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From the collection of the 

7 n 
_ z _ m 

o Prelinger 
v JJibrary 

San Francisco, California 

Lull J8, 



Its Methods and Uses 



Broadcast Consultant; Member Institute of Radio Engineers; 
Author, "Using Radio in Sales Promotion" 






All rights reserved. This book, or 

parts thereof, may not be reproduced 

in any form without permission of 

the publishers. 



The author of a book on television and in fact any 
book describing a technical science approaching com- 
mercialization has his choice of several points of view. 
He may disregard the present imperfect status of the 
science by simply not mentioning or totally disregard- 
ing existing shortcomings and problems; he may 
smother existing difficulties with rosy predictions and 
expression of enthusiasm; or he may treat them with 
the utmost frankness believing their conquest can 
come only through accurate understanding and com- 
prehending research. 

The present author has chosen the latter course, 
perhaps in reaction to the overabundance of optimistic 
treatments of television. He feels that a conservative 
attitude is particularly helpful at this time, because 
television has been treated to an excess of premature 
and unwarrantedly hopeful publicity. The author, of 
course, realizes that an exacting analysis of television 
as it exists today may be significantly altered by a 
development of tomorrow. 

Even as he examines the proofs of this book, the 
publisher inquires whether an invention, just 
announced with considerable gusto and rewarded by 
tremendous publicity, has not indeed made all the 
conclusions therein hopelessly obsolete. But this 
announcement, like so many of its predecessors, is 
accompanied neither by technical proof nor by open 
demonstration. I have the satisfaction of knowing 
that readers of this book (if any) will be able to place 
an accurate valuation upon any announced invention 


based entirely upon its potential contribution to the 
progress of television. 

My purpose in writing this book has been to develop 
a clear understanding of how existing television sys- 
tems work, the basic processes involved in any tele- 
vision system, the standards of performance essential 
to the rendition of a commercial service, the limitations 
of certain features of existing methods standing in the 
way of the attainment of commercial performance 
standards, and the nature of the developments still 
necessary to bring performance of public-service 

It is hoped that this volume will be of benefit to 
those desiring to establish television as a service, 
either by contributing to its technical advancement or 
by financing its progress, and to those planning to 
participate in its commercial development as manu- 
facturers and broadcasters. 

This volume has had the benefit of rather widespread 
collaboration and generous assistance from many of 
the leaders in the television field. I regret that my 
approach to the subject has not permitted me to give 
credit, but my purpose has been not so much to dis- 
tinguish the contributions of individuals as to set 
forth the present and future status of the science. 
The manuscript has been reviewed by competent 
physicists and radio engineers directly engaged in tele- 
vision problems. Unfortunately, because of their 
connections, it is not practicable to give them credit, 
but I am glad to be able to mention my appreciation 
to Professor Arthur Dickson of the College of the City 
of New York, who has reviewed the manuscript. 


June, 1931. 






Television Long Predicted Nature of Recent Developments 
When Will Television Arrive? Fundamental Definitions 



Communication Comparison of Sound and Visual Communica- 
tion Visual Information Essential to Television Reduction of 
Extent of Field of View Minimum Repetition Rate 



Scanning Light Sensitive Element Picture Signal Light 
Producing Element Receiving Scanning Disc Commercial 
Standards for Television of Entertainment Value A Typical 
Television System 



Purpose of Scanning Lines and Picture Elements Progressive 
Observation Progressive Illumination Effect of Large Scan- 
ning Disc Apertures Distortion Introduced by Coarse Scanning 
Progressive Illumination Installations Limitations of Scan- 
ning Disc Prismatic Discs Rating the Quality of Scanning 
Systems Scanning of Motion Pictures Television Compared 
with Motion Pictures 



Photoelectric Phenomena Selenium Photoelectric Tube 
Characteristics of Commercial Photoelectric Tubes Coupling 
with Amplifier Systems 



Limitations of Broadcast Receivers Transmission Band Re- 




quired by Television Contribution of High Frequencies to 
Reproduction Channel Requirements for Subjects of Various 
Dimensions and Detail Selective Fading Echo Images 



Broadcast Receivers for Television Special Requirements of 
Receivers for Television Radio Frequency Amplifiers Band 
Selector Superheterodyne Resistance Coupled Receivers 



Neon Glow Tube Water Cooled Neon Tube Concentrated 
Illumination Neon Tube Color Television Drum Scanner 
Neon Grid Screen Cathode Ray Tube Kerr Principle- 
Polarized Light Karolus Valve Projector 



Repetition Rate Limitations of the Scanning Disc Relation 
of Brilliance to Number of Picture Elements Required Pro- 
jection Reproduction of Color Television 



Essentials to Faithful Television Reproduction Effect of 
Inaccurate Synchronization Degree of Accuracy Required 
Manual Methods of Synchronizing Power Line Synchronization 
Effect of Voltage Changes and Load on Synchronous Motors 
Systems Depending upon Synchronous Motors Vacuum Tube 
Regulator Short Wave Synchronizing Independent Genera- 
tion of Control Frequency Framing the Image 



Adjustments of the Eye Viewing a Television Reproduction 
Effect of Color Brilliance Interfering Illumination Advan- 
tages Gained by Projection Necessity for Uniform Detail Eye 
Response to Motion Persistence of Vision 



Definitions Determining Eye Requirements for Detail Basis 




for Establishing Television Detail Requirements Resolving 
Power of the Eye Picture Elements Required for Scenes of 
Various Dimensions and Viewing Distances Detail in Motion 
Pictures Analyzed Channel Requirements for Television of 
Various Degrees of Detail Minimum Requirements for Perma- 
nent Entertainment Service 



Early Programs Restricted by Technical Limitations Television 
as a Supplement to Sound Broadcasting Possibilities of Crude 
Television Systems of Moderate Detail Broadcasting Studio as 
Source of Television Programs Possibilities of 100-Line Tele- 
vision Variable Repetition Rate and Detail Television of 
Motion Pictures 



How Advertiser Will Utilize Television Structure for Economic 
Support of Television Broadcasting Association of Television 
with Sound Broadcasting Advertising by Television New 
Types of Radio Advertising by Means of Television 



Development of Broadcasting Compared with Television 
Essentials to Television Meriting Public Support Early Tele- 
vision Compared with Early Broadcasting Taking Television 
out of Experimental Classification Handling Sporting Events 
with Systems of Limited Detail Association of Television 
Broadcasting with Reproducer Manufacture 


ANCES 247 

By-products for the Television Industry Sound Motion 
Pictures Facsimile Telegraphy Features of Various Systems 
Light Sensitive Devices in Industrial Control Photoelectric 






Public Standards for Television Service Broadcasting from 
Conception of Radio Telephony to Public Service Electronic 
Control of Reproduction Illumination Essential Channel Limi- 
tations Imposed by Continuous Progressive Scanning Methods 
of Circumventing the Channel Limitation 





Television is a fitting crown to the achievements of 
this age of electrical development. The magnitude of 
its potential contribution to human well-being is 
alluringly foreshadowed by the startling accomplish- 
ments in the broadcasting of sound programs. Tele- 
vision promises even greater fruits because the visual 
sense is so thoroughly trained to receive, correlate, 
enjoy and remember an infinite number of impres- 
sions with great rapidity, as a result of our educational 
methods and our instinctive heritage. 

The achievement of practical television is by no 
means a recent ambition. Since the human brain 
has been endowed with imagination, scientists have 
sought for the means to see beyond the range of the 
eye. The fundamental principles of modern television 
have been disclosed for more than half a century, and 
predictions that television would soon be an accom- 
plished fact have been an unfailing avenue to news- 
paper publicity. The development of radio broad- 
casting has merely stimulated the output of such 



A third of a century ago Charles H. Sewall wrote : 

The child born today in New York City, when in middle 
age he is visiting China, may see reproduced upon a screen, 
with all its movement and color, light and shade, a pro- 
cession at that moment passing along his own Broadway. 

A telephone line will bring to his ear music and the tramp 
of marching men. While the American pageant passes 
in the full glare of the morning sun, its transmitted ray 
will scintillate upon the screen amid the darkness of an 
Asiatic night. Sight and sound will have unlimited reach 
through terrestrial space. 1 

Nature of Recent Developments. 

The instrumentalities of television have recently 
improved tremendously by virtue of scientific prog- 
ress in other fields, such as the electrical transmission 
of audible frequencies, photoelectric tubes, vacuum- 
tube amplifiers and motion-picture projection. But 
the well-established principles remain unaltered; the 
basic methods are only superficially modified. It is 
to the new tools of exquisite responsiveness and 
accuracy, replacing the crude instrumentalities of the 
pioneers, rather than to new fundamental inventions, 
that we owe our recent progress. 

Realizing the enthusiasm with which the public 
will greet a television service offering visual programs 
of real educational and entertainment merit, a vast 
amount of inventive talent and enormous research 
facilities have been concentrated upon its develop- 
ment. Not only is the general public enthusiastic, 
but a highly developed radio industry eagerly awaits 

1 SEWALL, CHARLES H., The Future of Long Distance Communication, 
Harper's Weekly, December 29, 1900. 


the day when it may offer a newer and greater service 
by broadcasting television programs, and by making 
and selling the paraphernalia essential to their trans- 
mission and reproduction. The opportunity for serv- 
ice, distinction and profit, offered by television, has 
been clearly foreshadowed by the achievements of its 
predecessor, sound broadcasting. The long-awaited 
day of television's practical graduation to the status 
of a public service will indeed be a milestone in the 
lives of men and industries. It is natural that the 
most persistent question asked in regard to television 
is, when will television emerge from the laboratory 
and make its bow as an acceptable entertainment 
service ? 

When Will Television Arrive? 

That question is not easily answered. Year after 
year, those commercially interested in its future have 
stated that television is "just around the corner." 
Admittedly, the recent accomplishments of coordi- 
nated research in the field merit confidence and 
enthusiasm. The veritable avalanche of research con- 
centrated upon the solution of the remaining problems 
of television lends force to the prediction that practical 
television is soon to be an accomplished fact. 

How near we are to that achievement cannot be 
accurately determined from the exaggerated press 
reports which accompany each forward step. If we 
had a definite method of evaluating the performance 
of television transmitters and reproducers, it would 
be much easier to appraise the significance of each 
development and to determine how much of the road 
still remains to be traveled. However, the program 



and performance requirements of a television repro- 
ducer which will merit public support can be analyzed 
in fairly specific terms. With this standard defined, 
the capacity of the terminal apparatus and the magni- 
tude of the communication channel necessary to link 
transmitter and receiver can be determined. As one 
improvement follows upon another, we can then 
accurately appraise the significance of every step 
forward, and thus forecast more nearly when television 
will leave the confines of the laboratory and make its 
way into the home. 

Many Elements of Television Fully Developed. 

As we familiarize ourselves with accomplished 
developments, we shall find that an impressive part 
of the work of the laboratory technician has already 
been accomplished. Many of the elements of the 
television system are highly developed and fully 
capable of doing their part in producing and reproduc- 
ing detailed moving images. A major invention which 
effects a radical conservation of radio channel require- 
ments will provide a considerable impetus to the 
commercial progress of the art. In fact, if television 
is to be accomplished through radio broadcast trans- 
mission, such an invention is quite essential to material 
progress in the science. 

Every great invention has faced such obstacles. 
Radio telephony and broadcasting remained in the 
embryo stage for a decade, awaiting a practical system 
of carrier modulation; the automotive industry marked 
time until an adequate highway system spread its 
network over the land; long-distance telephony needed 
the vacuum-tube relay before it could come into 



its own. Each of these steps in progress was a major 
corner which had to be turned before real commercial 
success could be possible. The most important 
problem of television is a reduction of the burden 
which it places upon communication channels for 
sending an image of a given detail. Television will 
turn its corner when an image can be reproduced and 
maintained without requiring an extremely liberal 
communication channel for a given number of picture 
elements per second. 

Fundamental Definitions. 

Television is the remote and sustained reproduction 
of an active scene simultaneously with its presentation, 
transmitted to one or more remote points by means of 
electrical communication. It is the electrical trans- 
mission of what the eye sees at one point and its 
reproduction at one or more distant points. The 
subject of the transmission may be a news or sporting 
event, a specially staged entertainment or educational 
program, or a scene viewed from a vehicle on land, 
on sea or in the sky. The broadcasting of a motion 
picture is known as motion-picture television or 

The term television is now being used to refer specifi- 
cally to the transmission and reception of moving 
scenes by radio. It is considered misleading to use the 
term in connection with demonstrations in which 
transmission is accomplished through wire circuits. 
Such television, at least at this stage of the art, is 
properly referred to as wire television, in order to 
indicate that the problems of radio transmission and 
synchronization without the aid of special wire cir- 



cults or power interconnection are not involved. 
However, should general service be inaugurated, using 
guided radio-frequency transmission over power or 
telephone systems, this technical limitation of the 
term television to radio methods is not likely to be 

Picture transmission) phototelegraphy or facsimile 
transmission, is similar to television in that a reproduc- 
tion is made of a scene or illustration remote from the 
transmission point. The effect of continuous motion, 
however, is not secured, the elements constituting the 
reproduction being progressively assembled upon 
photographic paper over a comparatively long period 
of time. The reproduction with phototelegraphy is 
not viewed simultaneously with transmission, but 
must be built up by collecting light impressions through 
photochemical processes or by means of other chemical 
changes directly or indirectly controlled by light. 
The reproduced field of view cannot be observed in its 
entirety until after the transmission has been con- 
cluded, and the result constitutes a permanent photo- 
graphic reproduction of the field of view. 

The field of view is the area or space to which the 
television pick-up system responds and within which 
the subjects of a television transmission are confined 
in order to be within the purview of the light sensitive 
element or "eye" of the system. 


Since television is the radio transmission and repro- 
duction of a representation of the field of view of the 
human eye, or a substantial part of it, at a speed suffi- 
cient to permit its simultaneous and continuous 
reproduction in uninterrupted motion, it is a form of 
communication. Any communication is founded upon 
a sense impression as perceived by the human appara- 
tus; that impression is adapted by special equipment 
to the requirements of a transmission medium, such 
as a radio or wire channel, and, after propagation 
through it, the impression is reproduced in a form 
which appeals to the original sense response. 

Nature of Communication. 

Even simple speech between two persons involves 
a chain of conversions, each of which must be com- 
pletely and successfully carried out if the communica- 
tion is to be successful. Speech starts with a nerve 
impulse set up as a result of a thought in the brain of 
the speaker. The transmission medium in the case 
of speech is the air. The thought of the speaker is 
converted by the vocal system into variations of air 
pressure according to arbitrary sounds which con- 
stitute our language. If these air-pressure impulses 
fall upon a human eardrum, its mechanical response 
is perceived by the brain as sound. 



Inventive genius has greatly extended the range of 
our various forms of sense response. The limitations 
of our vocal system have been extended to the ends 
of the earth by the successful feat of converting the 
air-pressure impulses into electrical impulses, in 
which form they are readily distributed through two 
widely used transmission mediums, radio carriers and 
wire networks. It is essential in all communication 
systems that each conversion system and each trans- 
mission medium shall preserve the essential elements 
of the reproduction. Each link of the chain must be 
capable of carrying the entire burden of essential 
elements involved in reproduction. 

It is but natural that we should seek to communicate 
visual images through the same transmission mediums 
used for speech and message communication because 
of the comprehensive character of the information 
which can be conveyed by the sense of sight. The 
superior efficiency of our visual impressions over our 
auditory ones, so far as the rapid conveyance of 
comprehensive information is concerned, can be 
judged by considering the relative plights of a blind 
person and a deaf person. The former is strictly 
limited in his contacts with the world and robbed of a 
tremendous proportion of appreciation of it, while the 
deaf person can indulge in a great variety of occupa- 
tions without special training. 

The Problem of Sound Communication. 

The fundamental difficulty, which enormously com- 
plicates the consummation of successful television, is 
due to the complexity of the sense impressions which 
are successfully responded to, correlated and observed 



by our visual system. Response to sound is infinitely 
more simple. Air-pressure impulses bear a simple 
relation to time. The pressure exerted on the ear- 
drums at any instant is only a single value. Although 
an orchestra consisting of several scores of instruments 
may contribute to sounds at a given point, the air 
impulses which each instrument sets up combine with 
the impulse emanating from the others in a single 
impression which, at any instant, is represented by a 
single condition of air pressure. 

Therefore, in sound communication, we undertake 
the relatively simple task of transcribing a phenomenon 
which, at any one instant, consists of but a single 
condition. It is true that air-pressure variations may 
occur at a high frequency, requiring a communications 
system capable of accommodating a broad band of 
frequencies. A system, able to handle up to a maxi- 
mum of eight thousand accurately controlled impulses 
per second, is sufficient to permit remarkably faithful 
reproduction of the most complex music. That is 
why so simple a system as an electrical circuit, to 
which a fixed potential is applied, serves as a satis- 
factory transmission medium for sound impressions. 
With this facility, it is necessary merely to vary the 
resistance of the circuit in accordance with speech 
sounds at one point to transmit speech. A micro- 
phone is a type of resistance which varies according to 
the air pressure imparted to the diaphragm by a 
speaker. Now that we have vacuum-tube amplifiers 
at our service, there are no limits (other than economic 
or commercial limitations) within terrestrial space for 
any desired form of speech or musical communication. 



The Magnitude of Visual Communication. 

The problems of visual-image transmission are 
vastly more difficult. We do not deal with a single 
series of conditions, as with sound. The eye receives 
a separate and identifiable impression from every 
point within its field of view at all times. It is able 
to respond to a vast number of such impressions at 
the same time, discriminating their individual (1) 
frequency, which determines color, (2) intensity, and 
(3) direction. The eye is able to compare and corre- 
late the arrangement of these impressions geograph- 
ically not only so as to determine size, form and 
relation, but, by a comparison of what is seen by the 
right eye with what is seen by the left, their perspective 

If we attempt to resolve the constituents of a scene 
which the eye perceives and the brain collates into 
a comprehensive and intelligible scene into its separate 
elements, it would be found to comprise perhaps a 
hundred million elements. Indeed, so vast is the 
number of simultaneous impressions to which the 
visual system responds, that we have keen powers of 
visual concentration and selectivity, much keener 
than those possessed by any other sense. We habit- 
ually hear all the sounds within the range of the 
auditory system and, with the utmost difficulty, 
exclude sounds of major volume to give preference to 
others of minor volume. With vision, however, in 
order to relieve the brain of unessential discrimina- 
tions, we can readily reduce the field of view by adjust- 
ment of the lens system; we can consciously select 
definite frequencies or colors out of the vast maze of 



color and raise the sensitivity of the eye for any partic- 
ular image at will. 

Comparison of Sound and Visual Communication. 

The human vocal system can originate a vast range 
of sounds; specially trained persons can actually 
imitate almost any desired sound, the only real limita- 
tion being volume. It is significant that nature has 
evolved only a receiving system for visual impressions, 
and that there is no organism capable of originating 
visual impressions at will, as we can set up or imitate 
sound impressions. The equivalent of television 
transmission is a function which kindly nature decided 
to be non-essential, perhaps to avoid burdening the 
brain with too complex a labor, or perhaps in despair 
that such an apparatus could be included in the human 
anatomy without sacrificing the power of locomotion. 

One of the fundamental considerations which 
must be determined in evaluating television is what 
constitutes the communication of sufficient visual infor- 
mation to be practically useful. With such an enor- 
mous range of possibilities afforded through our visual 
sense, it is necessary, naturally, to make tremendous 
sacrifices in the scope of the field of view and the detail 
and character of impressions selected for television 
communication. With our present knowledge of the 
art, we can conceive only systems which confine 
themselves to the communication of but a small pro- 
portion of the elements naturally observed by the eye. 

The fundamental difference, then, between the 
magnitude of the problem encountered in the trans- 
mission of visual images and in that of speech is that 
visual images, as perceived by the eye, are, at any 



instant, numerous, and therefore require that we 
secure the effect of simultaneous transmission of 
parallel impulses. Furthermore, relative direction 
and elevation of these impulses must be accurately 
maintained to secure a comprehensive reconstruction 
of the field of view. Loss of direction in sound com- 
munication, such as that involved in reproducing an 
80-piece orchestra (ordinarily heard from a broad 
range of directions) from a single point, the loud- 
speaker, is accepted without comment by a satisfied 
audience of radio listeners. With sound transmission, 
we need only a simple linear system, consisting of a 
definite succession of impulses, accurate as to fre- 
quency and intensity. With television, we must 
accommodate a vast number of parallel impressions, 
convert them into a simple series, and then provide 
means of rearranging these numerous impressions in 
their proper parallel arrangement at the point of 
reproduction. Therefore, television imposes tremen- 
dous loads upon communication channels, whether 
wire or radio, if a reasonably comprehensive reproduc- 
tion is to be attained. 

Appraising Progress in Television Development. 

Television development can be readily evaluated 
by determining the extent to which the distant repro- 
duction differs from the original scene. Every tele- 
vision reproduction sacrifices many of the elements 
perceived by the eye. The specific character of the 
most important departures from faithful reproduction 
is readily classified into four major groups as follows: 

1. Color: Elimination of the distinction of wave frequency 
which results in color differences. 


2. Size: Restriction of the field of view to a small area. 

3. Detail: Reduction of the number of picture elements into 
which the field of view is broken down. 

4. Repetition: Reduction of the number of repetitions of the 
complete scene to the minimum which will secure the effect of 
continuous motion. 

It is necessary to understand the effect upon the 
potential entertainment and educational value of a 
television reproduction in the light of each of these 
curtailments. The fact that faithful reproduction 
is not attained by any television system is no reflection 
upon the progress of that science. Every artificial 
system of communication involves some sacrifice. 
The most advanced methods of artificial musical 
reproduction completely sacrifice the factor of direc- 
tion of sound which contributes to the enjoyment of 
orchestral music and simplifies the identification of the 
person speaking on the stage. A telescope, while not a 
system of communication, achieves its purpose of 
bringing distant images nearer by proportionate 
sacrifice in the extent of the field of view. The depar- 
tures from faithful reproduction in television must 
be studied only from the viewpoint that they may be 
too drastic to permit the final result, the distant 
reproduction, to justify itself by its educational or 
entertainment value in the light of the cost required to 
attain it. 

Elimination of Color Discriminations. 

Elimination of the distinction of frequency is very 
widely employed in all manner of visual image com- 
munication and recording. Photographic prints are 
records of the intensity of light as reflected from a 



given scene properly arranged. Loss of color involves 
but little sacrifice of usefulness in most types of illus- 
tration because the memory and imagination readily 
supply the missing element. Motion pictures, for 
instance, possess adequate entertainment and educa- 
tional value for most practical purposes, although 
they are rarely in color. A highly satisfactory tele- 
vision service is conceivable without introducing the 
element of color. 

Reduction of the Extent of the Field of View. 

Reduction of the field of view can effect a marvelous 
saving in the task imposed upon a television system, 
although that practice can also be carried too far. For 
example, a dramatic service rendered by television 
would possess little attractiveness if the capacity of the 
system were limited to the portrayal of the facial 
expression of a single individual. A television repro- 
duction of the busts of two persons would still possess 
little dramatic possibility, although it would represent 
a vast improvement over the transmission of a single 
face. The reproduction of three full-length figures 
with background in suitable detail, on the other hand, 
would permit the handling of many dramatic situa- 
tions and would therefore possess considerable enter- 
tainment value. 

Sacrifice of Detail. 

The detail required is somewhat influenced by the 
subject matter to be transmitted. The half-tone 
method of printing, as used for illustration purposes 
in books, is an excellent means of judging the effective- 
ness of television systems. With practically all 








systems of television reproduction, there is some 
improvement in detail over a half-tone of the same 
area and the same number of picture elements because, 
in a half-tone reproduction the shading of only half 
the area is controlled by the field of view, much of the 
scene being excluded by the screen, while the television 
reproduction is usually a composite of controlled 
picture elements covering the entire field. The usual 
newspaper half-tone consists of 55 to 65 screen, screen 
being used to indicate the number of dots per linear 
inch. Figures 1, 2 and 3 show the same illustration in 
various values of screen, illustrating the effective 
sacrifice in detail resulting from reducing the number 
of impressions which compose the illustration. 

Some indication of the striking sacrifice in detail 
which may be made without substantial loss in infor- 
mation value to the eye is given by comparing the 
number of picture elements in the reproductions on 
pages 15, 16 and 17. If a television reproduction, 4 
to 5 inches in size, of the same number of impres- 
sions as given in these half-tones were possible, 
visual programs of a wide range of interest would be 
entirely within its capabilities. The 133-screen picture 
of that size consists of 353,780 impressions. By reduc- 
ing to 100 screen, no vast sacrifice, the number is 
cut to 200,000. In the case of the rough newspaper- 
screen illustration, the number of impressions in a 4 
by 5 reproduction may be but 50,000. The marvelous 
detail of photographic processes, due to fine division 
by individual chemical action contributing to the tex- 
ture of a photograph, is indicated by the fact that a 4 
by 5 photographic reproduction may involve as many 
as 100,000,000 separate impressions of light and shade. 



The Repetition Rate. 

The number of times per second that the subject 
matter must be scanned to secure the effect of continu- 
ous motion is a function of the persistence of vision. 
The eye's perception is relatively instantaneous, but 
an impression made upon the retina endures up to as 
long as a tenth of a second after the disappearance of 
the subject. The intensity of illumination and the 
contrast in the subject influence the time period during 
which we may rely upon persistence of vision. The 
first motion pictures consisted of 16 reproductions per 
second. Although this number gives the impression 
of continuous motion, the flicker experienced is fatigu- 
ing. As a result, the modern standard is 24 reproduc- 
tions per second. Television transmissions with 12 
and 15 repetitions per second have been attempted, 
but 18 appears to be the minimum for slow-moving 
subjects, and a still higher rate is required for rapid 

Specifying Television Transmission Quality. 

In view of the variety of ways in which the informa- 
tion selected for television transmission may be cur- 
tailed, there is a wide latitude in the quality of the 
picture signal radiated by the television broadcaster. 
With its present knowledge, the public cannot analyze 
the factors controlling the attainable quality of repro- 
duction, but in the end the popularity of television 
features will depend as much upon the density of 
picture elements, the extent of field of view and the 
repetition rate as it does upon the subject matter 
being broadcast. Merely to state publicly that a 
scene from an airplane is to be "televised" or that the 



inauguration of a president will be served to a vast 
army of short-wave television observers is in itself 
quite meaningless unless we know how extensive is the 
field of view being televised, into how many picture 
elements the scene is resolved, and how high the 
repetition rate. Television promoters have freely 
taken advantage of the gullibility of the public and 
the press by promising to televise such events, pre- 
sumably with 24- or 48-line systems, although they 
must be aware that such comprehensive scenes, to be 
of the slightest service value, require a television 
system of vastly greater capacity than has yet been 
devised. The sooner the practical meaning of density 
of picture elements and extent of field of view is 
generally understood, the sooner will television public- 
ity be founded on honest representations. 


Before proceeding with a detailed study of the 
subject, it is of advantage to learn the general processes 
of television. We are as yet unable to transmit the 
entire subject of the field of view as a single electrical 
impulse. Therefore it is necessary to break down the 
subject of transmission into a succession of finite 
areas or picture elements. The total number of picture 
elements determines the detail attainable in reproduc- 
tion and the character of the subject matter which may 
be transmitted. 


