart a distance equal to their width. A.
motor synchronized with the projector reciprocates the grid back and
forth across the screen. Rear projection is employed, the grid being
between the audience and the screen. It is claimed that in this
manner the grid will select the pictures for the left eye and then for
the right eye in such a manner that each eye sees only the picture
intended for it.
William Alder, of Pasadena, Calif., has devised a method for which
much is claimed. The Alder method requires an ^attachment
that fits on the lens of an ordinary movie camera. It consists of a
group of mirrors revolved at high speed. By this means part of both
sides as well as the front of an object are recorded on the same film —
three images altogether. It is claimed that projection of the print,
through an ordinary projector and on any standard screen, results in
a three-dimensional effect without the necessity for using individual
viewers.
The Alder method is evidently capable of giving the screen image of
some subjects a certain plasticity absent in the ordinary two-dimen-
sional picture.
Since Mr. Alder's theories were expounded in a newspaper article, 8
I think it only fair to quote from that article :
"I have found," Alder said, "that the stereopticon photography
creates a false illusion of too much depth. I am trying to attain
'natural vision.' I want to show the human being and the landscape
with the same amount of depth it shows to the naked eye."
This broad statement does not explain that "too much depth" in
stereograms is the result of faulty technics in the use of the twin-lens
camera. Too much depth results when too wide an interocular is
employed. Varying the interocular controls the apparent depth and
for natural results it is often necessary to reduce the interocular to*
less than the normal 21/2 inches, as pointed out previously.
Quoting Mr. Alder from the same article again: "I have played
around with double lenses on a camera and with double rows of film,
attempting to equal the parlor type of stereopticon still photographs.
But while from one section of the theater there is the illusion of
depth and three dimensions, nevertheless any move from this limited
area means that the illusion vanishes and you have two pictures, in
two dimensions, running side by side on the screen."
Actually, when the proper conditions exist in the theater and the
spectators are furnished with proper means for viewing the three-
Nov., 1941] THREE-DIMENSIONAL PICTURES 523
dimensional picture it will remain a three-dimensional picture from
levery point in the house, with the qualification that a view from a
great angle will introduce marked distortion. But serious distortion
is present in any two-dimensional picture viewed from the same un-
favorable angle.
TRICKS OF THE NEW ART
Three-dimensional photography offers rare opportunities for special
stunts and startling effects. The possibilities are almost limitless.
First, we can change the depth of the scene — lengthen it exces-
sively or compress it if we so desire.
Second, it is possible to change the apparent size and shapes of ob-
jects and in that manner create startling and often amusing effects.
Third, it is possible to combine elements of one scene into another
to achieve striking effects.
CONCLUSION
John T. Rule stated, in the paper referred to above,4 "The present
stereoscopic movie, when intelligently taken and projected, is a very
good product, acceptable to even a critical observer."
That is the state of the art today. What it will be in the future,
with all the refinements that will be developed as the stereoscopic
movie becomes more widely used, can best be left to the imagination.
.GLOSSARY OF TERMS
Anaglyphs. Stereograms in which one image of a stereoscopic pair is printed
in one color and the other in another color.
Angular Distortion. The apparent distortion resulting from viewing a three-
dimensional picture from an unfavorable angle.
Bordering. The manipulation resulting in creating the "stereoscopic window,"
through which a three-dimensional picture seems to be seen.
Convergence Point. The point on the scene at which the optical axes cross.
Depth Distortion. The apparent distortion in the depth of a three-dimensional
picture.
Eclipse Stereoscopy. Methods of producing projected stereograms by inter-
mittent projection. One image of the pair is projected while the other image is
eclipsed; then the other image is projected while the first is eclipsed.
In- Front- of -the- Window. Term applied when objects apparently exist between
the stereoscopic window and the spectator.
Interocular. The distance between the optical centers of twin lenses or attach-
ments for single lenses.
Marginal Cut-Off. The effect at the side margins of the stereogram when ob-
jects in front of the "stereoscopic window" are cut off bv the "window."
524 J. A. NORLING
Polaroid. The material most commonly, used in making projection and viewing
filters for polarized-light stereograms.
Polarization Angle. The angle from the horizontal at which the axes of the
polarizing filters are set.
Stereoscopic Window. The border or frame around the stereogram and behind
which the three-dimensional scene appears to be.
Through-the- Window View. Applied to the stereogram when it appears as if
seen through a window.
REFERENCES
1 NORLING, J. A.: "Three-Dimensional Motion Pictures," /. Soc. Mot. Pict.
Eng., XXXHI (Dec., 1939), No. 12, p. 612.
2 LEVENTHAL-NORLING Audioscopiks, motion pictures released by Metro-
Goldwyn-Mayer since 1936.
3 WITHEROW, G.: "An Experiment in Three- Dimensional Movies," Home
Movies (April, 1941), p. 170.
4 RULE, J. T. : "The Geometry of Stereoscopic Projection," J. Opt. Soc. Amer.,
31 (April, 1941), p. 325.
6 LAND, E. H. : " Vectographs : Images in Terms of Vectorial Inequality and
Their Application in Three-Dimensional Representation," /. Opt. Soc. Amer., 30
(June, 1940), p. 230.
6 Moscow Pravda (March 7, 1940), and New York Times (Jan. 25, 1941).
7 U. S. Pat. No. 2,240,131, assigned to La Chronostereoscopic, Paris, France.
8 Gregg Toland, New York Times (Apr. 20, 1941).
SOLVING ACOUSTIC AND NOISE PROBLEMS
ENCOUNTERED IN RECORDING FOR
MOTION PICTURES*
WILLIAM L. THAYER**
Summary. — More and more attention is being given to the naturalness and
clarity of reproduction of sound in motion picture theaters. To accomplish these
it is necessary to improve not only the equipment in both the theater and the studio,
but also the acoustics, and to reduce noise in both the theater and in the sets where
the sound is recorded.
It is the purpose of this paper to describe the acoustic and noise problems encoun-
tered in recording, and to describe ways in which these problems have been met. This
includes a discussion of the ways of minimizing reverberation in outdoor scenes on a
sound-stage; of reducing sound resonance between ceiling and floor, and between
parallel walls of sets; of reducing reflection from concave surfaces, nearby hard walls,
windows, table and desk tops; of reducing resonance in small rooms such as telephone
booths, boat and train interiors. Also included is a discussion of the progress
recently made in reducing equipment noises such as those from cameras, background
projection machines, arc lamps, wind machines, treadmills, etc., and ways of re-
ducing noises caused by actors and horses on hardwood floors, gravel walks, and on
raised structures, such as artificial hills built of wood; and of noise created by artificial
rain. The control of outside noises such as those of traffic, aeroplanes, and wind
is discussed.
When looking at a motion picture one is interested mainly in the
story and thinks of the sound only when it is hard to understand or is
unnatural in quality; and then he becomes only slightly irritated
If this irritation keeps up throughout the picture he will not enjoy the
picture nearly as much as if the sound were so good that he would not
think about it. Surely no one in a motion picture audience is ever
consistently aware of the varying acoustic and noise problems that
confront the sound-recording engineers as the actors move about the
"set" and from one type of "set" to another. Neither are they aware
of the fact that when the picture was made a microphone was con-
tinually moved about just above the frame line of the camera and in
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received April
14, 1941.
** Paramount Pictures, Inc., Hollywood, Calif.
525
526
W. L. THAYER
[J. S. M. P. E.
front of each actor as he spoke, in order to record as much direct
sound as possible and consequently to minimize the acoustic and
noise problems that might become irritating to the listener.
Because of the necessity of maintaining beauty or realism in the
picture it often becomes impossible to build sets having good acoustics;
but through experience the sound engineers, working with the set
designers, have found numerous ways of avoiding poor acoustic condi-
tions without destroying the beauty or naturalness of the set. It is
the purpose of this paper to describe ways in which acoustic and
FIG. 1. Typical exterior set on motion picture sound-stage.
noise problems have been met, perhaps not perfectly in every case,
but adequately.
ACOUSTIC PROBLEMS
There is an endless number of types of sets encountered in making
motion pictures. They may be classed into exterior and interior sets.
The exterior sets are either natural outdoor settings or exterior sets
within a building or stage. The interior sets range from large rooms or
groups of large rooms to very small rooms, such as the interiors of
trains, aeroplanes, automobiles, boats, or even telephone booths.
Nov., 1941]
ACOUSTIC AND NOISE PROBLEMS
527
The general problem in exteriors on stages is to keep the reverbera-
tion sufficiently low to give the audience the impression that the re-
cording has actually been made outdoors in accordance with the illu-
sion established in the picture. Fig. 1 is a typical exterior set on a
motion picture sound-stage. Ori sets of this type, even though the
stage is relatively dead acoustically, the painted sky backing, which
usually covers about three-fourths of the stage wall area, is rather
hard and normally reflects enough sound to spoil the illusion of an
exterior scene. In recording scenes on such a set it is difficult to avoid
FIG. 2. A set requiring reduction of reflections.
reverberation in the "long shots" where the camera shows the entire
set; but in "medium shots" and "close ups" reflections and rever-
beration can be reduced by acoustic treatment on the portions not
shown in the picture. This is usually accomplished by hanging large
drapes or "sound blankets" (20 X 30 ft), to partition off the portion
of the stage not in use, or in hanging the "blankets" a short distance
in front of all "sky backing" not in the picture. Also, improvement is
sometimes obtained by hanging "blankets" overhead horizontally
just above the elevation of the lights. Considerable improvement can
be obtained by using a directional microphone such as the cardioid or
ribbon microphone. Jungle and forest scenes are less difficult than
528
W. L..THAYER
[J. S. M. P. E.
the set shown in Fig. 1 because the shrubbery and trees help to ab-
sorb the sound, while ocean or lake scenes are more difficult because
of sound reflections from the water's surface.
In interior scenes one expects to hear reverberation, and therefore
the presence of recorded reverberation is not disturbing to an audience
so long as the reverberation is not greater than what would be ex-
pected from the interior pictured on the screen. However, excessive
reveiberation, sound-wave resonance, or reflection from nearby hard
surfaces can distort the amplitude or phase of sound picked up by a
FIG. 3. Set with ceiling of acoustical board simulating concrete.
microphone to such an extent that dialog may become very difficult
to understand.
Fig. 2 shows a round set in which no resonance or long-period
reverberation occurred, but because of the hard materials and the
concave surfaces severe reflections occurred near the center of the
room. Luckily the set designer placed a round table in the center of
the room and a baffle between the piano and the glass tile. Further
reduction of the reflections was achieved by hanging heavy drapes
over the entire portions of the walls not included in the camera angles.
Parallel hard walls often set up undesirable resonance that is just
as objectionable as the direct concentrated reflections from concave
[ov., 1941] ACOUSTIC AND NOISE PROBLEMS 529
surfaces. Good quality in small rooms such as telephone booths, boat
cabins, train interiors, etc., can be obtained by the removal or draping
of one of the two opposite walls. Sometimes sufficiently good quality
can be achieved by building the room so that one wall is broken by an
open door or window or with sufficient other irregularities to avoid
resonance. Often the glass in windows may be completely removed
without being detrimental to the picture.
Where it is necessary to have ceilings in the sets, considerable care
in design is necessary in order to avoid resonance between the ceiling
FIG. 4. Set using removable sections of acoustic board for ceiling.
and the floor. Figs. 3 and 4 show ceilings that simulate concrete, but
were constructed of removable sections of soft acoustic board that had
been "aged" with a light water-color spray coat. Resonance and re-
flections were avoided in the air-raid shelter by removing all ceiling
sections not included in the camera angle, which in most cases in-
cluded the section immediately over the actors' heads. The acoustic
conditions in the cafe shown in Fig. 3 were sufficiently good to make it
unnecessary to remove ceiling pieces other than those over the table
of the principal actors.
An acoustically good ceiling that simulates a plaster ceiling can be
made by using a roll of muslin. Fig. 5 shows (at top of picture) a
530 W. L. THAYER [j. s. M. p. E.
muslin ceiling which can be rolled back to facilitate lighting and
placement of the microphone. When necessary a fair grade of re-
cording can be made through this ceiling, but such practice is not
recommended unless a much more porous cloth than muslin is used.
When hard ceilings are necessary, such as when the ceiling is the
under side of the upper deck of a steamer, acoustic improvement and
satisfactory appearance have been obtained by applying a layer of
sound-absorbing material covered with tightly stretched muslin.
NOISE PROBLEMS
Noises incident to the making of motion pictures may be classed
into equipment noises, noises created by actors and equipment in
scenes being photographed, and outside noises such as those from
aeroplanes, automobiles, industrial machines, wind, rain, people,
animals, and insects.
Equipment noises consist of noises made by cameras, background
projection machines, arc lamps, treadmills, vehicles for moving shots,
and effects-making equipment such as wave machines, wind machines,
lightning-making devices, artificial rain systems, artificial cloud, fog,
and snow-dispensing devices, and devices for jiggling, rocking, or
turning boats, aeroplanes, or vehicles.
Most of the camera and background projection machines now in use
require noise-absorbing enclosures. However, both cameras and
background projection machines have been developed that are
sufficiently quiet to be used without a "blimp" or "booth."
Arc lamps have been quieted considerably, but they are still noisy
enough to cause trouble when used in large numbers. It has been
found that arc "sing" or "whistle" can be almost entirely eliminated
by a line filter consisting of a series choke-coil (L = 0.15 jj.h) with
shunt electrolytic condensers of 2500 /z/ across the generator side of
the choke and 5000 /*/ across the line side. Arc motor noise has been
diminished by using a rubber motor mounting. Arc "boiling" noise
has been reduced by lining all lamp houses with woven asbestos and
by using an improved type of carbon. Tests are under way to reduce
further the "boiling" noise by baffling the lamp vents.
Treadmills are now available that are sufficiently quiet for normal
dialog recording. They are constructed with an endless rubber-on-
fabric belt, about 6 ft wide and 18 ft in total length, running on large
rollers driven by a variable-speed motor and variable-ratio belt trans-
mission system, both of which are enclosed in a "sound-proofed box."
Nov., 1941] ACOUSTIC AND NOISE PROBLEMS 531
In making moving shots on stages it is often necessary to use
'camera booms." Recently the largest-size booms have been
equipped with a motor-drive system made "silent" by enclosing it in
a heavy steel case lined with several layers of soft sound-absorbing
material.
Effects-making equipment is often extremely noisy and its use
makes it impossible to record satisfactory sound. Among these is
the high-velocity wind machine consisting of an aeroplane engine and
propeller, and the lightning-making device which consists of a hand-
FIG. 5. Set using muslin ceiling that can be rolled back.
ful of arc carbons mounted on each of the two points of a pair of
wooden scissors. The noise occurs when an arc is drawn between
the two groups of carbons. Relatively quiet flares are sometimes
used but the effect is not as good as that from the arc.
Effects-making machines that cause some disturbance but which
when run at moderate speed are tolerable, are wind "blowers,"
"silent fans," wave machines, and hydraulic or motor-driven rock-
ing and jiggling devices. The "blower" or "wind tunnel" consists of
a large ventilation-type centrifugal blower which is usually placed
outside the stage or at some distance from the set, and wind is de-
livered to the set through a canvas pipe about three to four feet in
532 W. L. THAYER [j. s. M. P. E.
diameter. The most satisfactory fan in use is the 48-inch diameter
fan having from three to eight wide overlapping blades. Fans 24
inches in diameter are useful where a fan must be small enough to be
hidden behind shrubbery, but fans smaller than that usually make
too much noise because of having to be run fast to deliver sufficient
breeze.
Artificial rain systems are normally very quiet except for the fall-
ing of the rain against the set and on the stage floor, or on hats or
umbrellas. Rain effects seen through the window of a room can be
kept quiet by putting a 6-inch layer of excelsior or rubberized hair on
the floor where the rain falls. Where heavy rain must strike window
panes the noise can be reduced considerably by placing a large piece of
glass on the outside of each window against the back of the set, leav-
ing a dead air-space of about 2 inches between the window and the
exterior glass. A layer of felt is often tacked to the outside of the set
to reduce the noise further when low-level dialog is to be recorded.
Thin metal roofs are usually avoided, but when they are necessary for
the correct pictorial effect they can be quieted some by coating with
tar. Umbrellas usually cause considerable noise because they are so
close to the microphone. The new type transparent umbrellas cause
more noise than cloth umbrellas and for that reason are usually
avoided where dialog is to be recorded.
Noises created by actors usually consist of footsteps on bare floors,
sidewalks, and gravel walks. Good substantial construction is neces-
sary on floors, stairways, and raised platforms in order to avoid
squeaks and drumminess. Dance floors must have a smooth lacquer
finish in order to keep foot-shuffle noise sufficiently low for recording
dialog. In close-up shots where the feet do not show, the dancers
wear window-dressing socks over their shoes to reduce the noise.
Sidewalks of cement sound natural, but when a sidewalk is built of
wood the surface is usually covered with a soft acoustic board to
avoid an unnatural sound. Noise of footsteps on gravel walks on
stage floors is lessened by spreading a thin layer of gravel on about one
inch of moist dirt. Gravel walks are sometimes constructed of chipped
cork, but this has the disadvantage of being dusty, and consequently
of being detrimental to photography. When artificial hills are
built, similar to that shown in Fig. 1, very sturdy construction and a
layer of dirt is necessary hi order to prevent drumminess. When
horses are to be ridden on the hills it is usually necessary to cover the
woodwork with four to six inches of moist dirt.
Nov., 1941] ACOUSTIC AND NOISE PROBLEMS 533
Outside noises are usually of little concern on a sound-stage inas-
much as the stages usually have from 40 to 60 db of attenuation in
the walls and from 10 to 30 db in the roof. Occasionally aeroplanes
and large trucks close to the stages cause some interference. When
shooting outside on the studio lot usually 6 to 12 "flagmen" are em-
ployed to keep down local noises. Some of the studios use an orange-
colored captive balloon about 400 feet in the air as a signal to aero-
plane pilots to keep away. A local ordinance specifies that pilots
seeing the balloon should avoid flying near it.
When choosing shooting locations a "noise check" is made in ad-
vance to make sure that there are no noises that can not be controlled.
Locations on boulevards are avoided unless heavy traffic noise can be
tolerated. In residential areas city traffic officers stop or re-route
traffic during "takes." No attempt is made to control aeroplanes on
locations; consequently areas where planes frequently fly overhead
must be avoided. The noise of ocean waves is generally too high for
satisfactory dialog recording, and it is nearly always desirable to
shoot beach scenes on the stage, using background projection. The
noise of the wind in the trees, the croaking of frogs, and the chirping
of crickets, and other similar noises of nature are detrimental to good
recording, mainly because the amount of noise varies from "take" to
"take," and when the takes are cut together the sudden jumps in the
volume of the noise become very distracting. Some success in con-
trolling the noises of locusts, crickets, and frogs has been obtained by
hiring boys to disturb them just prior to the start of each take.
Each problem that arises is somewhat different and each has to be
handled individually, but the problems discussed in this paper are
typical and of the type that the set designers and the construction
and operating crews have become familiar with to the extent that in
nearly every case steps are taken to eliminate possible acoustic or
noise troubles before commencing to "shoot" in the set.
The recent reduction in film background noise through the use of
fine grain films exposes recorded set noises and set reverberation to an
even greater extent than in the past, and efforts will be continued to
reduce them further. In order to gain full advantage of improve-
ments in the theater, the studios, through the Research Council of
the Academy of Motion Picture Arts and Sciences, are making an
effort to familiarize theater architects with the common acoustic and
534 W. L. THAYER
noise troubles that are evident in existing theaters, and to point out
ways in which these problems may be solved in existing theaters and
avoided in designing new theaters.
BIBLIOGRAPHY
"Recommendations for Reducing Acoustic Difficulties on Motion Picture
Sets," Acad. Mot. Pict. Arts & Sciences Tech. Bull. (May 19, 1941).
"Theater Acoustic Recommendations," J. Soc. Mot. Pict. Eng., XXXVI
(Mar., 1941), p. 267 (reprinted from Acad. Mot. Pict. Arts & Sciences Tech. Bull.).
"Report on Arc Lamp Noise Tests," /. Soc. Mot. Pict. Eng., XXXVI (May,
1941), p. 559 (reprinted from Acad. Mot. Pict. & Sciences Tech. Bull.).
HILLIARD, J. K. : "Theater Standardization Activities of the Research Council
of the Academy of Motion Picture Arts & Sciences," J. Soc. Mot. Pict. Eng.,
XXXV (Oct., 1940), p. 388.
REPORT OF THE STANDARDS COMMITTEE
Summary. — Letter ballots taken by the SMPE Standards Committee recently
gave approval to two projects, viz., (1} the designation of the direction of winding 16-mm
film perforated along one edge, and (2} the method of edge-numbering 16-mm motion
picture film. These projects have been approved by the Board of Governors and are
published here in accordance with the Standardization Procedure adopted recently by
the Board.
On two following pages are shown two SMPE Recommended
Practices recently approved by letter-ballot of the SMPE Standards
Committee, as follows:
(1) Designation of Direction of Winding 16 -Mm Film Perforated
along One Edge. — For a long time there has been some divergence of
practice among the various companies of the industry in designating
the direction of winding of 16-mm film, and it is the intent of this
SMPE Recommended Practice to establish a uniform method of
making such designations. The specification given on the following
page has been adopted by the large film manufacturing companies in
addition to approval by letter ballot of the SMPE Standards Com-
mittee, and subsequent ratification by the SMPE Board of Gov-
ernors.
(2) Edge-Numbering Interval for 16-Mm Motion Picture Film. —
Quite a number of proposals for edge-numbering 16-mm film have
come from various parts of the industry. One of the proposals was to
place numbers on the film at 16-frame intervals corresponding to one-
foot intervals on 35-mm film; at one-foot intervals; and at intervals
corresponding to seconds of screen time. After considerable study
and discussion with various companies of the industry, the Com-
mittee arrived at the specification shown on the following page. It
has been ratified by the SMPE Board of Governors.
These specifications are published in accordance with the Stand-
ardization Procedure for the Standards Committee adopted by the
Board of Governors. If after thirty days from the date of publication
of this issue of the JOURNAL, no adverse comments are received by the
Chairman of the Standards Committee from the membership of the
Society with regard to these two items, the specifications described
535
536
STANDARDS COMMITTEE REPORT
LT. S. M. P. E.
herein will be referred to the Board of Governors of the Society for
action upon them as proposals for either American Standards or
American Recommended Practices. Comments on these proposals
are invited from readers of the JOURNAL.
P. H. ARNOLD
H. BAMFORD
M. C. BATSEL
F. T. BOWDITCH
M. R. BOYER
F. E. CARLSON
T. H. CARPENTER
E. K. CARVER
H. B. CUTHBERTSON
L. W. DAVEE
J. A. DUBRAY
D. B. JOY, Chairman
A. F. EDOUART
J. L. FORREST
G. FRIEDL, JR.
A. N. GOLDSMITH
H. GRIFFIN
A. C. HARDY
P. J. LARSEN
C. L. LOOTENS
J. A. MAURER
G. S. MITCHELL
K. F. MORGAN
R. MORRIS
Wm. H. OFFENHAUSER
G. F. RACKETT
W. B. RAYTON^
E. C. RICHARDSON
H. RUBIN
O. SANDVIK
R. E. SHELBY
J. L. SPENCE
E. W. TEMPLIN
H. E. WHITE
Nov., 1941]
STANDARDS COMMITTEE REPORT
537
SMPE RECOMMENDED PRACTICE
For 16-mm Motion Picture Film
SMPE
July, 1941
DESIGNATION OF DIRECTION OF WINDING
OF FILM PERFORATED ALONG ONE EDGE
When a roll of 16-mm film, perforated along one edge, is held so
that the outside end of the film leaves the roll at the top and toward
the right, winding A shall have the perforations on the edge of the film
toward the observer; and winding B shall have the perforations on
the edge away from the observer. In both cases the emulsion surface
shall face inward on the roll.
The following sketch illustrates these definitions:
Winding A
Emulsion side in
Winding B
Emulsion side in
The above-given sketch shows reels having round holes on both sides. When
the film is wound on a reel having a square hole on one side and a round hole on
the other, the square holes in the illustrations shall be understood to be on the side
away from the observer.
538
STANDARDS COMMITTEE REPORT
SMPE RECOMMENDED PRACTICE
For 16-mm Motion Picture Film
SMPE
July, 1941
EDGE-NUMBERING INTERVAL
If 16-mm film is edge-numbered, the interval between consecutive
footage numbers shall be 40 frames.
NEW MOTION PICTURE APPARATUS
During the Conventions of the Society, symposiums on new motion picture appara-
tus are held in which various manufacturers of equipment describe and demonstrate
their new products and developments. Some of this equipment is described in the
following pages; the remainder will be published in subsequent issues of the Journal.
A NEW 13.6-MM HIGH-INTENSITY PROJECTOR CARBON*
M. T . JONES, W. W. LOZIER, AND D. B. JOY**
The condenser- type high-intensity carbon arc lamp, using 13.6-mm high-in-
tensity carbons at 125 amperes, has been used for a number of years by many of
the largest theaters in this country as the light-source for projection.1 On ac-
count of the large screens in such theaters, a 13.6-mm super-high-intensity carbon
for 180-ampere operation was developed about five years ago,2 providing at least
30 per cent more light than was obtainable from the regular 125-ampere carbon.
This "super" carbon has found usage in some of the largest theaters, and also for
background projection in process motion picture photography.3 However, the
necessary revisions in lamp and power-supply characteristics have prevented its
use in many applications where increased light is desirable.
Research and development work in the laboratories of National Carbon Com-
pany, Inc., has recently produced a new 13.6-mm high-intensity projector carbon
to fill this need. The new carbon, in most cases, can be directly substituted in
the present condenser-type lamps and operated with present auxiliary equipment,
although in a few instances minor changes may be necessary if the higher current
is used. This new carbon gives a substantial increase in light over the regular 125-
ampere carbon with considerably lower current than necessary for the 13.6-mm
super-high-intensity carbon. It also has other advantages of lower consumption
rate, greater latitude of carbon position, and improved resistance to the shocks en-
countered when striking the arc. The spectral composition and color of the
light on the screen is the same as with the regular and super-high-intensity car-
bons.
The new carbon has the same core size and outside diameter as the regular 13.6-
mm H.I. projector carbon. However, its design and composition allow it to be
burned equally well at the 125-ampere rating of the regular projector carbon and
* Presented at the 1941 Spring Meeting at Rochester, N. Y. ; received May 1,
1941.
** National Carbon Company, Fostoria, Ohio.
539
540
NEW MOTION PICTURE APPARATUS [J. S. M. p. E.
at higher currents ranging up to 150 amperes. In the higher part of the usable
current range of the new carbon, it is desirable that a 1/2-inch "Orotip" carbon
be used for the negative carbon since the 7/i6-inch "Orotip" commonly employed
with the regular carbon will be overloaded.
The burning characteristics of this new H.I. projector carbon are shown in
Table I in comparison with the regular and super H.I. projector carbons. The
new carbon at 150 amperes delivers a slightly higher crater candle-power than
the super carbon at the higher current of 180 amperes, while the consumption
rate of the new carbon is only little more than one-half that of the super carbon.
In comparison with the regular carbon at 125 amperes, the new carbon at 150
amperes delivers 45 per cent higher crater candle-power with only 20 per cent
1000
900
800
700
600
500
400
300
200
100
ISO AnP
/'
\
/
,,--"
' 125
AMP "
^
\
p
' — ..
p—
^"^
\
\\
125
AMP.
/
\
\
j
7
— NEW H.I. PROJECTOR
Vi
\
0
4 /
*
-SUPER H.I. PROJECTOR
"^
v\
7
REGULAR H.I. PROJECTOR
N
1
765432101234
RADIUS OF CRATER IN MILLI- METERS
567
FIG. 1. Distribution of intrinsic brilliancy across crater face of 13.6-mm
high-intensity carbons.
more current and 15 per cent increase in consumption rate. When both the new
and regular carbons are burned at 125 amperes, they give the same candle-power
but the consumption rate of the new carbon is 35 per cent lower.
Fig. 1 shows the distribution of intrinsic brilliancy across the crater face of the
above carbons at the currents given in Table I. The new carbon at 150 amperes,
despite a 40 per cent slower burning rate, has an intrinsic brilliancy of 870 candles
per sq-mm, which is slightly higher than that of the super carbon at 180 amperes.
At 125 amperes the new carbon has a lower intrinsic brilliancy in the center of
the crater than the regular carbon at the same current, but an essentially uniform
distribution over a much larger area, the importance of which will be discussed
later.
The data presented above indicate that in a motion picture projection system
the new carbon at 150 amperes would be expected to yield a considerable increase
in screen light, compared with the regular carbon at 125 amperes, and should in
Nov., 1941]
NEW MOTION PICTURE APPARATUS
541
fact equal that from the super carbon at 180 amperes. Comparative tests show
that this expected improvement is realized in practice. The performance of the
new carbon has been compared with that of the regular and super carbons in
several projection lamps and optical systems commonly used in theaters. The
importance of making screen-light comparisons at the same distribution of light
over the screen has been demonstrated in an earlier publication.2 Accordingly,
all measurements were made with the intensity at the sides of the screen 80 per
cent of that at the center. In order to place the measurements with the various
lamps and optical systems on a comparable basis, the screen light and efficiency
values for the various carbons and currents have all been expressed in Table I
and Fig. 2 on a relative basis, assuming the regular carbon at 125 amperes with
the same optical system to be 100.
TABLE I
Characteristics of 13. 6- Mm H.I. Projector Carbons under Typical Operating
Conditions
Carbon
Arc amperes
Arc volts
Positive consumption rate
(inches per hour)
Crater candle-power
Relative screen light at 80
per cent side-to-center
distribution ratio*
Regular H.I.
Projector
125
68
13
43,000
New H.
125
68
8.5
43,000
Projector
150
78
15
63,000
100 98-103* 128-147*
Super H.I.
Projector
180
75
25
60,000
122-136=
*The ranges given are due to variations between different conventional optical
systems employed.
