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Full text of "Journal of the Society of Motion Picture Engineers"

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. 
/.*Cirs 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 theaters 1 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 demonstrated 2 
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 Tuttle 3 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 Y 4 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 2 x /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 "J ust 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. 

3 TuTTLE, 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 y c 



EB&IE 




200%, 



100% 



30 20 10 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 

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 2 l /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 2 x /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 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, 
37Y 2 X 52 inches. Height of projection lens, 55 l /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 2 x /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 4700K. 

(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 
y 2 o 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 



= i s i ,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 
155F. 

(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 80F. 

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- 
r acturer 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. 



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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 



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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. 



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* C. P. = Construction Permit. 



96 



TELEVISION REPORT 



[J. S. M. p. E. 



Licensee and Location 


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sion of Hughes Tool Co., 




(Channel No. 2) 






C. 


P. 


Los Angeles, Calif. 














Hughes Productions Divi- 


W6XHT 


60,000-66,000 


10 


kw 


10 


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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 


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Inc., New York, N. Y. 




(Channel No. 8) 






C. 


P. 


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1 


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Company, Inc., Wash- 




(Channel No. 2) 






C. 


P. 


ington, D. C. 














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W3XPP 


102,000-108,000 


1 


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1 


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Company, Inc., Phila- 




(Channel No. 7) 






C. 


P. 


delphia, Pa. 














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78,000-84,000 


1 


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1 


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Inc. (Area of Los An- 




(Channel No. 4) 






C. 


P. 


geles, Calif.) 














WCAU Braodcasting Co., 


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84,000-90,000 


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SCHEDULE C 


EXPERIMENTAL TELEVISION BROADCAST STATIONS 




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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 


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1 


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System, Inc., Los An- 




(Channel No. 8) 


condl. 


C. 


P. 


geles, Calif. 














Farnsworth Television & 




66,000-72,000 


1 


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1 


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Radio Corp., Ft. Wayne, 




(Channel No. 3) 






C. 


P. 


Ind. 














General Electric Com- 


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60,000-86,000 


10 


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3 


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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 



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C. P. 
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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 R 3 . This condenser 
is connected in the grid circuit of a mercury-vapor thyratron T 2 with 
polarity such that the condenser voltage opposes that of the negative 
bias battery BI, reducing the negative grid potential of T 2 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 T 4 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. 



T 2 , 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 T 3 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 T 2 reaches a 
specific minimum value. For instance, the secondary voltage of the 
transformer TR, the magnitude of the resistor R 3 , 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 
T 2 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 R 3 constitute the essential timing cir- 
cuit as before, but now operating to raise the plate voltage of thyra- 
tron T 3 to its tripping point, so that the resulting discharge through 
the resistor R may trip T 2 . The tripping of thyratron T 3 , however, 
is also dependent upon the grid voltage pulse received each positive 
half -cycle through the transformer TR 2 . 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 T 3 
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 T 3 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 T 2 and ignitron T 4 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 /i 9 2 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 R 2 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 

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 



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19 

20 
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24 



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29 
32 



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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 
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. 



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 



i a 
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 I 1 /* 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 I 1 / \ 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. Mueller 1 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 
o f 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 systems 1 - 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- 
tice 6 the screen subtends an angle of about I6 l / 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 

o P C 
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 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 
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 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 

= 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 = 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 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 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 
y 2 4 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 = 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 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.M0.480 





SPROCKET TOOTH 

A 

TAN."0.2&/ 

FORCE OF TOOTH 
AGAmSTF.LMxO.995F ^/ \ Q ^ Qf 

TOOTH FRICTION 
FORCE 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 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 




6 
.00183 
.00008 


1. 



12 
00726 
. 00068 


1 



18 

.01625 
.00229 


1. 
0, 


24 
02863 
00539 


30 
1.04420 
0.01048 


36 
1.06272 
0.01797 


0,deg 
x 

y 


i 




42 
.08388 
.02831 


1 



48 
. 10732 
.04189 


1 




54 
. 13269 
.05902 


1, 



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 (DiG). 
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 S B 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 S c 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 (D l + 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, (DP)/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 S c 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 
S E 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 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 

S Ct per cent shrinkage accommodated = - X 100 = - ^T^ " 



100 - 1.23 



/ T\ Y _ w") _ (p Y _ T} 
S E , 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 




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 








T t0 













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 











0.5 








3130 


3 


2 





4 


5 


3 


3340 


42 


48 


5 


31 


40 


44 


3650* 


65 


82 


27 


59 


72 


80 


4050 





70 





1 


18 


14 


4358 





37 














5461 




















5770 




















5961 




















6908 





2 








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 

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New Motion Picture Apparatus 