Breaking up the field of view into an orderly suc- 
cession of picture elements is called scanning. Scan- 
ning is usually accomplished by a rotating disc in which 
a series of apertures has been cut in a spiral arrange- 
ment. Observed at any one point through a frame 
behind the disc (Fig. 4), the light reflected from 
the field of view becomes a series of horizontal sweeps 
which progressively break down the field of view 
in parallel rows. The entire field of view is scanned 
by a complete revolution of the disc. The horizontal 
dimension of the field at any specific distance from the 
scanning disc is determined by the spacing between 
the successive holes of the spiral and the vertical 
dimension by the depth of the spiral. Each hole of 



the scanning disc scans a line or row of picture elements 
horizontally across the field. A television system 
using a 24 -hole disc is therefore referred to as a 24-line 
system, a 48-hole disc as a 48-line system and so on. 
Speed of scanning must be sufficiently rapid to 
secure the effect of smooth motion. This requires 
at least 16 and preferably not less than 20 reproduc- 
tions of the field of view per second. Since one 
revolution of the conventional disc accomplishes one 
scanning of the subject, the customary scanning disc 
speeds lie between 960 and 1,200 revolutions per 

FIG. 4. 

The Light-sensitive Element. 

Having broken down the subject matter of the field 
of view into a succession of light impulses, arranged 
in a predetermined order, we then convert these 
impulses into electric current variations by means of a 
photoelectric cell or other light-sensitive device. The 
photoelectric cell is a vacuum tube, the output of 
which varies, when proper potentials are applied to 
it, in accordance with the amount of light reflected 
upon it. By directing the light reflected from the 



field of view to a photoelectric cell through the scan- 
ning disc, we secure a progressive electrical intensity 
record of the subject matter to be transmitted. An 
electric current, representative of a series of visual 
impressions, is called a picture signal. The coopera- 
tion of the scanning system and the light sensitive 
element secures an electrical counterpart of the field 
of view analogous to that obtained in sound trans- 
mission by the microphone at a broadcasting station. 

The Picture Signal. 

Picture signals may be transmitted over distances 
through wire circuits or radio transmission channels, 
provided they are not of such a high frequency as to 
impose loads too great for such channels to accom- 
modate. They may be recorded on phonograph discs 
or sound film for future transmission, if the recording 
device is of sufficient capacity to respond to the 
essential range of frequencies in the picture signal. 
Using our conventional methods of radio transmission 
and reception, we are able to reproduce the picture 
signal generated at the transmission point at any 
number of desired reception points. The final step 
in reproduction is to convert the picture signal thus 
intercepted into equivalent light variations arranged 
in their correct relative positions, so that they can 
be enjoyed by the observer at the distant point. 

The Light-producing Element. 

In most instances, a neon tube is used to restore 
these electrical impulses to equivalent light intensity 
variations. The illumination on the surface of the 
plate of the neon tube varies in accordance with the 



potentials applied to it. The size of the plate deter- 
mines the overall size of the reproduction secured. 
The surface or area upon which the reproduction is 
accomplished is called the field of reproduction. In 
order to reconstruct or reproduce the transmitted 
image, we reverse the scanning process. 

The Receiving Scanning Disc. 

Interposed between the eye and the relatively large 
plate is a scanning disc with the holes arranged in the 
same way as those of the scanning disc used for trans- 

FIG. 5. 

mission. It is essential, to secure an intelligible 
reproduction, that the reproducing scanning disc 
revolve at precisely the same speed as the scanning 
disc used to divide the subject of transmission into 
a series of picture elements. The method used to 
maintain transmitting and reproducing discs at the 
same speed is called synchronization. The purpose 
of the reproducing scanning disc is to restrict observa- 
tion of the illumination of the neon tube to the partic- 
ular area corresponding to the signal representative 



of the picture element being transmitted at the 

The transmission may start with the picture element 
in the upper right-hand corner of the field of view. If 
that particular area is the brightest value of the sub- 
ject, a maximum impulse is transmitted, causing the 
reproducing neon tube plate to brighten over its 
entire surface to its maximum intensity. Because 
the proper hole of the receiving scanning disc is then 
exposing only the upper right-hand corner of the 
plate, maximum intensity of illumination is observed 
only at that point, although the entire field of reproduc- 
tion is so illuminated. As the transmitting scanning 
disc revolves, its outermost hole passes progres- 
sively across the top of the field of view, while the 
corresponding area of the plate of the neon tube at 
the receiving point is likewise exposed. The light 
intensities reflected on the light-sensitive device are 
translated into corresponding light-intensity variations 
of the neon-tube plate. The scanning disc hole 
restricts the observation of these intensity fluctuations 
to their correct position in the reproduction. 

After the outermost hole of the scanning disc has 
passed over the edge of the neon-tube plate, the second 
hole, slightly lower, comes into position at the left, 
sweeping another row, slightly lower than the first, 
across the neon-tube plate. The succession of holes 
in the disc covers the entire surface of the plate in 
one revolution, thus completing one reproduction 
of the subject. The process is accomplished with 
such rapidity that the eye, because of the persistence 
of vision, instinctively collates these successive light 
impressions into a complete image. 



The Six Processes of Television. 

To sum up: Television transmission and reception 
involves the processes listed below. The elements 
accomplishing these processes are given in the second 

Function Device 

1. Resolution of the field of view Transmitting scanning disc 

in an orderly arrangement 
of picture elements 

2. Conversion of a series of light Light-sensitive element 

impulses thus secured into 
electric current 

3. Transmission through space Radio transmitter 

by radio 

4. Reception with radio receiver Radio receiver 

and restoration to electric 
current impulses 

5. Conversion of resultant im- Neon tube 

pulses to light 

6. Arrangement of light varia- Receiving scanning disc syn- 

tions in correct position chronized with transmitting 

scanning disc 

Each of these progressive steps in television has 
been demonstrated by at least several methods. The 
progress already achieved is sufficient to encourage the 
hope that practical commercial television will soon 
be evolved. Furthermore, public enthusiasm, even in 
the crudest preliminary demonstrations, indicates such 
a demand for practical commercial television that 
there has been tremendous concentration of scientific 
forces, working toward its ultimate accomplishment. 

Commercial Television. 

In order to aid in judging the merits of television 
systems as they are demonstrated, a special committee 



of the Radio Division of the National Electrical 
Manufacturers' Association, comprising some of the 
leading scientists working in this field, has prepared 
a definition of commercial television. This is not an 
attempt to set a standard but only to describe some 
of the principal elements of a television system which 
will have public appeal on the basis of the entertain- 
ment and educational service rendered rather than 
through its curiosity or experimental value. The 
definition is as follows: 

Commercial television is the radio transmission and 
reception of visual images of moving subjects, comprising a 
sufficient proportion of the field of view of the human eye 
to include large and small objects, persons and groups of 
persons, the reproduction of which at the receiving point 
is of such size and fidelity as to possess genuine educational 
and entertainment value and accomplished so as to give the 
impression of smooth motion by an instrument requiring 
no special skill in operation, having simple means of locat- 
ing the received image and automatic means of maintaining 
its framing. 

A Typical Television System. 

In order to clarify further the various steps in the 
process of television, consider the system shown in 
Fig. 6. The subject stands within the field of view, 
brightly illuminated by high-intensity arc lights in 
order that the maximum lights and shadows shall be 
reflected from his person. The scanning disc dis- 
integrates the field of view into successive impressions 
by progressively exploring the surface of the face. 
Light is thus reflected to the photoelectric cell from 
one picture element at a time. An amplifier builds 



up the electrical counter- 
part of the light inten- 
sities observed until they 
are of sufficient magni- 
tude to be combined with 
the carrier of a radio 
station and transmit- 
ted through space. The 
radio receiver, adjusted 
to the frequency of the 
carrier, amplifies and 
then isolates the picture 
signal impressed upon 
the carrier through the 
usual radio-frequency 
amplifier and detector 
circuits. The picture 
signal is then amplified 
to values sufficient to 
cause light fluctuations 
in the neon tube. The 
receiving scanning disc 
is rotated by a motor 
in synchrony with the 
motor rotating the scan- 
ning disc at the trans- 
mitting point. Some 
means of synchroniza- 
tion is employed to keep 
these two scanning discs 
in absolute unison so 
that, when the photo- 
electric tube is indicating 
the light intensity for a 



certain area of the field, that particular area is the 
one being reproduced at that instant. The eye 
collects the series of varying light impulses as a com- 
plete image, provided the system repeats the trans- 
mission and reproduction of the field at least sixteen 
times a second. 

The apparatus, so far described, is stripped of 
all but its bare essentials and is discussed at this point 
only in order that the separate elements of television 
systems may be defined and considered in detail as 
individual units. This procedure is followed in 
preference to describing the work of individual inven- 
tors because, undoubtedly, the first commercial tele- 
vision system which has any widespread vogue will 
be a combination of the devices of many inventors. 
We will consider separately such devices as light- 
sensitive units, of which the photoelectric cell is but 
one of several available types, scanning discs, amplifier 
systems, synchronizing methods and methods of 
reconstructing the image. The crude system already 
described will make it possible for the reader to con- 
sider each functional element in detail without losing 
sight of its relation to the television system as a whole. 


Purpose of Scanning. 

The purpose of scanning is to present small units 
of the field of view to a light-sensitive element in an 
orderly progression so that a series of electrical impres- 
sions of light intensity, suited to wire or radio trans- 
mission, may be secured. 

Lines and Picture Elements. 

Most scanning systems accomplish their purpose by 
dividing the field of view into a succession of parallel 
sweeps or lines, beginning at the top of the field and 
continuing progressively to its base. Such scanning 
systems are rated in "lines," e.g., 24-, 48- and 100-line 
systems. Each line is divided into an arbitrary num- 
ber of picture elements. The number of picture 
elements per line is calculated by dividing the effective 
diameter of the scanning aperture into the length 
of the arc through which it sweeps while scanning each 
line. If the field of view is square, the number of 
picture elements into which it is divided is the square 
of the number of lines. Thus a conventional 24-line 
system divides the field of view into 576 elements, a 
48-line system into 2,304 elements, and a 100-line 
system into 10,000 elements. 

The rapidity with which the scanning process 
is carried out is at the root of the major problems of 



television. The entire scene must be scanned within 
the short period that the eye can collate the separate 
picture-element impulses into a complete and unified 
image. As soon as we increase the number of lines 
to values approaching commercial standards, we are 
faced with the need for a tremendously rapid parade 
of picture elements to which the entire communication 
system must be responsive. For example, with a 
100-line system and 20 repetitions per second, we 
are concerned with 2,000,000 picture elements per 

Necessity for Scanning. 

This disintegration of the field of view by a scanning 
system is necessary because we have discovered no 
way of analyzing our complete subject matter simul- 
taneously in a manner that permits the retention of 
its detail. Were we to attempt the transmission of 
the light intensity of a field of view as a single impres- 
sion by our present methods, we would secure a result 
analogous to that obtained by looking at the whole 
field of view through a ground-glass window or any 
translucent but non-transparent material. The result- 
ant electrical impression of the light intensity of the 
field of view is only an average value of light which is 
reflected from it, and its reproduction would portray 
little or nothing of value to a distant observer. 

In the absence of such a convenient method of 
securing an impression of the entire field of view in 
all its detail in a single operation, we break up the 
field of view into a series of picture elements and 
transmit an electrical impression representative of 
the light intensity of each element in a regular and 



orderly progression. At the reproducing point, we 
control the intensity of a light source in accordance 
with these impressions and reconstruct the field of 
view by projecting or exposing the controlled light 
through the same progressive course followed in the 
original scanning or by limiting our observation of the 
light source so that the equivalent effect is secured. 

Progressive Observation and Progressive Illumination. 

There are two fundamental methods of submitting 
the subject to the light-sensitive element: first, the 
progressive observation of picture elements, one at a 
time, through a scanning disc, and second, the pro- 
gressive illumination of the field of view, picture 
element by picture element, by means of a moving 
pencil or beam of light. 

In the first case, the field of view is intensely illu- 
minated in order that the maximum light energy 
may be focused from each picture element of the field 
of view to the light sensitive element. The conven- 
tional spiral scanning disc, due to Nipkow, which has 
already been described, accomplishes the disintegra- 
tion of the field of view and restricts the response 
of the light-sensitive element to one picture element 
at a time. The amount of light reaching the light- 
sensitive cell is dependent upon the illumination of the 
subject and the effective apertures of the optical 
system and of the scanning disc or other device used to 
collect light from the subject matter. 

The alternative method is to explore the field of 
view by means of an intense beam of light. The light 
sensitive cells are placed in such a manner as to collect 
light from the entire field of view. When the exploring 



beam of light is focused upon a white part of the field 
of view, the maximum reflection occurs, causing maxi- 
mum output from the light-sensitive system; when the 
beam is focused upon a black part of the field of view, 
the light is absorbed, causing the light-sensitive sys- 
tem's output to fall to a minimum. Consequently, 
the exploring light has the effect of increasing the 
light falling on the photoelectric cell above the average 
reflection proportionate to the brilliancy of the picture 
element being explored at each particular instant. 

Progressive Observation of Picture Elements. 

With the progressive observation system, the 
amount of light actuating the light-sensitive element 
is limited to that reflected through a relatively small 
aperture. To secure an adequate response, it is 
necessary to illuminate the subject matter by means 
of extremely powerful lights and to paint the faces 
of actors in a grotesque manner. It is reported that 
Baird used arc lights of such intensity with his first 
experimental television transmitter that only dummy 
figures could be used as subject matter. 

The amount of light collected from each picture 
element can be increased by using a large collecting 
lens, but physical difficulties prevent increasing the 
diameter of such lenses with respect to focal length 
beyond a certain ratio. When the area of the image 
is increased beyond a certain point, a considerably 
larger scanning disc is necessary. Considerations 
of mechanical convenience therefore limit the size of 
the collecting system which may be employed. 

Experiments show that, with the best lens available 
to form an image 1 inch square, the subject matter 



must be illuminated with a 16,000 candle-power arc 
light at a distance of 4 feet in order to secure an image 
bright enough to cause a light-sensitive cell output 
sufficiently large to override the noise level of the 
associated amplifier system necessary to build up the 
picture signal for transmission purposes. A photo- 
electric cell of the usual type, with an opening 1 inch 
in diameter, will give a current of a microampere in 
response to a 25-watt lamp 4 feet distant. Improve- 
ments in the sensitivity of photoelectric devices may, 
of course, materially alter this situation at any time. 
However, so long as we are confined to light-sensitive 
devices of the present order of sensitivity, tremen- 
dously powerful illumination of a restricted field of 
view is necessary when a progressive selection system 
of scanning is used. 

Overriding Random Voltages in Coupling Resistors. 

When the light energy from the field of view is 
focused on the light-sensitive device in small units, the 
current variations resulting are naturally extremely 
small. Most light-sensitive devices, particularly of 
the photoelectric type, possess very high resistance, 
requiring that the output of the cell be coupled through 
a proportionately high resistance to actuate the first 
amplifier tube. In any piece of metal, the electrons 
are constantly in a state of rapid motion, resulting in 
minute and rapidly changing voltage differences at 
their ends. When extremely high amplification, of 
the order necessary when the output of the photo- 
electric cell is small, is applied to such resistances, 
these random voltages cause impulses which when 
reproduced in a television receiver resemble a phantom 



snowstorm. This limitation is overcome only by 
intense illumination of the field of view or by a scan- 
ning system which produces large photoelectric cur- 
rent. The photoelectric current must be sufficient 
to override the random voltage variations experienced 
in any type of coupling resistance suitable for use 
with light-sensitive cells, as well as the familiar tube 
and circuit noises. 

Effect of Large Scanning Disc Apertures. 

The larger the diameter of the holes in the scanning 
disc, the more light is admitted but with the attend- 
ant disadvantage of blurring the detail of the picture. 
Supposing the field of view consists of a checkerboard, 
the sides of each square of which are equal to the 
diameter of the area analyzed by the hole of the scan- 
ning disc. The maximum value of light is reflected 
while the scanning system focuses the light-sensitive 
element squarely on one of the white squares, as shown 
in a, Fig. 7. In consequence, the reproducing 
system may be expected to reproduce full brilliance. 
Likewise, when the light-sensitive element is focused 
through the scanning disc precisely on a black square, 
as in 6, Fig. 7, we transmit a signal equivalent to 
minimum light, causing the smallest light value at 
the reproducing point. But, midway between the 
extremes, instead of a sharp line of black as in the 
subject matter, we have a signal representing a half 
value, gray. When the scanning disc hole exposes 
an area one-fourth black and three-fourths white 
(c, Fig. 7), instead of securing a value of 75 per 
cent black or dark gray, as might be expected, the 
average light reflected is of a somewhat lower value, the 



area scanned being circular. Due to these causes of dis- 
tortion, the reproduction is far from a checkerboard, but 
rather a series of vertical bars, gray at the edge, darken- 
ing to black at the center and shading off to gray. 

Slightly more accurate reproduction will be secured 
by using square apertures in the scanning disc. The 
intensity observed is then directly proportionate to 
the average values of black and white, without losses 
due to the reduced area of the entering and leaving 

FIG. 7. Effect of scanning a 
checkerboard pattern of a texture 
equal to picture elemental area. 


FIG. 8. Effect of square- and 
round-disc apertures. A shows 
picture signal secured by scanning 
abrupt change from black to 
white through a square aperture; 
B, through the conventional 
circular-disc aperture. 

edges of the scanning area. But the gain accomplished 
is not very great, as will be observed by the curve of 
Fig. 8. The mechanical difficulties of making a 
square hole in a spiral scanning disc are hardly com- 
pensated for by the improvement in reproduction 
which it makes possible. 

In Fig. 7 we have assumed that the scanning 
disc is accurately focused upon each row of squares. 
Figure 9 analyzes the reproduction obtained when 
half the scanning line is on one row of checkerboard 
and half on another, while Fig. 10 shows the effect 



These distortions 
For instance, 


of the diagonal set at a slight angle, 
are observed in actual practice, 
scanning a close-up of a 
human face, the transi- 
tion from one row to the 
next, as shown in Fig. 11, FIG 9 _ Scanning a checkerboard 

will appear as a sharp jag pattern with the axis of the scanning 

in the reproduction of the P a * on the dividing line produces a 
. medium-gray texture in every position. 

lips when the mouth is at 

a small angle to the scanning line. 

There is no substitute for picture elements in scan- 
ning. If intelligible infor- 
mation is to be conveyed 
to the eye by the repro- 
duction, the scanning 
system must divide the 

FIG. 10. Scanning a checker- ^ 

board pattern at an angle to the Held OI V16W into a num- 

squares tends to produce a medium foer o f picture elements 

gray in every position. , . ,, , ,. 

sufficiently large to satisty 

the information requirements of the eye. The real 
recipe for securing accurate representation is to 
increase the number of 
picture elements ana- 
lyzed so that no element 
of the subject matter 

which must be discrimi- V s 

nated is less than the 

it-pa of * rnVtiirf* P!P FlG ' H Scanning the human lips 

area ol a picture ele- with a picture . element area equal to 

ment. This 1S equiva- their maximum depth generally pro- 
lent in printing to using duces a J a ^ ed line - 
a half-tone of fine screen so that the detail is better 

Some idea of the detail attainable by a given scan- 
ning system can be secured by considering the size of a 



unit-picture element in relation to the smallest detail 
which is essential to useful reproduction. In trans- 
mitting a page of print, for example, the dot of the i 
should be from two to four times the diameter of the 
unit-picture area in order to make it comfortably 
readable. If we transmit a close-up of a face, and 
desire to secure such details of facial expression as 
the wrinkles in the corner of the eye or those caused 
by the smile, such elements of the field of view must, 
in their shortest essential dimensions, be from four to 
eight times the unit-scanning element area. It 
seems unlikely that a permanent television enter- 
tainment service could be based upon a system which 
builds up a reproduction having less than a hundred 
thousand picture elements, regardless of the subject 

Progressive Illumination of Picture Elements. 

When the exploring ray method is employed, the 
light-sensitive cell responds to the total illumination 
reflected from the entire field of view. The subjects 
in it, however, are not so seriously inconvenienced by 
the light source used, because only a tiny, though 
powerful, pencil of light, moving through the field 
of view with great rapidity, is observed. A system of 
this character, set up by the Bell Laboratories in 1927, 
utilized a 40-ampere Sperry arc condensed by a lens 
in the path of the moving apertures of a conventional 
spiral scanning disc having 50 small apertures and 
rotating 18 times a second. The three photoelectric 
cells used presented an area of 360 square inches to 
collect light instead of the maximum of 7 square 
inches of lens which could be used to form an image 



through a 15 -inch scanning disc. As a result, a 
substantial current output was secured, somewhat 
above the noise level of the amplifier systems used, 
without inconveniencing the subject by the intensity 
of the light required. 

One of the features of the two-way television demon- 
stration, conducted by the Bell Telephone Labora- 
tories in June, 1930, was the use of a blue light beam 
for progressive illumination of the subject. The eye 






FIG. 12. Schematic diagram of equipment used in demonstration of two- 
way television conducted by the Bell Telephone Laboratories. 

is relatively insensitive to blue, while, by using special 
photoelectric cells particularly responsive to the light 
frequencies used, a substantial photoelectric output 
was secured with minimum inconvenience to the 
subject. Figure 13 shows the interior of the tele- 
vision booth with the photoelectric cell above and 
on either side of the speaker's seat. The pencil of 
light scanning the subject is directed through the 



circular aperture above the television reproduction 
of the speaker at the other end. 

A further improvement in the 72-line pick-up sys- 
tem, made subsequent to the public demonstration, 
was the addition of a deep-red component to the 

FIG. 13. The booth used in the two-way wire television demonstration. 
The scanning light is projected to the subject just above the reproduction. 
The photoelectric cells are at each side and above the subject. 

scanning ray, making the resulting light purple instead 
of blue. Two photoelectric cells of the caesium oxygen 
type very sensitive to red light supplement the sulphur 



vapor type cells, producing marked improvement in 
the shading of the reproduction. Utilizing only the 
blue ray in scanning tended to make the reds and 
yellows in the face too dark in comparison with the 
whites, such as those of a man's collar. An incandes- 
cent lamp shown in Fig. 1C also replaced the arc 
formerly used for scanning. 

FIG. 14. The three major units of the apparatus used in the two-way 
wire television demonstration. The terminals of the photoelectric pick-up 
system are in the unit at the right; the scanning discs for pick-up and 
reproduction at the center and the arc light for scanning are at the left. 

Advantages of Progressive Illumination. 

While light-sensitive devices remain in their present 
status, the progressive illumination system has definite 
advantages over the progressive selection of picture 
elements. So long as the exploring beam is called 



upon to reflect light from subjects close to a bank of 
photoelectric cells, relatively large outputs are secured. 
We are looking forward, however, to commercial 
television which does not necessarily contemplate 

FIG. 15. By the addition of two caesium-oxygen cells on either side 
and a deep-red scanning beam, much greater naturalness in shading was 
secured subsequent to the public two-way wire television demonstration. 

restriction of subject matter to close-ups and views of 
individuals. Inasmuch as the intensity of the light 
reflected to the light-sensitive unit as a result of the 



exploring beam falls off as the square of the distance 
through which it travels, the ratio of light changes 
produced by the beam to total reflection falls off very 
rapidly as we expand the field of view. Consequently, 
the present popularity of the exploring beam or 
selective illumination method of scanning may be 
maintained only while the lack of sensitivity of light- 
responsive elements continues to limit us to subjects 

FIG. 16. The incandescent-light source developed by the Bell Labora- 
tories for scanning by the progressive illumination method. 

placed close to such elements. When the ultimate is 
achieved in practical television, the scanning system 
and pick-up devices must reach the simplicity and 
practicability of the motion-picture camera, deriving 
sufficient illumination from the subject at a consider- 
able distance in natural daylight to produce an ade- 
quate picture signal free of disturbing background. 



Limitations of the Spiral Scanning Disc. 

Up to this time, I have confined myself to describing 
the conventional scanning disc for disintegrating the 
field of view into a progression of picture elements. 
The spiral scanning disc, however, is only one sort of 
mechanical and optical system which accomplishes 
this purpose. It is distinctly limited in its field. 

If the reproduction is to be observed without the 
aid of a lens system, the spacing between successive 
holes in the spiral is determined by the total width of 
the reproduction. As the number of lines and the 
width of the reproduction is increased to values 
approaching general service standards, we find that 
the scanning disc for direct observation quickly 
becomes of ungainly size and of dangerously high 
peripheral speed. A 100-line image of this character, 4 
inches wide, would call for a disc of over 400 inches in 
circumference, or nearly 12 feet in diameter. This 
would obviously be unsuited to home use, particularly 
when it is realized that it must be revolved at a speed 
of 18 to 20 revolutions per second. 

The diameter and peripheral speed difficulty is 
readily met by optical enlargement of a small image. 
But here we face a different set of difficulties. The 
more we rely upon optical enlargement, the smaller 
must the holes of the disc be and the greater is the 
effective distortion produced by any slight displace- 
ment of the holes. When a disc of small diameter 
having a spiral of several hundred holes is considered, 
its production in large quantities within the tolerances 
necessary constitutes an extremely difficult manu- 
facturing operation. 



The Prismatic Disc. 

One of the developments in this direction is the 
prismatic disc, the development of C. Francis Jenkins, 
a pioneer worker in television in the United States. 
The prismatic ring is the equivalent of a glass prism 
which progressively changes the angle between its 
faces. When rotated, it gives a beam of light having 
a fixed axis from side to side in one direction and a 
hinged or oscillating axis in the other. Figure 18 

FIG. 17. Jenkins' pris- FIG. 18. As the prismatic disk revolves, 
matic disc. the image oscillates along path EF. 

shows the area explored by a single prismatic disc. 
By employing two discs, one of which revolves many 
times faster than the other, the entire field of view 
may be explored. To secure a 100-line picture, 
for example, disc A may make 100 revolutions to 
each one of disc B. The subject matter of the field 
of view is thus covered by one complete revolution 
of disc B. 


Lens-scanning Discs. 

Jenkins has also constructed scanning discs employ- 
ing lenses. The effect of the lens is to collect a multi- 
tude of light rays from a single picture element, 
focusing them upon the light-sensitive device. John 
L. Baird, the Scotch inventor, has used a similar 
system. By means of a revolving disc, carrying 16 
lenses in stagger formation, images of the field of 
view are caused to pass over a second revolving ser- 
rated disc. This breaks up the light into a succession 
of impulses so as to suit them to electrical amplifica- 
tion systems. Behind this disc is another revolving 
at slow speed, having a spiral slot which passes in 
front of a longitudinal slot admitting light to a light- 
sensitive cell. The field of view is thus reduced to a 
series of impulses and is progressively explored. 

Scanning with Moving Mirrors. 

Several experimenters have worked with systems 
involving vibrating or revolving mirrors. One of the 
earliest of these systems was due to Szczepanik. With 
this device, the field of view is reflected on a mirror 
which is vibrated by an electromagnet, then reflected 
to a second mirror which is similarly vibrated at right 
angles to the motion of the first and, in turn, reflected 
to a light-sensitive device. Reception consisted of 
a similar system of electromagnets, the received 
impulse controlling the size of an aperture from a 
constant source of light. An oscillograph system, 
developed by Denes von Mihaly of Berlin, utilizing 
two oscillograph mirrors to accomplish exploration of 
the field of view, has been successfully demonstrated 
in Germany. 



Slower speeds of scanning disc revolution are also 
attainable by including several series of spirals in a 
single disc, as shown in Fig. 19. Senabria, of Chicago, 
has incorporated this feature in his scanning disc, 
staggering the apertures in such a manner that only 
a part of the field of view 
is scanned by each series 
of spirals. Thus, with a 
45 -line disc, the first spiral 
scans lines 1, 4, 7, 10 and 
13; the second, lines 2, 5, 
8,11 and 14 ; and the third, 
lines 3, 6, 9, 12 and 15. 
This distribution of pic- 
ture elements through the 
whole field of view makes 

... MI P .1 * IG - 19 - The staggered scanning 

it impossible for the eye to disc with three 15-hole spirals pro- 

become Conscious of the duces three images per revolution, 

f a third f the 

scanning pattern, and 
tends to equalize the 
flicker throughout the reproduction. 