The gain in screen light obtained with the new carbon at 150 amperes compared
to the regular carbon at 125 amperes ranges from 28 to 47 per cent, depending
upon the type of optical system employed. Similarly, at 180 amperes the super
carbon gives from 22 to 36 per cent more screen light than the regular at 125 am-
peres. It is therefore apparent that the new carbon at 150 amperes delivers
slightly more screen light than the super carbon at 180 amperes. The new car-
bon and the regular carbon produce essentially the same amount of screen light
when both are operated at 125 amperes. This may at first glance appear to
contradict the brilliancy data shown in Fig. 1, where it is seen that at 125 amperes
the new carbon has a lower center brilliancy than the regular carbon. This is
explained by the fact that the new carbon, with its larger area of uniform bril-
liancy, can be operated closer to the true focus in an optical system than can the
regular carbon.
As shown in previous publications,4'5 if the amount of screen light is divided
by the length of carbon consumed in unit time, there is obtained a measure of
the efficiency of utilization of carbon in terms of the total light energy derived
from a unit length of carbon. This efficiency is shown in Fig. 2. While the super
carbon gave higher light than the regular, this was accompanied by an efficiency
of carbon utilization only 63 to 71 per cent as great as with the regular carbon.
542
NEW MOTION PICTURE APPARATUS [J. S. M. p. E.
160
140
120
100
80
60
40
20
120
100
60
60
40
20
RELATIVE TOTAL SCREEN LIGHT ENERGY
PER INCH OF CARBON
NEW H.I.
AT 125 AMP.
REGULAR H.
AT 125 AMP.
.
NEW H.I.
AT 150 AMP.
SUPER H. I.
AT 180 AMR
RELATIVE SCREEN LIGHT PER ARC WATT
REGULAR H. I.
AT 125 AMR
NEW H.I.
AT 125 AMR
NEW H. 1.
AT 150 AMR
SUPER HI
AT 180 AMP.
FIG. 2. Efficiency of utilization of carbon and electrical power
with 13.6-mm high-intensity carbons. The ranges shown are due
to variations between the different conventional optical systems
employed.
One of the outstanding advantages of the new carbon is its greatly increased ef-
ficiency which at 150 amperes results in about 75 per cent more light energy per
inch of carbon than the super carbon at 180 amperes, and in fact is 11 to 27 per
cent better in this respect than the regular carbon at 125 amperes. In common
with past experience the new carbon has a higher efficiency at 125 amperes than
at 150 amperes, so that at the lower current it produces 50 to 57 per cent more
light energy per inch of carbon than the regular carbon at this same current.
Nov., 1941] NEW MOTION PICTURE APPARATUS 543
Fig. 2 gives also the relative efficiency of power utilization in terms of relative
screen light per arc watt. This shows that the new carbon at 150 amperes de-
livers about 20 per cent more screen light per unit of power consumed at the arc
than the super carbon at 180 amperes. At both 150 and 125 amperes, the new
carbon produces approximately the same amount of screen light per arc watt as
the regular carbon at 125 amperes. This new carbon offers a favorable combina-
tion of high light output and efficiency of utilization of carbon and power.
In addition to the advantages described above, the new carbon has greater
latitude in relative carbon positions at which steady burning may be attained.
In order to maintain steady operation with the regular carbon, a certain minimum
protrusion of the positive carbon from the jaws is required.1 The new carbon
will give a steady light at the optimum protrusion of the regular carbon and also
with the positive protrusion shortened by as much as about 0.1 inch. Carbon
efficiency, light, and life are slightly improved at the shortened protrusion possible
with the new carbon. This reduction in positive protrusion increases the dis-
tance from the crater to the condenser lenses, requiring a corresponding adjust-
ment of the condenser position toward the arc to maintain the desired screen-
light distribution. Maximum efficiency has been found when the arc length be-
tween the centers of the carbons is between 3/4 and 7/s inch. Table I and Fig. 2
were obtained with the optimum positive protrusion and arc length.
When an arc is struck, the positive crater is subjected to both thermal and
mechanical shock, particularly if the arc is struck at full current. Occasionally
this shock causes the lip of the crater to be cracked or a chip broken away so that
the burn-in period is increased by the time necessary to form a symmetrical
crater. This will occur more frequently when the contact is made on the lip of
the crater. The new carbon has improved resistance to these shocks and gives
greater assurance of freedom from chipping in case unfavorable conditions are en-
countered during the striking of the arc.
The new H.I. projector carbon possesses all the advantages of the super H.I.
carbon from the standpoint of light without requiring the high current and con-
sumption rate necessary for the super carbon, and in fact with very little increase
in consumption rate over the regular carbon. This new carbon therefore brings
to the great majority of the theaters now using the regular carbon an extremely
practicable means of increasing their screen brightness to give better projection.
REFERENCES
1 JOY, D. B., AND DOWNES, A. C.: "Characteristics of High-Intensity Arcs,"
/. Soc. Mot. Pict. Eng., XIV (March, 1930), p. 291.
2 JOY, D. B.: "A 13.6-Mm Super-High-Intensity Carbon for Projection,"
J. Soc. Mot. Pict. Eng., XXVII (Sept., 1936), p. 243.
3 JOY, D. B., LOZIER, W. W., AND NULL, M. R.: "Carbons for Transparency
Process Projection in Motion Picture Studios," /. Soc. Mot. Pict. Eng., XXXIII
(Oct., 1939), p. 353.
4 LOZIER, W. W., JOY, D. B., AND SIMON, R. W.: "A New Negative Carbon
for Low-Amperage High-Intensity Trims," J. Soc. Mot. Pict. Eng., XXXV
(Oct., 1940), p. 349.
6 LOZIER, W. W., CRANCH, G. E., AND JOY, D. B.: "Recent Developments in
8-Mm Copper-Coated High-Intensity Positive Carbons," /. Soc. Mot. Pict.
Eng., XXXVI (Feb., 1941), p. 198.
CURRENT LITERATURE OF INTEREST TO THE MOTION PICTURE
ENGINEER
The editors present for convenient reference a list of articles dealing with subjects
cognate to motion picture engineering published in a number of selected journals.
Photostatic copies may be obtained from the Library of Congress, Washington, D. C.,
or from the New York Public Library, New York, N. Y. Micro copies of articles
in magazines that are available may be obtained from the Bibliofilm Service, Depart-
ment of Agriculture, Washington, D. C.
American Cinematographer
22 (September, 1941), No. 9
Enlarging 16-Mm Kodachrome to 35-Mm Technicolor (pp.
414-415, 440)
A Three-Dimensional Exposure-Meter for Professional Use
(pp. 416-417, 440, 442)
What Should Tests Show? (pp. 418, 442)
Are We Making the Most of Modern Resources? Type-
writer Title Trickery (pp. 426-427, 444)
Communications
21 (September, 1941), No. 9
Standards for Electrical Transcriptions (pp. 10-11, 33)
"Add-A-Unit" Amplifiers Widen Application Scope (pp.
14, 16, 34-35)
Educational Screen
20 (September, 1941), No. 7
Motion Pictures — Not for Theaters (pp. 284-285, 292),
Pt. 29
International Projectionist
16 (July, 1941), No. 7
Reproducer Troubles Due to "Grounds" (pp. 7-9)
Recent Advances in Non-Reflective Lens- Coating Processes
(pp. 11-12, 15, 26)
Projector Factory Overhaul Procedure (pp. 18-19)
Motion Picture Herald (Better Theaters Section)
144 (September 20, 1941), No. 12
Accurate Calculation of Screen Size and Lighting Needs
W. STULL
W. LEAHY
L. WHITE
R. W. TEOREY
H. A. CHINN
H. PARO
A. E. KROWS
L. CHADBOURNE
W. C. MILLER
(pp. 33-34, 36, 38)
C. E. SHULTZ
544
JOURNAL
OF THE SOCIETY OF
OTION PICTURE ENGINEERS
Volume XXXVII
December, 1941
CONTENTS
Page
Proceedings of the Fiftieth Semi-Annual Banquet, Hotel Penn-
sylvania, New York, N. Y., October 22, 1941 547
A Compact Direct-Reading Reverberation Meter,
E. S. SEELEY 557
On the Playback Loss in the Reproduction of Phonograph
Records O. KORNEI 569
Analytic Treatment of -Tracking Error and Notes on Optimal
Pick-Up Design H. G. BAERWALD 591
The Specialization of Film Delivery J. H. VICKERS 623
Current Literature 629
Highlights of the Fiftieth Semi-Annual Convention, New York,
N. Y., October 20-23, 1941 631
Program of the Convention 636
Book Review 639
Society Announcements 640
Index, Vol. XXXVII (July-December, 1941)
Author 644
Classified 647
(The Society is not responsible for statements by authors.)
JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
SYLVAN HARRIS, EDITOR
BOARD OF EDITORS
ARTHUR C. DOWNBS, Chairman
JOHN I. CRABTREE ALFRED N. GOLDSMITH EDWARD W. KELLOGG
CLYDE R. KEITH ALAN M. GUNDELFINGER CARLETON R. SAWYER
ARTHUR C. HARDY
Subscription to non-members, $8.00 per annum; to members, $5.00 per annum,
included in their annual membership dues; single copies, $1.00. A discount
on subscription or single copies of 15 per cent is allowed to accredited agencies.
Order from the Society of Motion Picture Engineers, Inc., 20th and Northampton
Sts., Easton, Pa., or Hotel Pennsylvania, New York, N. Y.
Published monthly at Easton, Pa., by the Society of Motion Picture Engineers.
Publication Office, 20th & Northampton Sts., Easton, Pa.
General and Editorial Office, Hotel Pennsylvania, New York, N. Y.
West Coast Office, Suite 928, Equitable Bldg., Hollywood, Calif.
Entered as second class matter January 15, 1930, at the Post Office at Easton,
Pa., under the Act of March 3, 1879. Copyrighted, 1941, by the Society of
Motion Picture Engineers, Inc.
OFFICERS OF THE SOCIETY
** President: EMERY HUSE, 6706 Santa Monica Blvd., Hollywood, Calif.
** Past-President: E. ALLAN WILLIFORD, 30 E. 42nd St., New York, N. Y.
**Executivc Vice-P resident: HERBERT GRIFFIN, 90 Gold St., New York, N. Y.
* Engineer ing Vice-President: DONALD E. HYNDMAN, 350 Madison Ave., New
York, N. Y.
** Editorial Vice-P resident: ARTHUR C. DOWNES, Box 6087, Cleveland, Ohio.
* Financial Vice-P resident: ARTHUR S. DICKINSON, 28 W. 44th St., New York,
N. Y.
** Convention Vice-President: WILLIAM C. KUNZMANN, Box 6087, Cleveland, Ohio.
"Secretary: PAUL J. LARSEN, 44 Beverly Rd., Summit, N. J.
"Treasurer: GEORGE FRIEDL, JR., 90 Gold St., New York, N. Y.
GOVERNORS
**MAX C. BATSEL, 501 N. LaSalle St., Indianapolis, Ind.
*JOSEPH A. DUBRAY, 1801 Larchmont Ave., Chicago, 111.
*JOHN G. FRAYNE, 6601 Romaine St., Hollywood, Calif.
*ALFRED N. GOLDSMITH, 580 Fifth Ave., New York, N. Y.
*ARTHUR C. HARDY, Massachusetts Institute of Technology, Cambridge,
Mass.
**LOREN L. RYDER, 5451 Marathon St., Hollywood, Calif.
*TIMOTHY E. SHEA, 195 Broadway, New York, N. Y.
*RBEVE O. STROCK, 35-11 35th St., Astoria, L. I., N. Y.
*Term expires December 31, 1941.
**Tenn expires December 31, 1942.
PROCEEDINGS OF THE FIFTIETH SEMI-ANNUAL BANQUET
OF THE
SOCIETY OF MOTION PICTURE ENGINEERS
HOTEL PENNSYLVANIA, NEW YORK, N. Y.
OCTOBER 22, 1941
Nearly 200 members and guests of the Society assembled at the
Fiftieth Semi- Annual Banquet held at the Hotel Pennsylvania, New
York, N. Y., on October 22nd. This banquet commemorated the
twenty-fifth anniversary of the Society's founding.
Guests and officers at the Speakers' table were: President Emery
Huse; Mr. Otto S. Shairer, Vice-President of RCA Laboratories,
and Mrs. Shairer; Mr. Glenn L. Dimmick, recipient of the 1941
Progress Medal, and Mrs. Dimmick; Dr. John G. Frayne, one of
the recipients of the 1940 Journal Award; Mr. Ralph E. Farnham,
citationist for Dr. Frayne and Dr. V. Pagliarulo (who was not pres-
ent) ; Mr. and Mrs. Herbert Griffin; Mr. and Mrs. Donald E. Hynd-
man; Mr. and Mrs. E. Allan Williford; Mr. and Mrs. Paul J.
Larsen; Mr. and Mrs. George Friedl, Jr.; Mr. and Mrs. Edward
M. Honan; Mr. William C. Kunzmann; Mr. and Mrs. Reeve O.
Strock; Mr. and Mrs. Frank E. Carlson; Mr. John A. Maurer; and
Mr. Arthur C. Downes.
After introducing those seated at the Speakers' table, President
Huse announced the results of the election of officers and governors
of the Society for 1942,* which were as follows:
Engineering Vice-President: DONALD E. HYNDMAN
Financial Vice-President: ARTHUR S. DICKINSON
Secretary: PAUL J. LARSEN
Treasurer: GEORGE FRIEDL, JR.
Governor: FRANK E. CARLSON
Governor: JOHN A. MAURER
* For complete list of officers and governors for 1942, see p. 640 of this issue of
the JOURNAL.
547
548 PROCEEDINGS OF SEMI- ANNUAL BANQUET [J. S. M. p. E.
Governor: EDWARD M. HONAN'
Chairman, Atlantic Coast Section: ALFRED N. GOLDSMITH
Chairman, Pacific Coast Section: JOHN G. FRAYNE
Following this announcement, President Huse gave a brief descrip-
tion of the nature of the Progress and Journal Awards made each
year by the Society at the banquet of the Fall Convention, and called
upon Mr. Paul J. Larsen, Secretary of the Society, to report for the
Progress Award Committee in the absence of its Chairman, Mr. Ken-
neth F. Morgan. The Progress medal is awarded by the Society each
year to an individual in recognition of any invention, research, or
development, which, in the opinion of the Committee and the Board
of Governors, has resulted in a significant advance in the develop-
ment of motion picture technology. Mr. Larsen reported that the
Committee had selected as the 1941 recipient of the Progress medal,
Mr. Glenn L. Dimmick of RCA Manufacturing Company, Indianap-
olis, Indiana, and that the report of the Committee had been approved
by the Board of Governors at the meeting held on October 19th.
Thereupon, President Huse called upon Mr. Otto S. Shairer, Vice-
President of RCA Laboratories, to give an account of the work of the
recipient that formed the basis for the Progress Medal Award.
GLENN L. DIMMICK
RECIPIENT OF THE 1941 SMPE PROGRESS MEDAL
Otto S. Shairer
It is a real privilege to be accorded this opportunity to speak of the
accomplishments of the man whom this Society has chosen to receive
its Progress Award. The associates and friends of Glenn Leslie Dim-
mick believe that this honor is well deserved, and they salute him.
Born and reared in Missouri and educated in its public schools and
University, his ability was early recognized by his election to the
honorary engineering and scientific societies, Tau Beta Pi and Sigma
Xi. Almost immediately after graduation he developed a recording
galvanometer capable of modulating ten times more light than the
previous oscillographic type of galvanometer, and free from the ob-
jectionable requirement of oil damping. The increased light made
., 1941] PROCEEDINGS OF SEMI-ANNUAL BANQUET 549
ry other improvements possible, among which probably the most
iportant was the introduction of ultraviolet light for recording and
iting. While making no claim to priority in conception of the
sible advantages of ultraviolet light, Mr. Dimmick performed the
lly important service of "proving it in," which meant overcoming
numerous minor difficulties and problems needed to give the sys-
tem a fair trial, analyzing the factors contributing to its success, and
GLENN LESLIE DIMMICK
proving by actual results that the anticipated benefits were amply
afforded. Ultraviolet recording and printing are now practically uni-
versal where variable-area recordings are employed.
Time permits only a brief mention of a few of Mr. Dimmick's many
other developments, which include improvements in galvanometers;
advanced and refined designs of optical systems; new and improved
types of sound-tracks; a variable-intensity system for making den-
sity recordings; a system for making direct positives particularly low
in ground noise ; sound-powered telephones of high efficiency, in large
use in the Navy; the sound-recording system for a 16-mm camera,
550 PROCEEDINGS OF SEMI- ANNUAL BANQUET [J. S. M. P. E.
probably the smallest and lightest complete sound and picture re-
cording equipment ever built; optics for projection sound printers;
and optics to meet the unusual requirements for printing and re-
producing Fantasia; and the "class B" system in which the positive
and negatives half -waves are recorded on separate parts of the track.
The "class B" system, which requires no auxiliary ground-noise
equipment but is inherently the quietest of known methods of photo-
graphic recording, appeared to many to present almost insurmount-
able difficulties, but Mr. Dimmick found means of bringing all the
factors under control, not only for laboratory conditions but for prac-
tical field conditions, and the system is in wide commercial use.
For his contributions to science and engineering he has received
recognition as a Modern Pioneer by the National Association of
Manufacturers. Eleven of his technical papers have been published
in the JOURNAL of this Society. They are reports of accomplishments
and marvels of conciseness. His many patents are but by-products of
his original and imaginative thinking, rather than objectives in them-
selves. They represent an unusually high percentage of inventions
now in use. He is outstanding in his audacity in undertaking the
difficult and the seemingly impossible and in his ability to produce
practical results.
Mr. Dimmick would be the last person to wish to receive credit for
his many developments without acknowledging the assistance and
cooperation of his associates. However, they agree that it has been
largely through his energy, confidence, and enthusiasm that their
joint efforts have so often been brought to successful fruition. His
home life and his lovely family are an inspiration and an assurance
that the fine traditions already established will be carried on.
All of Mr. Dimmick's associates are grateful for the high honor this
Society is bestowing upon him tonight. In honoring one of us, you
honor us all. We congratulate Mr. Dimmick upon his award, and
this Society upon the fitness of its choice.
Following this account of Mr. Dimmick's work by Mr. Shairer,
and the presentation of the medal by President Huse, Mr. Dimmick
briefly thanked the officers and the Board of Governors and the
members of the Society for the honor thus bestowed upon him.
Dec., 1941] PROCEEDINGS OF SEMI-ANNUAL BANQUET 551
President Huse next called upon Mr. Ralph E. Farnham, Chair-
man of the Journal Award Committee to name the recipient or re-
cipients of the Journal Award for 1940 and to present a historical
account on the basis of which the award has been granted. Each
year at the Fall Convention of the Society a Journal Award cer-
tificate is presented to the author or to each of the authors of the
most outstanding paper originally published in the Journal of the
Society during the preceding calendar year. Mr. Farnham spoke as
follows :
JOHN G. FRAYNE AND VINCENT PAGLIARULO
RECIPIENTS OF THE 1940 JOURNAL AWARD
Ralph E. Farnham
President Huse has outlined the purpose of the Society's Journal
Award. In order to determine the paper deserving of this honor,
the Journal Award Committee had before it the job of studying and
analyzing some seventy-six papers published during 1940. These
papers were rated, first, on the excellence of presentation of the
material; second, the originality and breadth of interest; and last,
their technical merit and the importance of the material.
Members of the committee then voted first, second, and third
choices. It was felt that this method would result in a fair and ac-
curate appraisal of the paper meriting the Journal Award. The
nomination of the Committee was then approved by the Board of
Governors of the Society at its recent meeting.
It is my pleasure to announce that award has been granted to the
authors of the paper entitled, "The Effects of Ultraviolet Light on
Variable-Density Recording and Printing," by Drs. John G. Frayne
and Vincent Pagliarulo of Electrical Research Products, Inc., Holly-
wood, published in the June, 1940, issue of the JOURNAL of the Society.
This paper, in the opinion of the Committee, deserves the Journal
Award because of the excellence of organization of its material, the
originality displayed in the design of charts, and their arrangement,
j It is a relatively short paper, and yet it adequately covers an impor-
552 PROCEEDINGS OF SEMI-ANNUAL BANQUET [J. S. M. P. E.
tant development. This paper can well serve as a model for other
papers of this type. Of its two capable authors we have the following
brief information :
Dr. John G. Frayne, Superintendent of Methods Engineering, for
Electrical Research Products, Inc., was born in Ireland, and after
following the general arts and science courses at Kilkenny College as
well as Trinity College, came to the United States in 1914. He has
JOHN G. FRAYNE
to his credit experience as a miner, a farm-hand, and college instruc-
tor, and received a Fellowship in Physics at the University of Minne-
sota. He was a Lieutenant in the U. S. Signal Corps, stationed at the
Camp Vail Radio Laboratories during our participation in the World
War I.
He received his degree as Doctor of Philosophy from the University
of Minnesota in 1921 and organized the Physics department at An-
tioch College under Doctor Arthur E. Morgan. Dr. Frayne is a Fel-
low of the National Research Council, a Fellow of the American
Physical Society, a Fellow Member of the Society of Motion Picture
Dec., 1941] PROCEEDINGS OF SEMI-ANNUAL BANQUET
553
Engineers, a Research Council associate of the Academy of Motion
! Picture Arts and Sciences, and is Chairman of the Pacific Coast Sec-
tion of the Society.
Among his outstanding contributions, have been the introduction
of sensitometric controls in the processing of variable-density sound-
film, the introduction of noise-reduction systems in sound-recording,
and the development of an electrical densitometer that is becoming
VINCENT PAGLIARULO
me standard of the film industry. He has been a prolific contributor
to the JOURNAL of our Society.
Vincent Pagliarulo was born in Italy. He came to this country in
1900 and received his general school education in Chicago and is a
paduate of Armour Institute of Technology.
I His earlier experience was with the Kellogg Switchboard and Sup-
ply Co., in charge of automatic telephone development as well as
telephone equipment manufacture.
I Like Dr. Frayne, Dr. Pagliarulo likewise had a notable career in the
fJ. S. Signal Corps during the World War I. He was commissioned a
554 PROCEEDINGS OF SEMI- ANNUAL BANQUET [j. s. M. p. E.
Captain and spent a considerable period in the A. E. F. in France, in
charge of radio communication equipment, inspection, and supplies,
and was later Chief Signal Officer with the American Forces stationed
in Holland.
Following his war experiences he entered the University of Chicago
in advanced courses in physics and mathematics, and received a
Doctorate of Philosophy in 1924.
His entry into Electrical Research Products, Inc., was by way of
the Western Electric Co. Since 1928 he has been identified with
developments in sound recording, noise-reduction methods and fine-
grain film technics. He is a contributor to the technical literature of
the SMPE and is a member of the Society.
At the conclusion of Mr. Farnham's address, President Huse pre-
sented the Journal Award certificates to Dr. Frayne, who accepted
Dr. Pagliarulo's certificate in the absence of the latter. Dr. Frayne
responded with appropriate words of thanks.
At the meeting of the Board of Governors of the Society, held on
October 19th, action was taken to honor Mr. William C. Kunzmann
in recognition of his long service in behalf of the Society. President
Huse called upon Mr. E. Allan Williford, Past-President of the So-
ciety, to present to Mr. Kunzmann a testimonial certificate prepared
by the Board as a token of their deep appreciation. Mr. Williford
spoke as follows:
WILLIAM C. KUNZMANN
E. Allan Williford
As you have been told, this makes the twenty-fifth Anniversary of
the founding of our Society. Since that day, July 24, 1916, when six
men got together, recognizing the need for such a body as ours to
bring order out of the technical chaos existing in equipment and proc-
esses at that time, the Society has grown in numbers and in influ-
ence. As with all institutions, no matter how worthy, ours did not
grow of itself. Guiding the Society through these years have been
men giving of their spirit, their time, and their energies. Some have
Dec., 1941] PROCEEDINGS OF SEMI-ANNUAL BANQUET
555
been men in high office, some men in the ranks of our Society. Some
of these men have passed on from this earth, and some have lost inter-
est, or otherwise ceased activity in our Society. But the majority
of the hardest workers are still in harness, still working for the ad-
vancement of our Society.
There is one among us who has never missed a Convention. Upon
his shoulders have fallen the tasks of making preparations for each
Semi- Annual Meeting, and appointing and supervising the work of the
various convention committees. When the opening of a session was
upon us, if some piece of equipment was missing, it was to him that
WILLIAM C. KUNZMANN
we have all looked to get us out of the hole. The banquet arrange-
ments, including entertainment, are part of his responsibilities, and we
all know he has discharged them well.
During twenty-one of these twenty-five years, it has been my privi-
lege to be closely associated with him in business. It has been more
than just a business relationship, for during these years he has be-
come one of my close and most revered of friends. He has always been
as kind in looking after my interests, at the expense of his own con-
venience, as he has in looking after the interests of our Society — like-
wise, at the cost of his own convenience. And so it is with deep ap-
preciation of the privilege here given to me that I now ask Bill Kunz-
556 PROCEEDINGS OF SEMI-ANNUAL BANQUET
mann, our beloved Convention Vice-President, to stand while I read
to him and to you, this certificate which has been awarded to him by
unanimous vote of our Board of Governors as a special token of our
esteem :
"In recognition of his long and faithful service as a member of the Society since
1916, as a member of the Board since 1929, and as Convention Vice-President since
1933, the Board of Governors of the Society by unanimous action have on this
date presented this certificate to William C. Kunzmann as a testimonial of their
appreciation and esteem."
At the conclusion of Mr. Williford's address, Mr. Kunzmann re-
sponded briefly, and the banquet concluded with dancing and enter-
tainment.
COMPACT DIRECT-READING REVERBERATION METER5
E. S. SEELEY**
Summary. — Conditions surrounding widespread measurement of reverberation
time in theaters by a theater service organization require that the measuring equip-
ment stress economy in size, in cost, and in time for a set of measurements, and the
readings provided must be in such form that an acoustical specialist is not required
for their interpretation. These requirements must be satisfied even to the sacrifice
of information on secondary properties of the decay characteristic.
Several types of direct-reading reverberation-time meter circuits were devised and
one of these types, in trial quantity, is now giving service in the field. These instru-
ments integrate the decaying signal over approximately a 5-db interval beginning
after approximately 18 db of decay, and the result is translated into reverberation
time by meter scales. Thus the first 22 db (approx.) of the decay characteristic is
encompassed by the reading and it is shown that the contained energy includes essen-
tially all the reverberant energy important to quality.
Reverberation measurements are made with these instruments at the time that
acoustical response measurements are being made, and under these conditions 150
time readings may be made throughout the auditorium and over the audio spectrum
in a total added time of forty minutes. Practically no further treatment of the data
is required.
The requirements placed on reverberation-measuring equipment
are determined by the objective of the measurements and the condi-
tions surrounding the use of the equipment. For example, if it is
necessary to chart 60 to 90 db of decay or to reveal the fine detail in
the decay characteristic, the resulting complexity and high cost of
the measuring equipment may be readily acceptable. In other work,
the penalties paid for the unneeded exceptional delicacy of measure-
ment may exclude the equipment from use.
In the continuous effort to advance the art of theater service, it
appeared that the performance of reverberation measurements on a
broad scale offered considerable promise as a step toward the ob-
jective of uniformly best sound quality in all theaters. It would be
anticipated that any member of a large field personnel would be ex-
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received April
11, 1941.
** Altec Service Corp., New York, N. Y.
557
558
E. S. SEELEY
U. S. M. P. E.
pected to add the ability to make such measurements to his present
diversified skills. Special considerations attach to such a project, and
these have a dominant effect on the form and functions of the measur-
ing equipment selected. These considerations are :
(1) The equipment must be particularly compact and rugged.
(2) Work done in theaters during off-show hours involves expense to the
exhibitor for theater personnel and to the service organization for the engineer's
FIG. 1. Reverberation-measuring equipment.
time. Such cost makes it imperative that the time spent in the theater making
measurements be reduced to an acceptable level, that the equipment be operable
by a single engineer, and that the process of "working up the data" into a form
permitting proper interpretation be reasonably short.
(5) The technic for operation of the equipment and interpretation of the
results must be sufficiently simplified to secure satisfactory measurements without
inordinate personnel training.
(4) The equipment must be of modest cost since it is intended for use in a
non-revenue-producing service.
Dec., 1941]
REVERBERATION METER
559
(5) The equipment must be coordinated with other equipment carried by the
engineer.
In connection with requirement 5, a considerable part of the equip-
ment required for reverberation- time measurements is used also in the
measurement of acoustical response. Our organization, cooperating
in the work of the Academy Research Council Sub-Committee on
Acoustic Characteristics, described in the March, 1941, JOURNAL is
making many measurements of acoustical response in theaters. It is
FIG. 2. First form of circuit for providing data from which the decay charac-
teristic can be plotted and appraised in terms of reverberation time.
important that the reverberation-measuring equipment avoid dupli-
cation of microphone, amplifier, cables, etc., used for the response
measurement, since a number of sets of such equipment are already
distributed to the field. This equipment is shown in Fig. 1. An
essentially non-directional crystal microphone was selected for this
work. A small pre-amplifier together with its filament cell is built into
a flashlight case mounted on the tripod. The pre-amplifier permits
the use of a 100-ft unshielded cable to convey the signal at relatively
high level and low impedance to the measuring amplifier and meter.
This arrangement possesses several advantages including that of
virtually eliminating the effect of temperature on the characteristic of
560 E. S. SEELEY [j. s. M. p. E.
the microphone by terminating the latter with a very high impedance.
The measuring amplifier used is a high-quality emergency amplifier
carried by all engineers and designed to replace in an emergency any
entire theater amplifier system. The level indicator is likewise one
carried by all engineers in this organization for transmission measure-
ments.