Five New Models of 16-Mm Sound Kodascope 

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High Fidelity Headphones L. J. ANDERSON 319 

1941 Fall Convention at New York, October 20th-23rd 324 

Society Announcements 328 



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**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 recorder 1 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 printer 2 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-head 3 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. Crabtree 4 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 Pagliarulo 5 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 Herriott 6 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 recorder 1 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 printer 2 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-head 3 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. Crabtree 4 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 Pagliarulo 5 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 Herriott 6 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 





-60 


-44 


-60 







0.46 


1000 


-60 


-39.5 


-60 _ 


-2.5 




0.46 





-61 


-38.8 


-61 







0.61 


1000 


-61 


-38.4 


-61 


-5 




0.61 





-62 


-41 


-62 





Single 0.37 


0.08 


1000 


-64.5 


-64.5 


-64.5 


-7.8 




0.08 





-64.5 


-64.5 


-64.5 







0.38 


1000 


-64.5 


-64.5 


-64.5 


-2.4 




0.38 





-65 


-65 


-65 







0.56 


1000 


-66 


-66 


-66 


-4.2 




0.56 





-68 


-68 


-68 





Double 0.60 


0.20 


1000 


-65 


-45 


-65 


-3 




0.20 





-65 


-43 


-65 






0.56 


1000 


-65 


-45 


-65 


-2 




0.56 





-64.5 


-44 


-64.5 






0.78 


1000 


-66 


-45 


-66 


A 




0.78 





-67 


-46.8 


-67 




Single . 60 


0.18 


1000 


-70.3 


-70.3 


-70.3 


-3.2 




0.18 





-68 


-68 


-68 


. . . 




0.53 


1000 


-70.3 


-70.3 


-70.3 


-1.8 




0.53 





-70.3 


-70.3 


-70.3 






0.76 


1000 


-70.3 


-70.3 


-70.3 


-4.2 




0.76 





-74 


-74 


74 




Double 0.75 


0.26 


1000 


-63 


-44.4 


-63 


-2.0 




0.26 





-61 


-41.4 


-61 






0.66 


1000 


-64.4 


-46 


-64.4 


-1.3 




0.66 





-68 


-44 


-68 






0.92 


1000 


-68 


-46 


-68 


-3.0 




0.92 





-69 


-46 


-69 






1.15 


1000 


-68 


-47 


-68 


-8.0 




1.15 





-74 


-50.3 


-74 




Single 0.75 


0.26 


1000 


-68 


-68 


-68 


-1.0 




0.26 





-70 


-70 


-70 







0.64 


1000 


-71 


-71 


-71 


-1.0 




0.64 





-74 


-74 


-74 







0.86 


1000 


-71 


-71 


-71 


-2.2 




0.86 





-74 


-74 


-74 







1.09 


1000 


-74 


-74 


-74 


-7.2 




1.09 





-80 


-80 


-80 






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 65F. 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 68F. 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 

-66 
-76.4 
-70.5 
-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 65F. 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 




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 







0.64 


B&H 


Double 


-60 


-40 


-60 















0.60 


Non-Slip 


Double 


-62 


39-50 


-62 















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 





0.02 


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 





0.02 


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 





0.02 


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 





0.02 


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 





0.02 


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 





0.02 


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 





0.02 


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 






. 




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oj 

W.J? 

co'C 

rH CX 

o 






-M 

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> 








Pif/Nr C 


AMMA- 2.f*> 










U,r 
-4CYC1-E 0/Uo-PASS Flt-T 


<*HA Yiea 








; o ;;:::: 


&^ LEVEL 






-v 


6 **t"' *' Pe '^^+^~ 


$ ^^ 


^ 




f ' 












" fl " i r 








1 t 


x 






^j /Yf<f/inrf+ PX/NT spfretrrr-noi-E our of 4 


^/JrxMTT-o^ 




i 'i . A/CC o-/ 








* 


5 


X 


X.3 








< 

















-7.J3 





















i 












i 












e 










V 


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 





-68 


-68 


-68 


0.60 


0.76 





-74 


-74 


-74 


0.75 


1.09 





-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 





-68 


-58 


-68 


0.60 


0.76 





-68 


-58 


-68 


0.75 


1.09 





-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 tree 6 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 Strong 1 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 Blodgett 2 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 literature 1 ' 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 V 4 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 Y 4 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 paper 5 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 3 A~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 
Y 4 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 COLORS 1 
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 TOLERANCE 2 
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 COLORS 3 

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 
BACKGROUND 4 

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 

DIAGRAM 5 

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 STANDARDS 6 
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 TOLERANCES 7 
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 SPECTROPHOTOMETERS 8 

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 SPECTROPHOTOMETERS 9 

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 INDUSTRIES 10 

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 PULPS 11 " 
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 FURNISHES 12 

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 PAPER 13 

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 PAPERS 14 

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 R or R . t s 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 PAPER 15 
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 COLOR 16 

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 VISION 17 
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 DETERMINATION 18 
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 T c (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-RELATION 19 
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 DISCRIMINATION 20 
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) 






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 5 1 /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 2 3 /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 M L and R L 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 
R 2 , in which Mi is very large and R 2 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 
M D and C D are chosen so that their resonance occurs at about 800 cycles. The 
compliance C 2 back of the diaphragm is then made small enough to produce a 
resonance peak at the highest frequency desired. C 3 and M z 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 M 2 Ri 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 

HOTEL PENNSYLVANIA 
NEW YORK^ N. Y. 