Possibilities of Improvements in Scanning Systems. 
This by no means concludes the catalogue of the 
various systems used for exploring the field of view. 
A considerable number of experimenters have worked 
out their own favorite scanning progressions, all of 
which accomplish the same thing in a different way. 
In the last analysis, the fundamental values of a 
scanning system are determined by the number of 
picture elements into which the field of view is resolved 
and the number of repetitions of the complete field 
made per second. Modification of the progressive 



order of sweeps from top to bottom to more complex 
patterns has not yet proved sufficiently advantageous 
to offer any important improvement. 

There appear to be no insuperable difficulties to 
carrying on the process of scanning to almost any 
degree or number of picture elements which may be 
necessary to secure television of adequate quality. 
The available courses open to the designer are literally 
infinite in number. He may employ discs revolving 
in opposite directions or combinations of discs and 

D.C. Motor 

FIG. 20. The triple-disc scanner used by Baird in his earlier 

moving mirrors. The optical and mechanical prob- 
lems involved in such methods, however, increase 
tremendously as greater detail is attempted. While 
delicate equipment is allowable at the pick-up point, 
where skilled personnel may be employed, the impor- 
tance of simple and rugged equipment at the repro- 
ducing point is a major consideration if a large and 
scattered audience is to be served with home television. 



Rating the Quality of Scanning Systems. 

The human eye responds in a manner quite similar 
to the television pick-up apparatus. Two heavy 
black dots on a white card are distinguished as two 
separate dots when held close to the eye. As the 
card is moved away, a point is reached where they 
blend into one spot. The ability to distinguish 
two points as separate identities is determined by the 
resolving power of the eye. At the distance just 
short of the point that the two spots blend into one, 
the angle formed by two lines drawn from the extremi- 
ties of the spot to the retina of the eye is known as 
the resolving angle of the eye. 

The capacity of a television pick-up system for 
detail can be rated according to its resolving angle. 
All the detail within the resolving angle is blended 
and lost. If the resolving power of the television 
pick-up is less than that of the eye, blurring of detail 
becomes noticeable. The ratio of the resolving angle 
of the eye to the resolving angle of the television pick-up 
is a measure of the attainable fidelity of the system. 

As with any artificial system of reproduction, the 
television system involves a substantial sacrifice from 
perfect realism. For example, the average human ear 
responds to from 20 to 20,000 vibrations per second. 
A radio receiver of admirable faithfulness of reproduc- 
tion may reproduce from 50 to 5,000 vibrations accu- 
rately, or approximately 25 per cent of the range of 
the ear. The frequencies above 10,000 are not essen- 
tial to pleasing reproduction or to satisfactory enter- 
tainment service. In the same way, the entire 
response capacity of the human eye need not be 
duplicated for satisfactory visual reproduction. 



We have not had sufficient experience with tele- 
vision to determine what percentage of completely 
accurate reproduction is necessary to constitute an 
acceptable television service. It is of interest, how- 
ever, to evaluate some of the present systems. Con- 
sider the case of a pick-up system focused on the head 
of a single person which resolves the subject into 
2,304 picture elements by means of a 48-hole scanning 
disc. If we assume that the subject is 3 feet from the 
photoelectric cell and the field of view is 2 feet square, 
the picture-element area has one forty-eighth of the 
width of the field or % inch. The resolving angle of 
such a scanning system is one whose tangent is J^ 2 
or approximately 48 minutes. This is roughly 48 
times as large a resolving angle as that of the eye. In 
other words, the resolving power of such a television 
pick-up system is about 2 per cent of that of the eye. 

Scanning of Motion Pictures. 

When we attempt to televise or scan the field of 
view of a normally illuminated scene, particularly an 
outdoor subject, we deal with extremely small units 
of light energy and, consequently, extremely small 
outputs from the photoelectric cell. This limitation 
can be overcome by the use of photography. A 
motion-picture camera secures about twenty complete 
reproductions of the field of view in a convenient 
form, each consisting of perhaps millions of picture 
elements, vastly in excess of the number which we 
can successfully transmit over wire or radio channels. 
By focusing an exploring beam which oscillates from 
side to side through the film which is being reeled 
slowly before it, we secure substantial variations of 



light intensity to actuate the light-sensitive system. 
These variations may be somewhat greater than those 
required for making the film in the first place. The 
film is, in effect, a light amplifier because the light 

FIG. 21. Dr. C. Francis Jenkins and his motion-picture lens scanning 


variations secured by scanning the film with a moving 
light beam projected through it are greater than 
the light required to record the scene directly from the 
field of view. The reproduction embodied in the 
motion-picture film is of such texture as to constitute 
an ideal method of reducing the field of view to a form 



well adapted to the limitations of the available scan- 
ning and light-sensitive systems. The only sub- 
stantial sacrifice is in connection with spot news 
events which cannot be resolved into motion-picture 
form without an appreciable loss of time. 

The scanning of motion-picture film requires only 
horizontal scanning, the vertical element being con- 
tributed by reeling the motion picture before a light 
source oscillating from side to side. For example, if 
each frame of the motion picture is to be divided into 
100 lines, the light source must oscillate from side 
to side 100 times while the film moves the distance 
of one frame. 

Many students of television believe that the first 
transmissions for general public service will be confined 
to close-ups of individuals and the television of motion 
pictures because of technical limitations. This is 
no dismal prospect, considering the rapidity with 
which motion-picture news services reduce the events 
of the day to motion-picture film. The amount of 
illumination offered the light-sensitive element when 
motion pictures are used as the subject of a trans- 
mission depends on the brilliancy of the local source 
used and is therefore independent of the average 
brilliancy of the original scene. We have, therefore, 
in the film, a tool of tremendous importance in over- 
coming some of the difficulties occasioned by direct 
television of comprehensive and long-range scenes. 

Television Compared with Motion Pictures. 

There has been a tendency to compare television 
reproduction with motion-picture projection without 
taking into consideration certain important differences 
tending to invalidate the comparison. 



With motion pictures, every picture element of 
each field of view is exposed upon the screen for J4g 
second. It is estimated that 10,000,000 separate 
chemical actions, each controlled by the shading of 
the original field of view, contribute to the make-up of 
each frame. Persistence of vision carries over these 
10,000,000 picture elements distributed by projection 
on the screen for the J4s second interval during which 
the shutter interrupts projection so that the next 
frame may arrive at its correct position after that 
interval. These numerous picture elements are again 
projected with whatever changes are imposed by the 
motion of the subjects during the interval. The eye 
is thus supplied with 10,000,000 new picture elements 
each J^ 4 second, each persisting before it for J^g 
second; that is 240,000,000 picture elements each 
projected for J^g second per second. 

An imaginary television system, presenting 10,000,- 

000 picture elements per field of view, with the field 
repeated 24 times per second, offers decidedly less 
information to the eye than the equivalent motion 
picture. With conventional television, only a single 
picture element is projected at a time. The greater 
the number of picture elements, the less time each 
is offered the eye. W 7 ith existing television systems 
each individual picture element is illuminated singly 
for an interval equal to the total time required for 
the reproduction of the complete field of view divided 
by the number of picture elements constituting it. 
Accordingly, with the imaginary television system 
described, each picture element is illuminated but 

1 /240,000,000 second as compared with ^8 second 
in the case of the motion picture. 



With television, there is no interruption of the 
process of presenting picture elements, although in 
motion-picture reproduction they are presented but 
half of the total time. This fact has been stressed 
by television enthusiasts as a great advantage. As a 
matter of fact, there is no advantage to be gained 
by shortening the "blank" interval in motion-picture 
presentation to less than J4s second, because it imposes 
no hardship on the eye. On the other hand, the 
series procession of picture elements of present-day 
television has the very serious disadvantage of reduc- 
ing the total illumination as the number of picture 
elements increases. It appears, then, that with the 
light source of equal intensity, television of motion- 
picture detail offers to the eye only a five millionth 
of the light value of a motion-picture reproduction. 
With so short a period of illumination as 1 /240,000,000 
second per picture element, there is no known light 
source sufficiently brilliant to make an impression 
which will carry over through persistence of vision 
for J^4 second. 

We have considered only one problem in this com- 
parison, the brevity of the time available for illuminat- 
ing each picture element in reproducing television of 
motion-picture quality. As a matter of fact, an equal 
hardship is imposed on every element of the television 
system, from scanning to reproduction. Obviously, 
with present methods, we are forced to deal with a 
very much smaller number of picture elements than 
10,000,000 per frame. The most ambitious television 
device which has been demonstrated up to this time 
presents 72 lines, or approximately 5,000 picture 
elements per frame. Even with such a crude system, 



assuming 20 reproductions of the scene each second, 
each picture element is illuminated only about 1 /100,- 
000 second per frame as compared with J^g second 
with the motion picture. The results of the best 
laboratory television and ordinary motion pictures 
are still very far apart. 

Of course, the problems of television are not nearly 
so hopeless as this extreme comparison would indicate. 
Prospective developments in reproduction methods 
present a somewhat more hopeful view. For example, 
the illumination of each picture element at the repro- 
ducer may be made to continue for considerable 
periods beyond the time that the transmitted impulse 
controls it by utilization of a fluorescent screen. But 
television systems which do not depart from the con- 
ventional method of presenting but one picture ele- 
ment at a time, when projected to motion-picture 
quality, are nevertheless subject to the extraordinary 
problems herein indicated. 

The presentation of picture elements in a continued 
series, even with sufficient repetition to eliminate 
flicker, imposes great burdens on the eye. Were an 
average person compelled to watch a television repro- 
duction, even one of as few elements as 2,304 (48-line 
television) for a period of 2 hours, the eye fatigue 
would probably be sufficient to cause considerable 
discomfort. On the other hand, watching a motion 
picture for 2 hours causes only moderate eye fatigue. 
This is but one of the many problems of television 
which indicate that a great deal remains to be over- 
come before television becomes the equivalent of the 
motion picture. Fortunately, we need not await the 
approach of this high standard before television 
renders a useful and widespread service. 



The function of the light-sensitive element in tele- 
vision transmission is to set up current variations 
proportionate to the light reflected upon it from the 
field of view as it is disintegrated by the scanning 
system. The function of the light-sensitive system 
is, therefore, analogous to that of the microphone in 
the broadcasting of sound. The light-sensitive ele- 
ment is frequently termed the "electric eye." The 
rods and cones of the eye perform an identical function 
in a multiple fashion in the human nervous system, 
transforming the light energy reflected to the eye 
from the field of view into impulses suited for transmis- 
sion through the nerve system to the brain. The 
rods and cones of the eye are, in fact, light-sensitive 
units and the nervous system is analogous to, if not 
actually, a network of electrical communication 

Any substance which changes its electrical charac- 
teristics as a result of the influence of light is said to 
exhibit photoelectric properties. Some substances, 
such as selenium stibnite and cuprous oxide, change 
their resistance in response to light. The alkaline 
metals, such as zinc, potassium, caesium and rubidium, 
on the other hand, release electrons in response to 
light. It is interesting that Heinrich Hertz, who laid 
the foundations for radio communication by his funda- 



mental experiments with electric-wave phenomena, 
was the first to observe the influence of light on an 
oscillating spark discharge. Emission of electrons 
is facilitated by enclosing the light-sensitive material 
in a high vacuum. 

Requirements of Light-sensitive Elements in Television. 

The photoelectric phenomenon utilized for the 
light-sensitive element in a television system must 
operate with a speed so great that an adequate number 
of picture elements may be presented to it in extremely 
rapid succession without sacrificing the accuracy of 
the response. Its response must be linear or at least 
proportionate to the wide variation of light intensities 
which may be encountered in an average field of view. 
It should respond to equal light stimuli throughout 
the color scale in a manner somewhat similar to the 
response of the eye if natural subjects are to be the 
subject matter of transmission. Stability of opera- 
tion, mechanical ruggedness and uniformity are 
characteristics which will be increasingly important 
as light-sensitive devices are more extensively used. 

Properties of Selenium. 

Selenium was the first of the light-sensitive sub- 
stances to be the subject of extensive experimental 
work in picture transmission and television. If 
selenium is annealed at about 200 C. it becomes a 
conductor, the resistance of which varies as the square 
root of the light impressed upon it. In its usual form, 
selenium is deposited in a very thin coating on a 
non-hygroscopic insulating material with two elec- 
trodes forming the terminals. The Giltay cell is a 



well-known form of selenium cell. Two spirals of 
platinum wire, 6 millimeters apart, are wound upon 
a rectangular piece of steatite, about 3 by 6 centi- 
meters in dimensions. The platinum wires form 
the terminals. The thinner the layer of selenium, the 

higher the resistance and also 
the more sensitive the cell. 
A well-made Giltay cell of 
these dimensions has a resist- 
ance of about 250,000 ohms. 
Selenium has the disadvan- 
tage of being more sensitive at 
low frequencies or the red end 
of the color scale; but its out- 
standing shortcoming is the 
fact that resistance changes 
due to a light stimulus tend to 
continue for an interval after 
the light stimulus is removed. 
The heavier the deposit, the 
greater the lag effect. 
Although selenium, for these 
and other reasons, has been 
largely abandoned for televi- 

FIG. 22. A photovoltaic 
cell developed by S. Wein not- 
able for its large output and 
rugged qualities. (Courtesy of 
Radiovision Corporation.) 

sion purposes in favor of the photoelectric tube, great 
improvement in the performance of selenium is not an 
unreasonable expectation. Korn, in his picture-trans- 
mission system, tends to overcome the lagging effect of 
selenium by utilizing two cells on the opposite sides of a 
Wheatstone bridge with the result that the lag of one 
cell is counter-balanced by that of the other. T. Thome 
Baker has suggested the possibility of utilizing a high- 
frequency alternating current to overcome the lagging 



tendency of the cell. J. L. Baird, the famous Scotch 
television experimenter, has utilized the selenium 
cell in some of his earlier television transmitters. 
Another photoelectric phenomenon, the responsive- 
ness of cuprous oxide, first observed by Pfund, has 
recently been utilized by S. Wein in making a liquid 
light-sensitive element. A cuprous oxide electrode 
and a collecting electrode are sealed in a glass tube 
immersed in an electrolyte. This is termed by Wein 
a photovoltaic cell and has been successfully used 
in reproducing talking motion pictures. It is inexpen- 
sive to manufacture and uniform in its characteristics, 
two qualities not yet fully attained by other forms of 
light-sensitive cells. 

The Photoelectric Tube. 

For the exacting requirements of television service, 
the photoelectric tube or cell is used almost exclusively. 
It depends for its action on the quality possessed by 
some of the alkali metals of emitting negative electrons 
in a vacuum as a result of a light stimulus. The form 
of modern photoelectric tubes is due to the pioneer 
experiments of Elster and Geitel in 1889. It has 
been found that the alkali metals are photoelectrically 
sensitive to a range of light frequencies according to 
their electropositiveness as shown in the periodic 
table of the elements. Sodium, for example, in its 
pure state, is not photoelectrically sensitive for wave- 
lengths longer than 0.58 microns. The micron /z is 
equal to 10~ 6 meters; the Angstrom unit (A.U.) to 
10~ 10 meters. Potassium responds to longer waves, 
rubidium to still longer, and caesium to the longest 
visible light waves bordering on the infra red. 




Kind of radiation 



( ia in 11 i;i rays 

02 to 

0000000002 to 



07 to 

0000000007 to 

Shortest ultra-violet. 



Transmitted by quartz or rock salt .... 
Shortest solar waves that can pass 
through atmosphere 



Shortest waves transmitted by glass. . . 
Shortest visible waves 

4 000 












6 600 

. 00006 

Red (longest visible waves) 
Limit of solar spectrum 
Longest heat radiation 
Shortest Hertz waves: 
Harmonic overtone 





Harmonic with tuned receiver 
Hertz waves in practical wireless tele- 
graphy (recently lowered to 3 m.) . . 


3 to 20,000 



(From "Physics for Technical Students," by Anderson. 
Hill Book Company, Inc.) 

The McGraw 

A typical commercial photoelectric cell is shown in 
Fig. 23. A deposit of silver is distilled upon the 
inside of a spherical glass bulb. Upon this a photo- 
electric metal, usually potassium or caesium, is depos- 
ited, the silver forming the contacting element for 



the cathode connection, usually terminating in a 
pin similar to those of the conventional vacuum tube. 
The anode is in the form of a wire 
ring or ribbon which is carefully 
insulated from the cathode. 
Because of the great amplification 
to which the output of photoelectric 
cells is usually subjected, special 
precautions are necessary to prevent 
the slightest leakage between anode 
and cathode, which would manifest 
itself as distortion in the reproduc- 
tion. A window to admit light rays 
is formed by applying heat with a 
torch after the active coating has 
been distilled. This drives off a 
part of the silver coating. Fre- 
quently a small quantity of hyd- 
rogen is injected into the tube at ric cell suitable for 
the conclusion of the distilling pro- television and indus- 

. . trial purposes. 

cess to lorm potassium hydride, 

which responds more uniformly through the scale of 

light frequencies than potassium. 

Rarified Gas Photoelectric Tubes. 

As with the familiar vacuum tube, much greater 
output can be secured from photoelectric tubes by 
diluting the vacuum with a small quantity of inert 
gas to a value in the order of a few tenths of a milli- 
meter of mercury. Photoelectric emission is then 
effectively assisted by ionization. The density of the 
gas and the voltage applied to the cell must, however, 
be carefully adjusted to avoid unstable operation. 



The more rapidly the cell must respond, as determined 
by the number of picture elements scanned per second, 
the smaller the reliance which can be placed on the 
ionization phenomenon. While gas cells are satis- 
factory for use with systems involv- 
ing picture-signal frequencies of fifty 
thousand or so, it is probable that 
MMtife they will prove too erratic and 

unstable for considerably higher 
response frequencies, apparently 
necessary in high-quality commer- 
cial television service. 

HI Figure 25 shows the spectral re- 

sponse characteristics of the gas 
and vacuum types of light-sensitive 
tubes made by the Westinghouse 
Company. The VB cell is most 
responsive at the violet and the red 
ends of the visible spectrum but 
relatively unresponsive at the center, 
where vision is most active. The 
VA cell, on the other hand, more 
nearly approaches the range of the 
eye, but is only one-fifteenth as 

FIG. 24. A small 
Westinghouse photo- 
cell used extensively 

for industrial pur- sensitive as the VB type. 


Characteristics of Photoelectric Tubes. 

As we might expect, the photoelectric tube exhib- 
its characteristics similar to those of the familiar 
vacuum tube. As the voltage supplied to the anode 
is increased, the response for a given illumination 
increases. There is a definite saturation for a given 
illumination intensity with a fixed voltage, and that 
saturation varies according to the light frequency or 


color of the illumination impressed upon it. In 
general, commercial photoelectric tubes do not possess 
the same response characteristics to different light 
frequencies as does the eye. This characteristic 
makes necessary the grotesque make-up used by 
subjects of television transmissions. 

Now that extensive commercial uses are developing 
for photoelectric tubes, their variable response through 
the color range will undoubtedly be remedied, and 






Response Per Unit Energy 



'Sensitivity oF the human eye not 
scale with respect to other curve. 
For qualitative inFormation only 


i k 


















1 / 





















'For OB Cell, multiply 
Ordinate bv 5 












( / 














Htro. Vi 



- I// 












Limit oF 


















Wave Length in Angstroms 

I'iG. 25. Color sensitivity of average Westinghouse VA and VB cells. 

manufacturers will soon be advertising "straight-line 
frequency" light-sensitive cells. It is already possible 
to produce photoelectric cells, for laboratory pur- 
poses, exhibiting almost any desired color response 

For purposes of color television, the Bell Telephone 
Laboratories, under the supervision of Dr. H. E. Ives, 







FIG. 26. Characteristics of Raytheon 3GS photoelectric cell. 


have developed a special photoelectric tube, sensitive 
throughout the entire visible spectrum. The cathode 

jfo <*> 40 eo seo 


FIG. 27. Characteristics of Raytheon 3VS photoelectric cell. 

is of sodium, sensitized by a complex process, utilizing 
sulphur vapor and oxygen. Mention has already been 



made (page 39) of the cells built in the same labora- 
tories to be particularly responsive to blue light of a 
frequency least annoying to the eye, so that a human 
subject can be scanned by a blue scanning light at the 
same time that he is observing a television reproduc- 
tion in two-way television. This cell used a potassium 
cathode sensitized with sulphur. The delicate techni- 
que of photoelectric cell construction will undoubtedly 
keep pace with other developments in the art. 




FIG. 28. Spectral response curve of a typical Raytheon photoelectric 


Qualities of Photoelectric Cells. 

The most important feature of a properly made high- 
vacuum photoelectric tube, as it concerns the problems 
of television, is that its response is instantaneous. 
Therefore, we can present it successive picture ele- 
ments at a high rate and secure a response proportion- 




50 15 

Polarizing E.M.F., volts D.C. 



HIO 0.15 0.20 0.25 0.30 0.35 

Illumination Flux, lumen 

29. Characteristics of Jenkins SRI-16 photoelectric cell. 


ate to the light changes impressed upon it. The 
photoelectric cell is, moreover, exceedingly sensitive. 
Ferrie, for example, describes a photo-cell amplifier 
method of measuring light from the stars in a paper 
describing his experiments at the National Observa- 
tory in Paris. 1 Using a photoelectric tube of the 
potassium hydride type, coupled to a single-stage 
vacuum amplifier using a tube with two grids, he 
secured a plate-current variation of five microamperes 
from the star Capella as a result of the light collected 
from it through an astronomical telescope. 











e Ligf- 



Wave Length, Angstroms 
-l///ra V/'o/ef >|< V/o/ef -*j* fi/we >j Green >j< -Yellow 

FIG 30. Spectral-response curve for a Jenkins photoelectric cell. 

The principal shortcoming of the photoelectric 
tube is the exceedingly small output which can be 
obtained from it, although constant progress is being 
made in securing larger outputs for a given light 
stimulus. Nevertheless, tremendous amplification is 
invariably required, involving the usual difficulties 
inherent in such amplifiers. 

1 FERRIE, G., R. JOUAST and R. MESNY, Amplification of Weak Cur- 
rents and Their Application to Photoelectric Cells, Proceedings Institute 
of Radio Engineers, Vol. XIII, No. 4, August, 1925. 



The high impedance of photoelectric tubes requires 
a high-resistance coupling to the amplifier, introducing 
random voltages in its output. It was the discovery 
of random voltages in such coupling resistors, observed 
when the enormous amplification necessary was used 
in the Bell Laboratories television experiments, which 
led to the substitution of the selective illumination 
system for the selective observation system first used. 1 

Stimulated by the commercial requirements of 
talking motion pictures, constant progress is being 
made in the development of the photoelectric cell. 
Undoubtedly, its frequency or color characteristic 
will be improved; the difficulty of producing uniform 
photoelectric tubes will be remedied; and larger out- 
puts will be available, long before some of the more 
pressing problems of television are solved. The bur- 
den now placed upon the amplification systems asso- 
ciated with television systems will be materially 
lessened with a consequent reduction of the distortion 
and blemishes now experienced, for which the vacuum 
tube is responsible. 

Coupling the Photoelectric Cell with Amplifier Systems. 
The photoelectric tube, as used in television work, 
is usually coupled directly to the grid of the first 
amplifier tube. Figure 31 shows a circuit utilizing 
a common plate potential source for the first amplifier 
stage and the photoelectric tube. Figure 32 shows 
a two-stage resistance amplifier coupled with a photo- 
electric cell. A separate battery is used to furnish the 
polarizing potential to the photoelectric tube. The 
voltage drop across resistance RI is coupled through 

1 See pages 32 and 38. 



capacity Ci to the grid of the first vacuum-tube stage. 
In utilizing a circuit of this type for television pur 
poses, care should be taken that the resistance R 


Photo - 

Jo Succeeding 
Stages ' 

\ Plate Potential 

FIG. 31. Photoelectric cell circuit using common plate potential battery 
for amplifier and photoelectric cell. 

is not so large as to cause loss of higher frequencies 
through the capacitive reactance of the photoelectric 

To Positive +22 

Terminal of Photocell 
Polarizing Battery 


+200 Volts 

FIG. 32. Photoelectric cell with associated two-stage resistance coupled 


tube or to affect the polarizing voltage itself. The 
coupling capacity C\ together with the input capacity 
of the tube should not be sufficiently large to cause the 



possibility of appreciable losses by reason of its capac- 
ity to ground. Exceedingly short leads must be 
used and every precaution observed to prevent losses 
and the introduction of disturbances in the early 
stages of the photoelectric cell amplifiers. 

Photoelectric Tubes in Television Service. 

The task imposed upon the light-sensitive elements 
of a television system of commercial quality is exceed- 
ingly great. As we increase the number of picture 
elements presented to the light-sensitive device, we 
proportionately reduce the length of time during 
which each element is subjected to the observation of 
the light-sensitive element. 1 To secure accurate 
response, the light-sensitive element should be capable 
of causing a current fluctuation in its output circuit 
corresponding to a change from full white to full 
black or vice versa from one picture element to the 
next and, furthermore, for such responsiveness to be 
of value in the final reproduction, the transmitting 
amplifier system, the transmission medium, the 
reproduction amplification system and the unit con- 
verting the electrical impulse to light must also be 
capable of faithfully carrying out the response of the 
photoelectric cell to so rapid a change. Furthermore, 
as we increase the number of picture elements we, 
must, to gain the full reward of that improvement, be 
certain that the photoelectric tubes respond with full 
accuracy to finer discriminations of shading. Other- 
wise, the additional breaking down of the field of view 
into more picture elements is not restored in the 
reproduction and no material gain is accomplished. 

1 See page 53. 



Perhaps the greatest burden imposed upon the 
light-sensitive element in television of commercial 
quality is occasioned in the televising of outdoor scenes. 
The reason why experimental television devices have 
been limited in their scope to the viewing of close-ups 
of individuals is the tremendous increase in total 
number of picture elements per reproduction made 
necessary for a clear reproduction of an extensive field 
of view. The fact that many people prefer to take 
field glasses to a football game is an indication that 
even greater resolving power than is possessed by the 
eye is desirable in a television system designed to 
pick up such events. 


The light impulses constituting the field of view 
having been reduced to electrical impulses as a result 
of disintegration by the scanning system and the 
functioning of the photoelectric cell, the transmission 
of the resultant picture signal through space becomes a 
problem of communication engineering. For the 
purpose, established communication methods and 
instruments are employed, suitably modified to meet 
the special requirements imposed by the television 
signal. The first demonstrations of television made 
use of low-frequency amplifiers embodied in radio 
transmitters and receivers designed for broadcasting 
of speech and music. Such communication facilities 
have sufficient capacity for demonstrating the prin- 
ciples of television on a limited scale. 

It is beyond the scope of this book to deal with 
problems of communication. The design and con- 
struction of amplifiers, radio transmitters and receiving 
systems are the subject of an extensive technical 
literature. The special requirements which television 
transmission imposes upon communication facilities 
are, however, vital considerations, and the meeting 
of these requirements constitutes one of the major 
development problems to be solved before regular 
television service is a reality. 



Limitations of Broadcast Receivers. 

The highest rate of electrical communication involv- 
ing a band of frequencies which must be interpreted 
according to both frequency and intensity is encoun- 
tered in high-quality broadcasting of speech and music. 
An average broadcast transmitter and receiver respond 
faithfully to a band of audio frequencies extending 
from 100 to 5,000 cycles, or a total band width of 4,900 
cycles. Assuming 18 repetitions of the field of view 
per second, a transmitter with a maximum band 
capacity of 4,900 cycles could radiate a picture signal 
comprising 544 picture elements, or a square field of 
23 lines. If a broadcast transmitter of unusually 
good quality, handling up to 8,000 cycles, were used 
for the transmission of a television picture signal, the 
field of view could be broken down into 889 picture 
elements, or 30 lines. 

Quite obviously, the entertainment value of a 
television reproduction of 23 or 30 lines is so meager 
that we may conclude that neither broadcast trans- 
mitter nor receiver is capable of rendering any general 
television service. So long as we rely upon progressive 
and repeated scanning, a special communication 
system, involving much broader bands than those 
required for speech and music, is essential to television, 
and both transmission and receiving apparatus of 
special design must be utilized. 