Several designs of reverberation-measuring equipment more or less
fulfilling the foregoing requirements were considered. The first form
was one which provided numerical data from which the decay char-
acteristic could be plotted and appraised in terms of reverberation
time. The method was a variation of the variable-interval method,
but considered to have the advantage of being less susceptible to
local irregularities in the decay characteristic. It was also expected
to be fast enough to meet the requirements and to be sufficiently in-
expensive to build. Fig. 2 refers to this instrument.
The equipment contained a timing device in the form of a vibrat-
ing reed to divide the decay period into suitable intervals. The reed
produced electric pulses to operate a stepping relay. The relay car-
ried three contactors, each of which passed over its own series of con-
tacts and performed a separate function. The first was to interrupt
the electrical signal entering the speaker system. The second was to
control, through attenuation steps, the level delivered by the micro-
phone to the amplifier, beginning with maximum attenuation at start
of decay. The third switches the signal, amplified and rectified, to a
series of condensers through resistors of appropriate value. Thus, for,
let us say, the first one-fifth of a second the relatively strong signal was
considerably attenuated and made to charge the first condenser to a
value determined by the average acoustic pressure during that portion
of the decay period; during the second one-fifth of a second, the
weaker signal, less attenuated, was stored in the second condenser,
etc. The infinite-resistance voltmeter permitted evaluation of the con-
denser voltages without discharging them.
The resulting readings, corrected for the corresponding attenuation
values, were then plotted to reveal the decay characteristic. The
value of reverberation time was then approximated in the usual man-
ner by estimating the trend of the irregular decay characteristic and
extrapolating it to the 60-db ordinate.
To prove the accuracy of this instrument, an equipment was con-
structed which produced electronically a logarithmic decay of an
oscillator input signal. The decay rate could be set to any desired
Dec., 1941]
REVERBERATION METER
561
value. An example of such an accuracy check is shown in Fig. 2
along with a few typical decay curves taken in a theater.
A number of possible improvements in this equipment were evi-
dent on reexamining the design after considerable experience was
obtained with it in theaters. However, this experience proved also
that a point-to-point device would not fulfill the requirements sur-
rounding our reverberation measurements in theaters. Theater
working time was excessive, since each measurement required that a
Tim a -*•
FIG. 3. Circuit for evaluating the 60-db time abscissa of a pure exponential
which, plotted as db vs. time, has the same integral as the actual decay
characteristic.
number of readings be observed and recorded. When the theatre
work was finished the laborious task of correcting and interpreting the
data followed. Finally, the irregularity of most of the decay char-
acteristics, whether measured by this equipment or any other, made
the technic of interpreting them one for an acoustic specialist.
To close the gap between equipment performance and its require-
ments, it was decided that only a direct-reading instrument would
solve the problem. Two-slope decays, prevalence of small irregulari-
ties, and other secondary properties of the decay characteristic would
not be revealed by such a method, but the equipment could be simpli-
fied and its use speeded up so that the essential requirements could
562
E. S. SEELEY
[J. S. M. P. E.
be met. Furthermore, the labor and expertness necessary to convert
decay curves to reverberation-time values would be eliminated.
Three versions of a direct-reading reverberation meter were de-
vised and considered, one of them constructed and tested in "bread-
board" form and another built in substantial trial quantity and now
giving service in the field.
At this point it seems desirable to digress with a discussion of the
definition of reverberation time. The Acoustical Society of America
\ N«1- Ar«o -A.
A,v. Slop* To
™ Av. Slop*.
Rtforencei Rectifttr
Level 1
And
J
Control
1
L
FIG. 4. Circuit interpreting reverberation time as the 60-db abscissa of
a true exponential haying a slope corresponding to the average slope of the
actual decay characteristic slotted logarithmically.
has adopted the following definition for this quantity: the time re-
quired for the average sound-energy density to decrease to one-
millionth of its initial value. The usual method of measuring re-
verberation time is to obtain in some manner a curve representing
sound intensity versus time over a period beginning with the inter-
ruption of the steady-state signal at its source. This decay char-
acteristic is then assigned a value of reverberation time in more or less
accord with the definition given above. However, it is not common
practice to adhere rigidly to the definition. Difficulty in applying the
Dec., 1941]
REVERBERATION METER
563
definition results from the fact that decay in any but ideal acoustical
enclosures is not usually exponential and, as it will be pointed out
later, it is the earlier phases of decay that have the greatest signifi-
cance. It is an appraised average slope or, more accurately, trend, of
the db versus time plot that is mathematically converted by most
acoustical engineers to reverberation time. General trend of a
characteristic is a property not readily defined exactly.
Q. T.m« — -t, ta
I I
lRttti-1 lrv*«arqfinj| Mt+erina
I Fur | Cirtu-t I CK-C.V.I+
M,c
o
*mpl.
T ' J I/ fl
\
Vt
Lev*l
0 i
<
1
T T 1
/VW\A
!
4:
I Rr
n+oc-H
anVial
lance-
FIG. 5. Circuit for evaluating the 60-db abscissa of an exponential pass-
ing the origin and a point having coordinates x, y, where x = VW2 and y is
the intergrated acoustic pressure over the interval tih/(tz — 1{).
When developing an instrument to make a direct measurement of a
physical quantity like reverberation time it is necessary to adopt a
precise definition of the quantity to be measured. The three types of
direct-reading measuring device referred to performed the measure-
ment on the basis of three different interpretations of the term
"reverberation time."
The first circuit was devised to evaluate the 60-db time abscissa of
a pure exponential which, plotted as db versus time, has the same
integral as the actual decay characteristic.
564 E» S. SEELEY [j. s. M. P. E.
The circuit, as shown in Fig. 3, consists of a microphone and ampli-
fier, a level indicator to permit establishment of a steady-state refer-
ence level, a rectifier, a circuit the output voltage of which is pro-
portional to the logarithm of the input voltage, a d-c amplifier to in-
crease the amplitude of the signal and permit circuit isolation, an
integrating circuit consisting of a series combination of resistors and
condensers, and a metering circuit to permit reading the condenser
voltage, thus evaluating the integral. The dial of the meter is cali-
brated to read reverberation time corresponding to the 60-db abscissa
of the true exponential having the same integral as the actual decay
curve. It was required to develop a compact network the ouptut volt-
age of which was proportional to the logarithm of the input voltage,
and it was found that a silicon carbide unit in combination with re-
sistors would do this almost exactly over an adequate range.
The second circuit is one which was intended to interpret reverbera-
tion time as the 60-db abscissa of a true exponential having a slope
corresponding to the average slope of the actual decay characteristic
plotted logarithmically.
The circuit, as illustrated in Fig. 4, took the logarithm of the ampli-
fied microphone output, differentiated it to obtain the instantaneous
rate of decay, and then integrated the result to provide the average
rate or average slope of the decay curve. A direct-reading meter scale
interpreted that slope in terms of reverberation. However, it is not
average slope (a precise quantity) that represents the usual interpre-
tation of a decay curve, but trend slope (an inexact concept) which is
often a different number. The time constant of the differentiating
section of this circuit could be varied to bring about a result repre-
senting something between average slope and trend.
The so-called logarithm, differentiating, and integrating circuit
sections illustrated performed extremely well. When the last-de-
scribed circuit was assembled and tested with an artificial decay its
response agreed with predicted performance with an error slightly
over 1 per cent.
The third form of direct-reading circuit evaluated the 60-db
abscissa of the exponential which passed through the origin and a point
having coordinates x,y, where x equals the geometric mean time
•\/tiU, and y equals the integrated acoustic pressure over the interval
tih divided by k — t\. This relationship is illustrated geometrically
in Fig. 5 which shows also the circuit in rudimentary form. When the
circuit just described was assembled and tested with an artificial
Dec., 1941] REVERBERATION METER 565
decay, it was found to agree with predicted performance to 1 per cent.
The circuit of direct-reading instrument 3 shows the microphone,
amplifier, and level indicator in the same relation as in the previous
circuits. After the amplified signal is adjusted to reference value,
the time-measured sequence starts with the signal interruption.
When time ti has elapsed, the circuit is closed to the amplifier tube,
following which the signal is rectified and applied to an integrating
circuit. At time h this process is terminated. The metering circuit,
as before, responds to the voltage developed by the condenser charge,
and the meter scale is designed to express this voltage as reverberation
time.
If the decay characteristic were the simple exponential which it is
frequently considered, all the described measurement methods and a
variety of others would give identical results. Since almost any decay
curve obtained in theaters only approximates an exponential but is
evaluated in terms of the exponential deemed best to generalize its
erratic pattern, no method of evaluating reverberation time is precise.
Hence, in the practical case, all the existing methods may disagree in
their evaluations of reverberation time for a given decay. A direct-
reading instrument interprets a given decay characteristic in accord-
ance with some specific formula, and thus does not enjoy the element
of experienced judgment which is often an important factor in the in-
terpretation of a high-speed level recorder trace. This judgment must
therefore be applied in a broader way in selecting the formula to which
the direct-reading device is calibrated. One case in point is associ-
ated with the fact that many decay curves exhibit two or more general
slopes in different parts of the decay period and their evaluation is
influenced by the amount of decay taken into account. The selection
of the time-periods covered by the direct-reading device is therefore an
important one. Whereas reverberation determinations are sometimes
based upon measurements embracing decay extending to 40 db or
60 db, or even more, it was considered feasible to reduce these ranges
materially in theater measurements with considerable benefit in
operating convenience and equipment simplification and without
vitiating the significance of the readings.
In a paper by W. A. Mueller1 loudness measurements in a number
of theaters were reported which showed that the difference between
average dialog level and average audience noise level is 23 db. Thus
it appears unlikely that reverberation could have an important bear-
ing on intelligibility of speech after a decay of 20 db has developed,
566 E. S. SEELEY [J. S. M. P. E.
since at this time the speech energy Jhas dropped to the level of
audience noise.
However, reverberation has an important bearing on quality of
reproduction quite apart from its interference with intelligibility.
In this category we have the effects of reverberation on total energy
density, on the ability of the listener to distinguish direct from
reverberant sound, and the bearing of this factor on presence, and the
property termed "liveness" which in proper degree is essential to good
quality.
When a sound is originated the first wave- train that reaches an
observer is, of course, the direct wave, followed by the reflected wave-
train having the shortest path; and this in turn by that having the
next shortest path, etc. When the power of the sound-source stops,
the first wave-train to disappear at the observer is the direct wave,
the second is the shortest-path reflected wave, etc., hi the same
sequence as existed during growth. As a consequence, the time re-
quired for the energy-density to grow to half its steady-state value
is the same as the time required for it to decay 3 db. During the
period of intonation of the longer speech sounds, about 0.3 sec., and
in a room having a reverberation time of 1.5 sec., the reverberant
energy grows to 93 per cent of its final \alue, or, in the time-periods
decay will proceed to only 12 db. Thus the wave terminations that
pass an observer during the first 20 db of decay would seem to include
all the energy that has a significant bearing on the aural effects of
reverberation.
From the foregoing discussion it seems logical to conclude that
a decay of 20 db is adequate to form the basis of a measurement.
This is fortunate as it permits simplification of the measuring equip-
ment and facilitates the problem of providing a reading uncontami-
nated by the influence of noise or unaltered by the effects of noise-
suppressing filters. It is believed, however, that measurements of
short decay periods are less exactly reproduced than long-period
measurements due to the effects of phase of the warble cycle at which
the signal is interrupted. The facility with which data are obtained
and the absence of need for further treatment of the readings in the
case of the direct-reading reverberation meter permits a substantial
number of repetitions at a minor extension in the total time required
for a series of measurements.
It remained to select one of the three direct-reading circuits dis-
cussed. Study of a number of actual decay characteristics led to the
Dec., 1941] REVERBERATION METER 567
conclusion that, while none of the three circuits possessed a domi-
nant theoretical advantage over the others, the one finally selected
interpreted the more erratic curves in a manner somewhat more con-
sistent with analyses in which the element of judgment had full play.
The adopted circuit is perhaps as simple as a direct-reading circuit
can be, and the resulting greater stability of its calibration is expected
to make it somewhat to be preferred over the other circuits.
FIG. 6. View of the instrument.
The instrument in its existing form contains as a timing device a
small geared-down induction motor having negligible variation in
slip. The motor drives a series of cams and the cams operate leaf
switches. The complete sequence of events is started by pressing a
button which causes the motor to start and continue in operation for
one complete cam revolution. The first event is the excitation of a
relay located backstage or in the booth which interrupts the signal
568 E. S. SEELEY
to the horns. The next event is the closing of the measuring circuit to
start the integration of the decay signal. The third event is the
disconnection of the integrating circuit from the preceding equip-
ment to terminate the integration process after a particular time
interval. The meter on the panel of the instrument assumes a de-
flection as soon as the integrating condenser obtains a charge.
The instrument is shown in Fig. 6. There are three scales, covering
the overall range 0.6 to 3.5 seconds, although ranges extending into
shorter times or longer times could readily be built in.
For reverberation time falling at the middle of any scale, the instru-
ment timing is such that the 20-db point occurs at the middle of the
integrating period and the signal decays 5 db during the intergrating
period. These figures vary moderately over the scale range.
Due to the drooping frequency characteristics of most theater sys-
tems, consideration must often be given to room noise when high-
frequency measurements are made. The major portion of the noise
energy is confined to the lower frequencies, and means are therefore
provided for switching in a condenser at 1000 cycles or higher to at-
tenuate all components of the microphone signal below this frequency.
In application, reverberation measurements are made at the time
that acoustic response measurements are being made. Immediately
following a series of acoustic response readings for a given microphone
location, a series of reverberation measurements at 26 frequencies is
made without pausing to rewind the warble film. This procedure is
repeated in a number of microphone locations in the auditorium. As
a result of this method of procedure and the rapidity with which the
instrument can be operated, 150 reverberation- time readings may be
obtained in an auditorium by a single engineer in an added time of
about forty minutes including set-up. The work to be done with the
data after the theater work consists in averaging the readings at a
given frequency for the various microphone locations or for certain
groups of locations such as under balcony, front of balcony, etc., and
perhaps plotting the results on semilog paper. This economy of time,
the economy of equipment cost, and the compactness of the instru-
ment seem to fulfill the requirements set forth in the beginning of this
paper.
REFERENCE
1 MUELLER, W. A.: "Audience Noise as a Limitation to the Permissible
Volume Range of Dialog in Sound Motion Pictures," /. Soc. Mot. Pict. Eng.,
XXXV (July, 1940), p. 48.
ON THE PLAYBACK LOSS IN THE REPRODUCTION OF
PHONOGRAPH RECORDS*
O. KORNEI'
Summary. — A theory is set forth to explain the well known level losses, in par-
ticular of the upper frequency range, occurring in the reproduction of lateral-cut
records.
The performance of a pick-up stylus with a spherical point, riding in a laterally
modulated record groove, is discussed from the point of view of the elastic properties
of the record material. After introducing certain permissible simplifications, the
elastic deformations of the two supporting groove walls are calculated, under the
influence of the steady vertical pick-up force, the stylus inertia, and the stylus stiff-
ness. Due to the fact that both forces and geometry are different on the two walls
the respective elastic deformations are also found to be different for both walls. This
fact results in a displacement of the pick-up stylus from the position which it would
assume in an ideally rigid record groove and is responsible for the difference between
the reproduced amplitude and the recorded one. Playback loss and translation loss
are thus explained and quantitatively predicted.
The discussion of the loss equation leads to a number of conclusions. It is found
that in contradistinction to a theoretical pick-up with infinitely small vertical force
and stylus impedance, it appears advisable to provide a practical pick-up with a
definite stylus mass, in order to counteract effectively the playback loss due to the
steady vertical force. The translation loss can thus be reduced to zero in systems
with constant groove velocity if the pick-up constants — in particular, the stylus
mass — are properly chosen. In systems with variable groove velocity (standard disk
recording) the translation loss can not be made to vanish but an increase in the abso-
lute playback level of the upper frequency range can be achieved, thus improving the
signal-to-noise ratio.
INTRODUCTION
In any system for mechanically recording arid reproducing sound,
various undesired effects take place which tend to impair the tonal
quality by introducing both frequency discrimination and non-linear
distortion. Some of these effects are present even with inherently
perfect means of electromechanical conversion, since they are caused
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; revised ms.
received Sept. 29, 1941.
** The Brush Development Co., Cleveland, Ohio.
569
570 O. KORNEI [J. S. M. P. E.
by the geometry of the recording and reproducing means on the one
hand, and by the physical properties of the record material on the
other hand.
The first group of these phenomena is usually referred to as tracing
and tracking conditions. They comprise the kinematics of the re-
producing stylus scanning a record groove which is assumed to be cut
in a material of infinite stiffness. The related questions have been
dealt with extensively in the literature and a selection of the more
comprehensive publications is enumerated in the bibliography.1"8
The second group of the above-mentioned effects has received, so
far, but little attention. The few publications concern themselves
only with experimental investigations 9> 12 (compare also Discussion1)
and with attempts to compensate in the recording process for some
of the deficiencies incurred during the reproducing process.10-11
Only one publication,13 which came to the writer's attention after
completion of this paper, deals with an approximate qualitative ex-
planation of the encountered effects.
It is the purpose of this paper, therefore, to discuss one of the most
apparent effects caused by the physical properties of the record ma-
terial, namely, the frequency discrimination, or, more specifically, the
level loss of the upper frequency range in the reproduction of lateral-
cut sound records. (Basically, similar effects exist for hill-and-dale
records but shall not be treated here in detail.)
The following investigations are based upon sinusoidal tones only
and disregard non-linear distortions. This latter assumption is justi-
fied as long as the distortions do not exceed comparatively small per-
centages, a condition that must be met in a practical system in any
event if an intolerable reproduction is to be avoided. The permis-
sibility of this simplification within the errors of measurement has
furthermore been proved by extensive experiments.
THE PLAYBACK PROCESS
Definitions. — The fact has been known for several years that the
reproduction of the upper frequency range from lateral-cut sound re-
cordings is subject to a considerable level loss if the record groove
velocity, i. e., the recorded wavelength, is reduced. However, no
satisfactory explanation has thus far been advanced to account for
this effect. This case of decreasing groove velocity is realized in
the playback from normal record disks as the pick-up stylus travels
Dec., 1941] PLAYBACK Loss OF PHONOGRAPH RECORDS 571
gradually from the outside to the inside of the record. The corre-
sponding loss in the playback level has been termed translation loss.
A similar case of loss exists for a sound carrier with constant groove
velocity : If a frequency band of constant amplitude is recorded on
such a carrier, a certain deficiency in the reproduction of the upper
frequency range will usually be observed. This loss, too, shall be
included in the concept of the translation loss.
The definition of the translation loss as a level difference implies
the existence of an absolute playback loss for any given point of the
sound record. This loss, in turn, may be defined as the decrease of
the excursion of the reproducing stylus from the actual deviation of
the record groove from its neutral line.
The knowledge of the playback loss is, in practice only of secondary
interest; it is, however, of importance for the later considerations.
To avoid confusion in the terminology the two given definitions should
be kept in mind: The playback loss is the difference between the
recorded and the reproduced level in the very same point of a record;
the translation loss is the difference between the reproduced levels at
two different, but equally modulated points of a record, in other
words, the difference between the playback losses in the two points.
In the experimental determination of the translation loss, account
must be taken of any possible loss due to the recording process.
Experiments have shown that the record impedance to be overcome
by the cutter increases rapidly — for a given frequency and amplitude
— with decreasing groove velocity. Hence, a certain loss in the re-
corded level (generally about 1 to 3 db) is usually experienced toward
the record inside. The most convenient way to determine the
actually recorded amplitude is the well known light-pattern method.14
Geometry and Dynamics. — It has been pointed out before that only
sinusoidal wave-forms will be considered in the following investiga-
tions and that non-linear distortions will and can be neglected. The
problem of finding the level difference between recorded and repro-
duced signals is consequently reduced to the determination of the
amplitude differences; this means that a consideration of the con-
ditions at the crest of the wave only is sufficient.
It is helpful to start with the purely geometric aspect of the condi-
tions in a record groove. Fig. 1 shows the tip of a pick-up stylus,
represented as a sphere, riding at the crest of a sinusoidally modulated
record groove with infinitely stiff walls, i. e., deflected by the amount a
from its neutral position. The sphere is supposed to be supported by
572
O. KORNEI
[J. S. M. P. E.
the two groove walls only, * the left one forming part of a concave
cylinder, the right one part of a convex cylinder.
The engagement between the sphere (representing the stylus tip)
and either groove wall will be the more intimate the more closely the
curvature of the wall approaches the curvature of the stylus. The
support at the concave groove wall will consequently always be more
effective than the support at the convex side.
The two minimum radii of curvature of the groove walls, pi and p2,
respectively, are the radii of the intersections with the wallsjof the two
FIG. 1. Spherical stylus tip in rigid record groove.
planes perpendicular to them and passing through the center of the
sphere. Both radii are evidently equal but have opposite signs.
The magnitude of pi, 2 can be found from the general expression
for the radius of curvature
* This is the only positive support for the stylus. Any partial or total sup-
port by the rounded bottom of the groove, although sometimes encountered,
is mechanically not well denned and may lead to "chattering" of the stylus and
consequent distortion. It is, furthermore, mathematically hardly accessible.
Dec., 1941] PLAYBACK Loss OF PHONOGRAPH RECORDS 573
In the case under consideration the value is computed for a sine
wave, *
x
y — a sm co ~.
i. e., the path of the recording stylus in a horizontal plane. The values
of the first and second derivative at the crest of this sine curve are
dy _ , dzy aw2
-r- = 0 and 5-^ = — -^r
dx dx2 V2
Since the radii of curvature lie in planes inclined under the angle 0
to the horizontal plane, the values derived from the above formula
have to be divided by cos /? to give the radii in the considered planes.
Thus
V2 1 a V2
Pl'2 = * ^2 * 3370 = =*= ^Tp ' ~tf
In some cases it is preferable to introduce the frequency and to ex-
press the groove velocity by the turntable and record data. Equation
1 may then be written
Pl'2=
It is obvious that the absolute value |p| must never become smaller
than the radius R of the stylus tip if proper tracing is to be secured.
The forces acting upon the stylus tip in the indicated position are
the steady vertical force W and the lateral forces due to the stylus
stiifness va and inertia wao>2 (effective values for lateral motions only) .
In addition to the force W, there is another vertical force caused by
the pinch effect. This force, which reverses its direction with twice
the frequency of the recorded frequency, is due to the translatory
acceleration of the stylus-system mass as effective for vertical mo-
tion. It can be shown, however, that the influence of the pinch force
may be neglected** for most practical purposes, unless the steady
vertical force is made very small, as, for instance, below 10 grams.
Elastic Deformations. — All the effective forces may finally be split
up into two components F\ and F2 directed perpendicularly to the
groove walls at the points of tangency with the stylus tip. In spite
of the modest actual magnitude of these wall forces, it can be shown
* For notations see Appendix 1.
** See Appendix 2.
574
O. KORNEI
[J. S. M. P. E.
that the specific pressure between stylus and groove walls attains
considerable values, ordinarily of the order of 10,000 to 20,000 pounds
per square-inch. Under the influence of these pressures, the groove
walls will give elastically, and will, consequently, cause the stylus to
assume a position different from the ideal one, which is shown in Fig.
1. The assumption of elastic wall deformation implies, incidentally,
a limitation of the maximum permissible wall pressure: It must at
m a1 u)2 - a a1
FIG. 2. Elastic deformation of groove walls.
all times remain within the validity of Hooke's law in order to prevent
permanent deformation of the material.
Offhand, it can be predicted from Fig. 1 that for a given pressure,
the wall deformation at the convex side of the groove will be larger
than that at the concave side, because of the previously mentioned
more effective mode of stylus support in the latter case.
Fig. 2 serves to explain these conditions more clearly. It shows
the vertical section through the stylus tip and record groove as indi-
cated in Fig. 1. The solid circle (center at 0) again represents the
stylus tip in its ideal position, with no wall deformation ; the dotted
Dec., 1941] PLAYBACK Loss OF PHONOGRAPH RECORDS 575
circle (center in 0') shows the stylus tip in its new and actual posi-
tion after elastic deformation of the groove walls by the amounts 61
and 52. The resulting lateral deviation A of the stylus center from
its ideal position* is obviously equal to the difference between the
horizontal components of the deformations of the two groove walls :
A = (Si - 52) cos 0 (2)
It can be seen from Fig. 2 that the effective playback amplitude a'
is equal to the difference between the recorded amplitude a and the
resulting stylus deviation A
a' = a — A
The playback loss L, expressed in decibels, is therefore
L = 20 log a ^_ A db = 20 log - — - db (5)
a
The magnitudes of the wall forces FI and F* may be derived from the
vector diagram indicated in Fig. 2. Omitting the details of the simple
calculation, one obtains
W Or - mu*)a'
Fl' 2 = 2^3 2cos/S
Knowing these forces ajid the geometric shape of the groove walls, the
quantities di and S2 can be determined.
The general problem of the elastic deformation of two curved bodies
in mutual contact under the influence of a given force was treated and
solved by Heinrich Hertz. 15 His findings have been generally adopted
and convenient expressions for the computation of numerical values
may be found in any pertinent reference book.16- 17
From the last reference,17 for instance, the elastic deformation d be-
tween a sphere of the radius R and a cylinder of the radius p, under the
influence of the force F, is found
* The vertical displacement h is of no direct importance in this connection.
However, it should not reach a magnitude to cause "bottoming" of the stylus;
proper choice of the groove profile is therefore essential. Compare also Ap-
pendix 2.
576 O. KORNEI [J. s. M. P. E.
k is determined by the elastic properties of the two bodies and is given
by
3 1 - v? , 1 -
Since the pick-up stylus (usually sapphire) may be considered in
finitely stiff (Ei = oo ) as compared to the record material the ex
pression becomes (omitting the subscripts)
The quantity ^ can be found from tables, 15> 16> 17 by means of an
auxiliary function 0, which is for the system sphere/cylinder
arc cos
2P + R
The possible values of p range from pi = R — > oo for the convex side
and from pz = —R—^—cofor the concave side wall of the record
groove, if the extreme cases of maximum modulation and zero modu-
lation are considered, respectively. It can be seen from the quoted
references that the value of <p changes so little with 9, that it may be
considered a constant over practically the whole range of p. An ob-
jectionable error occurs only if p approaches — R very closely;
however, for p = — 1.5.R, for instance, the error is already down to
about 6 per cent and decreases rapidly with increasing p. Keeping
this limitation in mind, <p can be introduced as a constant, whose
magnitude is found to be 2.
Introducing this value and the value for k into equation 5, one
obtains
«?)
l_.tY^- \ *p/ _j
where
Equation 6 represents the required general expression for the deforma-
tion of the cylindrical and elastic groove wall of radius p against
which the spherical and rigid stylus tip of radius R is pressed with the
fofte F.
If the values for pi>2 are now introduced from equation 1, forF
from equation 4 and if sin (3 = cos 0 = I/ \/2 (according to the usual
Dec., 1941] PLAYBACK Loss OF PHONOGRAPH RECORDS 577
value 2/3 = 90°) one obtains for the deformation $1 of the convex
ide and 52 of the concave side of the groove wall :
- A)\V.
2V2F'
t should be noted specifically that in this equation the actual stylus
xcursion a1 = a — A (not the recorded amplitude a) has to be used.
Recalling equation 2 and setting a = mco02 one obtains finally
i , 1 - A)\V«
= 1
/ &o2(a - A)\V»/ , m(<o2 - coo2) (a- A)\V»H
V1 2V2F2 ) V ~W~ ~y J (
This equation contains implicitly the desired quantity A, that is,
the difference between the recorded groove excursion a and the
reproduced stylus excursion a' (compare Fig. 2). Since the rigorous
algebraic solution of this equation is, in general, not possible, resort
may be had to a graphical method, as will be shown later.
For most practical cases, however, an approximate solution is more
convenient and sufficiently accurate. A simple consideration of the
expression 8 shows that the second terms between the parentheses
are, in practice, usually small compared with unity. A series ex-
pansion of the right side of equation 8 may, therefore, be limited to
the linear and quadratic terms, which latter cancel by virtue of the
structure of the expression. The equation is thus transformed into
a - A 3 \E*RJ \4V2F2 W
Referring to equation 3 the final, approximate, expression for the play-
back loss is obtained
W
Discussion of Equation 9. — The detailed discussion of equation 9
is rather interesting. It should be borne in mind, however, that
any conclusions must not be extended beyond the range of validity
of this approximate expression — the range being determined by the
considerations in the preceding paragraph.
(a) In the first place, the important fact can be noted that the
playback loss is independent of the recorded amplitude. The maxi-
578 O. KORNEI [j. s. M. P. E.
mum permissible amplitude is, at the same time, determined by the
previously mentioned tracing condition
which yields (for 0 = 45°)
flmax ^ ^2 (10)
(b) Depending upon the numerical value of the difference term
in the parentheses, the playback loss may be positive, zero, or nega-
tive. The latter case of a "negative loss" is equivalent to a gain of
the reproduced over the recorded level. The frequency / at which
no loss occurs shall be called "zero-loss-frequency;" it divides the
range of lower frequencies suffering an amplitude loss from the higher
range gaining in playback level. The zero-loss-frequency / can be
calculated by equating the term between the parentheses of the
equation 9 to zero :
w(o>2 —
__ _
4V272 W
from which follows :
/2 = /o
- _ RW
It is interesting to note from this equation that the zero-loss-frequency
is independent of the record material; for a given groove velocity it is,
therefore, a characteristic of the pick-up only. A real value for /,
however, is obtained only as long as the denominator on the right
side in the above equation is greater than zero ; if it becomes zero or
negative, the zero-loss-frequency becomes infinite or imaginary.
This means that a positive playback loss prevails over the whole fre-
quency range.