APPLICATION FOR MEMBERSHIP 
APPLICANT'S RECORD 

\ 

Name Age 

Mailing Address 

Present Occupation 



Employer 

A complete account of the applicant's qualifications and accomplishments is 
required before an application may be submitted to the Board of Governors. 
The applicant should describe any inventions and improvements he has made 
in the art, as these are considered of more importance than a mere record of 
experience or the names of positions the applicant has filled. 



Education . 



Record of Accomplishments. 



Motion Picture Experience. 



Grade Applied For 

(Active or Associate) 

REFERENCES 

1. 3. 



2. 



The undersigned certifies that the above statements are correct, and agrees, 
if elected to membership, that he will be governed by the Society's Constitution 
and By-Laws so long as his connection with the Society continues. 



Date 19. .. Signed -. 

( Use a separate sheet of paper for complete record of accomplishments} 



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 papers 1 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 Seacord 3 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. * 

Hoth 4 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 Mueller 6 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 

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O O o O O C 


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LIGHT MODULATION^, 


x^ 












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** 


'-. 




X 


X 










x 


X 


k ^ 


AMPLIFIER 




X 


x 






iff 


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SOUND LEVEL IN DECIBELS 

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NC 


PLIFIER GAIN 
AND 
ISE LEVEL 


2^ 


X^ 







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. 



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ru U) . LJ 

C 


A 


LIGHT MODULATION v 


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,.^5^ AMPLIFIER GAIN 
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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 




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"\AMPLIFIER 
\ GAIN 


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/MODULATION 




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SOUND LEVEL IN DECIBELS 



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AMPLIFIER GAIN 
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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 







\\ 


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 



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 S 2 , 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 


n 














*- 






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. L 2 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, 




r 0.7 -0.6 -0.5 -0.4 -O.3 -0.2 -O.I 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 



6 

z 



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 t j ty o f n^fa f rom an ex citer 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 

i 50 

1 

j 40 
' 30 

I 

20 
1 10 

1 

- o 

-10 
-20 

-30 
1C 






























^ ^N 


s 


-'- 






--^, 


4- 


^_J^ 


















MAXIMUM PEAK LEVELS (ORCHESTRA) 