Whenever a new discovery in television is announced, 
or a publicity stunt successfully carried out, radio 
dealers report that prospective buyers of broadcast 
receivers hesitate because they fear that practical 
television will soon make existing receivers obsolete. 
But, unless television develops on radically new prin- 



ciples, accomplishing its service with a much lower 
rate of signaling than is now essential, the present 
radio-broadcasting system, from transmitter, through 
ether channel, to receiver, is about as useful for tele- 
vision as a small boy on roller skates for pulling a 
freight train. 

Transmission Band Required for Television. 

The theoretical maximum frequency encountered in 
a television picture signal produced by progressive 
and continuous scanning can be ascertained by cal- 
culating the frequency band necessary for transmitting 
a checkerboard pattern, each square of which has 
the dimensions of a picture element. The output 
of the photoelectric cell in that case rises from mini- 
mum to maximum in scanning a black square, and 
then falls from maximum to minimum to complete 
the scanning of the next succeeding white square. 
This pulse repeated continuously for each pair of 
picture elements produces a simple pulsating picture 
signal of a frequency equal to half the number of 
picture elements scanned per second. Any variation 
in the checkerboard pattern decreases the frequency, 
and no variation of the pattern tends to increase the 
frequency. The theoretical maximum frequency 
(without taking into consideration the possibilities 
of harmonics and their resulting influence in making 
possible accurate reproduction of the signal) can 
therefore be determined by the following formula: 

/ P XT 

Jmax Q 

where f max equals the highest frequency present in the 
picture signal, p the number of picture elements into 



which the field of view is resolved and r the number of 
repetitions of the entire process per second. 

Capacity of WQ-kilocycle Channels. 

The 100-kilocycle channels now being used for 
experimental television are therefore capable of accom- 
modating a television transmission of 10,000 picture 
elements with 20 repetitions per second, or of 12,500 
picture elements at 16 repetitions. These are equiva- 
lent to square fields of 100 and 112 lines, respectively. 

The nearest approach to television of this detail 
so far made was accomplished by the Bell Telephone 
Laboratories in the two-way wire television demon- 
stration of June, 1930. 1 A rectangular field was scan- 
ned to 72 lines at 18 repetitions, with a maximum 
frequency of 40,000 cycles. The amplifiers and wire 
channels used handled a band of from 10 to 40,000 
cycles. It is not yet practicable to transmit such 
broad bands of signal reliably over any known elec- 
trical communication channel, other than very short 
wire circuits especially treated to handle them. 

What the Maximum Picture Signal Frequency 


Some television experimenters have claimed that 
the amplifier systems and the communication channels 
used for television do not have to handle nearly so 
broad a band of signals as theoretical considerations 
indicate. If the theoretical maximum rate of change 
is not actually encountered in practice, that fact is of 
the utmost importance because the paucity of com- 
munication channels available for television purposes 

1 See also pages 39-41, 101, 104-106, 155-157. 



and the difficulty of greatly increasing the band width 
handled by amplifiers will inevitably impede commer- 
cial progress, unless methods much more economical 
of communication facilities are evolved. 

In describing the 50-line television system demon- 
strated by the Bell Laboratories several years pre- 
vious, 1 its designers state that the theoretical maximum 
frequency of 22,125 cycles was not actually utilized. 
By inserting filters and gradually decreasing the 
high-frequency cut-off, serious loss of detail was not 
observed until the maximum frequency transmitted 
was down to 15,000 cycles. However, the 72-line 
system of 1930, 2 with its greater detail, required a 
communication system capable of handling the theore- 
tical maximum frequency to secure the highest quality 
of reproduction of which the process is capable. In 
short, when fine scanning is employed so that real 
detail is embodied in the picture signal, any failure 
of the transmission and reproduction system to handle 
the highest frequency required nullifies the advantages 
gained by fine scanning. In those cases where com- 
munication facilities with a maximum capacity of 
less than the theoretical maximum frequency in the 
picture signal have appeared to be satisfactory, it 
has been due, very probably, to deficiencies in the 
optical and amplifier systems which themselves reduce 
the detail attainable in reproduction below the stand- 
ard merited by the scanning system. 

1 See also pages 38-39, 98-99, 111-112, 153-155. A series of papers 
describing the apparatus used in this demonstration appeared in the Bell 
System Technical Journal, Vol. IV, No. 4, October, 1927. 

2 See also pages 39-41, 101, 104-106, 155-157. Several papers de- 
scribing the apparatus demonstrated on this occasion appeared in the 
Bell System Technical Journal, Vol. IX, No. 3, July, 1930. 



From the checkerboard pattern illustration, it is 
clear that the highest frequencies in the picture signal 
contribute the abrupt changes of intensity in the 
scanned line. A sharp alteration from black to white 
or white to black is embodied in the picture signal 
only as sharply as the frequency characteristic of the 
television communication system will permit. In 
practically every field of view, such sharp contrasts 
exist. They are embodied in the picture signal, 
and consequently in the reproduction, as a gradual 
rise in intensity, spread over the distance scanned in 
the time required to cover half a cycle of the maximum 
frequency handled by the system. The lower that 
maximum frequency, the less sharply are the rapid 
changes in contrast accommodated. The amount of 
fine detail embodied in the picture signal therefore 
depends upon the maximum-frequency capacity of 
the television system. Any lowering of the maximum- 
frequency capacity is reflected in loss of the finer detail 
and blurring of the sharper contrasts in the reproduc- 
tion. When the fine dark lines and bright spots are 
blended with the background in a television reproduc- 
tion, making it appear out of focus, it is an indication 
either of coarse scanning or of fine scanning trans- 
mitted through a communication system incapable 
of handling a sufficiently wide band of frequencies. 

Maximum Frequency Required for Close-up Views. 
Utilizing the familiar methods, the equipment 
at both terminals tends to exceed all economic limita- 
tions when anything over ten thousand picture 
elements is considered. Nevertheless, considerable 
entertainment service is offered by a system with 



this limitation, provided that the observer secures a 
really accurate reproduction and portrayal of the 
face and of its expression. To accomplish such 
reproduction requires scanning and reproduction 
of very minute areas, in other words, a high density 
of picture elements. An expression of surprise, for 
example, created by a slightly increased opening of 
the eye and a small parting of the lips, means, in 
terms of television, alteration of the shading of very 
small areas corresponding to as little as a twenty- 
thousandth of the whole field of view. If such details 
are blended by rough scanning, the owner of a tele- 
vision reproducer will soon tire of watching the two 
or three faces performing before him. 

In a close-up of the face occupying the entire field 
of view, a heavy pair of bow-rim spectacles may be 
but one-fiftieth of the width of the head, the part of 
the hair one one-hundredth, and wrinkles about the 
eyes and mouth one five-hundredth. To be por- 
trayed in the reproduction, the area embodied in a 
picture element must be smaller than any item to be 
reproduced. Apparently the full capacity of 100- 
kilocycle channels is required for adequate portrayal 
of one or two faces in sufficient detail to possess 
sustained entertainment value. 

The Ideal Subject for Television. 

Many demonstrations of television have been 
offered with a considerably smaller number of picture 
elements than is suggested as the minimum for a 
limited entertainment service. No such demonstra- 
tion is considered complete without the reproduction 
of a person smoking a cigarette. The cloud of 



smoke is realistically portrayed because it is a simple 
area of white of indefinite size and shape. White 
space of such outline against an obscured background 
is the ideal subject for television purposes. It 
involves no sharp lines, and the main element is 
characterized in the picture sig- 
nal merely by absence of modula- 
tion. The very failure of such 
a system to handle fine detail is 
an advantage with this particular 
subject, because it heightens the 
nebulous haze which makes it 
realistic. The attainable detail 
for entertainment service can 
hardly be judged by such an 
unexacting subject. 

Low-frequency Requirements. 

FIG. 33. Scanning a 

subject of this kind pro- As important, though less diffi- 

duces a single pulse per ^ f th stan dpoint of 

line for approximately one- . . 

third of each frame. channel requirements, is faithful 

reproduction of low frequencies 

in the picture signal. Low frequencies are as essen- 
tial in the reproduction of simple shadowgraph 
subjects as in the most complex field of view. With 
horizontal scanning, the television signal required 
to transmit an image of a man standing before a 
white background in a dark overcoat is, for a 
major part of the transmission, as low as the picture 
repetition frequency itself. There is only one impor- 
tant change of tone from the plain white background 
to the black overcoat, producing only one change 
in signal intensity for each scanning line. Failure 



to respond to the lower frequencies tends to blend 
the man's figure with the background, a result which 
would make the whole procedure of transmission 
rather futile. 

Appraising the Channel Requirements of More Com- 
plex Subjects. 

It is unfortunate that any substantial improvement 
in television detail approaching commercial standards 
seems to involve increasing the signal frequency 
requirements somewhat beyond the capacity of 
existing amplifier systems and of available com- 
munication channels. In appraising the significance 
of improvements in television terminal apparatus, 
the added burden which they impose on communica- 
tion channels must always betaken into consideration. 

The light reflected to the light-sensitive system 
is, at all times, the average reflection admitted at 
each instant through the scanning aperture or illumi- 
nated by the exploring ray, regardless of the detail 
or sharp lines within the scanned area. A vertical 
line such as a wall, bar or post, in the field of view 
which occupies less than the width of the scanning 
area or picture element, blends with as much of the 
background as is scanned with it. If we move closer 
to the field of view with our television scanning 
apparatus, the dimensions of the important elements 
of detail become larger, but the scope of the field is 
proportionately reduced. 

Suppose we consider the subject presented for the 
delectation of the television audience to be the highly 
elevating and entertaining spectacle of a clown on a 
bicycle indulging in a head-on collision with a tele- 



graph pole. In order to observe the clown riding in 
blissful ignorance toward the pole, the resulting 
collision and the graceful spill which is to ensue, the 
field of view scanned should be about 15 feet wide and 
10 feet high. We will eliminate the element of 
varying distance of the subject matter from the 
photoelectric eye and consider the scene as a plane 
surface 10 by 15 feet. Because of the rapid motion 
involved, the repetition rate is to be 20 per second. 
By utilizing the maximum capacity of an experimen- 
tal television band 100 kilocycles wide, the field may 
be broken down into 10,000 picture elements. The 
area of the field of view being 21,600 square inches, 
each picture element is 2.16 square inches or roughly, 
a square with 1.47 inches to a side. Because the 
scene is rectangular, we cannot scan it to 100 lines as 
that would give us 150 picture elements per line or a 
total of 15,000 for each reproduction. By scanning 
82 horizontal lines of 122 picture elements each, we 
make maximum utilization of the channel, the prod- 
uct of these two figures giving approximately 
10,000 picture elements. The scanning spot will 
therefore travel 82 times across the scene each twen- 
tieth of a second. The scene being 180 inches wide, 
the scanning spot will cover the distance of 180 by 
82 inches, or 14,762 inches, each twentieth of a 
second, or 295,000 inches per second. Each hundred 
thousandth of a second, the distance covered by the 
scanning path is about 2.95 inches, or the width of 
two picture elements. 

Because of the limitations of our channel, the most 
information which we can convey through the tele- 
vision system in a single cycle is a change from white 



to black to white or the reverse, each having the 
width of a picture element. In other words, a 
series of white and black stripes, 1.5 inches wide, 
can be successfully reproduced. But a pattern of 
lines 0.75 inch in width would be blended inasmuch 
as they would be simultaneously scanned, and the 
resulting reproduction would be uniform medium 
gray. For the same reason, the frame of the bicycle, 
the sprocket, the handle bars, the details of the 
profile of the clown's face would be partially or 
wholly blended with the background. Maintaining 
this scene within a 100-kilocycle channel would 
confine response to details having a cross section 1.5 
inches, excluding many elements contributing signi- 
ficantly to the realism of the scene. 

Losses in Detail by Inadequate Transmission System. 
Furthermore, if the amplifier system at the trans- 
mitter handles no more than eighty thousand cycles, 
the most rapid changes are not effectively radiated. 
The detail lost because of failure of the amplifier to 
reproduce the necessary frequency range is of an 
identical character with that caused by increasing the 
elemental area observed through the scanning system. 
Obviously no television system is any better than 
the amplifier and transmission qualities of the com- 
munication medium used in connection with it. 
No improvement in the seaming or pick-up system 
can be reflected in reproduction unless the trans- 
mitting amplifier, the transmitting channel, the 
receiving amplifier and the controlled light-reproduc- 
ing system also respond to the increased rate of 
scanning. Conversely, improvement of the amplifier 



system to respond to higher frequencies than are 
analyzed by the scanning system contributes no 
improvement in the detail of reproduction. 

Significance of Television Demonstrations Not Involving 
Radio Channel. 

Demonstrations of television are frequently given 
to the press by inventors and by manufacturers of 
electrical and radio apparatus at radio shows and 
before societies. But unless such demonstrations 
involve the use of a radio channel between trans- 
mitter and reproducer, they merely show the capabil- 
ities of the terminal apparatus and not of television 
communication. Because it seems about as easy to 
transmit speech and music by radio as through wires, 
it is too readily assumed that a demonstration of wire 
television is clear proof of the possibilities of radio 
or long-distance wire transmission. A short-length 
wire channel is ideal from the transmission stand- 
point and successfully avoids some of the most 
baffling problems of television transmission. It is 
appreciation of the magnitude of the radio trans- 
mission problem which animates the prediction some- 
times made by authorities in the field that television 
is more likely to be distributed over wire lines for re- 
production in public places to which admission is paid. 

The most important difference between wire and 
radio transmission of television arises out of the 
peculiarities of high-frequency channels with which 
the broadcast listener in general is not familiar. 
High-frequency channels must be used for television 
purposes because broad bands of signals are not 
available elsewhere for television. 



Selective Fading in Short-wave Transmission of Speech. 

The two most important effects to be contended 
with are selective fading and radio echoes. Even 
with channels as narrow as 8 to 10 kilocycles, such 
as those used in transatlantic telephony, selective 
fading is readily observed, as indeed in almost any 
transmission of speech and music over long distances. 
For example, a transmitter utilizing a single 5,000 
cycle band from 3,000,000 to 3,005,000 cycles may 
at one instant find the signal swinging lower in 
intensity from 3,000,000 to 3,100,000 cycles but rising 
at the same time from 3,004,900 to 3,005,000 cycles. 
Broadcast listeners, being entertained by programs 
coming from across the seas through a short-wave 
radio link, have observed that the low notes come 
booming in for a few moments while the higher 
frequencies are fading out almost entirely. In 
speech, this gives the speaker a muffled and indistinct 
tone, difficult to understand. A moment later, the 
high frequencies are likely to rise, giving clear articu- 
lation but total absence of body. Many technical 
papers have been published describing some of these 
baffling effects. 1 

Selective Fading on the Transatlantic Circuit. 

Measurements made by the Bell System of the 
varying transmission characteristics of the short- 
wave transoceanic radio circuit between England 
and the United States indicate that selective fading 

1 POTTER, R. K., Transmission Characteristics of a Short-wave Tele- 
phone Circuit, Proc. Institute of Radio Engineers, Vol. XVIII, No. 4, 
April, 1930. 



and fading out of the carrier are likely to be exceed- 
ingly troublesome when picture signals occupying a 
wide-frequency band are utilized. The published 
reports concern themselves only with frequencies 
between 425 and 2,295 cycles, a considerably narrower 
band than would be required by a television enter- 
tainment service based on methods so far understood. 
The transmission characteristics of the radio tele- 
phone circuit were found to undergo exceedingly 
rapid changes, apparently due to wave interference 
between signals arriving over paths of a different 
electrical length and possibly combined with distor- 
tion produced by progressive change in the angle of 
rotation of the polarization plane with frequency 
over the signal band. The occasions when faithful 
transmission was observed were extremely rare. 
Considering the narrow band involved, as compared 
with that necessary for good television, the proba- 
bility is that wire rather than radio channels will 
be used for television-broadcasting service. 

In the effort to establish a reliable sound program 
service of features originating in England and on the 
continent, the National Broadcasting Company, 
through the Radio Corporation of America, has 
launched an elaborate series of experiments to combat 
the short-wave fading problem. Three extensive 
receiving aerial systems have been erected at River- 
head, Long Island. An automatic selector system 
connects the particular aerial which at each instant 
is delivering the most consistent and highest level 
with the circuit feeding the broadcasting system. 
An automatic volume control compensates for varia- 
tions in volume. The level indicators give a clear 



demonstration of the continual variations in signal 
encountered even with an extensive aerial system 
in an ideal location. 

The ambitious character of the equipment and the 
installation required to secure the semblance of a 
reliable signal indicate the gravity of the transmission 
problem involved in the high-frequency broadcasting 
of television signals. If we resort to the ultra radio 
frequencies higher than thirty megacycles, we may 
be able to deliver a more stable signal, but then the 
skip-distance effect is likely to make long distance 
reception of the signal a virtual necessity. The 
possibility exists, however, that the ultra high fre- 
quencies may yield a band suited to reliable television 

Effect of Selective Fading on Television. 

When channels 100 kilocycles in width are con- 
sidered, the liability to trouble from selective fading 
is enormously increased. Changes in the con- 
ductance of the transmission medium to various 
parts of the band being used for television transmis- 
sion result in peculiar distortion. As the higher 
frequencies fall out, detail disappears, but silhouettes 
increase in contrast. When the opposite condition 
obtains, major areas of uniform shading tend to 
merge with the background, while detail of fine 
texture improves. The difficulties of transmitting 
broad bands of signals on high frequencies are of 
such a fundamental character that they seem to be 
practically incurable. They are the play of gigantic 
forces of nature on the highly responsive transmission 
medium. The solution of the problem lies in narrow- 



ing the signal bands necessary to adequate television, 
rather than in any reliance upon broad frequency 
bands in the inherently unstable radio-transmission 

Echo Effects. 

The echo effect is caused by the arrival of the 
signal at the receiving point through different trans- 
mission paths. This may be caused by response to 
electromagnetic waves reflected from various levels 
of the Heaviside layer or arriving from various 
directions out of phase. 

Directional antennas at both the receiving and the 
transmitting ends may help to mitigate the reflection 
difficulties. Dr. Alexanderson mentions the ghost- 
like figures resulting from reproduction of the delayed 
signal when experimenting with transmissions from 
Schenectady at his home on Lake George. Ghosts 
were also observed in the famous Bell System demon- 
stration between New York and Whippany, the 
only known public demonstration of radio television 
over a considerable distance performed on schedule. 

Dr. Alexanderson describes his experiences with 
television reception during the summer of 1928 at 
Lake George, about fifty miles from Schenectady, as 
follows : 

A difficulty particularly brought out by the Lake George 
observations is a phenomenon which may be described as 
"mirage." It is analogous to the mirage that can be seen 
over lakes in the morning and evening and results in the 
distortion of images and sometimes in the appearance of 
several interwoven images. It appears as if the reflecting 



Kennelly Heaviside layer, which we assume to be located 
about one hundred miles over the earth, were broken up 
sometimes in several layers at different heights, each reflect- 
ing a separate image and sometimes giving an irregular 
and blurred image. 

The radio waves travel at the velocity of light, and though 
we are in the habit of thinking of this velocity as being 
almost infinite for anything that occurs on the earth, we 
find that these rays are too slow for television. Light 
travels at the rate of 186,000 miles per second, and yet we 
find that light will travel only about 200 miles in the time 
required for tracing one line in a television picture, and only 
50 miles in the time required to trace one-fourth of a line 
in a picture. Thus, if two rays have traveled from the 
transmitting to the receiving station through different 
paths and the length of these paths differs by only 50 miles, 
they will register separate images differing as much as one- 
fourth of the picture. Each of these rays will then trace 
its own picture and we will have two pictures displaced by 
that amount. On the other hand, a multiplicity of rays 
will arrive having traversed different paths, each tracing 
its own picture, with the result that all the details of the 
picture appear blurred. 1 

Reporting on experiments in radio transmission in 
New York City, particularly over short-wave chan- 
nels utilized in television experiments, C. W. Horn, 
general engineer of the National Broadcasting Com- 
pany, states: 

The massive steel structures of New York present an 
unusual problem in transmission, particularly over short- 
wave channels, such as are utilized in television experi- 

1 Statement by Dr. E. F. W. Alexanderson, released by General Electric 
Company Press Bureau, September 4, 1928. 



ments and facsimile. The shorter the wavelength or higher 
the frequency, the more these waves take on the properties 
of light waves, in that they are easily absorbed, reflected 
and refracted. Consequently, they literally bounce around 
among the steel structures of New York. The experi- 
ments and tests have shown the reception of three, four 
and more distinct signals coming into receivers from 
different directions and over different paths. The actions 
of these waves, bouncing to and fro, sometime create 
definite shadows behind buildings and other edifices in 
which little or no energy might be detected. This effect 
is not very noticeable, if at all, in the broadcast trans- 
mission band, so the radio listener need not be worried, 
but the greatest effect at present on radio signals is in the 
extremely short wavelengths such as are used for television 
experiments. 1 

It is quite possible that high-frequency television 
may be limited in scope to events comparable to 
those broadcast through international high-frequency 
circuits. When the news value of the subject matter 
is of sufficient interest to justify itself, even though 
it involves a loss in clarity of reproduction, the public 
will be content with a low quality standard but, for 
entertainment and educational purposes, the repro- 
duction must be stable and of adequate detail. 
Apparently, this can best be accomplished through 
wire transmission over telephone and power-line 
circuits through the carrier method. So long as 
the signal bears a proper ratio to the noise level 
background, the wire system possesses important 
advantages from the standpoint of stability and 

1 Statement issued by Press Relations Department, National Broad- 
casting Company, November 25, 1930. 



attainable frequency characteristics. Considering 
that we have vast wire telephone and power networks 
extending into a large proportion of homes and 
used only for low-frequency purposes, the possibility 
of utilizing these facilities for television transmission 
is far from remote. 



Conventional broadcast receivers are wholly un- 
suited to the reception of television signals, both 
because of the limited band width to which they 
respond and because television signals are not radi- 
ated in the broadcast band. The possibility of 
combining the requirements of television reception 
with broadcast reception in a single radio- and audio- 
frequency system are remote, specialized receiving 
methods being required for each of the two services. 

Special Radio Receivers Required for Television. 

For broadcast reception, the radio-frequency ampli- 
fier must admit a 10 kilocycle signal and exclude the 
signal of neighboring 10 kilocycle channels. A radio- 
frequency amplifier sufficiently selective to accom- 
plish such discrimination tends to cut off the higher 
audio frequencies involved in musical reproduction. 
For television reproduction, the receiver must respond 
to much broader signal bands, ranging from 20,000 
to 100,000 cycles in width, and consequently a tuning 
system of entirely different characteristics from those 
useful in broadcast reception is required to dis- 
criminate television signals. 

The comparative selectivity required for broad- 
cast music and television picture signals is indicated 
by the relation of the desired frequency band width 



to the signal frequency itself. At 1,500 kilocycles, 
the desired broadcast signal band of 10 kilocycles 
is 0.7 per cent of the signal frequency. A 100-kilo- 
cycle television band, at 2,000 kilocycles, is 5 per 
cent of the signal frequency. While television 
transmitters are not numerous and reception is 
restricted to short ranges, selectivity is not yet an 
important factor. But eventually the same stand- 
ard of selectivity now obtaining in broadcast recep- 
tion will be required for television service, although 
that standard, under the prospective conditions, will 
be somewhat more difficult of attainment. 

1550 1450 1360 1250 1150 1050 950 850 750 650 550 


3000 2900 2800 2700 2600 2500 2400 2300 2200 2100 2000 

FIG. 34. Selectivity requirements for television reception. Shaded 
area (a) shows response of an ideal broadcast receiver and the super- 
imposed curve is the average response. The lower curve (6) shows the 
same order of selectivity applied to the television band. Curve A shows 
the effect of admitting substantially the entire desired signal to be high 
response to cross-talk from the neighboring channels. Curve B shows 
that essential elements of the picture signal must be excluded if cross-talk 
is to be avoided. 

A receiving device employing an audio-frequency 
amplification system to be used for both television 
and broadcast reproduction and having two radio- 
frequency systems, one adapted to television and the 
other to broadcast purposes, also seems impracticable. 
When used for reproducing music an audio system 



responding to any band considerably more than 5,000 
cycles wide amplifies disagreeable heterodyne inter- 
ference which is excluded by an amplifier of suitably 
limited response. Consequently, neither the radio- 
frequency nor the audio-frequency amplifier of the 
broadcast receiver has characteristics suited to 
television purposes. 

A further barrier to combined broadcast and tele- 
vision receivers arises out of the remoteness of the 
present television band from the broadcast band. 
The broadcast spectrum lies between 1,500 and 550 
kilocycles (200 to 525 meters), while the experi- 
mental television channels lie between 3,000 and 
2,000 kilocycles (100 to 150 meters). Between the 
broadcast and the television band, is an amateur band 
(1,716 to 2,000 kilocycles), and a mobile band, devoted 
to aviation, maritime and police services. These 
assignments are the result of international agreement 
and cannot be readily altered. The probability that 
these mobile and amateur services will be shifted so 
that broadcast and television bands may adjoin is 
remote. Furthermore, the channels now assigned to 
experimental television are likely to be required for 
extensions of the band assigned to aviation purposes 
when that form of transportation is more extensively 
used. If still wider bands than 100 kilocycles are 
found necessary to television of commercial quality, 
it is more likely that the television band will be 
moved than that other services will be transferred to 
accommodate it. 

Television broadcasting, without accompanying 
sound, offers but a small proportion of the entertain- 
ment value of combined sound and visual programs. 



Although the popularity of sound motion pictures 
is substantiating evidence of this fact, it hardly indi- 
cates how essential accompanying sound is to the 
television program. The motion picture without 
sound offered a very high standard of reproduction 
and was produced by a specialized and capable 
technique. On the other hand, television is likely 
to represent a low standard of visual reproduction, 
at least in its initial stages, so that sound accompani- 
ment will be practically essential as a supplement 
to and embellishment of the entertainment offered 
the eye. The most likely development of television 
communication is therefore the use of broad signal 
channels which will not only transmit picture signals 
but will also handle synchronizing frequency and 
sound accompaniment as well. This combination 
signal will actuate receiving systems equipped with 
means of filtering each of the three signal components 
so that they may be employed for their respective 
purposes of visual reproduction, sound reproduction 
and speed control. 

Radio-frequency Amplifiers for Television Reproduc- 

The usual practice has been to employ single-tube 
short-wave radio receivers for intercepting experi- 
mental broadcasts, subjecting their output to three 
or four stages of resistance-coupled amplification. 
While this method overcomes the problem of broad- 
band interstage coupling, such receivers are almost 
invariably regenerative, whether equipped with 
regeneration controls and feed-back coils or not. 
Regeneration cuts off the higher frequencies so that, 



no matter how good the audio-frequency amplifier, 
fine shading details in the transmitted picture signal 
are certain to be suppressed before they can be 
reproduced. If the tendency toward regeneration 
is avoided, then the sensitivity of the single-stage 
radio-frequency system is of such a low order that 
reception range is greatly limited. The one-tube 
arrangements are strictly limited in their application 
to narrow-band television reception, in the order of 
10 kilocycles or less. 

For the reception of signal bands of 20 kilocycles 
or more, the conventional tuned radio-frequency 
amplifier must be substantially modified from the 
familiar broadcast designs. Having to admit a 
broad band of signals, the tuned radio-frequency 
receiver is likely to respond to signals radiated on 
neighboring channels, unless special design precau- 
tions are taken to overcome its inherent lack of 
selectivity when adjusted for broad-band response. 
Radio-frequency amplifiers, adapted to passing broad 
bands of signals are required for television purposes, 
such as band-pass filter systems and special super- 
heterodyne receivers. 

The Band Selector. 