The physical interpretation of the above considerations is quite
obvious: Referring again to Figs. 1 and 2, it can be seen that the
force FZ directed against the concave groove wall and causing the
deformation 52 tends to decrease the stylus deviation A and, conse-
quently, the playback loss. Since F2 is primarily determined by the
inertia reaction wa'co2, particularly for the upper frequency range, it
follows that for a given recorded frequency the reproduced amplitude
will increase with the stylus mass. For a given stylus mass, however,
the question of gain or loss of the playback level will depend upon
Dec., 1941] PLAYBACK Loss OF PHONOGRAPH RECORDS 579
whether the deformation due to the inertia force on the one hand, or
due to the steady weight and stylus stiffness on the other hand,
predominates. This, in turn, is entirely determined by the recorded
wavelength, i. e., by the record velocity V. Other conditions being
equal, the effect of the inertia force will be the more dominant at
the higher record velocities. In other words, the reproduced level
(particularly of the upper frequency range) will increase with the
record velocity.
(c) Another important conclusion can be drawn from equation 9.
If the term associated with the frequency is made to vanish, the loss
becomes independent of the frequency and is, for given pick-up con-
stants, a function of the groove velocity, V, only. In systems with
constant groove velocity, it is, therefore, possible to keep the playback loss
constant, provided the pick-up data were properly chosen ; this means
that the translation loss can, under such circumstances, be reduced to
zero.
The condition under which this happens is evidently given by
* -1 = 0
4V2F2 W
from which it follows that the mass m for zero translation loss is
RW
(12)
Instead of the mass, R or W could also be made to satisfy the con-
dition ; for practical reasons, however, they can not be varied within
wide limits.
From equations 9 and 12, the magnitude of the constant playback
loss L is thus defined by
L = 20 log [l + ^j^ (E*RW)~V> *]
db (13)
if a is substituted for mco02.
This expression is, incidentally, identical with the one which
can be found for the playback loss at very low frequencies (i. e.t for
co — > 0), where the loss is caused only by the stylus stiffness a. For
average pick-up constants, it is found that the playback loss remains
practically constant up to approximately 1500 cps. The numerical
value of this loss is ordinarily very small, in the order of less than 1
decibel.
580 O. KORNEI [J. S. M. P. E.
(d) It has been customarily postulated that the stylus mass of a
"high-fidelity" pick-up should be reduced as far as possible. Howr
ever, it could already be seen in section b that this is not correct for a
pick-up with finite vertical force, if the playback loss is taken into
account. The conditions can be even better understood if the stylus
mass m in equation 9 is made negligible (m — -> 0). We then obtain
for the playback loss L0 of a massless pick-up under the influence of
the vertical force W
It is seen that this loss vanishes only for very low frequencies; in
the upper frequency range, however, it may attain considerable mag-
nitudes. It is evident that this loss is due only to the unequal defor-
mations of the two differently shaped groove walls under the influence
of the steady vertical force W.
The postulate for a pick-up having negligible stylus mass is, there-
fore, justified only in the theoretical case of a simultaneously vanish-
ing vertical force. Since this latter requirement can not be met in
any practical design, it is necessary to provide a finite stylus mass in
order to attain favorable conditions with regard to the playback loss.
The magnitude of this mass is determined by the operating condi-
tions. For systems with constant groove velocity it is given by
equation 12; for variable groove velocity, by a certain compromise
which will be discussed later.
(e) Because of the interaction between the stylus mass and the
elastic groove walls, the question arises whether resonance may not
occur between these two quantities. This problem, however, is
rather involved, since it must be borne in mind that the elastic re-
actions of the groove walls are, in the first place, non-linear; and,
in the second place, unsymmetrical, because of the different shapes
of the two walls. A rigorous treatment of these conditions goes far
beyond the scope of this paper; but for the discussion of similar cases
the reader is referred to the pertinent literature.18' 19
However, an approximate idea of what may be expected can
again be obtained from equation 9. It is evident that for resonance i
or, rather, "resonance-like" conditions, the reproduced amplitude
must become very large; in other words, the stylus deviation A,
and with it, the playback loss, must attain very high negative values
(A— » — c° ; L = — >oo). In this case
Dec., 1941] PLAYBACK Loss OF PHONOGRAPH RECORDS 581
2V2K/
W»\ */,/
E*Rj V
3 \E*Rj V4\2F2 W
It can be assumed that resonance occurs at a very high frequency
, so that Wtf2 > >co02. Then
COB2
RW
4V2 F2
(15)
The numerator of the right side of this equation represents the
fictitious stiffness of an equivalent simple vibratory system, the de-
nominator, the corresponding fictitious mass. It is seen that the
stiffness is determined not only by the elastic properties of the
record material but also by stylus radius and vertical force, since
these quantities determine the "preloading" of the record material.
The equivalent mass turns out to be dependent upon the groove
velocity and approaches the actual stylus mass only for infinite
velocity, that is, for symmetrical conditions on both sides of the
stylus. For finite velocities, the fictitious mass decreases with the
groove velocity and finally vanishes for a certain value. Below this
value, the resonance frequency becomes imaginary; in other words,
it does not occur at all. Comparing equations 15 and 11 it is seen
that the limiting conditions for the occurrence of the resonance and
the zero-loss-frequency are identical because of the identity of the
denominators. Actually, no resonance can be expected in the range
below the zero-loss frequency where the reproduced amplitudes are
always smaller than the recorded ones.
The resonance between stylus and record material can be experi-
mentally verified and the findings are in reasonably good agreement
with the calculations. The observed increase in amplitude is, of
course, limited, since damping was neglected in the calculations;
however, peaks as high as 10 to 12 db could be measured. As an
example, it may be mentioned that a record resonance of approxi-
mately 20,000 cps was observed for the pick-up under consideration,
at the outside of a 12-inch cellulose nitrate disk at 78 rpm. The value
for the same pick-up, but with 3 times its original stylus mass, is
approximately 10,000 cps.
Graphical Solution of Equation 8. — It has been pointed out be-
fore that the approximate expression 9 is valid only as long as the
second terms between the parentheses of equation 8 may be considered
582 O. KORNEI [j. s. M. P. E.
small in comparison to unity. The permissible limit will be exceeded,
particularly if stylus mass and frequency attain very high values,
Under such extreme conditions a more rigorous solution of equation 8
has to be sought, preferably by a graphical method. This may be
accomplished in the following way :
Each side of equation 8 is considered a separate function of A, the
left side being denoted 3>i(A), the right side 3>2( A). Both functions
are plotted as ordinates in a system with A as abscissa. The solu-
tion of the equation, i. e., the value of A, is then represented by the
abscissa of the point of intersection between the two functions. The
graphical representation of <f>i( A) is, evidently, a straight line inclined
45 degrees and passing through the origin of the coordinate system.
In plotting the function <J>2( A), all quantities of equation 8 referring
to the pick-up system and the record groove have to be considered
as parameters, A being the independent variable. The choice of the
amplitude is not very critical, since it wa,s shown before that the loss is,
in first approximation, independent of the amplitude. It is, there-
fore, usually sufficient to plot the curves for only one single value of a.
This value, however, should preferably be so chosen as to be per-
missible under the most adverse conditions of the record under con-
sideration; that means for the highest frequency and the lowest
groove velocity to be expected. The magnitude of this amplitude
can be determined by means of equation 10.
Numerical Example. — An example for the graphic procedure is
shown in Fig. 3. The numerical values used here and later on
refer to a commercial light-weight pick-up with permanent sapphire
stylus (see Appendix 1). As to the magnitude of the modulus E of
the record material and its experimental determination, reference is
made to the Appendix 3. In order to select an extreme case for the
graphical example, the conditions as represented refer to a pick-up
with three times the original stylus mass. The symbol £ will be used,
hereafter, to denote the multiplying factor applied to the original
mass of the pick-up stylus.
The graph of Fig. 3 shows two groups of curves representing the
function $2 (A) and a straight line at 45 degrees for the function
4>i(A). The two groups of curves refer to the smallest and largest
practical groove radius of 5 cm above the A axis and 15 cm, be-
low the A axis, respectively. Within each group, three different high
frequencies are used as parameters. The recorded amplitude, a, used
for the computation of the graph was 10~4 cm. It is obvious that all
Dec,, 1941] PLAYBACK Loss OF PHONOGRAPH RECORDS
583
$2 (A) -curves must originate from the same point on the A-axis with
abscissa a, because A must equal a when ^(A) equals zero. All
cp2 (A) -curves lying above the A-axis lead to a positive loss, those
under the A-axis to a negative loss ( = gain) of the recorded amplitude.
The playback losses can now be found by proceeding according
to the previous explanations : By measuring the abscissae ( A- values)
of the intersection points of the<£i(A)-line with the 3>2( A) -curves and
finally computing by means of equation 3.
FIG. 3. Example for graphical solution of equation 8.
It is seen plainly that even under the rather extreme operating
conditions chosen foir this example all 4>2( A) -functions turn out to be
practically straight lines in the graphs of Fig. 3. This fact confirms
once more the previous statement that the simplified equation 9 can
be used for the great majority of all practical cases.
Fig. 4 (a) shows a set of curves for the playback loss as a function
of frequency, computed by means of equation 9 and verified by the
graphical method. The curves represent the conditions for both the
record inside and outside including a variation of the stylus mass (£ =
0, 1, 2, 3), all other parameters being held constant. Inspection of
Fig. 4 (a) yields in a pictorial way all the conclusions which have
584
O. KORNEI
[J. S. M. P. E.
Dec., 1941] PLAYBACK Loss OF PHONOGRAPH RECORDS 585
already been derived from the interpretation of equation 9 and which
can be summarized as follows :
The playback loss of a pick-up with given mechanical constants
depends primarily upon frequency and record velocity. For low fre-
quencies, the loss is always positive, very small and almost constant
up to a certain frequency (for the chosen example, approximately
1500 cps). From there on, the loss either increases continuously
with rising frequency, or decreases, passing through zero at the zero-
loss-frequency and changing into a gain at still higher frequencies.
Which one of these two trends actually materializes depends upon the
pick-up constants — primarily upon the stylus mass — and the record
velocity. High stylus mass and high record velocity favor, in
general, a gain, because of the easier deformability of the concave
groove walls under such conditions. Examples for the particular
case of constant playback loss (curve 5) and for the loss of a massless
pick-up (dashed curves) are also shown in the figure. In accordance
with the previous explanations, it is evident that the loss of the
massless pick-up with a finite vertical force must be always positive.
The curve of the pick-up with f = 3, for the record outside, shows
a very steep gain increase toward 10,000 cps, this frequency being the
resonance frequency between stylus mass and record material (com-
pare Fig. 3 and equation 15). The actually observed values were
lower, due to the damping; they are approximately indicated in
dotted lines (similarly for the case f = 2).
Fig. 4(b) shows the translation loss of the three pick-ups as derived
from Fig. 4 (a) by simply plotting the difference between the corre-
sponding playback loss values for the record inside and outside.
It can be seen that the translation loss rises rapidly with the
frequency but also with the stylus mass. This is not contradictory
to the decrease of the playback loss with increased mass, because the
amplitude gain at the record outside takes a steeper course than the
amplitude loss at the record inside. In spite of this fact, however,
it must not be overlooked that the absolute playback level for the
upper frequency range is always higher for a higher stylus mass (up
to the frequency of stylus-record resonance). This is of consider-
able practical significance, since it improves the signal-to-noise ratio.
Experimental Verification. — The findings and predictions of the
theory have been checked by a large number of experiments. They in-
cluded almost any possible variation of the involved quantities, such
as groove velocity, record material, frequency, stylus mass, and
586 O. KORNEI [j. S. M. P. E.
vertical force. The agreement between calculated and observed
values was very good throughout. A certain amount of variation
in the magnitude of the measured quantities has always to be ex-
pected, though, primarily because of the experimental difficulties in
the exact determination of the pick-up constants and because of the
influence of temperature and the aging and wear of the record mate-
rial. However, the deviations very rarely exceeded objectionable
magnitudes.
Practical Considerations. — Some considerations which have a direct
bearing upon the practical design of a pick-up will be~T)riefly re-
capitulated.
Because of the elastic deformation of the record material, any
pick-up construction is primarily determined by, and must conse-
quently start from, the steady vertical force. This force should be
kept below the limit which causes permanent deformation of the
record material (see Appendix 3), but above the vertical acceleration
forces to be expected on account of the pinch effect (see Appendix 2) .
Under average conditions, the vertical pick-up force should range
between 10 and 20 grams for lacquer records and if bottoming of the
stylus is excluded. The effective mass of the stylus system for verti-
cal motions should be reduced as far as possible in order to cut
down the influence of the pinch effect. On the other hand, the effec-
tive stylus mass for lateral motions should have a definite value to
counteract the inherently unavoidable deformation of the record ma-
terial caused by the vertical force. That means that the stylus mass
should be concentrated as closely as possible to the stylus tip.
The optimal stylus mass for the reproduction with zero translation
loss from records with constant groove velocity is given by the
theory and depends on vertical force, stylus radius, and record
groove velocity, but is independent of the record material (see equa-
tion 12).
In systems with variable groove velocity, as represented by the
standard disk-recording process, the optimal stylus mass is not un-
equivocally defined. It should, however, be chosen in such a way
as to give the best compromise between permissible translation loss
and signal-to-noise ratio under the particular operating conditions.
A fair choice is probably the case for which the playback loss under
the most adverse conditions (highest frequency at record inside)
does not exceed 5 to 6 db.
The stylus stiffness of a pick-up should be chosen as low as possible
Dec., 1941 ] PLAYBACK Loss OF PHONOGRAPH RECORDS 587
in order, to reduce not only the level loss for low frequencies (compare
etfttation 13), but also the required vertical force and the record wear.
A low stylus stiffness (low resonance frequency) also increases the
effectiveness of the stylus mass in counteracting the playback loss
(compare equation 9).
Some concluding remarks on the frequency performance of a pick-
up may be appropriate in this connection. The fact can not be
sufficiently stressed that the so-called "frequency response" as it is
usually supplied with a pick-up, is entirely without significance unless
the conditions pertaining to its establishment are exactly specified
From: the considerations of this paper, it is clear that the very same
pick-up can yield widely different characteristics if record material,
groove velocity, and steady vertical force are varied. It must be
borne in mind tliat no practical pick-up ever reproduces what is on
the record — the response will always be an overall effect determined
by the geometrical and physical constants of both the record and
the pick-up itself.
The only way to establish the "absolute" frequency response of a
pick-up consists in directly driving its stylus from a driver (for
instance a cutter) with an exactly known frequency characteristic.
Such procedure, however, is mainly of theoretical interest and,
therefore, not even desirable for the judgment of the actual pick-up
performance.
ACKNOWLEDGMENT
T" The author wishes to thank Mr. A. L. Williams, President of The
Brush Development Company, for his active interest in this project.
JEe appreciates also the cooperation of Mr. A. Dank in the numerous
experiments, and of Mr. A. Barjansky who computed the numerical
values of the equation for the groove wall deformation.
APPENDJ%1 , .„
(Notations, except for those defined in the text}
Numerical Values for
Symbol Meaning Dimension Chosen Example
f ' Radius of record groove cm 5 to 15
n Speed of turntable, rpm (60 sec)-1 78
• V Record groove velocity cm-sec"1 40.8 to 122.5
£ Young's modulus of record material dynes-cm~2 8.5 X 109
v Poisson constant of record material 0.3
2/3 Angle included by groove walls 90°
a Recorded amplitude cm
588
O. KORNEI
[J. S. M. P. E.
APPENDIX 1 (Continued)
Symbol
a'
V
Pi, «
Dimension
cm
cm-sec'
cm
cm
cm
W
/
Meaning
Reproduced amplitude
Stylus velocity
Radius of curvature of convex and
concave groove wall, respectively
Elastic deformation of convex and
concave groove wall, respectively
Lateral displacement of stylus center
due to groove elasticity
Radius of spherical tip of pick-up cm
stylus
Mass of stylus system as effective in grams
center of stylus tip for lateral
motions
Stiffness of stylus system as effective
in center of stylus tip for lateral
motions
Steady vertical pick-up force
Numerical Values for
Chosen Example
6.3 X iO~3
(2.5 mils)
3.5 X 10~»
dynes-cm-1 2.2 X 10*
dynes
Lateral resonance frequency of stylus sec"1
system
/ Frequency sec"1
2 X 10*
(approx. 20 grams)
4000
2*70, 2*7 sec"1
APPENDIX 2
(Pinch-Effect)
The pinch-effect may be defined as the magnitude of the up-and-down motion
of the tracing stylus tip caused by the periodic variation of the included angle
between the two modulated groove walls. The effect is caused by the different
geometric shape of recording and reproducing styli.1 Evidently, the extreme
positions of the stylus tip occur at the crest of the modulated groove (lowest
position) and at the point where the groove intersects the original neutral line
(highest position). The total vertical travel of the stylus tip may thus be con-
sidered the double amplitude 2ap of an approximately sinusoidal motion of twice
the recorded frequency. It can easily be shown7 that the "pinch amplitude"
ap is, neglecting the record elasticity,
R q»q)» /jgx
ap = 4v/2 ' V*
Remembering equation 10 for the maximum permissible lateral amplitude, it is
found
apmax = ^E« (17)
4
This means that the theoretical "pinch amplitude" under the most adverse
conditions can not exceed one-quarter of the lateral amplitude. The vertical
Dec., 1941] PLAYBACK Loss OF PHONOGRAPH RECORDS 589
and the lateral accelerations thus become equal (for the worst case) since the
pinch frequency is twice the lateral frequency.
The highest lateral inertia force is, therefore,
R
The numerical value of this expression amounts to 1.3 grams (at the record
inside) for the chosen example. The vertical acceleration force, due to the
pinch-effect, will ordinarily be several times this value, since the stylus mass
effective for vertical motion is — for reasons of a practical design — usually greater
than that for lateral motion. But even so, it remains small in comparison to the
vertical force of 20 grams and can, therefore, be neglected for an approximate
calculation. This premise is all the more justified, since the maximum per-
missible amplitudes are very rarely reached under practical operating conditions.
If the elastic deformations are taken into consideration, the conditions become
more complicated but are not materially affected as far as the vertical force is
concerned. The relative calculations may be omitted here because they are of
no immediate importance in regard to the problems treated in this paper. It
may be mentioned, however, that the analytical procedure is very similar to that
used for the lateral groove deformations. The analysis is based upon the fact
that the vertical stylus deviation h (Fig. 2), due to record elasticity, is h =
(81 + 82) cos /3, while the lateral one was found A = (5i — 82) cos /3 (see equa-
tion (2)).
APPENDIX 3
(Elasticity of Record Material)
Since no figures have been published for Young's modulus E of record mate-
rials, some values were specifically determined for this paper.
In a special test device a standard sapphire stylus of 2.5 mils tip radius was
pressed with an adjustable force against the surface to be investigated, and the
deformations determined under a measuring microscope. From the obtained
readings, E was calculated by means of the Hertz formula, equation 6. The
particular case at which plastic deformation (cold flow) of the material started
could also be found from the observations.
The values thus obtained are tabulated below. Under dynamic conditions,
the figures should be somewhat higher; experiments indicated, however, that
the deviations are comparatively small and may be neglected, considering other
errors involved in the experimental part of this paper.
E Limit for Plastic Deformation
Material 109 Dynes-Cm-1 (Vertical Force for 90° Groove)
Grams
Cellulose nitrate 8.5 20 to 25
Vinylite without filler 21 55
Vinylite pressings 25-32
Shellac pressings 54
590 O. KORNEI
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1 PIERCE, J. A., AND HUNT, F. V.: "Distortion in Sound Reproduction from
Phonograph Records," /. Soc. Mot. Pict. Eng., XXXI (Aug., 1938), p. 157.
2 LEWIS, W. D., AND HUNT, F. V.: "A Theory of Tracing Distortion in
Sound Reproduction from Phonograph Records," J. Acoust. Soc. Amer. XII
(Jan., 1941), p. 348.
3 LOFGREN, E.: "On the Non-Linear Distortion in the Reproduction of
Phonograph Records Caused by Angular Deviation of the Pick-Up Arm," Akust.
Zeits., m (1938), p. 350.
4BiERL, R.: "A Contribution to the Theory of Disc Records — The Play-
back Process," Akust. Zeits., IV (1939), p. 238.
6 DiToRO, M. J.: "Distortion in the Reproduction of Hill-and-Dale Re-
cording," /. Soc. Mot. Pict. Eng., XXIX (Nov., 1937), p. 493.
6 BAERWALD, H. G.: "Analytic Treatment of Tracking Error and Notes on
Optimal Pick-Up Design," J. Soc. Mot. Pict. Eng., XXXVII (Dec., 1941), p. 591.
7 FLEMING, L.: "Notes on Phonograph Pick-Ups for Lateral-Cut Records,"
/. Acoust. Soc. Amer., XH (Jan., 1941), p. 366.
8 OLNEY, B.: "Phonograph Pick-Up Tracking Error," J. Acoust. Soc. Amer^,
X (Nov., 1937), p. 19.
9 HASBROUCK, H. J.: "Lateral Disc Recording for Immediate Playback
with Extended Frequency and Volume Range," Pr.c. I. R. E., XXVH (March,
1939), p. 184.
^° SALIBA, G. J. : "Automatic Equalization in Disc Recording," Communica-
tions, XVTII (Aug., 1938), p. 15.
11 SALIBA, G. J.: "The Improved Automatic Equalizer for Disc Recording,"
A. T.E. Journal, VH (July, 1940), p. 18.
12 LEBEL, C. J.: "High-Frequency and Noise Level Characteristics of an
Instantaneous Recording Disc," A. T.E. Journal, VIII (Jan., 1941), p. 6.
13 GUTTWEIN, G.: "On the Linear and Non-Linear Distortions in Disc Re-
cording," Akust. Zeits., V (Dec., 1940), p. 330.
14 BUCHMANN, G., AND MEYER, E.: "A New Optical Measuring Method for
Phonograph Records," Elek. Nach. Technik, VII (1930), p. 147.
15 HERTZ, H. : "Gesammelte Werke," (Leipzig, 1895), pp. 155-196.
16 TIMOSHENKO, S.: "Theory of Elasticity," McGraw-Hill Book Co., New
York (1934), p. 344.
17 ROARK, R. J.: "Formulas for Stress and Strain," McGraw-Hill Book Co., >
New York (1938), p. 245. The expression for the factor k, erroneous in this
book, is given correctly in this paper (see eq. 5).
18 TIMOSHENKO, S.: "Vibration Problems in Engineering," D. Van Nostrand
Pub. Co., New York (1937), p. 137.
19 DENHARTOG, J. P.: "Mechanical Vibrations," McGraw-Hill Boo~k~Co.,
New York (1940), p. 403.
ANALYTIC TREATMENT OF TRACKING ERROR AND
NOTES ON OPTIMAL PICK-UP DESIGN*
H. G. BAERWALD**
Summary. — A complete analysis is given of the non-linear distortions due to the
tracking error of the pick-up mechanism in the reproduction of lateral-cut disk record-
ings. The separate treatment of tracking distortion is permissible as long as the
overall distortion of the reproduction is tolerable, the system being "almost linear,"
or the various distortion products superposable.
For the simplest case of a sinusoidal signal, it is possible to derive explicitly the
whole Fourier spectrum of the reproduced signal, the mathematical proposition being
the same as in the mechanical two-body problem. For general signals, an explicit
operational expansion of the distorted signal is obtained.
As the kinematical effect of tracking error consists of an amplitude controlled
advance and delay of the pick-up, the harmonic distortion may be characterized as
made up of the side-bands of frequency modulation of the signal by itself. Compared
with the ordinary type of non-linear distortion due to curved static characteristics,
which may be correspondingly characterized as amplitude automodulation, the spec-
tral character of tracking distortion stresses the higher frequency components. For
second-order distortion which is prevalent, the emphasis is proportional to frequency.
The analysis shows, that both absolute and nuisance effects of tracking distortion
are considerably greater than commonly assumed, published values usually being
underestimates, due to omission of rigorous procedure. Tracking distortion is given
approximately by the tracking error weighted with the inverse of the groove radius;
the weighted error is referred to the mean groove radius of the record. The recording
characteristic affects distortion products independently of their mechanisms.
Pick-up design as based on the analysis should reduce the weighted tracking error
as much as possible. For optimal design, Tchebyshev approximation, commonly
used in electric wave-filter design, is used. For straight arms, where only one design
parameter, i. e., the underhang, is available, optimal approximation of zero distortion
is of first order; for offset arms, where both offset angle and overhang are adjustable,
it is of second order and thus much closer. The influence of deviations from optimal
design due to errors of mounting is investigated as well as the combined effect of offset
angle and stylus friction on the lifting force and its reduction by suitably modified
design. The compromise design of multi-purpose arms is also treated. Simple de-
sign formulas are developed throughout, covering the various record sizes, speeds, and
arm lengths. It is found that offset arms are much superior to straight arms. Track-
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 1,
1941.
** The Brush Development Co., Cleveland, Ohio.
591
592 H. G. BAERWALD [j. s. M. P. E.
ing distortion can be reduced to negligible magnitude with properly designed offset
arms even under adverse conditions, such as short arm length and appreciable mount-
ing tolerance.
The tracking mechanism commonly employed for the reproduction
of disk recordings consists of an arm swinging about a vertical axis.
For lateral recordings, the center of motion of the pick-up stylus is a
horizontal axis pivoted in the head of the swing arm. By virtue of
the kinematics of this system, the direction of the stylus tip motion
does not, in general, coincide with the disk radius. The angular
difference, which is equal to that between the direction of the pivotal
axis and groove tangent, is known as "tracking error." Its magni-
tude and sign depend on the geometrical data of the system and on
the radial position.
It is well known that the tracking error gives rise to distortions.
Among the sources of non-linear distortion encountered in disk re-
cording and reproduction, the tracking error is usually considered as
entirely negligible. This, however, is not quite correct. The truth
is that tracking distortion can be effectively eliminated in a simple
way, i. e.t by proper geometrical design of the tone arm. Consider-
able deviations from optimum design, as are sometimes met in com-
mercial pick-ups, lead, however, to quite serious distortions.
As tracking error is obnoxious only by virtue of the distortion
caused, quantitative understanding of the effect is the necessary basis
for tone arm design. Optimal design should minimize tracking dis-
tortion over the entire playing range of the record, which, as analysis
shows, is by no means synonymous with minimizing the tracking
error itself. Although this may sound commonplace, there is the
fact that almost none of the numerous publications on the subject
gives an analytic investigation of the effect. This omission fre-
quently leads to erroneous results regarding optimum tone arm de-
sign, as in a recent paper by G. E. MacDonald;1 or it leads to con-
siderable underestimations of the magnitude of tracking distortion
and of its nuisance effect which depends on its spectral character.
This is the case in a frequently quoted paper by B. Olney2 who gives a
lucid qualitative description of the effect and considerable experi-
mental material. E. G. Lofgren,3 who first pointed out the error in
reference 2 is, as far as the writer is aware, the only author who at-
tacked the subject analytically and also discussed design questions on
this basis.
Dec., 1941] OPTIMAL PICK-UP DESIGN . 593
The present paper gives a rigorous analysis of tracking distortion
and develops the geometrical tone arm design on this basis.
Part I — Analytical Investigation of the Tracking Error Distortions
s = coordinate along the unmodulated groove
y(s) = laterally recorded signal
F(s) = stylus elongation or reproduced signal
77 = angular tracking error (radians or degrees)
distortion parameter \ defined in
weighted tracking error J
t = time
dy
v(t] = — = velocity of recorded signal
at
dY
V(f) = — = velocity of picked-up signal
at
fi = angular disk speed in sec"1 = — X speed in rpm
30
r = radius of an arbitrary groove
r\\ r2; rm = v fVa = inner; outer; mean groove radius of a disk record
co = recorded angular frequency in sec"1 ]
yo = recorded amplitude
vQ = co-yo = recorded velocity amplitude
of a sinusoidal signal
X = recorded wavelength
<p = recorded phase
\ff = picked-up phase
(a) General Considerations. — Fig. 1 gives a picture of the stylus
motion. The curve y_(s) represents the center line of the laterally
displaced groove or the recorded signal. Due to the angular error 77,
the instantaneous position of the stylus tip P becomes Sr instead of S,
i. e., its abscissa is displaced by As. The relation between the re-
corded elongation y(s), the picked-up elongation Y(s), and the in-
stantaneous shift As is evidently
F(s) = seci7-y(5 + As); As = tan 17-3^(5 + As) (1)
Kinematically, the effect thus constitutes an alternating advance
and delay of the reproduced signal with respect to the recorded one,
or a "frequency modulation" of the signal by itself. The associated
harmonic distortion can be interpreted as the "side-bands" of this
auto-modulation. This interpretation may prove helpful for the
understanding of the results of the analysis. It leads to the antici-
patory result that, due to the increased depth of frequency auto-
modulation, harmonic distortion of a given signal should increase with
decreasing groove velocity. For a given distortion limit, larger
594
H. G. BAERWALD
[J. S. M. P. E.
tracking error should thus be permissible at the outer grooves of a
record than at its inside. This is confirmed by analysis.
Fig. 1 represents idealized conditions, as the finite dimensions of
groove and stylus tip are neglected. As kinematic implication, the
effects connected with the groove geometry, which give rise to tracing
distortion,4 are thus ignored. As mechanical implication, the elastic
deformations caused by the bearing weight and the lateral stiffness
and inertia forces, are neglected. They lead to both linear and non-
linear distortions, but only the former have been investigated so far.5
They are referred to as "playback loss."5 While in the strict sense,
tracking distortion and harmonic distortion from other sources in-
herent in the playback process, are interdependent, they can be
FIG. 1. Idealized representation of tracking.
treated as superposable under practical conditions, i. e., as long as the
square of relative overall harmonic distortion is small compared with
1. This corresponds to "almost-linearity" as put forward by Feldt-
keller and others.6 Thus the idealized picture represented by Fig. 1
and equation 1 is usually adequate for the treatment of tracking dis-
tortion, if y(s) signifies the signal as modified by the linear effects.*
As the playback loss is usually positive, the neglected effects would
tend to reduce tracking distortion.
(b) Rigorous Solution for a Sinusoidal Signal; Bessel's Solution of
the Kepler Problem. — The relation 1 is an implicit equation involving
* It is further assumed that no tracking error is introduced by the cutter. The
dist jrtions due to angular error of the cutting tool which may be present in home
recorders, are of a more complicated character and will be treated in a separate
paper.