\ 


































^_ 




~~~ 


-' 


^ 


































































































^_^ 


REPRO 


DUCED 

1 1 


FILM NOISE LE 
111 


VELS 














*==^ 


1 _ 


m\ 
















ft 


UDIEN 


CE 


NOISE L 

I | 


EV 


:L5 


^^ 


^ 


































^N 


^ 


"V 


^ 


































" 


v. 


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 tests 3 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 C 


! 


























"-s, 
























^ 


/ 


^ 






5 
















^--- 


^^ 










\, 


"x 
























































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- 






x^ 




1 , 




~-^. 




















-^7 


^ - 


, ^ 






., 




<^ 


/ 


\ 


^^, 




*.- 




' 


*^' 


.-- 


,- 


" 











100 500 1000 5000 10000 

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 y 4 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 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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500 1000 5000 100< 



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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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. 



LJ 

1/1 1 5 


EQUALIZER NO. 1 PLUS 
30 DB COMPANDOR 


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EQUALIZER NO 2 PLUS 30 DB 
SELF ACTING 2:1 COMPANDOR 




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 cm 2 . 



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 A 12 to A u of Fig. 2. The pair of 
power amplifiers A u 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 demonstration 5 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 papers 1 - 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 P 3 
to the three modulator inputs. The modulators 1 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 
7V 3 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 A Q 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 
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 amplifiers 4 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 R s , if the impedance of the valve is Z v , and if the 
emf of the source is E'e jwt , the circuit diagram for the system is 
analytically equivalent to that shown in Fig. 1, where E is equal to 
E r R 5 /(Ri + R 5 ) and R is equal to RiR s /(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 



Ee jwt, 




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 w 2 where o> 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 co 2 x = T * 



If R v 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 = Ee jwi , 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 co , 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~ 9 k 2 /2(R + R^m. Substituting this value in equation 3, 
and setting 77 = co/co , we obtain the desired solution : 



IQ(R + 



- 01 



The greatest uniformity of response is obtained when co 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 co 
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 
co 2 = co 2 2 A 2 . The ratio of peak response to the response at oo = 
is, therefore, 




This ratio thus depends solely upon A/co . 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/co as large as is 
practically possible. The limit to which co 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 co 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 co 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/r 2 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 + Q 2 x (1 - kx) 2 

When x is small, this expression approaches unity, and when x is 
large it approaches l/k 2 x 2 . 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 co 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 



= and 



= 



d<2 2 

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. 



Q z = 



(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 w 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 co , 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 = and A 
dx 







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 co , 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_ 

k z x 



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, Q c 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 co 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 Q c . For instance, if Q is made equal to Q c /\/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|(R -/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) 




W : 



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 W Q 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 




M r 



R 
W, R 



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, W 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 Ri 2 - R 
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 
R Q = 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. W 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 



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. 



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 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. IIt 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 







4 TOTAL REPRO. FLUTTER - A.M.PA.S. , 1938 




o^ 
















UJ 


^<L 1 














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 
F r = l/(2-jr\/l/LC). Arrayed in a properly terminated filter they 
pass freely all frequencies below, and attenuate those above 2 F r . 
The attenuation, increasing rapidly at first, asymptotically approaches 




FRICTION 
BETWEEN 
FILM AND 
/ DRUM 

REFERENCE / FILM 

VELOCITY / COMPLIANCE 



SPROCKET 

TEETH 
VELOCITY 



J vW- 




FIG. 4. Filtered sprocket drum. 



a straight line through zero attenuation at F r 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, Zi m 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) d P/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 R c = 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/F r - 

FIG. 9. Characteristics of resistance-terminated niters. 
(7) Ideal L. P. filter section. (2) Peakless damping R = 
R c = V2L/C. (3) Underdamped R = OAR C . (4) Over- 
damped R = 2.5R C . 

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/F r - - 

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 R b 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 experiments 4 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, 



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 v z , 
then the transmission factor is 

R + 



F= v - 2 = _ _ , with co = 2ff (1) 



The absolute value of F is 






with 

n = Ci/co 2 (frequency factor) (5) 

and 

r = R 2 Ci/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 F P I = r l - 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 C 2 (dotted in Fig. 4) the transmission factor becomes 

C,) 



1 - /do; 2 + ;*(Ci + C 2 - 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 + 2C 2 



+ 2C 2 






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 n p . 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< r c (25) 

the transmission has a peak 

1 4J" 2 

Fp = r - 0.25r 2 = 4R*CJ - R*C Z 

which it reaches at the frequency 



r > r c (28) 



This curve is flat at low frequencies and droops smoothly near resonance fre- 
quency as shown in Curve 2, Fig. 9. 
For 

r > r c (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 

n p = 1 (33) 

and equals 

F p = 1 (34} 

At low frequencies F approaches 

| Fo | = RCu (35) 

which increases 6 db per octave. 
At high frequencies F approaches 

| F m | = (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 

F = 1 (43} 

At high frequencies it approaches 



j- 
kn 



The attenuation increases 12 db per octave. 
At the frequency 



2J S + 2J m 

(45) 



1 + k 2J + J m 

the second term on the right side of (39) vanishes, leaving F P independent of r 
and equal to 

F p = - 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 = n p . One finds for the "optimum 
drive-side" resistance r d the equation 

2(n p - 1) - V i^A* = (47) 

w p + r d 

and 

2fe 27, 

fd = = 



7? , _ 2J s J m 2 

' 



(6) Admittance to load-side disturbances: The impedance is 

v 1 T . Rjmjco 1 , , 

Z = -f^- + J&* + PIT' = "BY^ 7 " ^ 

Cju R + / m ja; rCjw 

At low frequencies the admittance approaches 

G = 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 n p defined by (45). 
The "common" value of admittance is 

Gc = _ j\ z = V%C(J m + 2J s )/J m (55) 

It is the "peak admittance" if one adjusts r to make the value of (55) an extreme 
for n = n p . One finds for this "optimum load-side" resistance r L the equation: 

, 1 _ p(l - W) 
an 



Wp tip ~\~ 

and 



(56) 



2 1 + 3fe _ 2/ + 2J 8 J m + 47, 

" 1 + * ' 3 + k " J m + 2J, ' 3J m + 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 

=-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 [6a 2 / - 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 ^-| 

[_4R 2 2R 2 D 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 3 1 /* 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 



j r ov., 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 /i 6 -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- 
r ul 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 published 2 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 



r v ' 



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\ 






L Snou-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 y 4 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 2 l / 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- 
glyphs 2 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 method 3 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 l l /% 
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