The band selector is a system of reactances so 
related to each other that they are balanced at any 
frequency within a desired band. Outside of the 
limits of this band, the reactances are unbalanced, 
offering a high overall reactance. Two reactance 
couples, Xi and X 2 (Fig. 35), are balanced within 
themselves at the same frequency and coupled to 
reactance X^ common to both. When the impressed 



frequency is that at which reactance couples X 1 
and X 2 are balanced within themselves, the overall 
reactance of the circuit is zero. The current at 
that frequency traverses Xi and X 2 without passing 
through X z . At any other frequency, however, 
there will be a potential difference between points 
a and 6, the terminals of the bridging reactance X z . 
If the frequency is lower than the balance frequency, 
the reactance is capacita- 
tive; if higher, it is induc- 
tive. Under the latter 
conditions, the inductive 
reactance tends to equalize 

the Unbalanced Capaci- FIG. 35. Schematic diagram il- 
tance of branches Xi and Crating principle of band-selector 

X 2 , provided that the com- 
bined reactance is no greater than that of X^. In that 
case, current will flow through X z of such amount that 
the reactive electromotive force across ab due to the 
currents in X s is equal to that due to the currents 
in Xi and X z . The energy delivered to the output 
circuit will then be equal (neglecting the resistances 
in the circuit) to that delivered when the applied 
frequency is that at which the circuits are mutually 
balanced. If the applied frequency is lower than 
the balance frequency, the capacity reactances in 
Xi, Xi and X 3 combine instead of tending to neutral- 
ize, thus offering a high reactance to the signal. 
By a proper selection of reactance X s , the width of 
band offering substantially zero reactance can be 
controlled to meet any specific requirements. 1 

1 VREELAND, DR. F. K., Distortionless Reception, Proceedings Institute 
of Radio Engineers, Vol. XVI, No. 3, March, 1928. 



A spaced amplifier may be used to amplify the 
output of the band selector. The spaced band ampli- 
fier consists of a number of stages of tuned radio- 
frequency amplification, each stage being slightly 
detuned in such a manner that the composite result 
is the amplification of a band of frequencies. Such 
an amplifier may consist of three 
tuned stages, each individually 
having the characteristic inverted 
V-type resonance curve but 
spaced slightly apart with refer- 
ence to the others, so that the 
overall amplification secured has 
the desired band characteristics, 
as shown in Fig. 36. 

The Superheterodyne Receiver for 
FIG. 36.-Overall re- Television Service. 

The superheterodyne principle 

of reception is especially well 
adapted to the needs of television reception because 
this type of circuit has the advantage of discriminat- 
ing strongly against signals on neighboring channels 
without any sacrifice of stability. Furthermore, 
the incoming signal can be altered to a frequency 
such that a 20 kilocycle band, for example, is 
reduced to a small proportion of the intermediate 
frequency. The Bell Telephone Laboratories em- 
ployed a superheterodyne receiver to pick up a 
1,575-kilocycle picture signal at New York, 22 
miles from the transmitter at Whippany, N. J. 
This signal was impressed upon the output of a 
6,575-kilocycle oscillator and the 5,000-cycle com- 



ponent isolated by a selector circuit. The advantage 
of this method is obvious when it is realized that a 
20-kilocycle band at the transmitted frequency of 
1,575 kilocycles represents 2.6 per cent of the band, 
while at 5,000 kilocycles, the same frequencies repre- 
sent but 0.8 per cent. It is therefore possible to 
employ materially sharper circuits without dis- 
criminating against the higher picture-signal fre- 
quency components. The 5,000-kilocycle circuit 
is combined with a 5,120-kilocycle oscillator which, 
in turn, feeds 120 kilocycles through a two-stage 
intermediate-frequency amplifier. 

The only commercial receiver so far developed 
especially for television purposes is made by the 
Jenkins Television Corporation. Its tuning range 
is from 100 to 200 meters (3,000 to 1,500 kilocycles). 
Three stages of screen-grid radio-frequency amplifica- 
tion are rectified by a non-regenerative power detec- 
tor. This feeds into a two-stage resistance-coupled 
audio amplifier, also utilizing screen-grid tubes, while 
the final power output stage employs a 245-type 
power tube. The audio-frequency amplifier handles a 
range of 15 to 30,000 cycles. 

Radio-frequency Amplification of Picture Signals. 

Regardless of the method employed to select the 
modulated radio-frequency signal and to amplify 
it before the picture signal is separated from the 
carrier, further amplification of the picture signal is 
necessary to actuate the light reproducer. The 
customary method is to employ resistance-coupled 
amplification for television purposes, although, as 
iron-core transformer design has evolved, the resist- 



ance amplifier has been virtually displaced in broad- 
cast reception. 

Figure 37 shows two tubes coupled by a resistance 
and capacity. The impedance of the plate circuit 
of the first tube is large compared with the plate 
resistance of that tube. In other words, the equiva- 
lent impedance of Ri shunted by R 2 and Ci must be 
large compared with the plate resistance R p . The 
coupling capacity should be the smallest which will 

transmit the lowest 
frequency required, 
because the imped- 
ance falls for all 
higher frequencies. 
The useful voltage 
output of the first 

FIG. 37. A resistance-coupled amplifier. tube IS that devel- 
oped across Ri im- 
pressed on the grid of the second tube. The larger 
the capacity of the coupling condenser, the more 
nearly the amplification of the lower frequencies 
approaches the maximum attainable for a tube 
of a given amplification factor. On the other 
hand, the larger the capacity of the coupling resistor, 
the smaller the amplification of the high frequencies. 
Consequently we are constrained to use small coup- 
ling condensers for broad-band response at the 
expense of gain per stage. Since substantial outputs 
are necessary for the television illumination system, 
the inefficiency of resistance-coupled audio-frequency 
amplifiers makes the use of three and four stages 
essential. Screen-grid tubes and, prospectively, pen- 
todes promise continued progress in amplifier design, 



certain to keep pace with the development of tele- 
vision service. The tendency is, therefore, to employ 
equalizing networks as a means of correcting high- 
frequency losses. 

Prospective Developments Affecting Television Receiver 


Recent developments in core materials for magnetic 
transformers foreshadow the availability of audio- 
frequency amplification systems capable of handling 
the broad bands necessary for television picture 
signals. For amplifying the 40,000-cycle picture- 
signal band used in the two-way television system, 
the Bell Telephone Laboratories employed output 
and input transformers with a newly developed 
chrome permalloy core material. The high gain 
per stage possible with such transformers makes 
them decidedly superior to any resistance-coupled 





Embodied in the transmitted picture signal are 
two elements of information: intensity, representative 
of shading, and relative time, representative of 
position. Reproduction of the television signal is, 
therefore, a twofold function; first, varying light 
proportionate to signal intensity, and second, the 
placing of each light impression in its proper position 
in the field of reproduction. 

The Neon-glow Tube. 

The neon tube is generally used for converting the 
picture signal into corresponding light. Its cathode 
is a flat metal plate, slightly larger in area than the 
field of reproduction. The anode is a similar plate, 
usually separated from the cathode by a distance of 
approximately one millimeter. With correct pres- 
sure of neon, and suitable spacing of the plates, the 
cathode dark space is equivalent to the separation 
of the plates, causing the luminous discharge to 
develop on the outer one. The illumination should 
be a thin, uniform, brightly glowing layer propor- 
tionate to the signal voltage. The neon-glow tube 
has proved itself to be adequately responsive for 


Kn MSI i, j||i 


television reproduction of the most advanced systems 
so far developed. 

The light output of a neon tube bears a linear 
relation to the applied current supplied over a wide 
range. The curve shows the characteristics of a 
neon tube, known commercially as the kino lamp 
made by the Raytheon Company. 
It produces approximately . 14 
candle per milliampere which, 
diffused over a plate of this size, 
offers a brilliance of 0.03 lambert 
per milliampere. The change is 
constant throughout a wide range 
of applied voltages. A suitable 
plate potential conveniently 
secured from radio B batteries, 
maintains a fixed potential differ- 
ence between anode and cathode 
of a value at which a slight increase 
in voltage produces substantial 
increase in illumination. The 
makers of the kino lamp recom- 
mend a minimum "dark" current 
of 10 milliamperes. Although the 
apparent resistance of the kino 
lamp is 500 ohms, it functions g ! w tu . be for u 

.V, observaton through 

satisfactorily in the plate circuit a scanning disc, 
of conventional radio output tubes, 
such as the 171-A or the 245. Better results 
are, of course, obtained by matching the output 
load of the vacuum tube with the impedance of the 
tube itself. It is customary to use a series resistor 
as a protective device in order to limit the current 


FIG. 38. A neon- 


flowing through the neon tube. The use of excessive 
direct-current output shortens the life of the tube 
and produces no proportionate gain in the contrast 
between minimum and maximum picture signal. 

'0 5 10 15 20 25 30 35 40 

FIG. 39. Brightness variation of Raytheon neon-glow tube with alter- 
nating-current change. 

Larger Neon Tubes. 

Neon tubes of this general design are satisfac- 
tory for small television outfits. However, as the 
illumination of the field of reproduction and its area 
increases, dissipation of the heat attendant upon 
making the neon luminous becomes a problem. 
Satisfactory 48-line reproducers have been made 
with the conventional type of neon tube, but water 
cooling of the neon tube was found necessary with a 
72-line system of the Bell Laboratories. Figure 40 
shows the rather complex neon tube developed for the 
Bell reproducer used in the first demonstration of 
two-way television. 1 The average plate current 

pages 39-41, 101, 155-157. 



through the tube is 200 milliamperes. In order to 
maintain stable operation, a source of hydrogen is 
continuously connected with the tube to maintain 
a carefully regulated pressure. 

FIG. 40. The water-cooled neon-glow tube used in the Bell Laboratories 
demonstration of two-way 72-line wire television. 

Every known means of producing controlled light 
requires the generation of a considerably larger 
amount of energy in heat. Dependence upon lumin- 
ous gas discharge through a simple tube requires that 
the entire field of reproduction be illuminated and 



that observation be restricted to the particular 
picture element being reproduced. Consequently, 
the tube used must be capable of generating the 
maximum light required for bright illumination of a 
single picture element, multiplied by the total number 
of picture elements in the field of view. Hence it is 
not difficult to arrive at a point where the heat 
generated is far in excess of that which can be radiated 
by a self-cooling device. As we become concerned 
with a larger number of picture elements, therefore, 
it appears to be practically essential that all of the 
light energy produced by means of the picture signal 
be concentrated upon the illumination of each pic- 
ture element in the field of reproduction only for the 
interval that it is to be observed. Dissipation of light 
over the entire area of reproduction when only one 
picture element is observed at a time constitutes a 
waste of energy which cannot be tolerated in com- 
mercial practice. 

A new^ type of neon tube developed by the Bell 
Telephone Laboratories has successfully circum- 
vented this wasteful practice. The tube has a small 
aluminum electrode rather far back from the front 
of the bulb. A lens mounted in front of the tube, 
together with a system of lenses mounted spirally in a 
scanning disc carries all of the light produced directly 
to the eye. This is indeed an efficient optical arrange- 
ment producing a brighter image with a much smaller 
current passing through the neon tube. In fact, 
the small aluminum anode is screwed into a large 
copper cylinder and this alone provides sufficient 
cooling to make it possible to dispense with the 
water-cooling system. 



Color of Neon Illumination. 

The luminous discharge of the neon tube is orange 
in character, a part of the color spectrum to which 
the eye is not particularly sensitive and which tends 
to cause fatigue. In experimenting with the possi- 
bilities of color television, Dr. Herbert E. Ives has 
produced an argon tube quite similar in design to the 
neon tube, which produces illumination rich in the 
green and blue elements of the spectrum. A picture 
signal is transmitted for each of the three prime 
colors and reproduced by two argon tubes and one 



FIG. 41. The Bell Laboratories color-television reproducer. Red, 
blue and green components are provided by one neon and two argon tubes 
with the assistance of suitable filters. Mirrors serve to blend the three 
controlled light sources into a single beam which is observed through a 
scanning disc. 

neon tube. The resulting images are optically 
blended to form a single image successfully combining 
the components of red, yellow and green in their 
correct proportions to produce very remarkable and 
vivid color television. This was demonstrated in May, 
1929, with a 50-line system. 1 Figure 41 shows the opti- 
cal system necessary to combine the three colored 
images into one for the observation of the looker-in. 

1 IVES, HERBERT E., Television in Colors, Bell Laboratories Record, 
Vol. VII, No. 11, July, 1929. 



The Drum Scanner. 

C. Francis Jenkins of Washington, D. C., has 
produced the drum scanner, which is more economical 
of energy at the reproduction point and also dispenses 
with most of the inconvenience of the scanning disc. 
The neon tube in Jenkins' drum scanner has four 
elements to which contact is made successively 
through a rotating commutator arrangement. The 
apertures through which the observer sees the light 
are arranged on a cylinder 7 inches in diameter, 3 

FIG. 42. Jenkins' drum scanner with multiple neon-glow tube. The light 
is conducted to the apertures through quartz rods. 

inches long and J-fe inch thick. Forty-eight scan- 
ning apertures are punched in the drum, four helical 
turns with the holes spaced 2 inches apart and with 
% inch between helices. Quartz rods which rotate 
with the drum conduct the light from the individual 
targets within the neon tubes to the drum apertures. 
The light loss in the inverse square law is thus avoided 
and the energy which would be dissipated is cut by 



three-quarters. Quartz conducts the light from the 
target to the aperture without loss of light energy. By 
using six helical turns and six targets, a 3-inch picture 
is produced, while a 10%-inch drum in six revolutions 
per picture produces a 4-inch square reproduction. 

One of the advantages inherent in the drum scanner 
is the fact that the apertures exposing the illumination 
sweep in a straight line before the eye. The image 

FIG. 43. A four-element neon tube used in Jenkins' drum scanner. 

produced by apertures in a disc are made up of curved 
sweeps, the radius of which is determined by the 
distance from aperture to disc center. Obviously, an 
image made up of straight parallel lines is more easily 
interpreted than one composed of curved lines. 

The television reproducers being offered experiment- 
ers are, more and more, of the drum design. The 



quartz rod and the multiple-element glow tube have 
not, however, made as rapid progress as the drum 
feature of the Jenkins design. The drum has the 

FIG. 44. The rotating switch system which distributes the picture 
signal to the 2,500-element neon tube and opens and closes approximately 
4 0,000 circuits per second. 

advantage of mechanical compactness, making possible 
60-line home reproducers of practical proportions. 

The Neon Grid Screen. 

One of the most interesting forms of picture-signal 
reproducer developed is the Bell System grid screen, 



which has the advantage of illuminating each picture 
element individually as the impulse for it appears in 
the picture signal. It consists essentially of a single 
long neon tube, turned back and forth upon itself, 
making 50 parallel sections. The interior electrode 
is a spiral wire while 2,500 external electrodes of 
metal foil, cemented outside the tube, each repre- 

Fio. 45. The brushing contactor which completes the circuit to the 2,500 
elements of the neon-grid screen. 

sentative of a picture element area in the field of 
reproduction, form the facing electrode. The pic- 
ture signal is distributed to the 2,500 external elec- 
trodes one at a time through a contactor arm, the 
motion of which is synchronized with the scanning 
of the transmitter. The heat energy produced is 
small because only one picture element is illuminated 



at a time and no power is utilized in light production 
except where it is observed by the eye. The contact 
distributor is naturally a rather costly device, and it 
is difficult to imagine it as an element of a single home 
receiver, particularly if considerably more than 2,500 
picture elements are produced. It was this device 
which was demonstrated by the Bell Laboratories 

FIG. 46. The complete grid screen in its frame. 

as screen projected television, a demonstration in 
which Herbert Hoover, then Secretary of Commerce, 
participated in June, 1927. 1 

The Cathode-ray Tube. 

The Braun cathode-ray tube has long been con- 
sidered as a possible source of controlled light for 

1 See pages 38-39, 77, 98-99, 153-155. 



television reproduction. The cathode-ray tube is a 
high-vacuum tube, the anode of which is punctured 
with a small pinhole. Electrons, released from a 
hot cathode, are directed, at an exceedingly high 
velocity, to the anode as 
a result of a high plate 
potential. Normally, the 
electron stream is con- 
ducted off the plate, with 
the exception of the incre- 
ment which falls to a pin- 
hole in the anode. This 
component of electrons is 
directed to a fluorescent 
screen, usually a coating 
of zinc sulphide, at the far 
end of the tube, which 
forms a second anode. 
The tiny pin-point stream 
of electrons is readily di- 
verted as a result of mag- 
netic and electrostatic 
influences. By applying FIQ 47 ._ close . up view of the 

voltages to two plates, Set neon grid screen, showing the con- 

at right angles to each tinuous grid element and some of the 

1 ir 2,500 elements of heavier wire con- 

Other, between the first nected with the switch points. 

and second anodes, the 

electron stream produces visible patterns on the 
fluorescent screen. There are no mechanical or 
moving parts and the device is sufficiently responsive 
to make possible observations of frequencies higher 
than a million. 



The use of the Braun tube for television purposes 
was suggested by Campbell Swinton as early as 1908. 
Various workers in Europe and America have experi- 
mented with cathode-ray television reproducers but 
their exceedingly high cost and short life have stood 
in the way of producing anything practical up to this 

FIG. 48. Dr. V. K. Zworykin examining the cathode-ray television tube 
which he developed at the Westinghouse laboratories. 

time. Dr. Zworykin of the Westinghouse Company 
has recently built a special cathode-ray tube for 
television purposes. It is of the usual conical form, 
playing the cathode ray upon a screen sufficiently 
large to produce an image 3 inches square. There 
are two anodes, the first being the controlling element, 



operating on 300 to 400 volts plate potential. The 
amplified picture signal is used to cause variations 
in the plate voltage which are translated into cathode- 
ray intensities. A second anode accelerates the pencil 
of electrons escaping through the first anode by means 
of a potential of 3,000 to 4,000 volts, focusing the 
beam to a sharp spot on the target which is 7 inches 
in diameter. The fluorescent material is Willimet, 
which is made slightly conductive so as to leak off the 
charge produced by the electron screen. 

Distinction between a Light Valve and a Variable Light 

While the neon tube and the cathode-ray tube 
appear to be adequately rapid to reproduce any tele- 
vision signal for which communication channels are 
available, they seem incapable of producing adequate 
intensities for projection purposes so that large audi- 
ences can be served. If illumination is restricted to 
one picture element at a time, as with the Bell tele- 
vision multiple-element screen, the distribution system 
becomes exceedingly complex and delicate, particularly 
when the number of picture elements necessary to a 
commercial entertainment service is considered. On 
the other hand, if we rely on total illumination of the 
field of view but restrict observation to a single ele- 
mental area, the neon system required for the equiva- 
lent of screen projection becomes of tremendous 

The desirable procedure is the use of a light valve 
having means of controlling the amount of light pro- 
jected on a screen from a powerful local source of 
light in response to a relatively small control voltage. 



The function of the amplified picture signal then 
becomes merely one of controlling the output of a fixed 
light source, a distinctly more reasonable task than 
that of being the energy source for the projected light 

The Kerr Principle. 

Any mechanically controlled shutter cannot be 
expected to operate at the high frequencies encoun- 
tered in television communication. This leads logi- 
cally to the conclusion that a non-mechanical method 
capable of high-frequency operation, such as one of 
the Kerr effects, can be utilized to advantage. John 
Kerr, a co worker of William Thomson (Lord Kelvin), 
the eminent English physicist, observed that double 
refraction occurs when a beam of polarized light is 
passed through a strong electrostatic field. The 
component polarized in the direction of the electric 
force acquires a wave velocity slightly different from 
that transversely polarized. The effect produced is 
proportional to the square of the force across the 

Kerr's conclusions were published by him in 1875. 
His discovery followed logically upon that of Faraday, 
who had observed that the direction of polarization of 
a polarized light beam can be rotated by submitting it 
to an intense magnetic field. The first practical 
television projector, depending on the Kerr effect 
and utilizing a light valve due to Dr. Karolus of 
Germany, was demonstrated by Dr. Alexanderson 
of the General Electric Company at Schenectady 
in May, 1930. It is a remarkable instance of the 
vision of Paul Nipkow, to whom is due the funda- 



mental principle of dissecting the field of view by 
scanning through a spirally apertured disc, that he 
outlined the method of utilizing Faraday's magnetic 
deviation of polarized light for projecting images 
on a screen. Without the aid of vacuum-tube ampli- 
fiers and experience in handling high-frequency mag- 
netic fields, it was obviously impossible for Nipkow 
to embody his disclosure in practical form. 

Inasmuch as the use of polarized light is destined 
to play a significant part in the development of prac- 
tical television service, the principles involved should 
be understood by the student of television. Undoubt- 
edly, new applications of great importance to television 
will be made of the phenomena of polarization. 

Polarized Wave Energy. 

A spot of light, projected on a screen, is produced 
by a stream of light energy set up by a myriad of tiny 
transmitters constituting the light source. Light 
from a candle is secured by oxidation of numerous 
tiny particles of the wick. The illumination of a 
tungsten filament of the conventional electric-light 
bulb is produced by uncounted millions of atoms, each 
heated to incandescence by an electric current. How- 
ever small a beam we secure even from a so-called 
point source of light, an immense number of separate 
sources contribute their quota to the total illumination 

Light is a wave motion which may be likened to 
the waves of the sea or to the wave which a man may 
impart to a rope by a swing of his arm. The rudi- 
ments of polarization can be demonstrated by means 
of a simple set-up consisting of a rope twenty or thirty 



feet long, fastened at one end to a post or wall and 
swung in various ways from the other. When the 
wave in the rope reaches the post, it tends to pull the 
post toward the man until the peak of the crest is 
reached and then the receding portion of the wave 
motion tends to push the post away from him. It 
makes no difference whether the wave is produced by 
swinging the arm horizontally or vertically or diago- 
nally, a similar effect is produced at the hitching post. 
In any case, the direction of propagation is toward the 
post and the wave motion is at right angles or trans- 
verse to the direction of propagation. 

If a fence with a vertical slit cut through it, just 
wide enough to permit the rope to swing freely, is 
erected halfway between the hitching post and the 
man swinging the rope, energy can still be imparted to 
the hitching post, so long as the man swings the rope 
vertically. The effect of the slit is to limit the type 
of wave motion which can be transmitted beyond 
it to a vertical wave form or, as it is termed, to verti- 
cally polarized waves. If, on the other hand, the 
slit is cut horizontally, only horizontally polarized 
waves can be made to reach the pole ; every other type 
expends its energy upon the obstructing fence. 

Polarized and Unpolarized Light. 

Unpolarized light, being produced by a tremendous 
number of uncoordinated transmitters or sources, 
includes wave motion of every conceivable type, 
horizontal, vertical and in every intermediate position 
between these limits. Certain crystals are of such 
construction, however, that light-wave motion in 
only one or two directions can be projected through 



them. When light is projected through a crystal 
of Iceland spar, its path is refracted into two rays. 
One ray obeys the regular optical laws and is called 
the ordinary ray; the other is called the extraordinary 
ray. Both rays remain parallel to the optical axis 
of the crystal. If the crystal is rotated about its 
optical axis, the ordinary ray will remain in a fixed 
position but the extraordinary ray will describe a 
circle. If the ordinary ray is projected through a 
second crystal, it will again divide into two rays, the 
ordinary and the extraordinary. The distribution 
of light energy between the two rays projected through 
the second crystal is determined by the angle of pro- 

/Extraordinary Ray 

FIG. 49. The Nicol prism, showing deflection of the ordinary ray. 

jection of the polarized ray to the axis of the crystal. 
By rotating the crystal through 90 degrees, each ray 
will go through a cycle from zero to maximum. The 
same effect is produced by changing the angle with 
which the polarized ray is projected on a fixed crystal. 
A Nicol prism is an optical device so designed 
that the ordinary ray is totally deflected and only 
the extraordinary ray remains. It is a convenient 
and efficient method of producing plane polarized 



The Karolus Valve Projector. 

The screen television projector built under the 
direction of Dr. Alexanderson of the General Electric 
Company, utilizes the Kerr effect and is known as the 
Karolus projector. A powerful light source, a stand- 
ard commercial motion-picture arc, is first concen- 
trated into a beam by means of a lens system. The 
water cell (Fig. 50) conducts off the heat of the arc. 
The light beam is then passed through a Nicol prism 
so that it is separated into two components at right 

Polarizing Prisms^ 
Condensing Lens- -^ \ 
Diaphragm, \ ! \ 

Condenser Lens 




FIG. 50. The Karolus cell projector lens system which produced a 6-foot 
image on the screen of a Schenectady theater. 

angles to each other. The plane polarized ray is then 
passed through a container of nitrobenzol, which serves 
as the dielectric of a condenser. The picture signal 
received from the television transmission station, 
suitably amplified, is applied to this condenser, with 
the result that the polarized light beam is doubly 
refracted. The applied field may be considered to 
produce a phase shift between the two components of 






FIG. 51. Schematic diagram of the transmitting equipment employed 
in the Karolus projector demonstration conducted by the General Electric 

Antenna Receiving 
Television Signal 

Antenna Receiving 
Voice Signal 


Projector Monitor 

FIG. 52. Schematic diagram of receiving end of television demonstration 
using the Karolus projector. 


the plane polarized light. The second Nicol prism 
is set at an angle such that no light is transmitted 
through it when there is no electrostatic strain in the 
nitrobenzol. Consequently the application of poten- 
tial on the condenser causes light to be transmitted 
through it. For small angles of phase shift, the 
transmitted light is proportional to the fourth power 
of the applied voltage. This relation being far from 
linear, it is necessary to work the device over the 
upper and somewhat restricted portion of its charac- 
teristic to avoid the distortion resulting from its non- 
linear properties. Such operation, in turn, involves a 
sacrifice of contrast. However, the system forms an 
entirely non-mechanical light relay capable of working 
at extremely high speeds, readily controlled by a 
television signal. The resultant light is projected 
to a translucent screen through a scanning disc which 
is synchronized with the scanning disc at the pick-up 

The results accomplished in the initial demonstra- 
tion of this device indicated that means have been 
developed for screen projection for a large audience 
of as good quality as that attained with individual 
peephole reproduction. The screen used at Schenec- 
tady was 6 feet square and the illumination attained 
about half that of the ordinary motion picture. The 
audience, seated 50 feet or more from the screen, found 
the reproduction of 48-line television to be about the 
same quality and intelligibility as if it were reduced 
to but 1 inch square and viewed 10 inches from the 
eye. Projection, however, made possible the serving 
of a large audience instead of a single observer, a 
fact of great economic importance. 





With such an extensive variety of methods for con- 
verting the picture signal to light, the designer of 
television reproduction systems has a wide latitude 
and a wealth of possibilities with which to work. Up 
to this time, the neon tube has been utilized almost 
exclusively but undoubtedly some of the alternative 
devices, such as the cathode ray and the Karolus 
valve, will ultimately prove their merit. 



The final process in television reproduction is the 
projection of the fluctuating light produced in response 
to the picture signal so as to form the moving image 
in the field of reproduction. It is the reversal of the 
scanning process, the reconstruction of the light 
impulses from their series to their original parallel 

The elementary method of accomplishing recon- 
struction is to reverse the scanning process by means 
of a disc identical in construction with that used to 
break down the field at the transmission point. For 
24- and 48-line television, discs of a diameter con- 
venient for home reproduction are easily made, and 
they have been used successfully for exposing a 72-line 

The signal controlled light is spread over the entire 
field of reproduction and viewed by the observer 
through the scanning disc so that his eye response is 
localized at each instant to the particular picture 
element being reproduced. This is at present the 
conventional method of television reproduction but, 
because of its limitations, the scanning disc neon-tube 
combination is likely to be displaced by more flexible 



Alternative methods of distributing the light impres- 
sions in the field of reproduction have already been 
developed, and others will undoubtedly appear. The 
cathode-ray tube, described in the preceding chapter, 
accomplishes control over illumination intensity by 
varying the voltage on the anode and distribution 
of the fluctuating signal so produced by electrostatic 
deflection. Another alternative method is the employ- 
ment of a separate source of illumination for each 
picture element, as with the Bell System neon-tube 
screen. The complex switching system necessary 
with a contact for each picture element reproduced 
militates seriously against its use for high-quality 
television in the home. 