Dec., 1941]
OPTIMAL PICK-UP DESIGN
595
the unknown shift As. In order to obtain from it the tracking dis-
tortion explicitly, the picked-up signal F(s) has to be expressed in
terms of the recorded one, y (s). The desired result will be obtained
in the form of an operational expansion, similarly as in other cases of
calculation of modulation products.7 Before, however, taking up
the general case of complex signals, we shall deal first with the sim-
plest c^tse, i.e., with a sinusoidal signal
y(s) = yo sin — 5 = y0 sin <f>, (2} "
. X
for which the solution of equation 1 can be obtained in closed form.
FIG. 2. Geometrical construction for sinusoidal signals.
The picked-up signal can be described correspondingly by means of a
phase 'angle ^ •
Kj -Y(s) - y0 sec 77 sin>(s) (5).
The simple graphical construction of 3 from 2 according to. equation
1 is carried out in Fig, -2 for an exaggerated case (77 = 30°), in order
to make the distortion plain. The corresponding implicit relation is
evidently :
'•?- e sin \f/ = <p] e = 2?r 2° tan 77 = distortion parameter.
'
(4)
Introducing time as the independent variable, by means of the rela-
tion
596
H. G. BAERWALD
[J. S. M. p. E.
it follows
and
y(t) = yo sin &>/ = yQ sin <p (2a)
distortion parameter « = ^ tan TJ == p° tan 7?, (4a)
rQ, rQ
where i>o denotes the recorded velocity amplitude while r!2 is the longi-
tudinal groove velocity.
As a matter of historical interest, it is worth pointing out that the
solution of eq. 4 or Fourier expansion of ^ in terms of <p, i. e., the track-
ing distortion of a sinusoidal signal proves to be the same mathemati-
cal proposition as the classical two-body problem of celestial mechanics
(Kepler problem) , which involves
the description of the (undis-
turbed) motion of a planet about
the sun in terms of the phase of
its period of revolution. This is
briefly illustrated in Fig. 3 which
shows the Kepler ellipse with the
half -axes a and b and the foci S
and S', representing the orbit of
a planet P about the sun S;*
the generating circle of radius a
is also shown. The numerical
FIG. 3.
Representation of planetary
motion.
IS
eccentricity of the ellipse
€ = V 1 — (b/a)2. The instantaneous position of the planet is
usually characterized by the focal phase $ or by the central angle ^
(both measured from the apex A), the so-called "true anomaly" and
"eccentric anomaly," respectively. Quantitative description of the
planetary motion requires an expression of these angles in terms of
time t or of the phase angle ("mean anomaly") <p = 2irt/T (T =
period of revolution). From the well known geometrical relations
between focal radii and anomalies :
r = a(l — « cos i/O; tan =
2
tan ; sin
2
-, etc..
1 + e sin
and from the dynamical theorem of the invariance of the momentum
* Actually, 5 represents the center of gravity of sun and planet; as, however,
the mass of the sun is usually very large compared to that of a planet, the differ-
ence **ill be negligible.
Dec., 1941] OPTIMAL PICK-UP DESIGN 597
vector (Kepler's second law), which requires the orbital area (shaded
in Fig. 3) to be equal to the mean anomaly :
/v i
TT I f2(t?)Jt? = 2lT — ^= <fl ,
Jo T
there follows, by elimination of #, at once the relation
\ft-e sin ^ = <p. (4')
This is identical with 4 and 4a; eccentricity, mean, and eccentric
anomaly correspond to distortion parameter, phase of the recorded,
and phase of the picked-up signal, respectively. Equation 4' was
first given by Lagrange8 in a famous memoir in 1770; he obtained
power expansions of the first three Fourier coefficients of sin ^. The
complete solution of the problem (including the Fourier analysis of t?
and r) was obtained by Bessel9 in 1824 in a classical investigation
where he introduced his well known integral representation, and
which is considered as the beginning of the modern theory of Cylinder
Functions. (For historical notes see Chapter I, Part 1.4 of Watson's
Treatise.10)
The solution of 4 follows at once upon application of the Bessel-
Sommerfeld integral (see10 Chapter VI, or,11 or 12); it may be found,
together with related results, in Chapter XVII (Kepteyn Series), No.
17.2, pp. 551-558 of Watson's Treatise;
Y = VJ* sec i\ • \ ^n ^ne' sin nut; — = V(f) = v 0 sec i\ • \ * ^n€' cos n wt (6)
n=l n = l
(Jn denotes the Bessel function of nth order.)
Apart from the factor sec rj, the relative amplitude #1 of the funda-
mental frequency is thus /i(e)/y2e, those an and bn of the nth har-
monic, Jn(ne)/1/zne and Jn(n€)/l/z€, for the picked-up elongation and
velocity, respectively. The relative harmonic distortion is defined by
the rms values
, respectively.
n-i
n-l
598 H. G. BAERWALD
By virtue of the well known relation ,
00 03
« = l C Y*(t)d<f> \^ b 2 = - f
^J(2x) **' 2Lt n rJc
[J. ' £ M. P. E.
(2ir)
we obtain
00
* = (^seci/Y - i f sin'^ = - f^sec^Y • i f si
\« / xj(2x) Vw / irJ(2T)
,by virtue of 4. Similarly,
-i f
* J(
,
(27T) 1 —
Therefore relative harmonic distortion
i - €2-V2(i + vr- v)
4567
18436
(7)
(c) Discussion — r/^ Prevalent Type of Distortion. — Equation 7
shows that if tne relative rms distortion is restricted to moderate
values, the 1st term alone : c/2 or c, on the basis of elongation or
velocity, respectively, gives a satisfactory approximation. The next
higher term is negligible for overall distortions even as high as 50 per
cent. The first term represents the relative amplitude of the second
order harmonic #2 or bz, respectively; this is seen from 6 upon substi-
tuting for the Bessel coefficients the initial terms of their power ex-
pansions. The relative amplitudes of the higher-order harmonics are
of corresponding order in e; they are
. respectively.
PA")"1 and (V« "'>-"
n! (n — 1)!
Under normal conditions, the distortion is therefore essentially of
second order. For complex signals, the 2nd-order cross-modulation
products will thus be the prevalent distortion components.
Dec., 1941]
OPTIMAL PICK-UP DESIGN
599
(d) Distortion Spectrum of Complex Signals. — For general signals, a
solution of 1 in closed form does not exist. Expansional solution is
thus called for and can be expected to converge satisfactorily, in view
of the rapid convergence of the expansions in case of sinusoidal signals.
The implicit form of equation 1 would require an approach by itera-
tion; it is possible, however, to obtain the final result at once by
means of a well known expansion theorem due to Lagrange13' 14* which
is a special case of Teixeira's Theorem.14 The simple intermediate
calculation is omitted. The result is analogous to that for the sinu-
soidal signal, eqs. 6 and 7, the relevant quantity, i. e., the distortion
parameter e = (ory0/^) being replaced by the distortion operator
tan rj/rtt— applied to y(f) :
dt
cos,-
cos -n'V(f)
y(t
nl
n =1
tan 77 / v
or symbolically:
cos i\-V(£) =
» = 1
rti
i tan i} d ;
I rQ dt;
(y(0)
(to)
(For sinusoidal signals, 8 is re-obtained from 6 by means of the power
expansion of the Bessel Functions — as it must be.)
Similar considerations as in the case of a sinusoidal signal show that,
for moderate overall distortion, the first term, which represents
second-order distortion is in general predominant :
cos,.F(0 -y(t) =
cosirF(/) -
tan
t2
dt2 \2
.
For the ordinary type of non-linear distortion as met in tubes, etc.,
the distortion terms corresponding to 8b would be const. yz/2 and
const. (d/dt)(y*/2), respectively. It will be appreciated that it is
* The convergence conditions are automatically fulfilled by virtue of the limita-
tion of the frequency spectrum inherent in the recording process: y(s) is, there-
fore, continuous and of bounded oscillation.
600 H. G. BAERWALD [j. s. M. P. E.
appropriate to interpret the two cases as amplitude and frequency
auto-modulation. In order to illustrate the difference between the
two associated distortion spectra, let us consider the simple two-
component signal*
y(t} = y\ sin wit + y2 sin w2t (9)
For the case of ordinary second-order distortion, we obtain, in addi-
tion to y itself, the following distortion components :
Frequency 0 2o»i 2wj J«i — w2| u>\ -f- w2
Amplitude -(yi2 + y22) -yi2 -yj cy\y* cyiy2
where c denotes the second expansion coefficient of the normalized
non-linear characteristic. For the second-order tracking error dis-
tortion of the same signal, we obtain from 8b
Frequency 0 2&?i 2o>2 |wi— o>2| o>i + co2
tan 77 yi* tan 77 yi2 tan 77, ,yiy2 tan 77 y^yz
Amplitude U — o>i — — o>2 — — o»i — o>2 — (ajj -p co2)
rO 2 rli 2 rO ' '2 rfi 2
It is seen that the amplitudes of the distortion components are
weighted with their respective frequencies, relative to the former case;
this corresponds to the application of d/dt. Comparison on the
velocity basis, which is more appropriate to the major part of the
conventional recording characteristic, gives the corresponding result
Frequency 2coi 2<a2 Ion — o>2| &>i + w2
-tt.mpiiLu.ue, oru. uisc.
Amplitude, track,
error
001
tan7V
tanV
c
tan 77 (c
•m
^1 — c^)2
COlC
tan 77 (wi
02
+ fc
IV*
2)2
rfl"1
m
rfl
2»,«,
rd S
S«ltt,
^1^2
* Lofgren3 carries out, and points out the salient results of, the multi-component
signal analysis, but he does not give the general expansion 8, 8a. Application of
the Laplace integral (spectrum analysis) to the general expansion 8 would yield
the spectra of the distortion components of wth order of signals with continuous
and/or line spectra. The character of such distortion spectra was studied and
discussed by Lewis and Hunt4 for the tracing distortions. It could easily be
carried out in the same way for tracking distortion, but this does not lead to any
fundamentally new conclusions pertinent to tone-arm design, beyond those based
on the simple case 9. It should be mentioned, however, that the spectrum of
-lateral tracing distortion, which is of odd order only, is weighted with a power of
frequency still higher — by one — than that of tracking distortion.
Dec., 1941] OPTIMAL PICK- UP DESIGN 601
(e) Nuisance Effect of Distortion; Influence of Recording Character-
istic; Permissible Size of Tracking Error; Weighted Tracking Error. —
The definition of the distortion parameter e (eq. 4a) can easily be ex-
tended to the two-component signal 9. In particular, for y± = yz :
y = y0(sin o>i* + sin w20> (9d)
we obtain
tan T/'Yocoo. ^/ - /V,N
eo = - L^ — ; Wo = V^icoz (4b)
rtl
Let us compare the second-order ordinary-type and tracking distor-
tions of 9a on a common basis, i. e., for equal rms values, whereby the
d-c component is not counted as it is without significance for electro-
acoustic purposes. The ratio of the distortion parameters c and €0
then becomes
c- = \/0.3(a + a
€0 Wl
This gives, apart from a common factor, the following values for the
amplitudes of the distortion components:
Fre-
quency 2coi 2&)2 |«j — coal («i + "2)
Ord. _ _ _ _
type Vo.Sfc + a-1) Vo.3(a + a~») 2Vo.3(a + a~^ 2\/0.3(a + a'1)
1 /- /_
Tr. err. — 7=. V a V a ~
If (coi, co2) represents a consonant musical interval, the second-order
modulation products are also consonant with it, with the possible
exception of the summation tone. For instance, for a Fourth with
a = 4/3 (in the natural scale; 25 /12 = 1.3347 according to equal tem-
perament), the difference tone is 2 octaves below o>2, i. e., consonant,
while the frequency of the summation tone is 7/e times that of the
octave of coi; this does not represent an interval of the musical scale
and is therefore dissonant. For unity rms value, the relative ampli-
tudes are in this case :
Frequency 2o>i 2o>2 |o>i — (at\ (wi + &*)
Ord. type distortion 0.316 0.316 0.632 0.632
Track, err. distortion 0.346 0.462 0.115 0.808
This is represented in Fig. 4.
602
H. G. BAERWALD
[J. S. M. P. E.
The preceding considerations show that the spectral character of
tracking distortion is roughly taken into account on the velocity basis.
This means that the distortion parameter e as originally denned for
sinusoidal signals gives a fair estimate of the relative tracking distor-
tion produced by complex signals, if o> signifies a dominant frequency
range and y a suitable average amplitude. (If the distortion were of
the ordinary type, c/2 would have to be used instead.)
The spectral character of all kinds of harmonic distortion produced
in the playback process is uniformly modified by the playback fre-
quency characteristic which is the inverse of the recording character-
MD/WTYPf WOW*
-•£
CJ,
2u,
FIG. 4. Comparison of tracking and ordinary distortion
spectra of a two-component signal.
istic. In case of the conventional constant- velocity characteristic,
for instance, distortion components are emphasized, in playback, pro-
portionally to their respective frequencies.* It can be generally
shown that, for any particular distortion term, the effects of the play-
back characteristic and of the distortion mechanism itself simply
superpose; the relative amplitude of any resulting distortion com-
ponent is thus the product of two mutually independent factors.
For instance, the relative amplitude of the second-order distortion
component of frequency up, which is produced by the two signal com-
* The emphasis of the constant-velocity characteristic on the higher distortion
components is stressed in Guttwein's paper.5
Dec., 1941] OPTIMAL PICK- UP DESIGN 603
ponents of frequencies ua and ub, is proportional to F(ua; ub; up) •
-^ where /(co) denotes the recording characteristic and F(wa;
ub; up) accounts for the distortion mechanism; in case of tracking
distortion, F is given by the second table following 9.
The physiological effect of harmonic distortion of a given rms value
is usually increased by high-frequency emphasis.15 It follows that
tracking distortion — in particular as produced by signal components
of large amplitude in the upper middle audio range — will have a rela-
tively high nuisance effect, not adequately accounted for on the
elongation basis.2
The preceding analysis and discussion of tracking distortion fur-
nishes the basis for rational pick-up design. In order to have a fixed
aim, it is useful to set a limit of permissible tracking distortion, in
terms of the distortion parameter e. This limit should be kept low,
for three reasons: (1) The nuisance value of tracking distortion is
likely to be increased on account of high-frequency emphasis; (2) an
amount of harmonic distortion too small to be troublesome by itself,
may become so when superposed upon distortion which is already
appreciable. This is the case here, due to the presence of harmonic
distortions obtained in cutting, pressing, and playback, more often
than not in objectionable amounts; (3) while, in the present state of
the art, the overall effect of these distortions, whose mechanisms are
still partly unexplained, can not be reduced below nuisance level
under commercial conditions, it is possible to eliminate tracking dis-
tortion substantially by proper tone arm design.
For these reasons, an upper limit of harmonic distortion — as repre-
sented by | e , the absolute value of the distortion parameter — of only
2 per cent is postulated for a signal level of 8 cm/sec velocity ampli-
tude, over the whole playing range of any record size and speed for
which an arm is designed. This corresponds to 1 mil elongation at
500 cps and represents approximately maximum conditions in trans-
cription and about half the permissible amplitude in commercial re-
cordings.
The two conventional speeds and associated playing ranges are
33 V3 rpm, n min ~ 3l/z inches, r2 max ~ 8 inches for transcription and
78 rpm, r\ min ~ 2 inches, rz max ~ 6 inches for commercial disks. The
associated minimum and maximum groove velocities are 31 and 41J/2
cm/sec, and 71 and 124 cm/sec, respectively. With the limit set
for | e , the maximum value of (tan rj\ occurring under any conditions
604 H. G. BAERWALD [j. s. M. p. E.
is then 0.3, according to 4a. It will be seen later that it is usually con-
siderably smaller. It is thus permissible to neglect r/2/2 against 1,
i. e., to put
tan 77 = 77 = sin 77, cos 77 = 1 : | e | = ' * L (10)
As tracking distortion is inversely proportional to the groove radius,
it is preferable to use for design purposes the
weighted tracking error 77' = — -ij (11)
which is referred to the mean groove radius rm, rather than 77 itself.
Distortion is then proportional to 77', independently on the radius r.
The factor of proportionality becomes 17 for transcription recordings
with umax ~ 8 cm/sec, 22 for commercial disks with z>max ~ 16 cm/sec.
Distortion | e |max(in per cent) ~ (17 to 22)- 1 77' | (in radians). (12)
For |c|max = 2 per cent at v = 8 cm/sec, it is thus (Vlmax ~ 0.12 and
0.18 for transcription and commercial conditions, respectively. For
the tracking error itself the following upper limits are obtained:
4*/2 to 10° for transcription, 6 to l7l/2° for commercial disks, for
f — ' 1 min LO T — TZ max'
PART H— PICK-UP DESIGN FOR MINIMUM TRACKING ERROR DISTORTIONS
Additional notations
L = length of tone arm (swing axis to stylus tip)
L + d = distance between turntable axis and swing arm axis \ (see Fig. 5)
(d is called "underhang" if >0, "overhang" if <0)
d
& = — = numerical under- or overhang
Li
x=- = numerical groove radius; Xi: 2;
JL
tj'j.j = weighted tracking error for r:; 2; 77'o^77/(x0) = extremum of the weighted
tracking error
7 = angle between groove tangent and line from stylus tip to swing axis
a = angle between direction of pivotal axis of stylus and line from stylus tip to
swing axis — "offset angle"
&»pf, dopt; ao; a0pt; cterif, a': explained in text
r»(x) = Tchebychev polynomial of nth order
Dec., 1941]
OPTIMAL PICK-UP DESIGN
605
P = bearing weight of stylus
F — longitudinal friction force between stylus and groove
Fh = horizontal centripetal component \
ft = vertical component / explamed ln tcxt
p = coefficient of friction between stylus and record groove
(a) Geometrical Relations; Tchebychev Method of Approximation. —
The geometrical conditions are represented in Fig. 5 for both straight
arms (left side) and arms with offset head (right side). In both cases,
C represents the axis of the turntable, A that of the swing-arm, P the
stylus point, and D the intersection of arm radius with the line CA .
FIG. 5. Geometry of straight and offset tone arms.
It is
sin 77 = sin y = r* ~ 2d ~ ^ = ** ~ 28 ~ 5* (5 > 0), for the straight arm
2r 2x
= y - at sn 7
(S < 0), for the offset arm
With the approximation 10, this gives for the weighted tracking error
T/', according toll:
,' = *zfl - 28 + 8>V for straight arms
(l - 2d + 5' - 2 sin "Y for offset arms
<V *» x
2 cos a'
An arm that is designed optimally for a single speed and record
size, will be called single-purpose arm; otherwise we speak of multi-
purpose arms. Single-purpose arms will be treated first.
The design should minimize tracking distortion over the whole
playing range by suitable choice of the design parameters. In case of
606 H. G. BAERWALD [J. s. M. P. E.
straight arms, only one parameter is available, i. e., the underhang,
whose optimal value turns out to be positive under any conditions
(Fig. 5, left). Offset arm design has two parameters available, i. e.,
offset angle and underhang; it turns out that for optimal design, the
latter is always negative, and thus constitutes an overhang (Fig. 5,
right). The result, namely the tracking distortion for optimal design,
will appear in terms of the arm length. This will yield the minimal
length compatible with a prescribed distortion limit.
The design reduces to elementary procedure as soon as a definition
of minimum distortion over the playing range is agreed lipon; in
other words, we have to decide, which function ri'(x) containing the
parameters a and 5 (or 6 alone in case of the straight arm) should
represent the "best" approximation of the ideal i\' = 0 in the playing
interval Xi ^ x ^ Xz. In general, the success of the approximation, in
a prescribed interval, of a given function by another that contains
adjustable parameters, can be judged by different criteria. For in-
stance, minimal rms value of the difference may be postulated (this
would imply the well known method of least squares). More gener-
ally, any monotonic increasing function of the difference, integrated
over the fundamental interval, could be chosen as criterion. In par-
ticular, this "weight function" by which the seriousness of local
deviation is gauged, could be chosen as zero below a certain limit
which should be made as small as possible, and very large above this
limit; in this case, that approximation is considered best, for which
the maximum of the absolute difference between the two functions or
their "tolerance" becomes minimum. This mode of approximation
was proposed and investigated by Tchebychev17 and has recently
found increasing application in engineering, e. g., in the design of elec-
tric wave-filters.18*19 Tchebychev's approximation is appropriate,
whenever deviation becomes rapidly objectionable beyond a certain
limit. This applies, more or less, to the nuisance value of harmonic
distortions. It seems therefore that the tone-arm design should be
carried out in the Tchebychev manner, provided that the associated
calculations are not unduly complicated. They will actually prove
to be of satisfactory simplicity. As a matter of fact, the relation 14
is so simple that almost any mode of approximation could be used on
that account. While the design is not greatly altered when applying
different types of approximation, it seems that the Tchebychev man-
ner is somewhat preferable to the minimal mean square suggested by
Lofgren.3
Dec., 1941]
OPTIMAL PICK-UP DESIGN
607
Tchebychev has proved that under rather general conditions the
smallest tolerance between the given and the approximation function
is obtained if the available parameters are so adjusted that the differ-
ence alternates as many times as possible in the given interval be-
tween the positive and negative tolerance. This is precisely the result
one would expect. The point is illustrated in Fig. 6, which shows the
graphs of the first four Tchebychev polynomials Tn(x); n = 1...4,
defined as those polynomials of nth degree with the coefficient 1 of xn,
which approximate the function x = 0 with the smallest tolerance in
the interval -1^*^+1. They are17'20
rn(x) = 2-(n~1)cos (warccosx);
\Z
V
/ I
-'it
FIG. 6. Plots of the first four Tchebyshev polynomials and their geometri-
cal construction.
i. e., Tn(x) oscillates between the tolerances =*=1/2W~1, reaches them
at the w + 1 locations Xk = cos kir/n; k = 0, 1, ..., n, and goes
through zero at the n points xm = cos (m — l/z)ir/n, m = 1, 2, . . ., n.
In the general case where the approximation functions are not
polynomials and this symmetry is no longer present, nth-order
approximation is still characterized by the fact that the tolerance is
reached (n + 1) times, including the ends of the interval. In general,
the order of the approximation is equal to the number of available
parameters. Consequently, the best approximation to be expected
for straight arms is, in general, of 1st order, with r/' running from
-Vmax at r = fi through 0 to +Vmax at r = r2, the distortion being
equal and maximum at the ends of the record; in this way, the small-
est maximum distortion is realized. For the offset arm, second-order
608 H. G. BAERWALD [j. s. M. P. E.
approximation should be obtainable with the tracking error passing
twice through zero and the distortion reaching the maximum value
three times, i. e., for r\ and rz and an intermediate radius r0. Con-
siderably smaller values of distortion can be expected for the offset
arm.
(b) Single-Purpose Straight Arm Design. — Tchebychev approxima-
tion of the underhang in eq. 14 for a = 0 gives the following results :
Optimal underhang
Maximal weighted tracking error
W-W.M.-
Maximal harmonic distortion, according to 12
|«|m« (%) ~ (8 tO 11)
. r-f
Weighted tracking error as function of groove radius
Distortion vanishes at r =
In Fig. 7, equation J?5d is plotted as y'/lv'lwi vs- x/xm = ?! *V/?vi f°r
the three values a = 2,3, and 4. When inserting the numerical values
of the playing ranges of transcription and commercial disks, as given
at the end of Part I, one obtains
This shows that for correct mounting, distortion can just be kept
within the limits set previously for commercial recordings with the
conventional arm length of 8 inches, while for high-fidelity achieve-
ment in transcription, L should be not less than about 15 inches for a
straight arm. For these conditions, the actual tracking error be-
comes about 4.5° at the extreme inner, about 10.3° at the outer groove
for transcription, and about 5.9° and 17.8°, respectively, for commer-
cial recordings.
Incorrect mounting may increase tracking distortions considerably.
The quantitative influence will be discussed later on for both straight
Dec., 1941] OPTIMAL PICK-UP DESIGN 609
and offset arms. It will be found that, for straight arm mounting,
conditions are much more critical at the inner end than at the outer
end of the playing range, and that therefore a mounting error which
reduces the under hang slightly below the optimal value 15a is much
less harmful than an increase by the same amount. For instance, for
an 8-inch arm and 12-inch disks, equation 15a gives dopt = 0.438 inch
= 7/i6 inch. It is found that an increase by only Vie inch would in-
crease the maximal distortion by 33 per cent. A decrease of d by the
same amount, however, would increase e|max by only 3.7 per cent.
It is thus safe to keep somewhat below rather than above the opti-
mal overhang.
(c) Single-Purpose Offset Arm Design. — The second-order Tcheby-
chev approximation gives the following optimal values :
Optimal offset
-»— -^i-rlP^) | y«o
Optimal overhang
Maximal weighted tracking error; maximal harmonic distortion
pz — 1 Xm _ fo — n)2
II Imax — pz _j_ 1 * £ COS «opt
1
Weighted tracking error as function of radial position
- 1. (16d)
'\ = jrj'lmaxatr = rlfr = r2, andr0
+
610 H. G. BAERWALD [j. s. M. P. E.
Fig. 8 shows V/IVlmax vs- r/rm> again for the three values a = 2,3, and
4. Comparison of the Figs. 7 and 8 with 6 shows that the two classes
of curves correspond, in their character, to the first two Tchebychev
polynomials TI and T2. With the values of TI and rz used previously,
it follows then
(%) -
3.7
, = (transcription)
V(Z,<inche8))2 - (53)2
5.5
/n, \t f?A
V CL(inches))2 - (3.4)
This shows that with an accordingly designed offset arm, it is possible
to obtain practically distortion-free tracking even with arms of the
shortest practicable length, which is somewhat more than the disk
radius.
This result is important as it implies a considerable flexibility of
pick-up design, which is necessary in order to meet a number of prac-
tical requirements which so far have not been taken into account.
One of these factors is the limited accuracy of mounting under prac-
tical conditions of production and service. This affects only one of
the two parameters, namely, the overhang. It is necessary that the
maximum distortion occurring within the playing range does not ex-
ceed the prescribed limit as long as the deviation from the optimal
overhang 16b is kept within reasonable tolerances. Another prac-
tical factor which has not been considered so far, is connected with
the tangential friction force of the stylus in the groove. This gives
rise to certain adverse conditions discussed below which depend on
the offset angle and are improved by decreasing it from its optimal
value 16a. Finally, multi-purpose tone arms must be designed on a
compromise-optimal basis for playing commercial as well as trans-
cription records. This implies deviations from optimal single-purpose
design and thus an increase of the maximal distortion. The result
16f shows that with the theoretical optimal design, tracking distor-
tion is still considerably below the permissible limit even for the short-
est practicable arm lengths. Thus it can be expected that sufficient
margin is left for taking into account the three factors just mentioned
by compromise design not requiring increased arm length, which is
undesirable for economic reasons. For straight arms, on the other
hand, this is indeed the only means to meet the situation, as seen from
15c and 15f. This flexibility of the two-parameter design demon-
strates the superiority of the offset head.
Dec., 1941] OPTIMAL PICK-UP DESIGN 611
It is now necessary to investigate the influence of the stylus friction
as well as the general dependence of the maximum distortion on a
and d on the basis of 14.
(d) The Influence of Stylus Friction. — Only part of the friction force
F between groove and stylus is taken up by the tone arm as this can
freely rotate about the axis A (Fig. 5). The remaining component
Fh = F tan 7 is taken up by the groove wall (Fig. 5). Evidently, 7,
and therefore Fh, increases with the offset angle, and for near-optimal
offset, Fh is centripetal throughout the play range. This gives rise to
an undesirable excess pressure on the inner groove wall which may
increase the linear translation loss and create even-order distortion
components due to asymmetry of wall deformation.5 Because of the
groove wall inclination, Fh creates a vertical component Fv, which is
directed upward. For light-weight pick-ups for high-fidelity repro-
duction,21- 22 where the bearing weight is kept to a minimum sufficient
to overcome the vertical components of tracking and tracing forces —
particularly due to pinch-effect23' 24 — and spurious accelerations due
to unevenness of the record and ambient mechanical vibrations, the
influence of the additional force Fv is sometimes considered so detri-
mental that return to the straight arm is advocated.
A fair estimate of Fv can be obtained for soft records where stylus
pressure is most critical. Measurements of the friction for cellulose
nitrate show that, from about 10 grams up to a "critical" bearing
weight P of 25-30 grams where record wear sets in, F increases lin-
early with P, accordingpto the empirical relation
F = 3 + 5 (F and P in grams) (17a)
(Ref. 21, Fig. 8, p. 215.) Fh is resolved into three components, i. e.,
one normal to the groove wall, one tangential (frictional), and the
vertical, Fv. For a groove angle of 90 degrees, Fv = Fh(l -- p)/
(1 + p), where p denotes the coefficient of friction between groove
wall and stylus. 7 = a + y- It will be shown below that j\ attains
its largest positive value at the outer radius for any design with a ^
<*opt (16a) and optimal overhang. This gives
Wmax ~ (3 + £) j-^-J ' tan (a + r,2) (grams)
It will be shown that under practical conditions (a + 772) will never
exceed 30 degrees appreciably, while p will not be smaller than 1/4, the
value of the corresponding coefficient in 17 a. With these assump-
tions,
612 H. G. BAERWALD [j. s. M. P. E.
(/VU* - (1.8 + 0.15P) • tan (<* + r,2) <. l\*5P (grams). (176)
(For offset angles a ^ <*opt, it will be found that |7?|max is small com-
pared with a; then tan a may be substituted in 17b.) For a bearing
weight of 15 grams — approximately the lowest commercially avail-
able—equation 17b gives (Fv)mSL^ = ll/2 and 2Va grams for a -\- rjz =
20° and 30°, respectively; for the "critical" weight of 30 grams, 2.3
grams and 3.6 grams, respectively. Considering these values, it must
be borne in mind that only the difference between tan 772 for the straight
arm and tan (a + 772) for the offset arm represents the increase of Fv
due to offsetting. As 772 is much larger for a = 0 than for a = aopt,
this increase of Fv is considerably smaller than Fv itself, usually one-
half or less of the values found, as will be shown by numerical ex-
amples. It appears therefore that even for the lowest bearing weights
commercially used, the increase of Fv due to offsetting is inconsider-
able, and that there is certainly no reason for giving up the offset
with its inherent advantages on that account. However, a choice of
a somewhat < aopt (16a) will give some benefit without exceeding the
permissible distortion.