The Minimum Repetition Rate. 

Regardless of the method employed to distribute 
the varying light corresponding to the picture-signal 
fluctuations in the field of reproduction, the same 
visual process is relied upon to collate the separate 
picture-element impressions into a complete image. 
The fluctuating light ray describes a path in the field 
of reproduction corresponding to the scanning pattern 
at the transmission point. The rapidly traveling 
ray brushes the reproducing surface in a series of 
parallel sweeps covering the entire surface within so 
short a time that the eye collates the sum of impres- 
sions into a single image. The scanning cycle must 
therefore be completed, regardless of the number of 
picture elements, within the period that persistence of 
vision makes possible observation of the first, last, 
and all the intervening picture-element impressions 
as virtually simultaneous. Although a bright impres- 



sion may affect vision for Ko second or more, the 
brilliance and contrast attainable in artificial repro- 
duction make ^o second about the maximum time 
which can be allowed for the reproduction of a single 
frame if no flicker is to be experienced. 

Limitations of the Scanning Disc. 

There are two important limitations to the conven- 
tional scanning disc neon-tube combination which 
tend to prevent its use in home reproduction of high- 
quality television. First, the entire field of reproduc- 
tion is illuminated for each picture element, although 
only one is actually observed at each instant; and 
second, for discs of reasonable proportions, good 
quality reproduction is limited to peephole size. 
Although the image produced by the conventional 
method may be optically enlarged, this involves loss 
of illumination and does not increase the information 
furnished to the eye. The amount of illumination 
available with existing neon tubes, particularly when 
the entire field of reproduction must be illuminated for 
each picture element, is too small to permit distribu- 
tion over a larger surface, with the attendant reduction 
in brilliance. 

If we attempt the use of neon tubes with larger 
reproduction areas for observation without the aid of a 
lens system, such as 4 by 5 inches in size, the scanning 
disc assumes unwieldy proportions. Each successive 
aperture in the spiral must then be separated by the 
total width of the field of reproduction. For a 4- by 
5-inch reproduction, broken down to 100 lines, the 
scanning disc must therefore have a diameter of nearly 
12 feet. With 20 repetitions, the rate of rotation of 



such an unwieldy disc is 1,200 revolutions per minute, 
giving it a peripheral speed of 10 miles per minute, 
a speed far greater than is tolerable in any home 
device. Obviously, we cannot use discs of such 
proportions in general practice and must rely on 
enlargement of a small image by means of a lens sys- 
tem. This, in turn, requires a high order of mechan- 
ical accuracy in manufacture and neon tubes of more 
than the ordinary surface brilliance. 

FIG. 55. A neon-glow tube producing highly concentrated illumi- 
nation sufficient for projection to the eye, developed by the Bell Telephone 

The concentrated illumination neon tube described 
on page 106 developed in the Bell Laboratories over- 
comes this inefficiency of the flat plate type tubes by 
projecting its entire illumination to the eye through a 
lens system arranged in a scanning disc. Thus the 
entire effective illumination of the neon tube is used 
to reconstruct the image. 

Utilizing the grid-type neon tube with a separate 
control electrode for each picture element, we over- 



come the illumination intensity problem, but find 
ourselves burdened with a mechanical switching 

FIG. 56. The optical system which projects the concentrated illumi- 
nation output of the neon-glow tube to the eye. A large collecting lens 
gathers most of the light flux which is projected to the eye through the 
lenses of the scanning disc. 

system which assumes discouraging proportions for a 
finely divided field of reproduction. The conclusion 



seems inescapable that any mechanical system of 
light distribution, such as scanning discs or switching 
methods, fails to meet the requirements of a system 
involving more than 5,000 or 10,000 picture elements 
per frame. The prospective solution is the control 
of a fixed powerful light source, as with the Karolus 
cell, or a flexible non-mechanical method of projecting 
a moving light ray, as with the cathode-ray tube. 

Relation of Number of Picture Elements to Brilliance. 

With any system of reproduction depending upon 
a moving ray sweeping over the field of reproduction, 
the maximum observed brilliance depends not only 
upon the intensity of the ray but also upon the length 
of time (within certain limits) that each element is 
illuminated. Since the illumination time for each 
picture element is, with conventional systems, reduced 
as the number of picture elements per second is 
increased, we must, to retain a given standard of 
brilliance, increase the attainable illumination in 
proportion to any increase in the number of picture 

If, however, a reliable means of maintaining illumi- 
nation of the picture element is developed which 
continues the illumination after the picture signal 
devoted to each element has ceased, a light source of 
much lower intensity is required. Suppose a phos- 
phorescent screen were developed which maintained 
the correct illumination one thousand times as long 
as the ray is projected upon each area. Then the 
impulse of one millionth of a second duration com- 
prising the illumination for each element in a 50,000 
picture-element signal with 20 repetitions would 
maintain illumination for 1/1,000 second, or 2 per cent 



of total time, as compared with 0.002 per cent without 

Factors Influencing Persistence of Vision. 

The degree to which reliance can be placed upon 
persistence of vision inherent in the eye is the com- 
posite of a considerable number of factors, such as the 
amount of detail present, rate of action involved, 
brilliance and contrast of projection and room illumina- 
tion. Motion pictures possess considerable detail, 
contrast and brilliance and are projected under favor- 
able conditions of low illumination. 

Although recognizability is readily obtained with 
considerably less than optimum conditions, such as 
those involved in motion-picture reproduction, failure 
to attain at least that standard inevitably introduces 
eye fatigue after more than brief observation. Con- 
sidering the prospective cost of television transmission 
and reproduction, continued commercial support 
cannot be expected for any system which causes eye 
fatigue to its followers. A convenient amount of 
room illumination must also be tolerated, certainly 
more than the average existing in a motion-picture 
theater. The television reproduction must be capable 
of being viewed clearly with the amount of room 
darkening readily secured under normal daylight 
conditions. The probabilities are that such brilliance 
and contrast will be possible only by means of a 
powerful local source of projected light subject to the 
control of the television signal. This requires dis- 
carding of the present method of viewing merely 
the amplified picture signal itself. The Karolus 
projector, demonstrated by Dr. Alexanderson of the 



General Electric Company is a development in this 
direction. 1 

From the foregoing discussion, it is obvious that 
we are far from the ultimate form of light reproducer 
in television. The cruder devices, depending upon 
illumination of a relatively large plate slightly larger 
than the field of reproduction and to be observed 
through a scanning disc, are limited in the number 
of picture elements which they can accommodate. 
Other forms of illuminators, more efficient from the 
energy distribution standpoint, are prohibitively com- 
plex and costly. Perhaps the reason that greater 
progress has not been made in this field is that the 
simpler means have been adequate for the most 
complex television devices built to date. 

Since we can depend upon so simple and fundamen- 
tal a phenomenon as discharge of electrons through a 
vacuum for producing luminosity, the flexibility of 
these facilities makes it probable that this element 
of television reproduction will keep pace with progress 
in other parts of the system. The ease with which 
an electron stream in a vacuum is controlled at high 
frequencies and the rapid increase in our knowledge 
of electron control in a vacuum foreshadow rapid 
developments and opportunities for the display of 
inventive ingenuity. 

So long as we are confined to small values of light 
controlled by the picture signal, we must content our- 
selves with peephole television. This means observa- 
tion of an image created by projecting light directly 
to the eye of the observer. When larger sources of 
light energy are available, the intensity of which can 

1 See pages 120-124. 



be directly controlled by the picture signal, we can 
then observe a projected image, similar to that made 
on a motion-picture screen. The only means so far 
available for producing light as a result of a varying 
picture signal is dependent upon such relatively feeble 
devices as the neon tube. Varying the intensity of 
the primary illumination source by means of electron 
discharge tubes may not yield a sufficiently powerful 
light source for screen projection. Dr. Alexanderson 
some years ago demonstrated the possibilities of 
television projection if we could project seven intensely 
powerful sources of light simultaneously throughout 
their range of illumination by means of seven received 
picture signals. 1 We are therefore constrained to 
controlling the projection of a powerful light source 
of fixed value, as is done with motion-picture projec- 
tion. But, electromagnetically controlled shutters 
for accomplishing this purpose appear to be hopelessly 
slow, being limited to 400 or 500 picture-signal impulses 
per second at the most. A multiplicity of such light 
shutters at once leads us to a relatively complex 
machine which may be suited only to the serving of 
large audiences paying admission. 

Magnifying the Image. 

By means of lenses and magnifying mirrors, it is 
possible to project the reproduced image to a con- 
venient size. It must be remembered, however, that 
every magnifying glass and prism used tends to reduce 
the illumination reaching the observer. The most 
convenient size for a television reproduction is best 

1 E. F. W. Alexanderson, at annual meeting of the Institute of Radio 
Engineers, New York, January 10, 1927. 



determined by the number of picture elements, the 
attainable brightness and the distance of the field 
of reproduction from the observer's eye. 

Figure 68, page 202, shows a form of projector 
developed by the Jenkins Television Corporation for 
enlarging a 48-line reproduction. The lens forms an 
image having about four times the area of the repro- 
duction formed on the plate of the neon tube as viewed 
at a convenient distance from the eye. 

The area of reproduction should never be so small 
that detail is produced which is not discriminated by 
the eye. On the other hand, it is preferable that the 
image be reduced to a size such that normal detail is 
actually reproduced. In the next chapter, the relation 
between density of picture elements and correct 
viewing distance is taken up. 

Reproduction of Color Television. 

The reproducing system of the Bell Laboratories 
color television system illustrates the flexibility of 
optical systems. Instead of a single reproducing 
light source, the image from three glow tubes is pro- 
jected to the eye through a semi-transparent mirror, 
two lenses and a scanning disc. One of the glow tubes 
is of the familiar neon type, which projects the "red" 
image. A filter excludes all but the red component of 
the glowing neon. Two argon tubes, rich in their 
output of green and blue, contribute those elements 
of color through suitable filters. Three signals are 
projected, one for each of the three glow tubes, and 
the resulting images superimposed for simultaneous 
exposure to the eye. The writer witnessed a demon- 
stration of this apparatus conducted under the super- 



vision of Dr. Herbert E. Ives. The coloring of the 
subjects was brilliant and realistic without a trace 
of distortion due to imperfect superimposition of the 
three images. 1 

The successful reproduction of color requires accu- 
racy of a somewhat higher standard than is necessary 
for black and white images. With the latter, if the 
light source fluctuates around too high or too low a 
value, the reproduction is too light or too dark, as the 
case may be. With color reproduction, however, 
each of the three light sources must fluctuate about 
precisely the correct value, or else marked distortion 
in color will result. Except in those few elements of 
the scene which correspond in color precisely to that 
of one of the filters, each patch of color is formed by 
the combination of the three colors in their correct 
proportion, and an incorrect blend results in a strik- 
ingly unnatural appearance, producing distortions 
which only a trained eye can analyze. The eye is 
trained to accommodate itself to a wide range of light 
intensities, but it cannot readily supply missing color 
elements. Furthermore, the disintegration of the 
image into colors, necessitating the projection of 
beams of colored light, requires light sources of con- 
siderably greater total luminosity to equal the effective 
brilliance of a white light producing a black and white 

1 See page 106. 



The conventional television picture signal conveys 
two kinds of information: (1) the intensity of light 
reflected from the picture element being scanned at 
each instant and (2) the relative position of the picture 
element with respect to the other elements of the field 
of view. The former information is conveyed by the 
intensity of the picture signal, the latter by the time 
relation to the beginning of transmission. Since 
scanning is conducted along a rigidly maintained 
pattern and at an accurately established rate, there 
is a specific instant, a definite interval after the begin- 
ning of each frame, assigned to the transmission and 
reproduction of each elemental area. 

Essentials to Faithful Reproduction. 

To secure a reproduction accurate enough to permit 
the reconstruction of the field of view at distant 
reproducing points without observable variation, the 
following elements must correspond: 

1. The reproducing light source must accurately 
follow the light intensities reflected to the light- 
sensitive element at the transmitter. 

2. The reproducing scanning disc or corresponding 
distributing system must follow out the same relative 
course as the scanning process at the transmission 
point, describing the same number of lines per scanning 



and bearing the same ratio of width of horizontal line 
to total height of field of reproduction. 

3. The reproducer disc or distributor must revolve 
at precisely the same rate as the transmitting disc or 
other scanning means. 

4. The reproducer disc must expose or illuminate 
the elemental area of the field of reproduction corre- 
sponding to the area of the field of view being scanned 
at all times. 

Each of these four elements contributing to faith- 
ful reproduction depends upon the electrical and 
mechanical similarity of transmitting and reproducing 
means. Taking each of these four elements in order, 
the performance requirements are: 

1. Amplification of photocell output, modulation 
of carrier, transmission through wire or radio channel, 
amplification, rectification and further amplification at 
the receiving point and reconversion of picture signal 
to light must be accomplished without discrimination 
in favor of or against any of the essential frequency 
range or, in absence of accurate transmission and 
reproduction of any part of that frequency range, 
suitable means of equalizing or correcting such inaccu- 
racy must be provided. 

2. Distribution of resultant light intensities in 
the field of reproduction must be carried on by an 
electrical or optical system having the same charac- 
teristics as the transmitting scanning system; for 
example, if scanning discs are used both at transmitting 
and reproducing point, both discs must have the same 
number and relative arrangement of apertures. 

3. The motive power controlling the electrical or 
optical distribution of the light produced by the 



picture signal must revolve at the same rate of speed 
as the corresponding motive power source affecting dis- 
integration of the field of view at the transmission point. 
4. The picture element in the field of reproduction 
controlled by the picture signal must correspond in 
position to the picture element of the field of view being 

Distinction between Framing and Synchronization. 

It is important to distinguish the difference between 
each of these factors because they involve different 
parts of the transmitting and reproducing system. 
For example, the transmitting and receiving system 
may work in precise harmony as to number and 
arrangement of apertures, the power source motivating 
both scanning discs may operate at precisely the same 
speed and the reproducing light may follow accurately 
the light variations reflected to the light sensitive 
element, but inaccurate reproduction may, neverthe- 
less, occur. The transmitting scanning disc may be 
exposing the top line of the field of view; the receiving 
disc, on the other hand, may be exposing a line near 
the center of the field of reproduction. In that case 
the observer will see the upper half of the field of view 
on the lower half of his reproducing area, while the 
lower half of the field of view will appear to him on 
the upper part of his field of reproduction. Correct- 
ing the position of the reproduction in the field is 
called framing. Maintenance of correct rate of 
distribution in the field of reproduction corresponding 
to the scanning rate is called synchronization. 

We have already considered the essentials to faith- 
ful transmission and reproduction of picture signal so 



that the reproducing light intensity corresponds to the 
light intensity reflected by the scanning disc to the 
light sensitive element. Correspondence of the dis- 
tribution system reconstructing the image at the field 
of reproduction to the evolutions of the system scan- 
ning or disintegrating the field of view requires, 
obviously, similar scanning discs or signal distributor, 
both as to number of lines and as to relative proportion 
of length of line to height of reproduction. It is 
desirable, as practical television services develop, that 
standards for numbers of lines and proportions of 
field of view be agreed upon, in order that reproducers 
in the hands of the public be adapted to reproduction 
of television images from any transmitter within 
range. If such standards are not observed, separate 
television reproducers will be required for each method 
of transmission employed. 

Effect of Inaccurate Synchronization. 

Before considering the various means of maintaining 
synchronization, it is advantageous to determine the 
accuracy required and the effect of lack of synchroniza- 
tion of varying magnitudes. The crudest conceivable 
television capable of reproducing simple objects in 
very slow motion is one of 20 lines exposed 10 times 
per second. The scanning disc speed is therefore 
600 r.p.m. Suppose the reproducing disc revolves 
one half of one per cent faster than it should, or 603 
r.p.m. The transmitting disc scans 20 lines per 
revolution or 12,000 lines per minute. In other words, 
it requires 1 /1 2,000 minute to scan each line. The 
reproducing disc also scans 20 lines per revolution, 
but makes 603 revolutions per minute. Therefore 



it requires but 1 /12,060 minute to expose a line of 
the field of reproduction so that it is already advanced 
in the second line when the transmitting disc begins 
to scan the second line. With each line, the disparity 
in the position of the two discs is increased. 

The effect of this gain is to shift the framing of the 
field of reproduction progressively from left to right 
or right to left (depending on direction of rotation of 
the scanning disc) at the rate of one complete dis- 
placement each 20 seconds. With 20 repetitions per 
second, instead of 10, the rate of displacement is 
naturally doubled. Many systems of synchronizing 
by means of governors or braking systems which 
have been proposed do not maintain sufficient con- 
stancy to keep speed even within the broad limit of one 
half of one per cent. While an image fading out of 
frame continually is not a serious matter with an 
experimenter, it is a sufficient annoyance to a non- 
technically inclined person seeking television enter- 
tainment to make such a system inacceptable. 

Accuracy of Synchronism Required. 

With a horizontal system of scanning of 50 lines 
from top to bottom, a representation of a picture 
element is transmitted each 1 /2,500 of the time 
required for each repetition. Assuming a repetition 
rate of 20 per second, each picture element is assigned 
1 /50,000 second for transmission. Each element is 
provided for in systematic order according to the 
design of the scanning system. With the conventional 
spiral arrangement of apertures, the fourth picture 
element of the top row is scanned 3 /50,000 of a second 
after the first element; the first element of the second 



row, 50 /50,000 or 1 /1, 000 second after the first element 
of the frame and the last picture element on the 
bottom row, 2,499 /50,000 second after the start. 

Obviously both the disintegration and the recon- 
struction of the field of view must be accomplished by 
an exceedingly stable mechanism which carries out its 
regular program accurately as to rate and position. 
The maximum deviation from synchrony which is not 
automatically corrected by changes in the speed of 
the reproducer should not exceed the width of half of 
a single picture element per frame. In the case 
cited, the scanning disc motor should hold its speed 
to within 1 part in 5,000 for each revolution. With 
a 50-hole disc, the holes are spaced 7.2 degrees apart, 
each picture element being 0.02 of the total sweep per 
line. The angle represented by a picture element is 
0.1440 degree. Since the maximum deviation which 
can be tolerated is half that angle, synchrony must be 
within 0.07 degree. 

Since the accuracy of synchronization required is 
a function of the picture elements per revolution of the 
scanning disc, experience will no doubt establish a 
definite minimum ratio of synchronizing frequency to 
maximum picture-signal frequency necessary to hold 
the reproduction steadily before the observer. When 
reliance is placed on synchronous motors at both 
terminals, it appears that the synchronizing signal, 
however incorporated in the system, requires a fre- 
quency which is in the order of 5 per cent of the maxi- 
mum picture-signal frequency. 

Early Attempts at Synchronization. 

The first experimental television kits distributed for 
amateur use depended upon manual speed control for 


maintaining synchrony. The most popular method 
was the employment of a fairly constant speed direct- 
current motor which rotated the scanning disc through 
a friction clutch. C. Francis Jenkins first described 
such a system to encourage amateur experimentation. 
Another system attempted by Pilot's engineers, who 

FIG! 57. Side view of Pilot television reproducer demonstrated in 
October, 1928. It produced a 48-line image, one and a half inches square. 

experimented for some time through WRNY in New 
York, employed a magnetic clutch arrangement shown 
in Fig. 58. 



Dr. Alexanderson built a number of television 
receivers for home use, employing synchronous motors 
with a resistor connected at the motor terminals 
through a push button. By skillful manipulation, 
deviation from synchrony is corrected by allowing 
the rotor to slip out of "mesh" as the field is weakened 
with the aid of the resistor. The same principle is 

FIG. 58. One of the first complete television reproducers offered by 
the Pilot Electric Company in 1929. A magnetic clutch was intended to 
maintain synchrony. 

employed with direct-current motors. While such 
devices seem to meet the requirements of experi- 
menters, they fall far short of providing care-free 

Although experience has demonstrated that manual 
synchronization is a feat calling for an order of dexter- 



ity and an exhibition of patience beyond the capacity 
of the average experimenter, reliance on manual 
methods has persisted. One system after another 
has been exploited only to be wrecked on the shoals 
of unsatisfactory synchronization. The hope once 
reposed in manual synchronization is indicated by 
the statements issued in connection with Dr. Alexan- 

FIG. 59. Rear view of the Pilot friction-drive television reproducer. 

derson's television reproducer when it was demon- 
strated to the press on January 13, 1928. 

Whenever television has been discussed in the past there 
has always been some pessimist who has wound up the 
discussion by asking "how are you going to synchronize?" 
The answer has always been that we will have a synchro- 





nous motor and transmit a special synchronizing wave or 
synchronize to the picture frequency or to a tuning fork. 
But all these devices mean higher cost, special amplifiers, 
and more things that may get out of order. We, therefore, 
simply decided to leave out all this complication. We took 
a standard electric motor made for household use and are 
manipulating its speed by an electric hand control. With a 
little practice and coordination between the eye and the 
hand, it is possible to hold the picture in the field of vision 
as easily as one steers his car on the middle of the road. 
In special cases, when the transmitting and receiving sys- 
tems are on the same power network, the machines may be 
operated by 60-cycle synchronous motors. 

After describing the facilities for transmitting 
television signals, Dr. Alexanderson continued: "We 
feel that the inauguration of this new development 
will be the starting point of practical and popular 
television." 1 

David Sarnoff, President of the Radio Corporation 
of America, declared on this occasion, 

While this is an historical event, comparable to the early 
experiments in sound broadcasting, the greatest signif- 
icance of the present demonstration is in the fact that the 
radio art has bridged the gap between the laboratory and 
the home. Television has been demonstrated both in 
this country and abroad prior to this event, but it did not 
seem possible within so short a time to so simplify the elabo- 
rate and costly apparatus of television reception that the 
first step might be taken toward the development of 
television receivers for the home. 2 

1 Statement released by Information Bureau, Radio Corporation of 
America, January 14, 1928. 

2 Statement released by News Bureau, General Electric Company, 
January 14, 1928. 



A few months later, representatives of the General 
Electric Company, testifying before the Federal 
Radio Commission, stated that no conclusive evidence 
of regular and successful reproduction of their trans- 
missions had been reported and cessation of the 
television schedule had been accepted without protest. 
A more convincing test of the possibilities of populariz- 
ing television relying on manual synchronization can 
hardly be conceived, combining the most distinguished 
scientific auspices, the finest technical publicity and 
exploitation organization and an outstanding broad- 
casting station of considerable power and coverage, 
all cooperating at a time that public interest in tele- 
vision was at a maximum. But Dr. Alexanderson 
and his associates have not in the least relaxed their 
researches and, as we have already seen, are responsible 
for demonstrating the only fundamentally new method 
of television reproduction since the neon-glow lamp, 
for which Dr. D. MacFarlan Moore, also of the General 
Electric Company, is responsible. 

Power-line Synchronization. 

The simplest method of synchronization which does 
not require a special communication channel for trans- 
mission of a synchronizing signal is dependence upon 
the uniform 60-cycle frequency supplied through power 
lines for driving synchronous motors. In many parts 
of the country, there is sufficiently widespread inter- 
connection of power lines to provide a reference 
frequency within the service range of a television 
station. On the other hand, there are many populous 
centers served by non-interconnected power services. 
For example, in the vicinity of New York City, elec- 



trically independent power systems in Queens, Brook- 
lyn, New Jersey and the direct-current districts of 
Manhattan do not offer an early possibility of general 
synchronization through power lines, although the 
widespread availability of television service would 
probably force interconnection. The point is fre- 
quently made that synchronously driven electric clocks, 
accurate within a few seconds a day, prove that even 
non-interconnected systems adhere closely to the stand- 
ard 60-cycle frequency. However, it is customary to 
check total cycles every hour or two and to make 
corrections by running power supply alternators 
above or below 60 cycles to compensate for deviations 
which may have occurred. Therefore, while there 
may be an average of 216,000 alternations every hour, 
the minute to minute variations may be sufficient in 
some instances to make television synchronization 
among non-interconnected systems difficult. 

Effect of Voltage and Load on Synchronous Motors. 
Utilizing the conventional four-pole synchronous 
motor, the difficulty of maintaining a stabilized phase 
relation also complicates the power-line synchroniza- 
tion method. Such a motor, when operating at full 
load, with a unity power factor, has an angular phase 
displacement of about 20 electrical degrees between 
the impressed and back potentials. This corresponds 
to 10 mechanical degrees, taking into account the 
fact that the motor has two pairs of poles. If the line 
voltage varies, the phase angle decreases as the volt- 
age increases and vice versa. Likewise, if the load 
varies, the angle increases as load increases and vice 
versa. These variations, particularly in areas having 



industrial loads, are sufficient to introduce serious 
irregularities in television reproduction. Therefore 
synchronous motors with many pairs of poles are 
necessary to maintain constancy of speed between two 
or more remotely located synchronous motors on the 
same power circuit. 

To maintain a 50-hole scanning disc in satisfactory 
synchrony, within the limit of half a picture elemental 
area, requires maintaining the correct position of the 
scanning disc within 0.07 degree. To indicate how 
precise a proposition this is, the angular twist of a 
1-inch steel shaft 6 feet long operated at rated load is 
practically that amount. As the number of picture 
elements increases, the precision required becomes 
increasingly delicate. 

Synchronizing Systems Depending on Photoelectric Cells. 
A scheme for synchronizing proposed by Paul L. 
Clark utilizes a photoelectric cell. The intensity of 
the neon lamp is controlled by the picture signal and 
projected to the eye through a form of prismatic 
disc. A small part of the reproducing light is diverted 
through a grating of slits to a photoelectric cell. When 
the motor falls out of synchrony, the light, instead of 
reaching the cell, is intercepted by the grating, causing 
a change in the output of the cell, which, duly ampli- 
fied, weakens the field strength of the motor through a 
special control field coil, stepping up its speed. There 
may be a possibility of developing some such system, 
when neon tubes of illumination sufficient to actuate 
light-sensitive cells are available. The system, as 
disclosed, makes no provision for discriminating 
between reduced illumination due to the light-sensi- 



live device caused by non-synchrony and that caused 
by a dark part of the field of view. If a part of the 
field of view, for instance, the first picture element of 
each line, is devoted to synchronization by this means, 
the gain is questionable, because the same frequency 
space might be devoted to the transmission of a timing 
signal for controlling the motor directly without the 
need for diverting the all too scarce illumination 
from the field of reproduction. In view of the prob- 
able improvement in controlled light sources and light 
response elements, the general principle of the use of 
photoelectric synchronizing control depending on the 
reproduction illumination source may, however, 
become of practical value in the future. 

An inherent difficulty in all systems depending upon 
correction of deviations from synchrony by mechanical 
or electrical means is due to their normal cycle of 
operation. The speed of the device they are intended 
to govern inclines to "hunt" within the limits of 
control employed. The higher the frequency of 
control the narrower these limits and, likewise, the 
higher the frequency of fluctuation; the fundamental 
principle is not altered. For example, the tendency 
of the system may be to permit the motive source to 
revolve slightly above correct speed and to check that 
tendency by a remotely controlled means of slowing it 
down to the correct speed. In actual practice, all 
such governor systems tend to go through a cycle 
of overspeed, which is then overchecked; the momem- 
tum gathered to build up to correct speed again brings 
the motor to overspeed, and this cycle tends to main- 
tain itself. 



When the picture signal itself is used for synchro- 
nization purposes, difficulty is usually experienced 
because of the influence of picture-signal variations on 
the control system. To avoid this the natural tend- 
ency is to isolate a small part of the picture elements 
or field of reproduction illumination in order that it 
may be devoted exclusively to speed control. This 
brings us logically to a separate synchronizing signal 
filtered out from the frequency band devoted to 
television transmission. 