(e) Compromise Design and Influence of Mounting Error. — It has
been shown that for practical reasons, it is partly desirable, partly un-
avoidable to deviate from the theoretical optimal design values 16.
In order to see how far one can go in this respect without infringement
on the distortion limit set, the dependence of distortion on a and 5
according to 14 must be investigated. For this purpose, it is useful
to introduce the numerical radius
_ _»_,.
sin a
where 77' attains its minimum 77 V In terms of XQ and sin a, it is
, = x^_ ( _ sina / _ *A> }
2 cos a } x V x ) \
Starting from a = 0, let us first increase the offset continually under
adjustment of the underhang or overhang d for Tchebychev approxi-
mation. Between a = 0 where 5 > 0, and o: = aopt where d < 0,
there must be an angle o^ for which 5 = 0, i. e., where the stylus tip
passes through the center C (Fig. 5). It follows
Xm XzX\ r%r\ . . fia\
"n - - = = : s(aa) = a
Dec., 1941]
OPTIMAL PICK-UP DESIGN
613
This angle is convenient from the point of ease of mounting. For aQ,
the radius #9 (18) becomes zero; it is negative, i. e., has no significance,
for a < (XQ. For a0, XQ is thus still outside the playing range xi ^ x ^
Xz, which implies that the rj f(x) is monotonic increasing over the play-
ing range, like the curves of Fig. 7. With further increase of a, how-
ever, xQ becomes = x\. Then the curve rjf(x) has a horizontal tangent
at x = Xi. The corresponding value of a is called acrit, and it follows
Sin acrit =
For a > txcrit> the curve rj'(x) has a negative slope at x = Xi up to the
/
I
1 '
it
f I
FIG. 7. Relative weighted tracking error
relative groove radius for straight arms.
minimum at x = XQ; then it increases again to 17 '2 .
approximation is then obtained by making
not by r;'i
r?0 = fli —
Tchebychev's
ax. and
• which would give a higher value of r?'max. (This
is readily shown and follows also from continuity with the domain
a < Ocrff) The value rj'i, which in this range of values of a. does not
play any part in the Tchebychev design, lies between Vo = — Vmax
and 77 72 = +r/ 'max and increases, with increasing a, monotonically
from -Vmax at a = acrit to +Vmax for a = aopt (eq. 16a). This case,
which has already been dealt with in equation 16 and Fig. 8, repre-
sents the second-order approximation. Increasing a beyond aopt leads
into the domain where -rj'0 = r/i = U'Lax and i\\ < rj'i- Finally,
614
H. G. BAERWALD
[J. S. M. P. E.
there should be an upper critical value of a where 77 '2 has decreased to
— 17 V and XQ has increased to xz. For this offset angle,
sin a
-K(f)'-1)
(20a)
For practical values of xi and x2, this leads to imaginary a, i. e., under
actual conditions, x0 < Xz for a up to 90°.
The design formulas for different offsets follow.
J 4-
FIG. 8. Relative weighted tracking error vs.
relative groove radius for offset arms.
For a ^ acrjt. we obtain:
(x2* + *i2) cos a
5L+«!-sma[
- *i) (x + Xi - 2sin a) -
X
x- 2sina)
-25 - 52 = 2
(*2 — Xi) (x2 + Xi — 2 sin a)
sinflt-l 5 = Q for sin ao =
~
(21)
For a = 0, this reduces, of course, to equation 15. For acrit ^ «
2 cos a
^7^ A/ — j Vsin2 a + (xz — sin a)2 — sin a -
Vsin2 a + (x2 — sin a)2 — sin a
sin a I
- sin a)2 - (*2 - sin a) | I -26 - 52 = .v0 sin a
Dec., 1941] OPTIMAL PICK-UP DESIGN
For a £ aopt :
615
t] 1 = — •
2 sin
in2« + (sina "
to'lmax \/sin2 or + (sin a — Xi)2 — sin a.
t
Xo = shhxlv/sm2a + ^sina ~*i)2+ (sin« -*i)( I"25"52 = *osin«
(23)
FIG. 9. Numerical example: Plots of weighted tracking error and distortion
of optimally hung arm vs. groove radius for various offset angles.
Conditions are illustrated in Fig. 9 which shows plots of weighted
tracking error and associated harmonic distortion for 2-mil elongation
at 500 cps vs. the groove radius, for an 8-inch arm and 12-inch
disks, with r\ = 2 inches, r2 = 6 inches. The straight line / refers
to tne case of zero offset and underhang, which gives, according to
eq. 14, i]r = xm/2 = 0.217 rad. = 12.4° or 4.8 per cent distortion,
according to 12. Curve // represents rj'(x) for a = 0 and optimal
underhang 5 = 9/ieo; d = 0.45 inch, according to eq. 15, resulting in
hi max = 0.173 rad. = 9.9° ; the curve is the same as in Fig. 7 for a =
3, with different scales. The angle a0 where 5opt = 0, becomes
sin (XQ = 3/ie, «o = 10.8°, according to eq. 19; the associated rj'(x)
with
= 0.11 rad. = 6.43° is plotted as curve ///; it leads to
616
H. G. BAERWALD
[J. S. M. P. E.
y2 = a + 772 = 22°, which is only 4l/z° 'larger than for a = 0. Curve
IV belongs to an angle a which is still <, but close to, 0^ and > OQ:
sin a = 2/7, « = 16.6°; [77^^ = 4.44°; a + 772 = 24.3°. According
to eq. 20, sin acrit = 9/28, «crit = 18.75°. Curve V refers to
sin a = 5/i4, a = 20.9° which is > acrit. As different from IV, the
minimum occurs no longer at x = xi, but at XQ = 0.290, according to
equation^; -Vi/hlma* = -0.863; |7?'|max = 0.0538 rad. = 3.08°;
a + m = 26.2°. The curve VI, the same as in Fig. 8 for a = 3,
represents the second-order Tchebychev approximation, with sin aopt
FIG.
Numerical example
Plots of various parameters vs. offset
angle.
= 3A, «opt - 25Vs
- 0.0343 rad. - 1.97°. The
improvement over the straight arm, curve //, is striking. aopt +
^("opt) = 283/4°. Finally, curve VII pertains to sin a = l/z, a =
30° > «opt, with r/'2 < -i?'0 = r,f max = 0.059 rad. - 3.38°; XQ =
0.405; i/Vh'Uax = Vs» according to eq. 23. It is seen that the in-
crease of XQ and decrease of vVJv'Inn is comparatively slow above
«opt, in accordance with the fact that the critical angle (eq. 20a) is
usually non-existent; in the present example, xi = l/z, ^Vji?' max =
— 7/9i°r a = 7T/2, according to eq. 23.
Fig. 10 shows the dependence of the optimal numerical underhang
or overhang, of the associated maximum weighted tracking error,
and of the angle (a + rjmax) which occurs in 17b, on the offset angle a.
Dec., 1941] OPTIMAL PICK-UP DESIGN 617
The curves refer to 16-inch disks and 33 1/3 rpm, with L = 12 inches,
r\ = 3.6 inches, r2 = 8 inches (xi = 0.3, #2 = 2/s)- The characteristic
angles are «0 = 12°, acrit = 20.7°, and aopt = 26.5°. The kinks
of the curves at a = aopt are due to the Tchebychev condition. The
minimum of i? '(max represents, according to 12, a maximum distor-
tion of only Ysper cent, while about 2l/2 per cent is obtained for
the straight arm, which is somewhat more than the permissible limit.
It is seen that a may be chosen considerably below a0pt> without in-
fringement on the distortion limit, but that the resulting reduction
of the vertical force 17b is comparatively modest, because (a + 772)
increases only slowly with a for a < a:opt. If a > aopt, r}'2 becomes
rapidly < rj\ (curve VII of Fig. 9), and for a certain angle a', r/2 be-
comes = 171; f or a > a', rjmax = rji. According to eq. 23, it is
*• -J2f 1 +^1 - 1 („ = 7,2)
and
(24)
Thus the curve (a + r?max) nas 2 kinks: at a = aopt and a = a'.
The line a is shown for comparison; it is seen that for a ^ txopt,
>7max < «, as expected for ?7max = 771. Therefore a may be used in-
stead of a + r7max = Tmax in the estimate 17b.
In order to obtain the influence of inaccurate mounting on tracking
distortion, it is necessary to supplement the preceding calculations
which concerned the case of optimal overhang —5 for variable offset
angle, by those for variable 8. It has already been mentioned in con-
nection with the straight arm design that distortion increases rapidly
if the underhang is increased beyond the optimal value, while a de-
crease is much less harmful; it can be shown that the ratio of the two
effects is (r2/ri)2. This is easily understood on the ground that the
design is based on the weighted tracking error. Conditions are there-
fore most critical at the inner groove radius. This is true not only
for straight arms but for all under-critical offset angles. An illustra-
tion is given by Fig. '11, which shows |T7'|max versus —d for the same
numerical example as used in Fig. 10. The curves are plotted for the
four cases a = 0, a = a0, a = acrit, and a = aopt. The unsym-
metry noted for a = 0 persists up to a^t, while the mounting be-
comes more and more critical with increasing offset angle due to the
618
H. G. BAERWALD
[J. S. M. P. E.
decrease of |V|max(5opt). For a > ac?lt, the steepness of ascent for
5 < 5opt increases rapidly, due to the appearance of the minimum 770.
At a = cfopt, the inclination on the side of positive (5 — 6opt) is still
substantially unaltered, but the side of negative (d — 5opt) is now
the steeper one. At the same time, the influence of the mounting
becomes most severe, which is understood, as the optimal approxima-
tion is achieved through compensation. In the present numerical
example, a deviation \S — 3opt| of about 4.10~3 corresponding to a
mounting error of only 3/64 inch, already doubles the value of |r;'|max.
The full realization of the second-order approximation would thus
call for an accuracy of a few tenths of a millimeter. But as the dis-
-a/ -AM -0.01
FIG. 11. Numerical example: Distortion vs. mounting error for characteris-
tic offset angles.
tortion associated with the minimum of |V|max is far below the per-
missible limit, requirements can be considerably relaxed, under prac-
tical conditions. Liberal mounting tolerances of about l/% to 1 per
cent of the arm length or =±= Vie to Vs inch will be permissible in most
practical cases. They are easily determined by calculating the
slopes of the |e|max-5 curves at both sides of 6 = 5opt (a). For all
designs with a < aopt, which were found preferable on account of the
decrease of Fv (17), an overhang 5 somewhat larger than 5opt should
be prescribed, which is likewise found in terms of the two slopes.
(/) Design of Multi-Purpose Arms.— When it is desired to play
records of different sizes and/or speeds with the same tone arm, this
should be designed on a compromise basis so as to render as small as
possible the maximal tracking distortion occurring under any condi-
tions thus included. As the mean radii rm of different types of disk
Dec., 1941]
OPTIMAL PICK-UP DESIGN
619
records are different, the design is based on the original relations 10
and 14 or 18a combined:
*„
sin a
(25)
It should furnish for a and XQ such values that minimize \e max simul-
taneously for the different types of records which are characterized by
their values of ft, rb and r2, v being considered as constant (i. e., 8
cm/sec at 500 cps for permissible |e|max = 2 per cent). In practice,
only the following combinations are used
(1) 78 rpm; n ~ 2 inches, r2 "^ 6 inches
(2) 78 rpm; r\ ~ 2 inches, ^2 ~ 8 inches
(5) 33l/s rpm; r\ ~ 3x/2 inches, rz ~ 8 inches
For design purposes, (Jf) can be disregarded as it is fully included in
the range of (2). Using upscripts in referring to (2) and (5), the
following six values have to be considered as potential maxima of
distortion
i _ sin «/2 - *° M-
x^2) \ *i<2>/r
1 -
-TO
sin
x0
_ sin_a/2 -
^i(3) V
1 -
; A0(3) = 2.34 1 -
=2.34
1 _ !E.?/2 - -^
#2(3) \ X2(3)/
Here, A stands as abbreviation for (2L122 cos a) \ e \ /v with
122 = 787T/30 sec"1; 5.34 = ratio of the two speeds. A0(Af) has to be
omitted if xQ(k} ^ #i(A:) (k — 2 or 3) as being outside the playing range.
The optimal numerical values of XQ and sin a are those which minimize
the largest of the four to six A's, i. e., which make the three largest of
them equal. A2(2) can obviously be omitted from the comparison, but
not necessarily A0(2), as XQ may be < #i(3) but > #i(2).
As an example, the design of a double-purpose 12-inch arm is given.
The numerical limit radii are *i(2) = Ye, #i(3) = 7/24, and *2(2) = #2(3)
2/3. It is found that for | A/2) = Ai(3)| = | A2(3)| = Araax, A0(2'| <
Amax and *i(3) > ^o = 0.288B; sin a = 0.344; a = 20.1°. d = 0.0509,
d = 0.61i inch. Fig. 12 shows the resulting distortion e, according
to 25, f or v = 8 cm/sec at 500 cps; its maximum is only 1 per cent.
This is only half the permissible limit and leaves thus a safety mar-
gin for inaccurate mounting. It is seen that x0 is only slightly < #i(3)
0.292; i. e., for the speed 33 Y», the design is close to that for a = o^.
For the speed 78 rpm, on the other hand, a > aopt, and the overhang
620
H. G. BAERWALD
[J. S. M. P. E.
— 6 < — 5opt(a). If the same arm were to be used for 78 rpm alone,
the optimal design as given by equation 16 would be: aopt = 19°,
— dopt = 0.532 inch, |e|max = 0.39 per cent; if it were designed for
use at 33 Vs rpm alone: o^t = 26°, -dopt = 1.023 inches, c|max =
0.36 per cent. It is seen that the double-purpose design lies closer to
the first case.
OTHER EFFECTS OF TRACKING ERROR
The harmonic distortions due to tracking error depend, as shown,
only on the distortion parameter e (4a), i. e., on the weightecf tracking
error 17' (11). Consequently, the design was based on this quantity.
i.o * io
-0.2,51
T -075;
2.5
3.5
5 676
T IN INCHES
FIG. 12. Numerical example for multi-purpose arm design:
Tracking distortion over playing range.
There are, however, effects which depend on the tracking error 77 itself
rather than on 77'. Although they are in general unimportant, they
should at least be mentioned here.
Going back to the rigorous expressions 6, 7, and 8 for the picked -up
signal, it is seen that this contains the factor sec 77. This implies an
increase, not only in signal amplitude, but also in the lateral reaction
force (both stiffness and inertia), by sec 77. This increase, however, is
of negligible magnitude for all practical purposes although the design
minimizing IV^x does not minimize r7|max. The largest value
which 77 may take occurs at the outer rim for straight arms. It was
shown (Part II, b) that even for maximal permissible distortion —
Dec., 1941] OPTIMAL PICK-UP DESIGN 621
which is nearly obtained with a properly underhung 8-inch arm for
12-inch records— 772 = 18°, i. e. (sec 772 - 1) = 0.05 <T 1. This is in
line with the assumption *72/2 <^C 1 on which the design procedure
was based.
It has been claimed that tracking error may cause appreciable
record wear. Again, this supposed wear would not depend on r?' and
therefore would not be minimized by the proposed design. But, as in
case of the signal amplitude, it does not seem that, within the per-
missible range as based on the presented design method, tracking
error could have any noticeable effect of this kind. No clear experi-
mental evidence of additional record wear caused by tracking error
of usual magnitude has ever been presented. Careful listening tests
undertaken by Olney2 did not reveal any clear effect. Besides, it is
hard to understand how record wear could ever be produced by track-
ing error of permissible magnitude. For permanent stylus points,
it is certainly ruled out as they are surfaces of revolution. All com-
mercial light-weight pick-ups have permanent styli. In case of steel
needles, on the other hand, high stylus tip pressure and motional im-
pedance cause appreciable record wear, quite independently of the
tracking mechanism. Wear due to tracking error is supposedly
caused by the rate of change of tracking error along the groove spiral :
the needle is initially ground to fit the groove and is therefore no
longer a surface of revolution ; turning about its axis due to change of
77 therefore entails regrinding of the projecting edge. It is certainly
hard to see how this regrinding which occurs very gradually as com-
pared with the initial grinding in the first few grooves, could possibly
cause any wear noticeable against the background of that due to ex-
cessive stylus pressure and impedance, as met in cheaper grade pick-
ups.
I wish to tender my acknowledgment to the Brush Development
Company for making this work possible. I am also obliged to Dr.
S. J. Begun, head of the recording department, for hints and en-
lightening discussions.
REFERENCES
1 MACDONALD, G. E.: "The Reduction of Pick-Up Tracking Error," Com-
munications, XXI (Jan., 1941), p. 55.
2 OLNEY, B.: "Phonograph Pick-Up Tracking Error vs. Distortion and Re-
cord Wear," Electronics, X (Nov., 1937), p. 19.
3 LOFGREN, E. G. : "On the Non-Linear Distortion in the Reproduction of
022 H. G. BAERWALD
Phonograph Records Caused by Angular Deviation of the Pick-Up Arm," Akust.
Zeits., m (1938), p. 350.
4 LEWIS, W. D., AND HUNT, F. V. : "A Theory of Tracing Distortion in
Sound Reproduction from Phonograph Records," /. Acoust. Soc. Amer., XII (Jan.,
1941), p. 348.
5 KORNEI, O.: "On the Playback Loss in the Reproduction of Phonograph
Records," /. Soc. Mot. Pict. Eng., XXXVII (Dec., 1941), p. 569. After completion
of the manuscript, a paper by G. Guttwein, Akust. Zeits., V (Dec., 1940), p.
330, on linear and non-linear distortions in recording and playback, was received.
6 FELDTKELLER, R., AND WOLMAN, W. : "Almost-Linear Networks," Tele-
graph, u. Fernsprech. Tech., XX (1931), p. 167.
7 BARTLETT, A. C.: "Calculation of Modulation Products," Phil. Mag., Part
I, XVI (Oct., 1933), p. 834; Part II, XVII (Mar., 1934), p. 628.
8 LAGRANGE, J. L. : Hist, de I'Acad. R. des Sci. de Berlin, XXV (1771), p. 242.
* BESSEL, F. W.: Berliner Abh. (1826).
10 WATSON, G. N. : "A Treatise on the Theory of Bessel Functions, " Cambridge
(1922).
11 MCLACHLAN, N. W. : "Bessel Functions for Engineers," Part I, Oxford
(1934).
12 JAHNKE-EMDE: "Tables of Functions," Third Ed., Leipsic and Berlin
(1938), p. 147.
13 LAGRANGE, J. L. : Mem. de I'Acad. de Berlin XXIV, Oeuvres, n, p. 25.
14 WHITTAKER, E. T., AND WATSON, G. N.: "A Course of Modern Analysis,"
Fourth Ed., Cambridge (1935), 7:31 and 7:32, p. 131.
15 MASSA, F. : "Permissible Amplitude Distortion of Speech in an Audio
Reproducing System," Proc. IRE, XXI (May, 1933), p. 682.
16 LYNCH, T. E., AND BEGUN, S. J. : "General Considerations of the Crystal
Cutter," Communications, XX (Dec., 1940), p. 9.
17 TCHEBYCHEV, P. L. i Mem. Acad. Sc. Petersb., Series 6, VII (1859), p. 199;
Oeuvres, I, p. 271.
18 GUILLEMIN, E. A. : "Communication Networks," n (1935), p. 386.
19 CAUER, W.: "An Interpolation Problem with Functions with Positive Real
Values," Math. Zeits., XXXVHI (1933), p. 1.
20 COURANT, R., AND HiLBERT, D.: "Methods of Mathematical Physics,"
Berlin, I (1931), p. 75.
21 WILLIAMS, A. L. : "Further Improvements in Lightweight Record Repro-
ducers and Theoretical Considerations Entering into Their Design," /. Soc. Mot.
Pict. Eng., XXX (Aug., 1939), p. 203.
22 LYNCH, T. E. : "Some Considerations in Phonograph Pick-Up Design,"
Brush Strokes, m (April-June, 1939), p. 3.
23 PIERCE, J. A., AND HUNT, F. V.: "Distortion in Sound Reproduction from
Phonograph Records," /. Soc. Mot. Pict. Eng., XXXI (Aug., 1938), p. 157; /.
Acoust. Soc. Amer., X (July, 1938), p. 14.
24 FLEMING, L.: Notes on Phonograph Pick-Ups for Lateral-Cut Records,"
/. Acoust. Soc. Amer., XH (Jan., 1941\ p. 366.
THE SPECIALIZATION OF FILM DELIVERY*
J. H. VICKERS**
Summary. — The problem of transporting film and of distributing it to the
thousands of theaters in the country is a considerable economic problem. Approxi-
mately ninety per cent of all film shipped between exchanges and theaters is handled
by trucks operating out of thirty-two film-distributing centers shattered throughout the
country.
The paper describes in considerable detail the truckman's routine in picking up
and delivering film between the exchanges and the theaters.
In order to discuss film transportation with an audience such as
this it is not necessary to review the history of the development of the
motion picture business ; however, it is well to remember that a com-
plete program for the old nickelodeon had a film weight of about
thirty pounds, whereas today the average small town theater often
uses in excess of one hundred pounds of film in building its program.
In fact, there is very little difference between programs offered the
patrons of small towns and those shown in the palatial theaters of the
large cities.
The millions spent annually in advertising motion pictures natu-
rally create a desire to see the pictures and stars as soon as possible
and, as with any other useful product, a public demand for it is
created. Due to radio, magazines, and newspapers reaching all the
byways and small hamlets of the country, the demand for pictures
soon after release date is no longer confined to the large centers of
population. Supplying this rural demand is an economic problem, as
good business limits the number of prints available for exhibition;
therefore fast and efficient transportation is a necessary bridge be-
tween the economics of furnishing pictures and the public demand
for seeing them.
Throughout the country truck companies specializing in the de-
livery of film have been organized to furnish this fast and efficient
transportation. At most of the thirty-two film distributing centers
throughout the United States you will find one or more trucking firms
* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received April
14, 1941.
** National Film Carriers, Inc., Philadelphia, Pa.
623
624 J. H. VICKERS [J. S. M. P. E.
performing special film delivery service, commonly known as film
carriers. Approximately 90 per cent of all the film shipped between
exchanges and theaters are handled by trucks.
To understand better how film carriers vary from ordinary truck
operators, let us look in detail at the film carrier's operations. As the
film carrier's work is an endless chain, a convenient beginning will be
with a truck loaded with film ready to start on its nightly pick-up
and delivery journey. The driver has a key for every theater on his
route, and, by prearrangement, each theater has a Designated
place to leave the film ready for return and a place for the incoming
film. On a route making a loop from and back to the exchange
center, the first stop will be made after the first theater on the route
closes; here the film for the next day's use will be delivered and the
film just shown picked up. After working the theaters on this loop
route, the truck will be back to the exchange center early the follow-
ing morning, having delivered all the film for that day's use and re-
turned all that used the previous day. This film is delivered im-
mediately to the various exchanges for inspection and reshipment.
Much of this film will probable be booked that day in nearby subur-
ban theaters. Immediate inspection makes the film ready for ship-
ment to these theaters before opening time of that day. Such films
will be picked up by the film carrier and delivered to the suburban
theater with the print having been inspected and no time lost between
play-dates. During the day shipments are made ready for the next
day's play-dates and late in the afternoon the film carrier picks up
these films from the exchanges and again loads the truck ready for
the next day's delivery. This is the complete chain in its simplest
form, but there are many more details and much more complicated
operation in a complete film delivery service.
In addition to the early morning suburban delivery previously
mentioned, there is usually a late pick-up after the suburban
theater closes at night. This is especially true in the distributing
centers located in large cities where some film carriers maintain
a night inspection service as well as some of the distributors. The
film picked up from suburban theaters after closing time is rushed to
the inspection rooms, and those booked for the next day's showing
are inspected and made ready for shipment on trucks leaving around
2:00 A. M. These late routes are usually loop routes.
In order to reach the more distant and off-route points, small
towns, and hamlets not on the main line, junction points are neces-
Dec., 1941] SPECIALIZATION OF FILM DELIVERY 625
sary. At these junction points relay trucks meet the main-line truck.
These relay trucks leave the last town on the line at the theater's
closing time, picking up film from this and other theaters on the re-
lay. At the junction point all the pick-up is delivered to the main-
line truck, and the film for the next days' showing is received. Very
often a relay has one or more feeders, or sub-relays, which serve off-
route points. When more than two or three relays meet at a common
point a sub-terminal is usually set up by the film carrier. The man-
agement of this sub-terminal supervises the proper transfer of film
from one route to another and maintains a refueling station and in-
spection service where emergency repairs and adjustments can be
made. As a rule main-line trucks run directly out to the junction
points and return over the same route, which enables the truck to
reach more distant junction points than if it were making a loop.
This type of run has the advantage of an early leaving time over
a loop run, as it does not have to wait until the theaters on the route
are closed before passing. On the return trip pick-up and delivery are
made to the theaters that were not closed on the outgoing trip.
It can readily be seen that a system of this kind easily lends itself
to the circuiting of product from one theater to another from the
closing time at night to the opening time next day. Products so cir-
cuited can not be inspected between play-dates; however, it is of
great advantage to both exhibitor and distributor to have this cir-
cuiting possibility available, as it saves many dark houses or un-
warranted cost when some unforeseen circumstance puts a print out
of service, such as damage by bad mechanisms, hold-overs, fire, or an
error in booking. Prints having a specific time value, such as news-
reels, and not requiring a great deal of inspection, seldom ever see the
film exchange until they are worn out or are out of date, as they are
set up on circuit from town to town during their useful life.
There is a large amount of detail involved in the proper execution of
a specialized film-delivery system, and in addition there is the question
of personnel and equipment with which to do this exacting job. One
of the great railroad systems of the country was justly proud in pub-
lishing a statement regarding their crack train, saying that it main-
tained its schedule over a period of a year of 92*/2 per cent on time.
In order to maintain good service and to avoid missouts, it is nec-
essary for the average film carrier to maintain a schedule at least 95
per cent perfect; the country's average among film carriers is higher
than 95 per cent.
626 J. H. VICKERS [J. S. M. P. E.
Road accidents are one of the greatest hazards to a schedule. Ex-
cessive speeds greatly increase road accidents, therefore the film car-,
rier must use moderate speeds and at the same time maintain a fast
schedule. This makes it imperative that the right kind of equipment
be selected for the job to be done. To do this all factors affecting the
minimum speed of the truck must be considered ; namely, the aver-
age weather conditions, the load to be hauled, the number and per-
centage of grades to be encountered, and all other road conditions.
In order to maintain a consistent average speed without high top
speeds it is necessary that the lower speeds on grades be near the
average speed desired. Sufficient available horsepower is the only
remedy for this equation. When analyzing an engine Sufficient
Available Horsepower is a much broader term than Maximum
Available Horsepower as it takes into consideration the maximum
torque of the engine and the speed at which this torque is developed,
and refers only to the horsepower developed at an engine speed prac-
tical for continuous operation. These facts must be taken into con-
sideration, as unfortunately most internal combustion engines de-
velop their maximum horsepower at a speed considerably higher than
the speed that will give an economical motor life that is practical for
continuous operation. Some truck units have as many as ten for-
ward speeds hi order to give the operator a chance to do the most
with the horsepower available at a safe engine speed. The next
great enemy to a schedule is road breakdown. The secret of elimi-
nating road breakdowns is to have the equipment as nearly perfect
on every trip as possible. This can not be done without allowing the
mechanical force all the time necessary to make inspections, adjust-
ments, replacements, and repairs, which often calls for a truck's
being in the shop longer than the hours between runs, and necessi-
tates a large amount of spare equipment. Some companies have as
much as 75 per cent spare equipment.
One of the greatest horrors to a film carrier is a fire on a film truck.
Aside from the great physical loss there is the miss-out loss and dis-
appointment of the customer and the public. To reduce this hazard
to a minimum gas tanks are equipped with safety devices against
spillage on overturn or collision, and the ignition is automatically
cut off when the truck reaches any excessive angle from the horizon-
tal. Of course, every precaution such as fire extinguishers properly
located, exhaust pipes away from wood and gas lines, and no smoking
rules, are in effect. Another enemy to film is excessive cold; to over-
Dec., 1941] SPECIALIZATION OF FILM DELIVERY 627
come this the film carriers in the northern part of the country use
insulated trucks.
In order to render the industry better service and to standardize
the film delivery service by truck, a number of the leading film car-
riers met in New York early in 1933 and formed an Association known
as National Film Carriers, Inc. The only requirement for member-
ship is that a member be a truck carrier specializing in the handling
and the delivering of film and doing the job in a reliable and depend-
able manner. The Association has attracted the leading firms en-
gaged in the transportation of film and includes some thirty mem-
bers. The distributors and exhibitors have met with the members
of the Association from time to time and great progress has been made
through this cooperative effort. Uniform rules and regulations for
carrying the proper insurance and bonds, and other standards of
practice have been devised. Standardization of equipment and regu-
lations as to fire prevention and safety have been promoted. Through
its representatives the Association has met with the representatives
of the government in order to formulate plans and regulations that
will benefit and not hinder this type of service. The Association has
set up a fire-prevention bureau which issues rules and regulations to
reduce the danger of fire to a minimum at the terminal as well as on the
road. The Association maintains membership in the Bureau of
Explosives' and thereby keeps abreast of all the latest rules and regu-
lations which promote safety of operation. The American Trucking
Association has recognized that the film delivery service is a highly
specialized business and has set up a division in its organization known
as the Film Carriers Division.