Magnitude of the Synchronization Problem. 

Important as is accurate synchronization to reliable 
television service, comparatively little attention has 
been given its development up to this time. Many 
public demonstrations have been made under condi- 
tions eliminating this vital factor essential to regular 
service or utilizing methods which cannot be provided 
practically for home use. When both scanning motor 
and reproducing disc are mounted on the same shaft 
without the intermediary of the communication chan- 
nel, service conditions are not simulated and the 
capabilities of the system under service conditions 
cannot be conclusively determined. 

Some television stations are radiating picture signals 
without any provision for synchronization. Owners 
of reproducing apparatus are expected to synchronize 
their equipment by the haphazard process of manual 
control, a juggling achievement which appeals at 
first because of its sporting character but sooner or 
later becomes utterly boring to the most hardened 
experimenter. Perhaps the only instance in which the 
synchronizing problem has been fully .met in a demon- 


stration of radio television was in that conducted 
by the Bell System in 1927 between New York and 
Whippany, N. J. The apparatus required is fully 
described in the October, 1927, issue of the Bell System 
Technical Journal and the reader who would pursue 
this subject in greater detail is advised to make a 
thorough study of that issue, which includes com- 
plete descriptions of all the apparatus used in that 

The Bell Telephone Laboratory's Synchronizing System. 

In order to maintain an accuracy of half a picture 
element, this system required synchrony within 0.07 
degree. A special synchronous motor with 120 pairs 
of poles was devised, which, with 20 degrees normal 
full-load displacement, maintained synchrony within 
Y degree of mechanical phase displacement. This 
motor (when supplied with an alternating current of 
2,125 cycles) revolves at 1,062.5 revolutions per 
minute (the product of 17.7, the number of repetitions, 
multiplied by 60). It is of the variable reluctance 
type, giving one cycle per tooth. This type was 
selected because it simplifies the coil arrangement, 
there being but eight armature coils instead of one 
for each coil, as required with the usual synchronous 

In order to make unnecessary an amplifier system 
of sufficient output to furnish power for driving a 
relatively large scanning disc, the amplified 2,125- 
cycle synchronizing current is used solely for speed- 
regulation purposes. The major driving power for 
the scanning disc is secured from a direct-current 
power having inherently poor regulation, while the 



2,125-cycle synchronous motor is used to maintain 
accurate synchronization. 

Inasmuch as the synchronous motor may interlock 
in any of 120 positions, all but one of which are out 
of synchrony, a .special 17.7-cycle synchronizing signal 
is used to establish the position of rotors of the trans- 
mitting and reproducing motors in correct angular 
displacement. This frequency is supplied to the field 


FIG. 61. Schematic diagram of synchronizing circuits used in the 
Bell System Washington-New York 50-line radio and wire television 

through a pair of slip rings tapped to two opposite 
commutator bars of the direct-current motor. When 
the two motors have been interlocked by this syn- 
chronizing signal, the 2,125 cycle is then thrown in 
to establish accurate synchrony. Because 17.7-cycle 
pulses are not readily transmitted through communica- 



tion channels, it is necessary to cause this frequency 
to modulate a 760-cycle carrier. Both the 760- and 
2, 125 -cycle synchronizing currents are transmitted 
through the same communication channel and isolated 
through suitable filter systems. 

While this system of synchronization proved effec- 
tive during the series of demonstrations conducted, it 
is obviously complicated and costly. Two synchro- 
nizing frequencies are required and two successive 
operations are necessary before actual synchronization 
is effected. 

The Improved Bell System Synchronizing Method. 

For the 72-line television system, first demonstrated 
in June, 1930, a somewhat simpler synchronizing 
system was developed. 1 The scanning disc was 
motivated by a four-pole, compound-wound, direct- 
current motor, having an auxiliary regulating field, 
controlled by a vacuum regulator. A special damping 
winding was developed which permitted the total 
flux of the motor to increase or decrease as required 
by the regulating circuit, but which opposed any 
tendency of the flux to shift back and forth across the 
pole face. A synchronizing frequency of 1,275 cycles 
was supplied from a vacuum-tube oscillator, the out- 
put of which was made available at both terminals 
of the system through a special wire line circuit. 
On the shaft of the direct-current motor was a small 
pilot generator which delivered 1,275 cycles at the 
desired speed of operation. The outputs of the control 
oscillator and of the pilot generator were both coupled 
to the grids of a push-pull amplifier system which 

1 See pages 39-41, 101, 104-106. 



impressed its output on a bank of three regulator 
tubes. If the controlling frequency and the pilot 
generator were delivering the same frequency which 
occurred when synchrony was established, the detector 
tubes delivered the maximum output to the regulator 

To start the system, the switch was closed, applying 
three-phase power from the slip rings of the motor 
to the transformer. As the motor reached the operat- 
ing point, a beat frequency between the output of the 
pilot generator and that of the oscillator was set up, 
observable as a movement of the needle of the meter 
through which the current passed to the regulating 
field. When the exact operating speed was obtained, 
the beat frequency in the regulating field fell to zero. 
Then when the motor tended to speed up, the phase 
relationship between the pilot generator and the oscil- 
lator reached a point tending to give maximum strength 
to the regulating field. Further increases in the motor 
speed were checked by the increased field, so that 
the speed tended to fall until the phase of the pilot 
generator with respect to the oscillator had reached 
the equilibrium value, after which the motor continued 
to operate at a constant speed. 

Instead of obtaining the 1,275-cycle current direct 
from the transmitting station by wire or radio, it may 
be supplied from an independent local oscillator of 
sufficient stability to maintain synchrony between the 
transmitting and receiving motive systems. 

As a further measure to insure stability of rotation 
of the scanning disc, a hydraulically damped coupling 
between the motor shaft and the disc was provided, 
consisting of a flexible metal bellows filled with oil 



and connected by a small pipe with a needle adjusting 
valve. This synchronizing system proved remarkably 
effective, no swinging of the subjects from side to 
side being observed during the demonstrations. Com- 
plex as it appears, the method was a marked simplifica- 
tion over its predecessor. 

Short-wave Synchronizing Signal. 

Various proposals have been suggested for distribut- 
ing synchronizing signals on a national scale by means 
of a standard transmitted through a centrally located 
short-wave station, so that each television broad- 
casting .service will not be compelled to radiate a special 
synchronizing signal for those utilizing its programs. 
So long as such synchronizing signals are received 
with constant volume, such a reference frequency is 
prospectively useful. Any marked fluctuation in 
the signal, however, is reflected in the power delivered 
to the synchronous motor and this, in turn, affects 
its phase displacement. Considering the fading and 
fluctuations experienced with short-wave transmis- 
sions, a separate system of synchronizing signal broad- 
casting is likely to be as extensive as that required 
for the broadcasting of the picture signals themselves. 
Since it is a great operating convenience to combine 
the transmission of television programs with their 
own synchronizing signals, the latter method rather 
than national distribution of a reference frequency is 
likely to be the practice for at least the initial stages 
of television service. 

Because of the varying character of the transmission 
quality of the ether medium between any two points 
at different frequencies, it is preferable to utilize a 



frequency for synchronizing purposes as nearly adja- 
cent to the picture-signal frequency as possible. A 
separate synchronizing band removed from a picture- 
signal band by any appreciable amount is likely to 
introduce difficulties which are not readily compen- 
sated for because the fluctuations in the transmission 
quality of the separated frequencies will not carry 
out the same cycle of variations. A reference fre- 
quency of variable amplitude may nevertheless be 
exceedingly useful in checking the accuracy of an 
independent synchronizing frequency source. 

Independent Sources of Stable Frequencies. 

The possibility that a stable source of high-fre- 
quency currents for driving synchronous motors can 
be developed which will eliminate the necessity for 
remote control has received some attention from 
authorities in the field. Piezo-crystal oscillators have 
been built which maintain their frequency to 1 part 
in 1,000,000, but none so far developed is sufficiently 
simple in character to show much promise as units 
in a commercial home receiver to be sold at a reason- 
able price. The quartz crystals must be accurately 
ground; their temperature must be accurately main- 
tained to a fraction of a degree centigrade and the 
power supply to the vacuum tube associated with them 
must be exceedingly constant. There are so many 
uses for a constant and accurate source of high-fre- 
quency currents that considerable research effort is 
being expended in this direction. There is always 
the hope that some day a new and practical method 
will emerge from the laboratories for setting up high- 



frequency currents of such stability and accuracy that 
synchronization in television will become a completed 

Judging from the results now being obtained with 
crystal-controlled vacuum-tube oscillators under labo- 
ratory conditions, it should be possible to maintain 
frequencies constant to 1 part in 10,000,000. It is 
not possible to adjust the frequency of a crystal 
oscillator to an absolute value and maintain it to that 
value within these limits, but two or more independ- 
ently controlled frequencies may be made to retain 
their original values within these limits. Two crystal 
oscillators having nearly the same frequency were 
set up in the Bell Laboratories so that variations in 
the low difference frequency could be determined 
accurately. By the method used, the relative accu- 
racy, assuming one oscillator to be constant, could be 
determined to 1 part in 500,000,000 during each 
5-second interval. Slow variations, having the period 
of the thermostat operation and amounting to a few 
parts in a hundred million occurred, but, over a period 
of 4 hours, the total variation resulting was less than 
1 part in 10,000,000. J These results were obtained 
under most exacting laboratory conditions with 
precise temperature control and under the supervision 
of experienced engineers. They foreshadow the possi- 
bility of stable oscillators of somewhat lower yet 
adequate standards for television synchronization 
purposes, which require only initial adjustment at the 
beginning of the program. 

1 HARRISON, W. A., Frequency Measurements, Bell Laboratories Record, 
Vol. VI, No. 6, August, 1928. 



Combination of Local and Remote Synchronization. 
Dr. Y. K. Zworykin, of the Westinghouse Company, 
in his system of high-speed still-picture transmission 
and reproduction, has employed a synchronizing 
method which combines the use of a stable local source 
of a controlling frequency with a synchronizing pulse 
from the picture-signal source, effecting an economy 
of channel space required for a synchronization signal. 
The method is readily adapted to television purposes. 
The source of frequency at both terminals is a 70-cycle 
tuning fork in a constant temperature box. The 
transmitting and receiving terminal forks are adjusted 
so that there is not more than one beat between them 
in 20 seconds, an accuracy of about 1 part in 1,500. 

Osci/Mor ' ^'I'l > "Shunt Held 


FIG. 62. Synchronizing system used by Dr. V. K. Zworykin in his 
facsimile system. 

The fork at the receiving machine is corrected at every 
revolution of the picture-recording drum by an impulse 
of about y% cycle duration. This synchronizing pulse 
is transmitted over the same channel as the picture 
but on the margin of the paper to avoid interference 
with the picture signal. A direct-current motor 
provides the torque, while the output of the tuning- 
fork controlled oscillator is supplied to a second wind- 
ing on the same rotor, after suitable amplification. 



The final stage of the tuning-fork amplifier consists 
of two UX 250 tubes in push-pull arrangement. While 
this principle has not yet been applied in television, it 
may be susceptible of development for this more 
exacting service. 

Framing the Image. 

For reproduction at the proper position in the field, 
it is necessary not only to maintain synchrony between 
transmitting and receiving systems, but also to assure 
that the reproduction is properly framed. 

If we could place the neon-glow tube in any position 
along the periphery of the scanning disc, rotating in 
synchrony, we could invariably find one position at 
which the image is correctly placed in the field of 
reproduction, so that the outermost hole of the disc 
exposes the neon tube as the top line of the field of 
reproduction is being scanned. In all other positions 
of the neon tube, however, the image would be improp- 
erly framed. The top of the scene, for example, might 
be at the middle, with the bottom just above it, 
producing an effect occasionally observed in carelessly 
conducted motion-picture projection. 

In practice it is convenient neither to change the 
position of the neon tube or to observe the image 
upside down or on its side. The reproduction must 
not only be correctly framed, but it must be so framed 
at the top of the scanning disc, where provision is 
made for convenient observation, sometimes with the 
aid of a suitably mounted lens. One method which 
might be used is to shut off the power from the syn- 
chronous motor for an instant, so that it would lose 
just enough speed to slip into the correct position. 



Naturally, it would be next to impossible to time this 
interval so accurately that reliable framing could 
be accomplished by this method. On the other hand, 
the method of utilizing a synchronizing pulse corre- 
sponding to the picture signal, for placing transmitting 
and reproducing discs in phase, already described on 
pages 153-157, has the disadvantage of requiring an 
extra synchronizing signal, a special starting operation 
and considerable apparatus at both terminals. 

Fortunately, a practical manual system of great 
simplicity is available which has been widely used by 
many experimenters. Instead of revolving the light 
source around the periphery of the scanning disc to 
find the position of correct framing, the frame of the 
motor is mounted in a helical gear, so that the entire 
motor can be revolved into any position by means 
of a worm gear meshing with the helical gear. The 
scanning disc continues to revolve in synchrony while 
the framing adjustment is made. 

This chapter concludes the description of some of 
the principal instrumentalities of television as they 
have been developed up to this time. Many of them 
now universally used are doomed to discard; others, 
perhaps the least conspicuous, will expand in utility; 
finally, new devices and methods will come forward 
to displace the weak links in the system. But funda- 
mentally, television is bound to retain, in one form 
or another, the basic elements of the systems described. 
The field of view must be disintegrated by a scanning 
process so that it may be reduced to an electric counter- 
part by a light-sensitive system, or as many light- 
sensitive systems as there are parallel communication 
channels to be used; the scanning process must be 


carried on with sufficient rapidity to give a smooth 
reproduction of subjects in motion; the resulting signal 
or signals must then be impressed on the radio trans- 
mission system, to be picked up by an antenna system 
and associated receiving apparatus, restoring it to 

FIG. 63. The 2,125-cycle synchronous motor, with 120 pairs of poles, 
used to drive the reproducing scanning disc. The worm drive is used to 
adjust framing. 

electrical form; the counterpart of the original picture 
signal or signals must control a light source for pro- 
jection or observation; and the scanning process must 
be reversed to restore the original arrangement of light 



The final reproduction is the ultimate purpose of the 
entire process. The impression which that reproduc- 
tion makes upon the observer is clearly a function of 
the visual system. Therefore, the information which 
the eye requires to discern the elements of a scene 
without conscious effort or strain must now be 




The human eye, the instrument of vision, consists 
essentially of an adjustable lens system which focuses 
an image of the field of view on the retina. Disposed 
on the surface of the retina are a vast number of 
light-sensitive elements called rods and cones. The 
lens system of the eye is quite similar to that of a 
photographic camera, having means both of adjusting 
the focus to adapt it to fields of varying depth and of 
increasing or decreasing the aperture according to the 
available illumination. In place of a photographic 
film which makes a single permanent impression of 
the image focused upon it, the photoelectric elements 
of the eye form a continuous communication circuit 
with the brain by means of a separate nerve com- 
munication channel for each rod and cone. The eye 
is, therefore, quite similar to a television transmission 
apparatus except that it requires no scanning device 
because there is always available for each picture 
element a complete communication system to main- 
tain a continuous contact with the brain. 

Adjustments of the Eye. 

The eye is capable of a considerable variety of 
adjustments which are being made almost continuously 



without conscious control. The muscles of the lens 
system focus the eye according to the extent of the 
field of view being observed and its distance from the 
eye. The iris controls the size of the aperture accord- 
ing to conditions of illumination, protecting it from 
excessive light or admitting every feeble ray within 
range. The muscles of the eyeball, sometimes assisted 
by the neck, control the direction which is viewed. 
The six extrinsic muscles rotating the eyeball are the 
most active and responsive muscles of the human 
system. The field of view to which the eye responds 
extends more than 90 degrees outward, 70 degrees 
downward, 60 degrees inward and 50 degrees upward. 
The two eyes combined, therefore, have a field of 
nearly 200 degrees in extent horizontally and 120 
degrees vertically. 1 The eye is highly sensitive to 
motion in this extensive field, although it perceives 
practically no detail or color, except for a restricted 
angle near the center of the field. Mechanically the 
eye is most remarkable, but optically it is deficient 
in many ways. 

Viewing a Television Reproduction. 

With so extensive a range of possible adjustments 
of focus, admitted illumination, sensitivity and direc- 
tion, it is of interest to determine what adjustment the 
eye assumes when viewing a television reproduction. 
Naturally it receives the most concentrated attention 
of the eye which, therefore, positions the eyeball so 
that the image of the television reproduction is formed 
on the most sensitive part of the retina, known as the 

1 COBB, P. W., Physiological Optics, "Illuminating Engineering," John 
Wiley & Sons, Inc., 1928. 



fovea. Here the eye is sensitive both to the maximum 
detail and to the finest discriminations of color. 

Regardless of whether the television reproduction 
is viewed a few inches from the eye through a peephole 
device or projected on an extensive screen, it is sub- 
jected to the most exacting scrutiny of which the 
eye is capable. When viewing a screen projection, 
the extent of the image may be too large to be com- 
prehended in its entirety without progressive explora- 
tion by the aid of the extrinsic muscles. In that case, 
the eye selects that part of the field of reproduction 
of the greatest interest at the moment and subjects 
it to the most exacting scrutiny, within the limits of the 
resolving power and sensitivity of the visual system. 
Apparently the television reproduction must meet 
most exacting requirements if the eye is not to be 
conscious of defects. 

Color of Reproduction. 

The sensitivity of the eye to color varies with the 
intensity of illumination. If the entire spectrum of 
colors is observed under very weak illumination, no 
color can be discriminated. The spectrum under such 
conditions appears gray, but varies in brightness, being 
brightest at 0.53//, 1 corresponding to green. As 
illumination is increased, the eye becomes rapidly 
adapted and the brightest part of the spectrum is 
then at 0.58/i 1 in the yellow. This indicates that a 
screen or field of reproduction feebly illuminated by a 
green light source will appear brighter to the eye than 
if any other part of the range of light frequencies is 

1 See page 59. 



utilized. This phenomenon! has not yet been used to 
advantage in any television reproduction system. 

If green or blue light is gradually increased in 
intensity from zero, there is a considerable interval 
before the color can be discriminated. This is called 
the photochromatic interval. With red light, the 
photochromatic interval is practically non-existent. 
Consequently a field of reproduction illuminated by 
a neon-glow lamp appears red if it is perceived at all. 
On the other hand, if the light source were a feeble 
green, the eye would not be conscious of the color. 
Since red is a rather unnatural color for most scenes, 
it is unfortunate that the neon-glow lamp gives rise 
to red rather than to green, because the observer would 
not be conscious of an unnatural color in a feeble 
green reproduction. While glow tubes giving rise to 
green and other colors are available they require 
considerably more power to produce a given brilliance 
than the neon type. 

Extensive experiments with flickering images indi- 
cate that color does not affect the duration of persist- 
ence of vision. Therefore, selection of the most 
favorable color frequency from the standpoint of 
sensitivity and contrast does not affect favorably or 
unfavorably the phenomenon of persistence, upon 
which the collation of picture elements into a single 
image depends. 

Brilliance of Reproduction. 

In view of the difficulties of securing adequate 
illumination of the reproduction, the television image 
is most favorably viewed in total darkness. Under 



such conditions the iris gradually relaxes so as to admit 
the greatest possible amount of light. 

The light-sensitive elements on the retina are 
described as rods and cones. It is believed that the 
cones are excited only by bright illumination, while 
the rods are the sensitive elements used for observa- 
tion at low intensities. Twilight or night vision, 
presumed to be accomplished by the rods, is known 
as "scotopic" vision. Everyone has noticed the 
feeling of temporary blindness experienced on entering 
a motion-picture theater. At first it is difficult to 
find one's way about or to recognize objects until the 
eye has adapted itself to the conditions of night or 
scotopic vision. After 5 or 10 minutes in darkness the 
eye has become quite well adjusted, but does not 
reach its maximum sensitivity until some 30 or 40 
minutes in comparative darkness. When the eye 
has fully adapted scotopic vision, it is 1,500 to 8,000 
times more sensitive than in full daylight. The 
contrast required for a good television reproduction 
viewed in conditions of approximate darkness is 
therefore small. 

The sensitivity of the eye to contrast varies under 
different conditions, detecting a minimum change in 
relative contrast of from 0.5 to 1.7 per cent. The least 
difference in brightness which can be perceived is 
not a fixed quantity, but is more nearly a constant 
fraction of the brightness itself. Under average 
conditions contrast of 1 per cent is just sufficient to 
be perceivable; that is, two areas varying in brilliancy 
by a ratio of 1 : 1.01 will appear as just different. This 
corresponds to the decibel in measurement of sound. 
The contrast sensitivity of the eye is reduced if the 



areas compared are seen on a background much 
brighter than themselves or if relatively large light 
flux reaches the eye from any direction within its 
field of view. 

A reproduction can be analyzed to determine the 
range of contrast required for realistic representation. 
A silhouette is reproduced by only two shades, which, 
to be barely discernible, need only vary by 1 per cent 
in intensity. The other extreme is the availability 
of the entire range from black to white, offering 
approximately 100 to 200 discernible shades, according 
to the maximum brightness. Clearly a much more 
satisfactory reproduction, as far as shading is con- 
cerned, can be secured merely by improving the maxi- 
mum brilliance attainable from the reproducing light 
source, even though no new detail is imparted to the 
original picture signal. The feeble illumination so 
far used in television reproduction obviously fails to 
portray to advantage the degrees of shading embodied 
in television signals of the order now employed. Any 
judgments made on the service value of television are 
therefore subject to modification when more effective 
methods of illuminating the reproduction are evolved. 

Either contributing toward or furnishing the reason 
for clearer distinction of contrast when viewing a 
brilliant reproduction is the fact that the iris reduces 
in diameter according to the brilliancy of the field 
observed. If the aperture admitting light to the 
retina is reduced, the image is sharpened, exactly in 
the same way that the greatest detail is obtained with 
a camera by reducing the diaphragm opening. With 
a large opening of the iris, occurring under conditions 
of feeble illumination, a point in the field of view is 



diffused on the retina over a relatively large area, 
blurring the detail of the image. Obviously there is a 
substantial gain in sharpness and contrast as the 
brilliancy increases in viewing a given field of reproduc- 
tion composed of a given number of 'picture elements. 

Interfering Illumination. 

A factor influencing the responsiveness of the eye 
to small contrasts is the interfering influence of neigh- 
boring illumination. The eye, as has been said above, 
is sensitive to light over a range extending 200 degrees 
horizontally and 120 degrees vertically, although its 
concentrated attention is focused only upon a small 
conical space directly before it. The presence of a 
relatively powerful light source within this broad angle 
of response at once increases the brightness and con- 
trast necessary to secure a discernible image in the 
center of the field of view. 

While it is possible to darken a room to a certain 
degree without inconvenience, there are practical 
limits to such darkening. If the television perform- 
ance is to be continuous and available without inter- 
fering with other activities in the home, then a fair 
amount of general room illumination must be tolerated. 
In a motion-picture theater, the minimum illumination 
generally considered satisfactory is one-tenth of a 
foot-candle while the film is being shown, and the 
preferred practice in modern theaters is to provide at 
least five times this value of illumination intensity. 
If one member of the family watches a television 
reproduction as others are occupied with different 
pursuits, a minimum of one foot-candle of general 
room illumination is necessary for comfort and safety. 



At the same time, the difficulty of darkening an 
average room to one foot-candle or less during bright 
daylight hours must also be taken into account, 
because the most attractive television programs, such 
as sporting and news events, will be available prin- 
cipally during daylight hours. The amount of inter- 
fering light with projected television is likely to be 
fairly substantial, making quite brilliant reproduction 

One simple method of disposing of any problem of 
surrounding illumination is to provide a peephole 
arrangement which obscures from the observer's eye 
all light except that from the television reproduction. 
Such arrangements introduce two problems which 
tend to counteract the advantage gained. First, 
unless the arrangement is exceedingly bulky, the 
reproduction is at a distance from the eye such that 
it is most perceptive of minute detail, and, therefore, 
most critical to defects. Second, the observer is 
forced to attempt to enjoy his television reproduction 
in a rigidly fixed position, not only of the head and 
body, but more particularly of the eye itself, which 
becomes rapidly fatigued if vision is concentrated 
upon the same part of the retina continuously. 

Advantages Gained by Projection. 

All of these factors make the conclusion inescapable 
that projected television, even though the screen 
area be small, is a minimum requirement to general 
home service. This at once makes us subject to 
the interfering influences of general room illumina- 
tion, and in turn requires considerable contrast and 
brightness in the field of reproduction. The average 


16-millimeter home projector uses a 200- watt incandes- 
cent lamp directing a ray of 50 to 60 lumens to the 
surface of the screen. The picture width is about 4 
feet and the throw about 20. If the illuminating 
source were withheld from the motion-picture screen 
for an average time as short as would be encountered 
for a television system of 100,000 picture elements, the 
power of the light source would have to be one of 
immense magnitude. 

The effect of projection is to increase the dimen- 
sions of the field of reproduction. The actual size 
of the field so far as the eye is concerned is not 
measured by its physical dimensions, but by the 
angle formed by its extreme dimensions with the eye. 
From the standpoint of visual effectiveness, an image 
may be greatly increased in size without reducing its 
realism, provided the observer moves proportionately 
far away and the proper amount of illumination is 
maintained. So long as the overall angle which the 
eye must explore to view the whole field is suitably 
limited, the entire field may be comprehended by a 
single observation. 

How the Eye Explores a Scene. 

In practice, the eye prefers a field of reproduction 
sufficiently large to enable it to roam so as to intro- 
duce the proper rest periods to different areas of the 
retina. Even in such an apparently restrictive 
operation as reading, the eye obtains its necessary 
periods of rest. Concentrated attention is restricted 
to a small area. The eye in reading a single line 
jumps to four or five positions and rests at each for 
from Ko to Y second according to the speed of the 



reader. In other words, the eye assumes a rapid 
succession of fixed positions, requiring not more than 
Kooo second to make the changes from one to the 
next, and it rests in each position only a fractional 
part of a second. 1 Yet each such change is resting a 
part of the highly sensitive and responsive fovea. 

It appears then that we do not view a scene in a 
motion picture in a complete comprehensive sweep, 
but rather take a series of impressions over a rela- 
tively constricted area. The eye naturally focuses 
its attention upon the element of the scene where the 
crucial action which is of the greatest interest to the 
observer is taking place. For example, anyone hav- 
ing presented to him an airplane view of his home 
town will almost automatically select familiar land- 
marks and will attempt to identify his own residence 
as viewed from above. This concentration upon 
small parts of a comprehensive field establishes 
exacting requirements for the detail which must be 
present throughout the reproduction so that the 
observer may select any part of it for his attention 
at any instant with complete satisfaction. 

Entire Field Must Possess Good Detail. 

The selective properties possessed by the eye can 
not be conveniently transferred to the television 
scanning or reproducing system. So as to simplify 
the communication system required, it is conceivable 
that high-quality scanning might be restricted to a 
small area at the center of the field where the most 
intense action takes place. Imagine an observer in 
a plane looking at a vast field of view below him 

footnote p. 166. 



through a powerful spyglass which limits his observa- 
tion to one spot in the field of view. He may sweep 
the spyglass throughout this field according to his 
desire for information, but unless he could precede 
his selective operation by a comprehensive glance at 
the whole field, he would be likely neither to find the 
points of special interest to him nor to secure a 
comprehensive understanding of the relation of the 
various parts of the scene which he is able to observe. 
Suppose a television scanning apparatus were 
substituted in place of the spyglass. The reproduc- 
tion would then consist only of the area selected with 
the spyglass scanning system. Considerable detail 
could be conveyed to the observer without an exces- 
sive use of the channel facilities, but he would not be 
able to form a vision of the whole scene as a single 
unit. It appears then that, in order that the eye 
may enjoy its habitually employed selective powers, 
satisfactory high-quality portrayal of scenes through 
television means a communication system even more 
comprehensive than that possessed by the eye itself. 