As stated before film carriers handle approximately 90 per cent of
all film shipments between exchanges and theaters, but no exact
statistics have been compiled on the number of miles required to do
this tremendous job. To get some idea of the magnitude of film-car-
rier operations the analysis of one exchange center will throw some
light on the subject. Charlotte, North Carolina, is a 2 per cent dis-
tributing center serving the States of North and South Carolina.
The film carriers in this territory handle slightly more than 95 per
cent of the film between exchanges and theaters, and the miles in-
volved to do this job weekly would encircle the world twice. This
illustration is probably a low average for the miles traveled out of the
thirty film distributing centers in the United States.
To summarize the outstanding requirements of an efficient film
628 J. H. VICKERS
delivery service : an infallible system for receiving, listing, checking,
circuiting, and delivering the many items required in building each |
program, from a one-sheet to the feature ; the selection and training
of the highest type driver personnel, as this is the key to maintaining
schedules, safety, and dependability; the selection of the proper road
equipment, and as nearly perfect maintenance as possible; and last,
management that can keep a business that must necessarily stay in a
groove out of a rut.
Behind these cold facts there is a lot of romance in the develop-
ment and operation of the film delivery business. The service ren
dered and the successful operation of this "backstage" branch of the
motion picture industry can largely be attributed to the fact that all
its personnel are as thoroughly imbued with the spirit that "the show
must go on" as is any actor.
CURRENT LITERATURE OF INTEREST TO THE MOTION PICTURE
ENGINEER
The editors present for convenient reference a list of articles dealing with subjects
cognate to motion picture engineering published in a number of selected journals.
Photostatic copies may be obtained from the Library of Congress, Washington, D. C.,
or from the New York Public Library, New York, N. Y. Micro copies of articles
in magazines that are available may be obtained from the Bibliofilm Service, Depart-
ment of Agriculture, Washington, D. C., at prevailing rates.
Acoustical Society of America, Journal
13 (October, 1941), No. 2
The Stereophonic Sound-Film System — General Theory
(pp. 89-99)
Mechanical and Optical Equipment for the Stereophonic
Sound-Film System (pp. 100-106)
The Stereophonic Sound-Film System — Pre- and Post-
Equalization of Compandor Systems (pp. 107-114)
Phase Distortion in Electroacoustic Systems (pp. 115-
123)
The Acoustic Wattmeter, and Instrument for Measuring
Sound Energy Flow (pp. 124-136)
A Re-Examination of the Noise Reduction Coefficient
(pp. 163-169)
The Flutter Echoes (pp. 170-178)
A Cinematographic Study of the Conduction of Sound
in the Human Ear (pp. 179-181)
American Cinematographer
22 (October, 1941), No. 10
What a Modern 16-Mm Business-Film Studio is Like
(pp. 470, 496)
Remember to Light the Background (pp. 478, 498)
Communications
21 (October, 1941), No. 10
Cathode Design (pp. 5-8, 28)
Educational Screen
20 (October, 1941), No. 8
Motion Pictures— Not for Theaters (pp. 333-335), Pt. 30
H. FLETCHER
E. C. WENTE, R.
BIDDULPH, L. A.
ELMER, AND A. B.
ANDERSON
J. C. STEINBERG
F. M. WIENER
C. W. CLAPP AND
F. A. FIRESTONE
J. S. PARKINSON AND
W. A. JACK
D. Y. MAA
H. G. KOBRAK
I. B. DYATT
G. MEEHAN
O. W. PIKE
A. E. KROWS
629
630
CURRENT LITERATURE
Electronics
14 (October, 1941), No. 10
Research Beats the Priorities (pp. 27-30, 78, 80, 82-83) C. J. LEBEL
Storage in Television Reception (pp. 46-49, 115-116) A. H. ROSENTHAL
Institution of Electrical Engineers, Journal
88 (September, 1941), No. 3, Pt. Ill
Acoustics of Cinema Auditoria (pp. 175-190)
C. A. MOSON AND
J. MOIR
International Projectionist
16 (August, 1941), No. 8
Mechanics of the Modern Projector (pp. 12-14)
New RCA Lens-Coating Process Available (p. 14)
Effect of Static on Sound Systems (p. 15)
H. B. SELLWOOD
Motion Picture Herald (Better Theaters Section)
145 (October 18, 1941), No. 3
How Visual Angles Affect Image Size (pp. 33-34)
C. E. SHULTZ
BACK NUMBERS OF THE TRANSACTIONS AND JOURNALS
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Beginning with the January, 1930, issue, the JOURNAL of the Society has been
issued monthly, in two volumes per year, of six issues each. Back numbers of
all issues are available at the price of $1.00 each, a complete yearly issue totalling
$12.00. Single copies of the current issue may be obtained for $1.00 each.
Orders for back numbers of Transactions and JOURNALS should be placed through
the General Office of the Society and should be accompanied by check or money-
order.
HIGHLIGHTS OF THE 1941 FALL CONVENTION
The attendance at the 50th Semi- Annual Convention at the Hotel Pennsylvania
in New York was remarkably good and no great difficulty was experienced in se-
curing papers of good quality and interest in spite of the National Emergency.
These facts show the wisdom of the decision of the Board of Governors some
time ago to continue to hold the usual two conventions per year. On succeeding
pages of this issue of the JOURNAL will be found the program of papers as actually
followed at the sessions.
The Convention opened formally at 10 A.M. on Monday, October 20th in the
Salle Moderne of the Hotel Pennsylvania, Mr. Herbert Griffin, Executive Vice-
President of the Society, presiding. The first part of the morning was occupied
with the reports of the Financial Vice-President, the Engineering Vice-President,
and a welcome by the President of the Society, Mr. Emery Huse. Then followed
the announcement of the successful candidates for office for 1942, the ballots hav-
ing been counted on the previous day by a committee of tellers appointed by the
Board. The new officers and governors of the Society for 1942, who are to as-
sume office on January 1st, are as follows:
Engineering Vice-President: Donald E. Hyndman
Financial Vice-President : Arthur S. Dickinson
Secretary : Paul J. Larsen
Treasurer: George Friedl, Jr.
Governor: Frank E. Carlson
Governor: John A. Maurer
Governor : - Edward M . Honan
The terms of other officers and governors of the Society whose names are not listed
above have still one year to run. They are as follows :
President: Emery Huse
Past President: E. Allan Williford
Executive Vice-President: Herbert Griffin
Editorial Vice-President: Arthur C. Downes
Convention Vice-President: William C. Kunzmann
Governor: Max C. Batsel
Governor: Loren L. Ryder
In addition, Drs. Alfred N. Goldsmith and John G. Frayne have been elected
Chairmen of the Atlantic Coast Section and Pacific Coast Section, respectively,
by virtue of which they become members of the Board of Governors. The re-
sults of the elections of the Mid- West Section are not yet available.
The Monday morning session continued with a series of four papers of a general
nature, Mr. Richard Griffith of the Museum of Modern Art Film Library begin-
ning the series with a paper on "Adventures of a Film Library." Mr. Robert
631
632 HIGHLIGHTS OF THE CONVENTION [J. s. M. P. E.
Russell, formerly with the Training Film Production Laboratory at Fort Mon-
mouth, N. J., discussed what he called the "Dynamic Screen," pointing out that
within its present limits various phases of the motion picture have been brought
close to technical exhaustion and artistic satisfaction. However, competition
with color television and other forms of entertainment require another "sudden
impact of novelty" similar to the previous ones of sound, montage, and color.
One great frontier remains; "the selective delimitation of the screen," making
the screen area the entire proscenium wall, and selectively limiting projected pic-
tures within this potential area. An interesting discussion of "Motion Picture
Cant," meaning the jargon of (technical) motion pictures, was presented by Mr.
Barry Buchanan, lexicographer of New York, and Mr. Terry Ramsaye of Quigley
Publications presented a brief dissertation on the extremes to which-the motion
picture industry goes to produce effects not nearly so extreme in scope. The
title of his talk was "Lots of How, a Little What."
The usual informal luncheon was held at noon of Monday, October 20th. Ap-
proximately a hundred and fifty persons came out to listen to the four well known
invited speakers. The Honorable Newbold Morris, President of the Council
and Acting Mayor of the City of New York, extended the official welcome of the
City of New York to the delegates of the Convention, and referred to the im-
portance of motion pictures in our defense program during these troublous times.
The second speaker, Mr. Sol A. Rosenblatt, formerly Administrator of the Motion
Picture and Broadcasting Industries, during the NRA, and now General Counsel
of the National Democratic Committee, spoke vigorously against the Wheeler-
Clark-Nye investigation of alleged propaganda in American motion pictures.
Mr. Francis S. Harmon, Assistant to the President of Motion Picture Pro-
ducers and Distributors of America, was the third speaker. Mr. Harmon dis-
cussed at some length the importance of the motion picture in contributing to the
general public morale, and also elaborated on the broad extent to which the
motion picture industry is involved in the question of priorities, including very
large quantities of such homely materials as typewriter paper, pens, and ink, not
to speak of the more technical materials such as are required in the production of
film and equipment.
The afternoon session opened with a talk by Alan H. Morgensen on the ques-
tion of "Work Simplification — Essential to Defense." Mr. Morgensen's paper
was illustrated by a 16-mm picture depicting the way in which motion pictures
may be used to analyze the motions of industrial workers and to simplify the pro-
cedure and render it more efficient.
Next followed two papers on 16-mm production problems. Mr. Lloyd Thomp-
son of the Calvin Company discussed at considerable length "Some Equipment
Problems of the 16-mm Producer," pointing out that the direct 16-mm method
is now definitely out of the experimental stage. Mr. William H. Offenhauser,
Jr., of Precision Film Labs, presented "A Review of the Question of 16-mm
Emulsion Position." When a 16-mm sound-film is properly threaded in a 16-mm
projector the emulsion on the film may face the screen (which position is called
the "standard" position) or it may face the projector light-source (the "non-
standard" emulsion position). The well designed 16-mm sound projector of to-
day should be capable of projecting either "standard" or "non-standard" film.
The Monday afternoon session concluded with a talk by Lt. Col. M. E. Gillette
Dec., 1941] HIGHLIGHTS OF THE CONVENTION 633
of the U. S. Signal Corps, Fort Monmotith, N. J., on "The How and Why of
Army Training Films," illustrated by several films produced in the Fort Mon-
mouth Laboratory.
The Monday evening session opened with a paper by Mr. Glenn L. Dimmick
of the RCA Manufacturing Company, Indianapolis, in which was described "A
New Dichroic Reflector and Its Application to Photocell Monitoring Systems."
Certain crystals have long been known to transmit light of one color and reflect
light of another color, and some thin metallic films also exhibit the same phe-
nomenon. Such films evaporated on glass have been successfully employed in
high-level photocell monitoring systems for sound recorders. Nearly all the ac-
tinic value of the modulated light is transmitted to the photographic film, while
a large part of the red and infrared is reflected to a caesium photocell for monitor-
ing. V
Dr. Peter C. Goldmark, of the Columbia Broadcasting System, gave a brief
history of color television, and described the general theory of the system, in-
cluding color, flicker, and electrical characteristics. Next followed a paper by
Dr. Alfred N. Goldsmith of New York on "The IR System: an Optical Method
for Increasing Depth of Field." The system was devised for the purpose of at-
taining greater depth of field than is attainable by existing methods of utilizing a
lens system for image formation. The depth of field of a corrected lens system is
determined by its focal length, its effective aperture, and the permissible diameter
of the in-focus image of a point-source. This limited depth of field restricts free-
dom of action on motion picture sets, and dictates a stylized, protracted, and
costly studio procedure. The IR system is based on a division of the set into
optically appropriate regions, each region having identifiable illumination, with
the identification and differential focusing at the camera of all regional images
within a single exposure.
The evening session was concluded by a paper on "Mobile Television Equip-
ment," by R. L. Campbell, R. E. Kessler, R. E. Rutherford, and K. V. Lands-
berg of the Allen B. DuMont Laboratories.
The morning session of Tuesday, October 21st, was devoted to a series of papers
on projection and lighting. Mr. W. Hotine described "A Constant-Torque Fric-
tion Clutch for Film Take-Up" which, when adusted initially to deliver a given
safe torque to the take-up spindle, maintains the torque at a constant value which
can not be exceeded. Messrs. E. L. Boecking and L. W. Davee of the Century
Projector Corp discussed new developments in the design of projector mechan-
isms, and the session concluded with reports of three technical committees of the
Society — the Theater Engineering Committee, the Studio Lighting Committee,
and the Standards Committee.
The Theater Engineering report included an account of an investigation by the
Projection Practice Sub-Committee into the use of hand fire extinguishers in pro-
jection rooms. The sub-committee recommends that such fire-fighting equip-
ment be not required in projection rooms in view of the policy of the Committee
expressed last year to the effect that in the event of film fire in the projection
room the projectionist should immediately shut down the equipment and leave
the room, in which case he would not be inside the room to use such hand extin-
guishers as might be included therein. The presence of hand extinguishers in a
634 HIGHLIGHTS OF THE CONVENTION [j. s. M. P. E.
projection room might provide a temptation ^f or the projectionist to remain in
the room and attempt to fight the fire.
Another part of the Report of the Theater Engineering Committee, that of the
Sub-Committee on Screen Brightness, outlined the admirable work that is being
done by that group in attempting to discover or to design, or to induce instru-
ment manufacturers to build, suitable instruments for measuring the light inci-
dent upon and reflected from the screens in motion picture theaters. Specifica-
tions for such meters were proposed, and several methods of achieving the desired
measurement were described. The Sub-Committee on Theater Design des-
cribed some tests made recently to determine preferred seating areas in theaters.
During the afternoon session of Tuesday, October 29th, Messrs. M. R. Null, W.
W. Lozier, and D. B. Joy discussed "The Color Quality of Light on the Projection
Screen," and Messrs. Lozier, Joy, and M. T. Jones described the characteristics
of "New 13.6-Mm Carbons for Increased Screen Light." The four authors men-
tioned are engineers of the National Carbon Company at Fostoria, Ohio. The
session closed with a brief discussion by W. Scanlon of Larry Strong, Inc., on the
question of "How Safe Are Safety Devices," referring especially to devices employ-
ing photoelectric cells.
The opening paper on the morning of Wednesday, October 22nd, dealt with
"A New Electrostatic Air Cleaner and Its Application to the Motion Picture In-
dustry," by Henry Gitterman of the Westinghouse Electric and Manufacturing
Corp. The presentation was attended by a demonstration of the principle of
electrostatic precipitation, and in the paper were described a number of installa-
tions of such equipment of particular interest to motion picture engineers.
Mr. M. H. Sweet, of Agfa Ansco, described "A Direct-Reading Photoelectric
Densitometer," in which a logarithmic amplifier circuit had been modified to pro-
vide an accurately linear output, with excellent stability.
Two papers by R. M. Evans, W. T. Hanson, Jr., and P. K. Glasoe of the East-
man Kodak Company discussed "Iodide Analysis in an MQ Developer" and
"Synthetic Aged Developers by Analysis." The technical part of the session
was concluded by a paper by Messrs. J. I. Crabtree, G. T. Eaton, and L. E.
Muehler of the Eastman Kodak Company on the "Effect of Composition of Proc-
essing Solutions on Hypo Removal from Motion Picture Film."
On the evening of Wednesday, October 27th, was held the Fiftieth Semi-An-
nual Banquet of the Society, commemorating the Silver Anniversary of the found-
ing of the Society, which occurred on July 24th, 1916.
The proceedings of the banquet opened with the introduction of the officers
and governors elect for 1942, after which occurred the granting of the Journal
and Progress Awards. Mr. R. E. Farnham, Chairman of the Journal Award
Committee, presented the report of the Committee. The recommendation for
the award was the paper entitled "Effects of Ultraviolet Light on Variable-Den-
sity Recording and Printing" by Drs. John G. Frayne and Vincent Pagliarulo of
Electrical Research Products, Inc., Hollywood, Calif., which appeared in the
June, 1940, issue of the JOURNAL. Mr. Farnham's report included brief his-
torical sketches of the two authors.
Mr. Paul J. Larsen, member of the Progress Award Committee, reported on
the Journal Award for the Chairman of the Committee, Mr. Kenneth F. Morgan
who was unable to be present. The Society's medal was awarded to Mr. Glenn
Dec., 1941] HIGHLIGHTS OF THE CONVENTION 635
L. Dimmick of the RCA Laboratories at Indianapolis, in recognition of his out-
standing contributions to the advancement of the motion picture art. An his-
torical account of Mr. Dimmick's achievements was given by Mr. Otto S.
Schairer, Vice-President in charge of RCA Laboratories.
The formal proceedings of the banquet included the presentation of a testi-
monial certificate to Mr. William C. Kunzmann, Convention Vice-President of
the Society. The certificate read as follows :
"In recognition of his long and faithful service as a member of the Society since
1916 and a member of the Board of Governors since 1929, and as Convention
Vice-President since 1933, the Board of Governors of the Society on this day pre-
sent this certificate to William C. Kunzmann as a testimonial of their apprecia-
tion and esteem."
The Thursday morning session was devoted to a symposium of three papers
on fine-grain film. C. R. Dailey of Paramount Pictures, Inc., Hollywood, Calif.,
discussed "Production and Release Application of Fine-Grain Films for Variable-
Density Sound Recording"; Messrs. J. R. Wilkinson and F. L. Eich, Paramount
Pictures, Inc., Hollywood, Calif., discussed "Laboratory Modification and Pro-
cedure in Connection with Fine-Grain Release Printing"; and V. C. Shaner of
Eastman Kodak Company, Hollywood, discussed the question of "Hollywood
Processing Procedures for Eastman 1302 Fine-Grain Release Positive."
The final session of the Convention, on Thursday afternoon, October 23rd, in-
cluded four papers devoted to sound. Messrs. John G. Frayne and F. P. Herrn-
feld, of Electrical Research Products, Inc., Hollywood, discussed "A Frequency-
Modulated Control-Track for Movietone Prints"; in which a 5-mil frequency-
modulated track is located between the sound and picture areas to control re-
production in the theater from one or more sound-tracks. Messrs. R. R. Scoville
and W. L. Bell, also of Electrical Research Products, described the factors under-
lying the design and use of biased recording systems, showing how, in order to
minimize noise and "shutter bump," special precautions must be taken in filter-
ing. The paper dealt In some detail with the "Design and Use of Film-Noise
Reduction Systems."
The third paper of the afternoon was by W. J. Albersheim and L. F. Brown, of
Electrical Research Products, Inc., of New York, on "A Feedback Light-Valve"
and the Convention concluded with a paper by S. L. Reiches of the Brush De-
velopment Company, Cleveland, Ohio, on "The Quarter-Wave Method of
Speaker Testing."
ACKNOWLEDGMENT
The Society wishes to acknowledge its indebtedness and appreciation to all
those who contributed in time and effort toward the conduct and success of the
Convention. This includes the various chairmen and members of the convention
committees, various officers of the Society, and a number of companies of the
Industry.
In addition, the Society acknowledges the kindness of the Capitol Theater,
Radio City Music Hall, Warner's Strand Theater, Roxy Theater, and the Para-
mount Theater in issuing courtesy admissions to the convention delegates.
PROGRAM OF THE CONVENTION*
MONDAY, OCTOBER 20th
10:00 a.m. General Session, Herbert Griffin, Chairman.
Report of the Convention Arrangements Committee: W. C. Kunz-
mann, Convention Vice- President.
Report of the Financial Vice-President ; A. S. Dickinson.
Report of the Engineering Vice-President ; D. E. Hyndman.
Welcome by the President; Emery Huse.
Election of Officers and Governors for 1942.
"Adventures of a Film Library;" R. Griffith, Museum of Modern
Art, New York, N. Y.
"Dynamic Screen — a Speculation;" R. W. Russell, New York, N. Y.
"Motion Picture Cant;" Barry Buchanan, New York, N. Y.
"Lots of How, a Little What?" Terry Ramsaye, Quigley Publishing
Co., New York, N. Y.
12:30 p.m. Informal Get-Together Luncheon; Emery Huse, Chairman.
Addresses by:
The Honorable Newbold Morris, President of the Council and Acting
Mayor of the City of New York.
Mr. Sol A. Rosenblatt, New York, N. Y.
Mr. Francis S. Harmon, Assistant to the President, Motion Picture
Producers & Distributors of America, Inc., New York, N. Y.
Mr. Claude Lee, Director of Public Relations, Paramount Pictures,
Inc., New York, N. Y.
2:00 p.m. General and 16-Mm Session, C. R. Keith, Chairman.
"Work Simplification — Essential to Defense;" A. H. Mogensen,
New York, N. Y
"Some Equipment Problems of the Direct 16-Mm Producer;" L.
Thompson, The Calvin Company, Kansas City, Mo.
"A Review of the Question of 16-Mm Emulsion Position;" Wm. H.
Offenhauser, Jr., Precision Film Laboratories, New York, N. Y.
"The How and Why of Army Training Films;" M. E. Gillette, Lt.
Col., Signal Corps, U. S. Army, Fort Monmouth, N. J.
8:00 p.m. General Session, Paul J. Larsen, Chairman.
"A New Dichroic Reflector and Its Application to Photocell Moni-
toring Systems;" G. L. Dimmick, RCA Manufacturing Co., Inc.,
Indianapolis, Ind.
"Color Television;" P. C. Goldmark, J. N. Dyer, E. R. Piore, and
J. M. Hollywood, Columbia Broadcasting System, Inc., New
York, N. Y.
* As actually followed at the sessions.
636
PROGRAM OF THE CONVENTION 637
"The IR System: An Optical Method for Increasing Depth of
Field;" Alfred N. Goldsmith, Consulting Engineer, New York,
N. Y.
"Mobile Television Equipment;" R. L. Campbell, R. E. Kessler,
R. E. Rutherford, and K. V. Landsberg, Allan B. DuMont Labora-
tories, Passaic, N. J.
TUESDAY, OCTOBER 21st
10:00 a.m. Projection Session, G. L. Dimmick, Chairman.
"A Constant-Torque Friction Clutch for Film Take-Up;" W.
Hotine, Rotovex Corp., Newark, N. J.
"Recent Developments hi Projection Mechanism Design;" E. L.
Boecking and L. W. Davee, Century Projector Corp., New York,
N. Y.
Report of the Studio Lighting Committee; R. Linderman, Chairman.
Report of the Standards Committee; D. B. Joy, Chairman.
Report of the Theater Engineering Committee; Alfred N. Gold-
smith, Chairman.
2:00 p.m. General Session, Arthur C. Downes, Chairman.
"Film Production for Education;" Floyde E. Brooker, Defense
Training, U. S. Office of Education, Washington, D. C.
"The Color Quality of Light on the Projection Screen;" M. R. Null,
W. W. Lozier, and D. B. Joy, National Carbon Co., Fostoria,
Ohio.
"New 13.6-Mm Carbons for Increased Screen Light;" M. T. Jones,
W. W. Lozier, and D. B. Joy, National Carbon Co., Fostoria,
Ohio.
"How Safe Are Safety Devices?" W. Scanlon, Larry Strong, Inc.,
Chicago,-Ill.
WEDNESDAY, OCTOBER 22nd
10:00 a.m. Laboratory and Business Session, D. E. Hyndman, Chairman.
"A New Electrostatic Ah- Cleaner and Its Application to the Motion
Picture Industry;" Henry Gitterman, Westinghouse Electric and
Manufacturing Corp., New York, N. Y.
"A Precision Direct-Reading Densitometer;" M. H. Sweet, Agfa
Ansco, Binghamton, N. Y.
Society Business.
"Iodide Analysis in an MQ Developer;" R. M. Evans, W. T. Han-
son, Jr., and P. K. Glasoe, Eastman Kodak Company, Rochester,
N.Y.
"Synthetic Aged Developer by Analysis;" by R. M. Evans, W. T.
Hanson, Jr., and P. K. Glasoe, Eastman Kodak Company,
Rochester, N. Y.
"Effect of Composition of Processing Solutions on Hypo Removal
from Motion Picture Film;" J. I. Crabtree, G. T. Eaton, and
L. E. Muehler. Eastman Kodak Company, Rochester, N. Y.
638 • PROGRAM OF THE CONVENTION
8:00 p.m. Fiftieth Semi- Annual Banquet and Dance.
Introduction of Officers-Elect for 1942.
Presentation of the SMPE Progress Medal.
Presentation of the SMPE Journal Award.
• Entertainment and Dancing.
THURSDAY, OCTOBER 23rd
10:00 a.m. Fine-Grain Film Symposium, John G. Frayne, Chairman.
"Production and Release Applications of Fine-Grain Films for
Variable-Density Sound-Recording;" C. R. Daily, Paramount
Pictures, Inc., Hollywood, Calif.
"Laboratory Modification and Procedure in Connection with Fine-
Grain Release Printing;" J. R. Wilkinson and F. L. Eich, Para-
mount Pictures, Inc., Hollywood, Calif.
"A Note on the Processing of Eastman 1302 Fine-Grain Release
Positive in Hollywood;" V. C. Shaner, Eastman Kodak Co.,
Hollywood, Calif.
"Stereophonic Sound and the Realistic-Width System;" R. G.
Camp, New Philadelphia, Ohio.
2:00 p.m. Sound Session, John A. Maurer, Chairman.
"A Frequency-Modulated Control -Track for Movietone Prints;"
J. G. Frayne and F. P. Herrnfeld, Electrical Research Products,
Inc., Hollywood, Calif.
"The Design and Use of Film-Noise Reduction Systems;" R. R. Sco-
ville and W. L. Bell, Electrical Research Products, Inc., Holly-
wood, Calif.
"A Feedback Light-Valve;" W. J. Albersheim and L. F. Brown,
Electrical Research Products, Inc., New York, N. Y.
"The Quarter-Wave Method of Speaker Testing;" S. L. Reiches,
The Brush Development Co., Cleveland, Ohio.
Adjournment of the Convention.
BOOK REVIEW
Acoustics. Alexander Wood, Interscience Publishers, Inc. (New York, N. Y.)
1941; 575 pp., 310 illustrations; $6.00.
This book is intended for students who desire a more detailed treatment of
acoustics than they can find in a book on general physics. A student who has
mastered this book should have an exceedingly good fundamental knowledge of
modern acoustics. The book is also valuable to the engineers who wish to form a
basis for specialization in any of the numerous applications of acoustics. The con-
tents by chapters are Wave Motion; Analytical Discussion of Wave Motion;
Forced Vibration; Resonators, Filters, and Horns; Dissipation of Energy of
Sound-Waves; Reflection of Sound-Waves ; Refraction of Sound- Waves; Super-
position or Interference; Diffraction; Measurement of the Velocity of Sound;
Vibrations of Strings; Organ Pipes; Intensity of Sound; Pitch and Frequency;
Analysis of Sound ; Rods, Membranes, and Plates; The Ear and Hearing; Re-
cording and Reproduction of Sound; Acoustics of Buildings; Name and Subject
Index.
As indicated by the contents, this book covers the entire gamut of both classical
and modern acoustics. This book is valuable and unique in the wide exposition of
the classical side of acoustics in a modern style. Due to the wide scope of subjects
covered in this book, the treatment is in certain instances quite brief. However,
this is compensated for by the inclusion of references to more comprehensive ex-
positions. The book is adequately and appropriately illustrated.
In summary: "Acoustics," by Wood, is a contribution to acoustic literature
which is of value and interest to the advanced student, applied physicist, and
acoustical engineer.
H. F. OLSON
SOCIETY SUPPLIES
The following are available from the General Office of the Society, at the prices
noted. Orders should be accompanied by remittances.
Aims and Accomplishments. — An index of the Transactions from October,
1916, to December, 1929, containing summaries of all articles, and author and
classified indexes. One dollar each.
Journal Index.— An index of the JOURNAL from January, 1930, to December,
1935, containing author and classified indexes. One dollar each.
Motion Picture Standards .—Reprints of the American Standards and Recom-
mended Practices as published in the March, 1941, issue of the JOURNAL; 50 cents
each.
Membership Certificates. — Engrossed, for framing, containing member's name,
grade of membership, and date of admission. One dollar each.
Journal Binders— Black fabrikoid binders, lettered in gold, holding a year's
issue of the JOURNAL. Two dollars each. Member's name and the volume
number lettered in gold upon the backbone at an additional charge of fifty cents
each.
Test- Films. —See advertisement in this issue of the JOURNAL.
SOCIETY ANNOUNCEMENTS
OFFICERS OF THE SOCIETY FOR 1942
The results of the recent election of Officers of the Society for 1942, are as
follows:
** Engineer ing Vice-P resident: DONALD E. HYNDMAN
** Financial Vice-President: ARTHUR S. DICKINSON
^Treasurer: GEORGE FRIEDL, JR.
* Secretary: PAUL J. LARSEN
**Governors: FRANK E. CARLSON
JOHN A. MAURER
EDWARD M. HONAN
Officers and Governors of the Society, whose terms do not expire until De-
cember 31, 1942, are as follows:
""President: EMERY HUSE
* Past-President: E. ALLAN WILLIFORD
* Executive Vice- President: HERBERT GRIFFIN
"•Editorial Vice-President: ARTHUR C. DOWNES
* ''Convention Vice-President: WILLIAM C. KUNZMANN
^Governors: MAX C. BATSEL
LOREN L. RYDER
The three remaining governors are the chairmen of the three local Sections.
Dr. Alfred N. Goldsmith* has been elected Chairman of the Atlantic Coast
Section and Dr. John G. Frayne, * re-elected Chairman of the Pacific Coast Section.
The results of the Mid-West Section elections will be announced as soon as they
are available.
Atlantic Coast Section
The results of the election of officers and managers of the Atlantic Coast Section
for 1942 are as follows:
* Chairman: ALFRED N. GOLDSMITH
* Past- Chairman: REEVE O. STROCK
* Secretary-Treasurer: HARRY B. CUTHBERTSON
** Managers: CECIL N. BATSEL
MERVIN W. PALMER
* Term expires December 31, 1942.
** Term expires December 31, 1943.
640
SOCIETY ANNOUNCEMENTS 641
EARL I. SPONABLE
*Managers: PETER C. GOLDMARK
H. E. WHITE
WM. H. OFFENHAUSER, JR.
Pacific Coast Section
The results of the recent election of officers and managers of the Pacific Coast
Section for 1942 are as follows:
* Chairman: JOHN G. FRAYNE
* Past- Chair man: LOREN L. RYDER
* Secretary-Treasurer: CHARLES W. HANDLEY
** Managers: HOLLIS W. MOYSE
RAY WILKINSON
JOHN HILLIARD
*Managers: BARTON KREUZER
SIDNEY P. SOLOW
The new Officers and Managers assume their positions on January 1, 1942.
PROGRESS AND JOURNAL AWARDS OF THE SOCIETY
It is a requirement each year after the presentation of the Progress Medal and
the Journal Award certificate at the Fall Convention of the Society to publish
in the JOURNAL a list of the names of all those who have thus far received these
awards. The list follows:
Progress Medal
1935 E. C. WENTB
1936 -C. E. K. MEES
1937 E. W. KELLOGG
1938 H. T. KALMUS
1939 L. A. JONES
1940 WALT DISNEY
1941 G. L. DIMMICK
Journal Award
1934 P. A. SNELL
1935 L. A. JONES and J. H. WEBB
1936 E. W. KELLOGG
1937 D. D. JUDD
1938 K. S. GIBSON
1939 H. T. KALMUS
1940 R. R. McMATH
1941 J. G. FRAYNE and V. PAGLIARULO
Term expires December 31, 1942.
Term expires December 31, 1943.
642
SOCIETY ANNOUNCEMENTS
ADMISSIONS COMMITTEE
At a recent meeting of the Admissions Committee, the following applicants for
membership were admitted into the Society in the Associate grade:
ALCORN, E. F.
3126 Peachtree Drive,
Atlanta, Ga.
COOK, R. O.
1956 Myra Avenue,
Los Angeles, Calif.
CREWS, R. F.
607 N. La Jolla,
Los Angeles, Calif.
DEMANN, J. L.
c/o Warner Bros. Projection Dept.,
Clark Building,
Pittsburgh, Pa.
DOWNING, WM.
Mills Novelty Co.,
4100 Fullerton Avenue,
Chicago, 111.
FRANKLIN, R. C.
Popular Science Publishing Co., Inc.
353 Fourth Avenue,
New York, N. Y.
GULLO, J.
93 West Ferry Street,
Buffalo, N. Y.
HlNSHAW, R. M.
Box 294,
Weiser, Idaho
JONES, WATSON
RCA Manufacturing Co., Inc.,
1016 N. Sycamore Avenue,
Hollywood, Calif.
LACKOFF, S. K.
Cineola Corporation,
152 West 42nd Street,
New York, N. Y.
LARGEN, F. C.
Creighton,
Nebraska
LARIME, L. H.
Jam Handy Picture Service, Inc.,
Detroit, Mich.
MOL, J. C.
Multinlm Batavia, N. V.,
Bidara Tjina 125,
Batavia, D. E. I.
OSTINELLI, L. U.
238a, High Street,
Uxbridge, Middlesex, England
REHM, L. H.
7327 Holly Court,
River Forest, 111.
ROBIN, H. L.
Panoram Soundies Connecticut Co.,
86 Meadow Street,
New Haven, Conn.
ROSSOMANDO, P. T.
Fitzer Amusement Co.,
218 W. Fayette Street,
Syracuse, N. Y.
SMITH, H. L.
P. O. Box 304,
Bound Brook, N. J.
TURNIPSEED, R., JR.
Reagan Visual Education Co.,
614-615 Rhodes Bldg.,
Atlanta, Ga.
In addition, the following applicants have been admitted to the Active grade :
KERNS, E. F.
Museum of Modern Art Film
Library,
11 West 53rd Street,
New York, N. Y.
SCHAFFERS, T. W. M.
Philips Export Corp.,
Hotel Roosevelt,
New York, N. Y.
JOURNAL
OF THE SOCIETY OF
MOTION PICTURE ENGINEERS
AUTHOR AND CLASSIFIED
INDEXES
VOLUME XXXVII
JULY-DECEMBER, 1941
AUTHOR INDEX, VOLUME XXXVII
JULY TO DECEMBER, 1941
Author
ALBERSHEIM, W. J.
(and MACKENZIE, D.)
ANDERSON, A. B.
(and R. BIDDULPH,
L. A. ELMER,
and WENTE, E. C.)
ANDERSON, L. J.
ARNOLD, J.
BAERWALD, H. G.
BAKER, J. O.
(and DREW, R. O.)
BIDDULPH, R.
(and ANDERSON, A. B.,
and ELMER, L. A., and
WENTE, E. C.)
BIDDULPH, R.
(and WENTE, E. C.)
BOWDITCH, F. T.
(and DULL, R. B.,
MACPHERSON, H. G.)
BURRIS-MEYER, H.
CHANDLER, J. S.
CHANON, H. J.
(and FALGE, F. M.)
DREW, R. O.
(and BAKER, J. O.)
DULL, R. B.
(and BOWDITCH, F. T.,
and MACPHERSON, H. G.)
ELMER, L. A.
(and ANDERSON, A. B.
BIDDULPH, R., and
WENTE, E. C.)
ELMER, L. A.
FALGE, F. M.
(and CHANON, H. J.)
644
Issue Page
Analysis of Sound-Film Drives Nov. 452
Mechanical and Optical Equipment for
the Stereophonic Sound-Film System Oct. 353
High-Fidelity Headphones Sept. 319
M-G-M's New Camera Boom Sept. 278
Analytic Treatment of Tracking Er-
ror and Notes on Optimal Pick-Up
Design Dec. 591
New and Old Aspects of the Origins of
96-Cycle Distortion Sept. 227
Mechanical and Optical Equipment for
the Stereophonic Sound-Film Sys-
tem Oct. 353
A Light- Valve for the Stereophonic
Sound-Film System Oct. 397
Characteristics of Intermittent Carbon
Arcs July 98
Development and Current Uses of the
Acoustic Envelope July 109
Some Theoretical Considerations in the
Design of Sprockets for Continuous
Film Movement Aug. 164
Black Light for Theater Auditoriums Aug. 197
New and Old Aspects of the Origins of
96-Cycle Distortion Sept. 227
Characteristics of Intermittent Carbon
Arcs July 98
Mechanical and Optical Equipment for
the Stereophonic Sound-Film Sys-
tem Oct. 353
A Non-Cinching Film Rewind Machine Oct. 418
Black Light for Theater Auditoriums Aug. 197
INDEX
645
Author
FLETCHER, H.
GARITY, W. E.
(and HAWKINS, J. N. A.)
GOLDSMITH, L. T.
(and LEVINSON, N.)
GOODMAN, A.
HANSON, H.
HAWKINS, J. N. A.
(and GARITY, W. E.)
HAYS, WILL H.
HILL, W. G.
(and SCHAEFER, C. L.)
HUSE, EMERY
JONES, F. L.
JONES, M. T.
(and JOY, D. B.,
and LOZIER, W. W.)
JOY, D. B.
(and JONES, M. T.,
and LOZIER, W. W.)
JOY, D. B.
(and LOZIER, W. W.,
and ZAFFARANO, D. J.)
KORNEI, O.
LAMBERT, K. B.
LESHING, M.
(and ROBINSON, B.)
LEVINSON, N.
(and GOLDSMITH, L. T.)
LOZIER, W. W.
(and JOY, D. B.,
and ZAFFARANO, D. J.)
LOZIER, W. W.
(and JOY, D. B.,
and JONES, M. T.)
MACKENZIE, D.
(and ALBERSHEIM, W. J.)
MACPHERSON, H. G.
(and DULL, R. B.,
and BOWDITCH, F. T.)
MACPHERSON, H. G.
Issue Page
The Stereophonic Sound-Film System
— General Theory Oct. 331
Fantasound Aug. 127
Vitasound Aug. 147
Factors Affecting Sound-Quality in Nov. 510
Theaters
Resume of an Extemporaneous Address Nov. 449
Fantasound Aug. 127
Salute to the SMPE July 5
A Method for Designing Film Sprock-
ets Aug. 177
Another Milestone July 7
Some Properties of Polished Glass Sur-
faces Sept. 256
A New 13.6-Mm High-Intensity Pro-
jector Carbon Nov. 539
A New 13.6-Mm High-Intensity Projec-
tor Carbon Nov. 539
Improved Methods of Controlling Car-
bon Arc Position Nov. 485
On the Playback Loss in the Reproduc-
tion of Phonograph Records Dec. 569
An Improved Mixer Potentiometer Sept. 283
New Gadgets for the Film Laboratory Sept. 274
Vitasound Aug. 147
Improved Methods of Controlling Car-
bon Arc Position Nov. 485
A New 13.6-Mm High-Intensity Pro-
jector Carbon Nov. 539
Analysis of Sound-Film Drives Nov. 452
Characteristics of Intermittent Carbon
Arcs July 98
A Suggested Clarification of Carbon
Arc Terminology as Applied to the
Motion Picture Industry Nov. 480
646
INDEX
Author
MERRIMAN, W. E.
(and WELLMAN, H. C.)
MILLER, W. C.
MORIN, E. R.
MULLER, A. H.
(and WENTE, E. C.)
NORLING, J. A.
PRATER, J. R.
REISKIND, H. I.
RICHARDSON, F. H.
ROBINSON, B.
(and LESHING, M.)
SEELEY, E. S.
SEFING, J. J.
SCHAEFER, C. L.
(and HILL, W. G.)
SNOW, W. B.
(and SOFFEL, A. R.)
SOFFEL, A. R.
(and SNOW, W. B.)
STEINBERG, J. C.
TASKER, H.
THAYER, W. L.
VICKERS, J. H.
WELLMAN, H. C.
(and MERRIMAN, W. E.)
WENTE, E. C.
(and ELMER, L. A.,
BIDDULPH, R., and
ANDERSON, A. B.)
WENTE, E. C.
(and BIDDULPH, R.)
WENTE, E. C.
(and MULLER, A. H.)
ZAFFARANO, D. J.
(and JOY, D. B.,
and LOZIER, W. W.)
Issue Page
Five New Models of 16-Mm Sound
Kodascope Sept. 313
Recent Improvements in Non-Reflec-
tive Lens Coating Sept. 265
Air-Conditioning Safety Device for
Theaters Sept. 307
Internally Damped Rollers Oct. 406
Progress in Three-Dimensional Pic-
tures Nov. 516
The Projection Booth — Its Location
and Contents Nov. 506
Multiple-Speaker Reproducing Sys-
tems for Motion Pictures Aug. 154
Twenty- Five Years of Service July 9
New Gadgets for the Film Laboratory Sept. 274
A Compact Direct-Reading Reverbera-
tion Meter Dec. 557
Projection Room Equipment Require-
ments Nov. 502
A Method for Designing Film Sprock-
ets Aug. 177
Electrical Equipment for the Stereo-
phonic Sound-Film System Oct. 380
Electrical Equipment for the Stereo-
phonic Sound-Film System Oct. 380
The Stereophonic Sound-Film System
— Pre- and Post-Equalization of
Compandor Systems Oct. 366
Improved Motor Drive for Self-Phas-
ing of Process Projection Equipment Aug. 187
Solving Acoustic and Noise Problems
Encountered in Recording for Mo-
tion Pictures Nov. 525
The Specialization of Film Delivery Dec. 623
Five New Models of 16-Mm Sound
Kodascope Sept. 313
Mechanical and Optical Equipment for
the Stereophonic Sound-Film Sys-
tem Oct. 353
A Light-Valve for the Stereophonic
Sound-Film System Oct. 397
Internally Damped Rollers Oct. 406
Improved Methods of Controlling Car-
bon Arc Position Nov. 485
CLASSIFIED INDEX, VOLUME XXXVH
JULY TO DECEMBER, 1941
Acoustics
Development and Current Uses of the Acoustic Envelope, H. Burris-Meyer,
No. 1 (July), p. 109.
Solving Acoustic and Noise Problems Encountered in Recording for Motion
Pictures, W. L. Thayer, No. 5 (Nov.), p. 525.
A Compact Direct-Reading Reverberation Meter, E. S. Seeley, No. 6 (Dec.), p.
557.
Air-Conditioning
Air-Conditioning Safety Device for Theaters, E. R. Morin, No. 3 (Sept.), p.
307.
Amendments, Constitution and By-Laws
Amendments to the By-Laws, No. 4 (Oct.), P- 443.
Anniversary of the Society
Twenty- Fifth Anniversary of the Society of Motion Picture Engineers, No. 1
(July), p. 3.
Salute to the SMPE, Will H. Hays, No. 1 (July), p. 5.
Another Milestone, Emery Huse, No. 1 (July), p. 7.
Twenty-Five Years of Service, F. H. Richardson, No. 1 (July), p. 9.
Apparatus
Improved Motor Drive for Self -Phasing of Process Projection Equipment,
H. Tasker, No. 2 (Aug.), p. 187.
New Gadgets for the Film Laboratory, B. Robinson and M. Leshing, No. 3
(Sept.), p. 274.
M-G-M's New Camera Boom, J. Arnold, No. 3 (Sept.), p. 278.
An Improved Mixer Potentiometer, K. B. Lambert, No. 3 (Sept.), p. 283.
Air-Conditioning Safety Device for Theaters, E. R. Morin, No. 3 (Sept.), p.
307.
Five New Models of 16-Mm Sound Kodascope, W. E. Merriman and H. C.
Wellman, No. 3 (Sept.), p. 313.
High Fidelity Headphones, L. J. Anderson, No. 3 (Sept.), p. 319.
Mechanical and Optical Equipment for the Stereophonic Sound-Film System,
E. C. Wente, R. Biddulph, L. A. Elmer, and A. B. Anderson, No. 4 (Oct.),
p. 353.
Electrical Equipment for the Stereophonic Sound-Film System, W. B. Snow
and A. R. Soffel, No. 4 (Oct.), p. 380.
A Light-Valve for the Stereophonic Sound-Film System, E. C. Wente and R.
Biddulph, No. 4 (Oct.), p. 397.
647
648 INDEX [j. s. M. P. E.
A Non-Cinching Film Rewind Machine, L. A. Elmer, No. 4 (Oct.), p. 418.
A New 13.6-Mm High-Intensity Projector Carbon, M. T. Jones, W. W. Lozier,
and D. B. Joy, No. 5 (Nov.), p. 539.
Arcs
Characteristics of Intermittent Carbon Arcs, F. T. Bowditch, R. B. Dull, and
H. G. MacPherson, No. 1 (July), p. 98.
A Suggested Clarification of Carbon Arc Terminology as Applied to the Motion
Picture Industry, H. G. MacPherson, No. 5 (Nov.), p. 480.
Improved Methods of Controlling Carbon Arc Position, D. J. Zaffarano,
W. W. Lozier, and D. B. Joy, No. 5 (Nov.), p. 485.
A new 13.6-Mm High-Intensity Projector Carbon, M. T. Jones, W. W. Lozier,
and D. B. Joy, No. 5 (Nov.), p. 539.
Cameras
M-G-M's New Camera Boom, J. Arnold, No. 3 (Sept.), p. 278.
Committee Reports
Non-Theatrical Equipment, No. 1 (July), p. 22.
Standards, No. 1 (July), p. 76; No. 5 (Nov.), p. 535.
Theater Engineering, No. 1 (July), p. 78.
Constitution and By-Laws
Amendments to the By-Laws, No. 4 (Oct.), p. 443.
Disk Reproduction
On the Playback Loss in the Reproduction of Phonograph Records, O. Kornei,
No. 6 (Dec.), p. 569.
Analytic Treatment of Tracking Error and Notes on Optimal Pick-Up De-
sign, H. G. Baerwald, No. 6 (Dec.), p. 591.
Distortion in Sound Reproduction
New and Old Aspects of the Origins of 96-Cycle Distortion, J. O. Baker and
R. O. Drew, No. 3 (Sept.), p. 227.
Educational Motion Pictures
(See also Non-Theatrical and Sixteen- Millimeter Motion Pictures.)
Recommended Procedure and Equipment Specifications for Educational
16-Mm Projection — A Report of the Committee on Non-Theatrical Equip-
ment, No. 1 (July), p. 22.
Emulsions
Recent Advances hi the Theory of the Photographic Process, C. E. K. Mees,
No. 1 (July), p. 10.
Fantasound
Fantasound, W. E. Garity and J. N. A. Hawkins, No. 2 (Aug.), P- 127.
Fluorescent Lamps in Theater Illumination
Black Light for Theater Auditoriums, H. J. Chanon and F. M. Falge, No. 2
(Aug.), p. 197.
Dec., 1941] INDEX 649
General
Twenty-Fifth Anniversary of the Society of Motion Picture Engineers, No. 1
(July), p. 3.
Salute to the SMPE, Will H. Hays, No. 1 (July), p. 5.
Another Milestone, Emery Huse, No. 1 (July), p. 7.
Twenty-Five Years of Service, F. H. Richardson, No. 1 (July), p. 9.
Recent Advances in the Theory of the Photographic Process, C. E. K. Mees,
No. 1 (July), p. 10.
Television Report, Order, Rules, and Regulations of the Federal Communica-
tions Commission, No. 1 (July), p. 87.
Fantasound, W. E. Garity and J. N. A. Hawkins, No. 2 (Aug.), p. 127.
Vitasound, N. Levinson and L. T. Goldsmith, No. 2 (Aug.), p. 147.
Multiple-Speaker Reproducing Systems for Motion Pictures, H. I. Reiskind,
No. 2 (Aug.), p. 154.
Black Light for Theater Auditoriums, H. J. Chanon and F. M. Falge, No. 2
(Aug.), p. 197.
Report on the Activities of the Inter-Society Color Council, No. 3 (Sept.), p.
292.
Resume of an Extemporaneous Address by H. Hanson, No. 5 (Nov.), p. 449.
Progress in Three-Dimensional Pictures, J. A. Norling, No. 5 (Nov.), p. 516.
Solving Acoustic and Noise Problems Encountered in Recording for Motion
Pictures, W. L. Thayer, No. 5 (Nov.), p. 525.
Proceedings of the Fiftieth Semi-Annual Banquet, Hotel Pennsylvania, New
York, N. Y., October 22, 1941, No. 6 (Dec.), p. 547.
The Specialization of Film Delivery, J. H. Vickers, No. 6 (Dec.), p. 623.
Headphones
High-Fidelity Headphones, L. J. Anderson, No. 3 (Sept.), p. 319.
Historical
Twenty-Fifth Anniversary of the Society of Motion Picture Engineers, No. 1
(July), p. 3.
Salute to the SMPE, Will H. Hays, No. 1 (July), p. 5.
Another Milestone, Emery Huse, No. 1 (July), p. 7.
Twenty-Five Years of Service, F. H. Richardson, No. 1 (July), p. 9.
Illumination in Projection
Report of the Theater Engineering Committee, No. 1 (July), p. 78.
Characteristics of Intermittent Carbon Arcs, F. T. Bowditch, R. B. Dull.
and H. G. MacPherson, No. 1 (July), p. 98.
A Suggested Clarification of Carbon Arc Terminology as Applied to the Motion
Picture Industry, H. G. MacPherson, No. 5 (Nov.), p. 480.
Improved Methods of Controlling Carbon Arc Position, D. J. Zaffarano, W. W.
Lozier, and D. B. Joy, No. 5 (Nov.), p. 485.
A New 13.6-Mm High-Intensity Projector Carbon, M. T. Jones, W. W. Lozier,
and D. B. Joy, No. 5 (Nov.), p. 539.
Illumination in Theaters
Black Light for Theater Auditoriums, H. J. Chanon and F. M. Falge, No. 2
(Aug.), p. 197.
650 INDEX [j. s. M. p. E.
Index
Author: No. 6 (Dec.), P. 644.
Classified: No. 6 (Dec.), p. 647.
Instruments
Report of the Theater Engineering Committee, No. 1 (July), p. 78.
A Compact Direct-Reading Reverberation Meter, E. S. Seeley, No. 6 (Dec.), p.
557.
Inter-Society Color Council
Report on the Activities of the Inter-Society Color Council, No. 3 (Sept.), p.
292.
Journal Award
Proceedings of the Fiftieth Semi-Annual Banquet, Hotel Pennsylvania, New
York, N. Y., October 22, 1941, No. 6 (Dec.), p. 551.
Laboratory Apparatus
New Gadgets for the Film Laboratory, B. Robinson and M. Leshing, No. 3
(Sept.), p. 274.
Lenses
Some Properties of Polished Glass Surfaces, F. L. Jones, No. 3 (Sept.), p. 256.
Recent Improvements in Non-Reflective Lens Coating, W. C. Miller, No. 3
(Sept.), p. 265.
Light-Valves
A Light-Valve for the Stereophonic Sound-Film System, E. C. Wente and
R. Biddulph, No. 4 (Oct.), p. 397.
Multi-Speaker Systems
Multiple-Speaker Reproducing Systems for Motion Pictures, H. I. Reiskind,
No. 2 (Aug.), p. 154.
Non-Theatrical Equipment
Recommended Procedure and Equipment Specifications for Educational 16-Mm
Projection — A Report of the Committee on Non-Theatrical Equipment,
No. 1 (July), p. 22.
Five New Models of 16-Mm Sound Kodascope, W. E. Merriman and H. C.
Wellman, No. 3 (Sept.), p. 313.
Optics
Some Properties of Polished Glass Surfaces, F. L. Jones, No. 3 (Sept.), p. 256.
Recent Improvements in Non-Reflective Lens Coating, W. C. Miller, No. 3
(Sept.), p. 265.
Photography
Recent Advances in the Theory of the Photographic Process, C. E. K. Mees,
No. 1 (July), p. 10.
Process Projection
Improved Motor Drive for Self -Phasing of Process Projection Equipment.
H. Tasker, No. 2 (Aug.), p. 187.
Dec., 1941] INDEX 651
Progress
Recent Advances in the Theory of the Photographic Process, C. E. K. Mees,
No. 1 (July), p. 10.
Progress in Three-Dimensional Pictures, J. A. Norling, No. 5 (Nov.), p. 516.
Progress Award
Proceedings of the Fiftieth Semi-Annual Banquet, Hotel Pennsylvania, New
York, N. Y., October 22, 1941, No. 6 (Dec.), p. 548.
Projection, General Information
Recommended Procedure and Equipment Specifications for Educational 16-Mm
Projection — A Report of the Committee on Non-Theatrical Equipment, No. 1
(July), p. 22.
Report of the Theater Engineering Committee, No. 1 (July), p. 78.
Projection Room Equipment Requirements, J. J. Sefing, No. 5 (Nov.), p. 502.
The Projection Room — Its Location and Contents, J. R. Prater, No. 5 (Nov.),
p. 506.
Factors Affecting Sound-Quality in Theaters, A. Goodman, No. 5 (Nov.), p.
510.
Projection Practice
Report of the Theater Engineering Committee, No. 1 (July), p. 78.
Projection Room Equipment Requirements, J. J. Sefing, No. 5 (Nov.), p. 502.
The Projection Room — Its Location and Contents, J. R. Prater, No. 5 (Nov.),
p. 506.
Projection Screens
Report of the Theater Engineering Committee, No. 1 (July), p. 78.
Projectors
Recommended Procedure and Equipment Specifications for Educational 16-
Mm Projection — A Report of the Committee on Non-Theatrical Equipment,
No. 1 (July), p. 22.
Rewinds
A Non-Cinching Film Rewind Machine, L. A. Elmer, No. 4 (Oct.), p. 418.
Screen Brightness
Report of the Theater Engineering Committee, No. 1 (July), p. 78.
Seating, in Theaters
Report of the Theater Engineering Committee, No. 1 (July), p. 78.
Sixteen-Mm Motion Pictures
Recommended Procedure and Equipment Specifications for Educational 16-
Mm Projection — A Report of the Committee on Non-Theatrical Equipment,
No. 1 (July), p. 22.
Five New Models of 16-Mm Sound Kodascope, W. E. Merriman and H. C.
Wellman, No. 3 (Sept.), p. 313.
Sound Recording and Reproduction
Fantasound, W. E. Garity and J. N. A. Hawkins, No. 2 (Aug.), p. 127.
652 INDEX [j. s. M. P. E.
Vitasound, N. Levinson and L. T. Goldsmith, No. 2 (Aug.), p. 147.
Multiple-Speaker Reproducing Systems for Motion Pictures, H. I. Reiskind,
No. 2 (Aug.), P. 154.
New and Old Aspects of the Origins of 96-Cycle Distortion, J. O. Baker and
R. O. Drew, No. 3 (Sept.), p. 227.
An Improved Mixer Potentiometer, K. B, Lambert, No. 3 (Sept.), p. 283.
High Fidelity Headphones, L. J. Anderson, No. 3 (Sept.), p. 319.
The Stereophonic Sound-Film System — General Theory, H. Fletcher, No. 4
(Oct.), p. 331.
Mechanical and Optical Equipment for the Stereophonic Sound-Film System,
E. C. Wente, R. Biddulph, L. A. Elmer, and A. B. Anderson, No.-4 (Oct.),
p. 353.
The Stereophonic Sound-Film System — Pre- and Post-Equalization of Com-
pandor Systems, J. C. Steinberg, No. 4 (Oct.), p. 366.
Electrical Equipment for the Stereophonic Sound-Film System, W. B. Snow
and A. R. Soffel, No. 4 (Oct.), p. 380.
A Light- Valve for the Stereophonic Sound-Film System, E. C. Wente and R.
Biddulph, No. 4 (Oct.), p. 397.
Internally Damped Rollers, E. C. Wente and A. H. Muller, No. 4 (Oct.), p. 406.
Analysis of Sound-Film Drives, W. J. Albersheim and D. MacKenzie, No. 5
(Nov.), p. 452.
Factors Affecting Sound-Quality in Theaters, A. Goodman, No. 5 (Nov.), p.
510.
Solving Acoustic and Noise Problems Encountered in Recording for Motion
Pictures, W. L. Thayer, No. 5 (Nov.), p. 525.
On the Playback Loss in the Reproduction of Phonograph Records, O. Kornei,
No. 6 (Dec.), p. 569.
Analytic Treatment of Tracking Error and Notes on Optimal Pick-Up Design,
H. G. Baerwald, No. 6 (Dec.), p. 591.
Sprockets
A Method for Designing Film Sprockets, W. G. Hill and C. L. Schaefer, No. 2
(Aug.), p. 177.
Some Theoretical Considerations in the Design of Sprockets for Continuous
Film Movement, J. S. Chandler, No. 2 (Aug.), p. 164.
Standardization
Recommended Procedure and Equipment Specifications for Educational
16-Mm Projection — A Report of the Committee on Non- Theatrical Equip-
ment, No. 1 (July), p. 22.
Report of the Standards Committee, No. 1 (July), p. 76.
Report of the Standards Committee, No. 5 (Nov.), p. 535.
Stereoscopic Motion Pictures
Progress in Three-Dimensional Pictures, J. A. Norling, No. 5 (Nov.), p. 516.
Stereophonic Sound Reproduction
The Stereophonic Sound-Film System — General Theory, H. Fletcher, No. 4
(Oct.), p. 331.
Dec., 1941] INDEX 653
Mechanical and Optical Equipment for the Stereophonic Sound-Film System,
E. C. Wente, R. Biddulph, L. A. Elmer, and A. B. Anderson, No. 4 (Oct.), p.
353.
The Stereophonic Sound-Film System — Pre- and Post-Equalization of Com-
pandor Systems, J. C. Steinberg, No. 4 (Oct.), p. 366.
Electrical Equipment for the Stereophonic Sound-Film System, W. B. Snow
and A. R. Soffel, No. 4 (Oct.), p. 380.
A Light- Valve for the Stereophonic Sound-Film System, E. C. Wente and R.
Biddulph, No. 4 (Oct.), p. 397.
Studio Equipment
Improved Motor Drive for Self-Phasing of Process Projection Equipment,
H. Tasker, No. 2 (Aug.), p. 187.
M-G-M's New Camera Boom, J. Arnold, No. 3 (Sept.), p. 278.
Studio Practice
Solving Acoustic and Noise Problems Encountered in Recording for Motion
Pictures, W. L. Thayer, No. 5 (Nov.), p. 525.
Television
Television Report, Order, Rules, and Regulations of the Federal Communica-
tions Commission, No. 1 (July), p. 87.
Theater Design
Report of the Theater Engineering Committee, No. 1 (July), p. 78.
Theater Equipment
Air-Conditioning Safety Device for Theaters, E. R. Morin, No. 3 (Sept.), P
307.
Vitasound
Vitasound. N. Levinson and L. T. Goldsmith, No. 2 (Aug.), p. 147.
S. M. P. E. TEST-FILMS
These films have been prepared under the supervision of the Projection
Practice Committee of the Society of Motion Picture Engineers, and are
designed to be used in theaters, review rooms, exchanges, laboratories,
factories, and the like for testing the performance of projectora.
Only complete reels, as described below, are available (no short sections
or single frequencies). The prices given include shipping charges to all
points within the United States; shipping charges to other countries are
additional.
35-Mm. Visual Film
Approximately 500 feet long, consisting of special targets with the aid
of which travel-ghost, marginal and radial lens aberrations, definition,
picture jump, and film weave may be detected and corrected.
Price $37.50 each.
16-Mm. Sound-Film
Approximately 400 feet long, consisting of recordings of several speak-
ing voices, piano, and orchestra; buzz-track; fixed frequencies for focus-
ing sound optical system; fixed frequencies at constant level, for de-
termining reproducer characteristics, frequency range, flutter, sound-
track adjustment, 60- or 96-cycle modulation, etc.
The recorded frequency range of the voice and music extends to 6000
cps.; the constant-amplitude frequencies are in 11 steps from 50 cps. to
6000 cps.
Price $25.00 each.
16-Mm. Visual Film
An optical reduction of the 35-mm. visual test-film, identical as to
contents and approximately 225 feet long.
Price $25.00 each.
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