Eye Response to Motion. 

The eye cooperates very favorably to encourage 
appreciation of contrast when motion is portrayed 
in the reproduction. If the eye is focused on a white 
spot in a black field and the white spot is then 
removed, that area actually appears blacker to the 
eye. The eye, being accustomed to lagging behind 
major alterations in shading, tolerates sluggish 
operation of a television reproducer in portraying a 
scene in which black and white are shifting rapidly. 
Furthermore, the eye is extremely sensitive to changes 



in a scene, observing them more acutely than sta- 
tionary elements. A person can be seen moving, 
"out of the corner of the eye," even though the eye 
discriminates nothing of the stationary elements of 
that part of a scene. The existence of active motion 
in a scene commands the concentrated attention of 
the eye, to the detriment of the stationary elements 
of the field. When motion ceases, the eye tends to 
review or refresh its memory with respect to the 
stationary elements, which it has temporarily neg- 
lected so as to concentrate on the motion. A blurring 
of the detail in the stationary elements of a field 
while following a rapidly moving figure or object, 
may escape unobserved, particularly if detail is 
restored when the motion ceases. The allowable 
distortion of a field involving rapid motion is there- 
fore greater than one of stationary character. 

Persistence of Vision. 

The most familiar characteristic of vision is 
persistence. Without reliance upon persistence of 
vision, there could be no television and no motion 
pictures. An impression directed to the retina is 
impressed upon the consciousness some time after the 
source of the impression is removed. The duration 
of persistence is influenced by the brilliance and the 
duration of the originating impression and by the 
presence of interfering light. There is no magical 
quantity, such as Jf Q second, which has been popu- 
larly assumed as the precise duration of persistence 
of vision. Prior to the advent of sound motion 
pictures, the standard rate of filming was 16 per 
second, but reproduction was carried on at almost 



any speed ranging from 20 to 24 frames per second, 
the former if the program was ahead of schedule and 
the latter if the management was cramped for time. 
Naturally, the action in the film is a bit hasty if the 
higher speed is maintained, but only then is the 
observer conscious of any variation in the reliance 
placed upon his persistence of vision. With sound- 
on-film, overspeeding produces a substantial rise in 
pitch of the accompanying sound, with the result 
that standard speeds of reproduction are more 
closely adhered to. Sixteen frames per second is 
the minimum which makes the observer unconscious 
of the interval between frames and, with the order of 
illumination and brilliance obtaining in motion- 
picture reproduction, gives a smooth blend of 
successive frames. A different quality and intensity 
of illumination would call for a different optimum 
repetition rate. 

In a general way, an image formed on the retina 
persists >^ to %o second. If the brilliance and 
duration of the original image is such that persistence 
is only J<5 second, 25 repetitions is the minimum 
which gives the impression of smooth motion; on the 
other hand, under suitable conditions, 15 repetitions 
would be sufficient with a bright image, each element 
of which remains illuminated for a major part of the 
interval between frames. 

If we assume a brilliance of reproduction and dura- 
tion of originating impression such that maximum 
reliance can be placed on persistence of vision, the 
minimum repetition rate giving smooth motion is 
determined by the rate of the action in the scene. 
Persons in ordinary controlled motion and views 



from a slow-moving vehicle would be satisfactorily 
reproduced with the aid of a picture signal represent- 
ing the slowest repetition rate, that is, 15 per second. 
But rapidly moving figures, such as are involved in 
athletic sports or scenes taken from a high-speed 
vehicle, require a higher repetition rate. 

On the other hand, the briefer the period each 
element is presented to the eye, the greater the 
brilliance necessary to produce an impression. Con- 
versely, as the speed of motion is reduced, the 
necessity for rapid scanning falls off, up to the point 
where the whole mass of picture elements constituting 
one reproduction can no longer be collated as a 
single image through persistence of vision. Con- 
sidering the slow-motion subjects so far presented 
for television programs, the use of higher repetition 
rates than the minimum required by the eye is 
induced by inadequate brilliance of reproduction and 
short duration of originating image rather than by 
any inherent inability of the eye to form a smooth 
succession of images from the reproduction. 

The ease with which an object is perceived is 
influenced by its size as measured by the angle which 
its extreme dimensions present to the eye. The ease 
of perception increases from the minimum angle 
discerned by the eye up to two degrees, beyond 
which no further increase in ease of perception takes 
place, because the eye begins to explore the elements 
of a larger object. There is a reciprocal relation 
among the size of the object viewed, the time of 
exposure, and the brightness, which determines the 
ease of perception. Any reduction in one of these 
three features must be accompanied by a correspond- 



ing increase of one of the others up to certain limits 
to secure equally good perception. 

When portraying a scene in which there is no 
motion, the comprehension of each reproduction 
improves proportionately to the length of time that 
the reproduction endures. Leisurely examination 
permits exploration for desired detail, which is not 
indulged in when the eye is supplied with subjects of 
interest by reason of their motion. When the 
activity of the subject is a routine operation, readily 
taken for granted, such as speaking and singing, the 
motion is not of an arresting character and the eye 
therefore calls for more detail. It should be clear 
then, particularly in view of the further investigation 
of detail in the succeeding chapter, that detail 
requirements vary greatly according to the subject 

Necessary Detail in Television Reproduction. 

The amount of detail necessary to an enjoyable 
television reproduction of a scene of given character 
is a matter which seems to vary according to the 
optimism of the observer. The foregoing explanation 
of the characteristics of visual response indicates that 
no fixed formula can be established which describes 
quantitatively exactly the detail necessary to a 
perfect illusion in a reproduction. The brilliance of 
the reproduction, the amount of interfering light and 
the amount and character of the action in the scene 
materially alter the requirements as defined by 
number of picture elements per reproduction. The 
psychological influence of accompanying music and 
vivid speech also modifies any requirements which 



may be set up. Ingeniously planned visual features 
will take the fullest advantage of the psychological 
responses of the observer. The more clearly the ear 
is satisfied by the information delivered to the brain 
through that channel of communication, the less 
exacting are the demands of the eye* Dreamy music 
suggests an indefinite reproduction, while a lecture 
describing and depending upon elements in a scene 
which are not reproduced emphasizes the shortcom- 
ings of television. Nevertheless, it is of advantage to 
analyze the resolving powers of the eye because they 
give one measure, though only one, of several deter- 
minants of the information which must be com- 
municated to the eye of the television observer. 




The term detail is used in television parlance to 
describe the degree to which a reproduction reveals 
the fine particulars and small elements of the scene 
which it represents. Quality has a broader meaning, 
designating the composite of all the factors, such as 
detail, brilliance, contrast and size, which contribute 
to the fidelity of the reproduction and the realism of 
the illusion created. 

Satisfying the Information Requirements of the Brain. 
Detail is defined by the smallest element of a scene 
which can be discriminated as a definite identity by 
the eye. If the angle formed by drawing two 
imaginary lines from the extremities of any object to 
the retina of the observer is larger than the resolving 
angle of the eye, the detail can be discriminated, 
provided the illumination is sufficient. The closer 
one moves to an object, the larger the angle formed 
by two lines reaching from its extremities to the 
retina. When an object in which the observer is 
interested is not clearly distinguishable, he instinc- 
tively moves closer to it, in order that he may enjoy as 
much detail as the information requirements of his 
brain demand. He is rewarded with more detail 
until he is within ten inches of the object. Closer 



than this, distortion is introduced by reason of the 
sharp angle formed from the object to the retina. 

So that more detail can be discriminated, we have 
developed means of "fanning out" the light rays 
reflected from an object so that finer elements of 
which it is composed exceed the resolving angle of the 
eye. The microscope is such a device. With natural 
objects we can spread out or enlarge the image which 
an object forms on the retina of the eye up to the 
point where each source of a light wave forms an 
angle to the neighboring wave which is less than the 
resolving angle of the eye. This point is the limit 
of magnification of the microscope. If greater 
detail is to be observed, rays having a wavelength 
shorter than light waves must be used. X-ray 
analysis, which has enabled physicists to establish 
the arrangement of electrons within the atom, 
depends precisely upon this phenomenon. 

Conversely, when viewing an extensive scene, we 
tend to move away from it until all the elements 
composing it are conveniently observed simultane- 
ously. When we are interested in the relationship 
of objects, one to the other, we would rather move to 
such a distance that all of them may be observed at 
the same time. If it is physically impossible to 
station ourselves at such a distance, we draw a map 
or chart so as to bring all the essential elements of the 
scene within the limits of simultaneous observation. 

Requirements of the Eye Are Definite. 

All of this leads to a fundamental conclusion: For 
a given subject, there is an amount of detail which 
satisfies the information requirements of the brain. 



The detail required is dictated by the subject. If the 
demand for detail is not satisfied, we tend instinctively 
to alter our position with relation to the object so that 
the required detail is observed. 

When we are indulging in more than casual obser- 
vation, as, for example, visiting an art gallery, we 
willingly go to considerable effort to secure satisfac- 
tion of the information requirements of the eye. We 
move closer to enjoy the precise detail of a Dutch 
master, and further away to sense the comprehensive- 
ness embodied in the work of a modern landscape 
painter. Apparently the information requirements 
of the eye are definitely dictated and must be satisfied 
if the attention is to be held. Since television is 
designed primarily for satisfying the visual sense and 
not merely for casual observation, we are quickly con- 
scious of any lack of satisfaction of the information 
requirements of the eye. 

Detail in a Reproduction. 

When viewing a natural object, we can usually 
observe it as closely as we desire within the limits of 
the optical capacities of the eye. When viewing a 
representation or reproduction such as a painting, a 
photograph or a motion picture, we are, however, 
limited to observation of the actual detail embodied 
in the reproduction. For example, a designer of 
fine hardware may look at paintings in a gallery in 
order to find suggestions from the craftsmen of the 
middle ages. He may see a painting of a Venetian 
doorway. Enthusiastically, he takes a notebook out 
of his pocket, moves close to it and sketches such 



details as he desires. By moving close to the 
painting he has satisfied his information require- 
ments. At another point in the gallery, he sees a 
painting of the ancient Hotel de Ville in Bruges. 
Again he hopes to obtain a suggestion. He examines 
the painting closely, but the door hinges, in this case, 
are merely a tiny stroke of the artist's brush. He 
takes a small magnifying glass from his pocket, but 
finds that instead of fine detail, enlargement merely 
shows the ridges and depressions formed by the 
brush. No amount of magnification of a reproduction 
will impart detail which is not embodied in it. 

Establishing the Requirements for Television. 

The principles of detail and enlargement which 
have been discussed control the following questions 
respecting television reproduction: 

1. What is the minimum detail necessary to 
secure a satisfying reproduction of subjects of various 
kinds ? 

2. What is the correct viewing distance for a repro- 
duction of a given size composed of a given number 
of picture elements? 

3. To what extent may a reproduction of given 
detail be enlarged to advantage? 

The detail embodied in a television reproduction 
of adequate brilliance and contrast is directly propor- 
tional to the number of picture elements. The size to 
which a reproduction of a given number of picture 
elements may be enlarged is determined by the angle 
formed by imaginary lines drawn from the extremities 
of each picture element to the eye. 



Resolving Power of the Eye. 

One method of estimating the requirements in 
picture elements per field of reproduction for com- 
mercial television of the highest useful quality is to 
use the resolving power of the eye as the basis of 
calculation. The resolving power of the eye, as we 
have seen, is the minimum angle by which two points 
may be separated without losing their identity as two 
separate points. The resolving power of the eye is 
therefore a measure of the maximum useful detail 
with which a reproduction may be endowed. 

A television reproduction having a wealth of detail 
such that each picture element has a dimension so 
much smaller than can be discerned within the 
limitations of the resolving power of the eye cannot 
be improved upon by increasing the density of the 
picture elements. Since the resolving power of the 
eye is a measure of an angle, the maximum dimen- 
sions of a picture element giving the highest quality 
of detail which the eye can take advantage of can be 
determined if the distance of the plane of the field of 
reproduction from the observer's eye is known. 
Considering that this reproduction will possess 
perfection of detail, it may be quite small and 
still portray subjects of considerable complexity 

A field of reproduction 9 by 12 inches viewed at a 
distance of 6 feet would make a small but clear 
reproduction of both indoor and outdoor scenes fully 
capable of holding sustained interest, provided 
illumination and contrast were adequate. This 
dimension is, of course, considerably smaller than the 
4-foot width to which 16 millimeter home motion 



pictures are usually projected, but two prize fighters 
in a ring or even a football game clearly reproduced to 
this size could hold a fascinated television audience. 

The resolving power of the eye is by no means an 
arbitrary or fixed angle. It is influenced by such 
factors as the diameter of the pupil, the brightness 
level to which the retina is adapted, the illumination 
intensity and the contrast of the subject. I am 
indebted to Lloyd A. Jones of the Physics Depart- 
ment of the Eastman Kodak Research Laboratories 
for the conclusion that a fair average value for the 
resolving power of the eye under the conditions under 
which motion pictures or television reproductions 
are viewed is 60 seconds of an arc. The resolving 
power of the eye ranges from 35 to 75 seconds, but 
extensive researches in the Eastman Kodak Labora- 
tories have established 1 minute of an arc as a fair 
average value for the resolving power of the eye 
under the conditions of brightness, contrast and visual 
adaptation obtaining with artificial reproduction. 

In sweeping across a path 12 inches in length at a 
distance of 6 feet from the eye, the angle covered is 
9 degrees 28 minutes, which at 1 minute per ele- 
ment requires 568 picture elements per line for the 
field of reproduction proposed. Since the height of 
the reproduction is three-fourths its width, the 
number of lines is 426. Hence a total of approxi- 
mately 245,000 picture elements is required to 
produce a field of reproduction of perfect texture. 
With twenty repetitions per second, the frequency 
band required with double side-band transmission is 
2,450,000 cycles, assuming our conventional methods 
of continuous progressive scanning. 



The quality of reproduction with this texture and 
with the dimensions and distance from the observer 
cited is theoretically perfect, no improvement in 
detail being attainable either by increasing the 
density of picture elements or by having the observer 
move closer to the field of reproduction. The 

FIG. 64. A telephotograph consisting of 250,000 picture elements, 
or approximately the definition of a 500-line television reproduction. 
(Courtesy of Bell Telephone Laboratories.) 

conditions suggested are therefore quite practical, the 
optimum distance for perfect quality being sufficient 
to permit a family group to view the television 
reproduction, at the same time permitting increase of 
the size of the reproduction either by moving closer 
to it or by projecting it to a larger size, with propor- 
tionate loss of detail. 

Peephole Television Requires as Much Detail as 

Projected Television. 

For "peephole" television, suited only to observa- 
tion by one individual the equivalent of headphone 



reception of broadcasting perhaps the best value to 
select is with the field of reproduction 10 inches from 
the eye. This is the approximate distance at which 
the average eye observes the greatest detail. If the 
field of reproduction is 4 by 5 inches, the eye is 
capable of observing a scene of considerable wealth 
of detail and discerning items constituting but a 
small proportion of the whole field of reproduction. 
Fully as much information is imparted to the eye 
with such a reproduction system as by the finest 
quality of rotogravure printing. 

In sweeping across a distance of 5 inches in a plane 
10 inches from the eye, the angle traversed is 26 
degrees 34 minutes, requiring 1,594 picture elements 
per line. The height of the field of reproduction 
requires 1,308 lines, making an approximate total of 
2,080,000 picture elements for a reproduction of 
perfect texture. This is nearly ten times 245,000, 
the number required by the 9 by 12-inch reproduction 
to be viewed at a distance of 6 feet. Obviously, the 
reduction in dimension is not sufficient to compensate 
for the added density of picture elements made 
necessary by the reduction of the distance at which 
the reproduction has been placed from the eye. This 
calculation leads us to the conclusion that so far as 
channel requirements are concerned, projected tele- 
vision is less difficult of accomplishment than high 
quality peephole television. 

If, however, we content ourselves with a reproduc- 
tion % by 1 inch, the dimensions of the familiar 
motion-picture film negative, to be viewed 10 inches 
from the eye, the reduced area results in a marked 
conservation of communication requirements. For 



perfect texture, we require 257 lines, each consisting 
of 342 picture elements, or a total number of 87,894 
per field of reproduction. At 20 repetitions per 
second, a 900 kilocycle channel is required for the 
transmission of the resulting picture signal. 

The following table shows the dimensions of 
reproduction to which an image consisting of 87,894 
picture elements may be enlarged to retain theoreti- 
cally perfect texture at various distances from the 

87.894 Picture Elements 

Dimensions of 

Distance Projection, 

from Eye Inches 

10 inches . 75 by 1.0 

2 feet 1.8 by 2.4 

4 feet 3 . 6 by 4.8 

6 feet 5.4 by 7.2 

8 feet 7.2 by 9.6 

10 feet 9.0 by 12.0 

20 feet 18.0 by 24.0 

40 feet 36 . by 48 . 

80 feet 72.0 by 96.0 

100 feet 90 . by 120 . 

Mere Enlargement of Present Systems of Limited 


The claim is often made that, the principles of 
television having been demonstrated successfully 
with a system involving relatively few picture 
elements, it requires merely improvement of the 
system to bring it up to commercial entertainment 
requirements. Logical as this reasoning is, the full 
significance of the problems involved in such improve- 
ment is not appreciated without calculations such as 



the foregoing. The successful accomplishment of a 
reproduction % by 1 inch is too readily accepted as 
proof that the image can be increased to 4 by 5 
inches without changing the viewing distance, for 
example, by reasonable increase in the size and 
capacity of the apparatus involved. 

A better index to the problen is secured by compar- 
ing the maximum picture-signal frequencies involved. 
The 87,894 picture elements forming a % by 1-inch 
reproduction, which would certainly be considered no 
more than an interesting demonstration by the 
public, require a channel facility of 900 kilocycles for 
20 repetitions; to increase the size of reproduction to 
4 by 5 inches without reducing density is a matter of 
somewhat more than twentyfolding the capacity of 
the apparatus; it calls for a system of terminal and 
channel equipment capable of handling some 2,340,- 
000 picture elements, presenting a problem of about 
250 times the magnitude of the first. 

Two hundred and fiftyfolding the speed of any 
process, particularly one involving such delicate 
accuracy as television, is not to be considered without 
the deference which is its due. Automotive engi- 
neers, for example, do not hesitate to build racing 
automobiles capable of speeding 150 miles an hour, 
but they hardly consider an order to build a car to 
travel 250 times as fast, as a reasonable demand. It 
is no more likely that such a speed will be attained by 
merely increasing the power, speed, or dimensions of a 
motor vehicle built along present-day conventional 
lines than that a high-quality commercial television 
reproducer of 1,000,000 picture elements per frame 

will be perfected merely by Detailed improvements of 



FIG. 65. The pictures on the left are the equivalent of what could be transmitted over ten 
broadcast channels; those at the right, over twenty. (Courtesy of Bell Telephone Laboratories.) 



established instruments and methods. Fundamen- 
tally new principles must be applied, involving 
ingenuity of a far higher order than is required for 
mere enlargement or general speeding up, or else the 
hope of large reproductions of perfect texture must 
be abandoned. 

Picture Elements in Motion Picture. 

A motion picture viewed in a theater at a distance 
of 50 feet is an example of commercial reproduction 
of high quality. The usual dimensions of such a 
screen are 18 by 24 feet. Based on the resolving 
power of the eye at a distance of 50 feet, perfect 
texture is secured with approximately 1,800,000 
picture elements, a number smaller than is required 
for the 4 by 5 peephole television system of maximum 
useful quality. The chemical processes which con- 
tribute to the making of the % by 1-inch negative 
from which the image on the screen is projected are 
so finely divided that they accomplish a proper 
relation of picture elements to this number; in fact, 
they go considerably further than this requirement. 
It is clear, then, that stepping from commercial 
peephole television to full projected television repro- 
duction requires only solution of the projection 
problem itself. The terminal apparatus and the 
channel requirements for projected television will 
have been developed with the accomplishment of 
really good peephole television. 

Effect of Enlargement. 

However, it must not be concluded that a % by 
1-inch image of perfect texture can be projected to 



full screen size merely because that is accomplished 
in motion-picture projection. Such a field of repro- 
duction of perfect texture, viewed at a distance of 10 
inches, consists of not less than 87,894 picture 
elements, according to the calculation made. The 
image can be readily enlarged by lenses or merely by 
increasing the area viewed as a picture element. 
This process will at once sacrifice clarity of detail 
which can, however, be compensated for by moving 
the observer further from the scene. If the field of 
reproduction is moved 6 feet from the eye, it can be 
increased to 7.2 by 5.4 inches without sacrifice 
of texture. Greater enlargement without further 
separation of the reproduction from the eye subtracts 
from the detail. 

Apparently there is no substitute for picture 
elements. The eye acts upon the information 
imparted to it. A given density of picture elements 
per square inch can be viewed without a conscious- 
ness of texture at a certain distance, according to 
the accompanying curve. Television receivers can 
readily be rated according to the number of picture 
elements per square inch of reproduction. Their 
fidelity can then be rated according to the percentage 
of picture elements per square inch by which they 
deviate from the minimum dictated by the resolving 
power of the eye when viewed at the distance from 
the eye intended by the designer. For example, 100 
per cent potential fidelity is attained when the total 
density of picture elements per square inch is the 
quality which just satisfies the resolving power of the 



How Closely Perfect Texture Must Be Approached. 

Highly useful and entertaining television can be 
expected from a so.mewhat smaller percentage of this 
exacting optimum quantity, just as we enjoy radio 
receivers which reproduce only up to 4,000 or 5,000 
cycles, while the average ear can respond to as high 
as 17,000 or 20,000 cycles. The eye is more exacting 
than the ear, however, because it does observe 
exceedingly fine detail when viewing objects at rest. 
When the subject is in rapid motion, however, the 
eye is decidedly less exacting. In every type of 
activity there are periods of rest and it is only during 
such periods that the eye demands full detail. In 
waiting for an automobile race to start, for example, 
the eye is instantly conscious of lack of accurate 
detail in the fixed background of the scene, a condi- 
tion which might be simulated by wearing glasses 
which put the scene out of focus. But once the cars 
start whirring around the track, the eye is quite 
willing to tolerate very poor detail. Consequently 
we may expect satisfactory television involving 
subjects in rapid motion at far below 100 per cent 
fidelity. Indeed, it might be established that the 
detail requirements bear an inverse relation to the 
rapidity of the action in the scene. 

Just what percentage of perfect fidelity will serve 
in practice is hard to estimate, because such elements 
as contrast, degree of illumination and surrounding 
illumination modify the requirements substantially. 
For home projection, moderate illumination of the 
room must be tolerated, and that condition requires 
brilliance in the illuminated reproduction. Existing 
systems rely largely upon the neon tube, the illumina- 



Fia. 66. The three pictures on the left are representative of the definition attainable if the 
picture signal is confined to a single broadcast channel; those at the right consist of 1,250 picture 
elements, whioh can be transmitted through 46,971, the equivalent of two broadcast channels. 
(Courtesy of Bell Telephone Laboratories.) 


tion of which is the product of the amplified signal. 
It would be desirable to develop economical light 
valves of a size suited to the home, as the source of 
illumination for screen projection. A motion-picture 
film acts as a valve intercepting and admitting an 
exceedingly powerful light source to a screen. Suffi- 
cient illumination is provided to produce from 3 to 22 
foot-candles on the screen in commercial motion- 
picture theaters; the majority use 5 to 14 foot- 
candles. For 16-millimeter home projectors 1 to 15 
foot-candles are used, with 7 to 8 foot-candles as a 
good average value. Persistence of vision varies 
according to the illumination and the weak light 
sources now used for television require a higher 
repetition rate to secure clear reproduction than would 
be necessary if the field of reproduction could be 
illuminated to higher intensity and deeper contrasts. 
Inasmuch as reduction of the repetition rate effects 
enormous channel economy for any given number of 
picture elements, the gain which would be accom- 
plished by brighter illumination sources would be 

Varying Repetition Rate Requirements. 

Although we have determined the theoretical 
requirements of perfect television upon the basis of 
the resolving power of the eye, it appears that the 
practical requirements of picture-element density are 
modified by the nature of the subject matter. In 
viewing a reproduction of a panorama viewed from a 
plane in flight, the eye is extremely exacting in its 
demand for detail, searching down to the very limits 
of its resolving power. The action, under the cir- 



cumstances, is quite slow. We can, therefore, 
conceive of a low rate of repetitions as a means of 
compensating for the greater density in picture 
elements called for by such a scene. On the other 
hand, a slow rate of repetitions, such as 15 or 16 per 
second, would result in a blur in viewing an athletic 
event. We would hardly tolerate less than 20 
repetitions for such rapid activity. And again, we 
would call for less detail while the activity is rapid. 
Thus these requirements are complementary and 
therefore tend to simplify the ultimate problem. 

Because of these varying requirements imposed by 
the character of the subject matter, practical tele- 
vision may be somewhat more flexible in character 
than is now the practice. The control operator may 
have facilities for changing the repetition rate and 
the detail as required by the subject matter being 
televised, and the reproducer will have to respond 
automatically to these changes. Television programs 
will be monitored to suit them to the limitations of 
terminal equipment and channel, just as sound 
programs are today. Only until we have a com- 
munication system which has an ample margin of 
capacity, so that we can have both detail and speed 
without coming up against channel limits, will a 
fixed scanning speed and a fixed scanning density be 
anything other than a handicap to the development 
of the art. In this absence of flexibility, the tele- 
vision system will be limited to reproduction of 
scenes of a given character, either scenes of great 
detail and slow motion or those of little detail and 
rapid motion, depending upon the standards estab- 
lished. It is undesirable to consider standardization 



of repetition rate or elemental density until the 
future of the art is more clearly defined, except for 
the purpose of encouraging cooperative experiment. 

Advantages Gained by Projection. 

The curve of Fig. 67 brings out quite clearly the 
advantages of projected television over the more 




L 200 






240 300 360 420 
Viewing Distance, inches 

480 540 600 

FIG. 67. Lines per frame and viewing distance for perfect texture. 

familiar peephole variety. As we approach the 
distance from the eye at which it perceives the 
greatest detail, the requirement in picture-element 
density rises substantially. At approximately 5 or 
6 feet, the curve straightens out, bringing us to 60 
lines per inch at 5 feet, or 3,600 picture elements per 
square inch of reproduction for absolutely perfect 




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Inasmuch as various television systems have 
already been developed using 24-, 48- and 72-line 
scanning, it is of interest to establish the size of 
reproduction of perfect texture which can be attained, 
shown in the following table. The calculations have 
also been made for 100, 200, 500 and 1,000 lines. 
The communication channel requirements for trans- 
mitting television signals of these values at various 
repetition rates are also given. 




Viewing distance 

10 in. 




25 ft. 



















































All of these calculations as to density requirements 
are based upon the assumption of perfect texture. 
The degree to which we may depart from perfect 
texture is dependent upon the information which is 
logically sought by the eye in observing the particular 
subject being offered. Obviously, we do not have to 
await the attainment of such exacting standards to 
render a useful service. The detail required in actual 
practice is debatable. In the early stages of commer- 
cial exploitation, considerable departure from theo- 
retical perfection will be tolerated. As the novelty of 



television wears off, the public will demand improved 
quality, but whether the ultimate standard will be 10, 
50 or 80 per cent of theoretical perfection will not be 
easily determined until somewhat better quality than 
is now attainable can be subjected to public criticism. 

Estimating the Value of Existing Systems. 

Assuming that we must rely upon a mere extension 
of present methods, we can analyze the present 
status of television quality. The simplest entertain- 
ment service which can be conceived is the televising 
of a single human face to accompany the broadcasting 
of an artist's voice. A clearly recognizable close-up of 
a face, but lacking somewhat in detail, it is true, is 
secured by 50-line television. Considerably better 
reproduction is obtained with 72 lines, the capacity 
of the system used in the Bell System two-way wire 
television demonstration. 

However, the 72-line standard still leaves much to 
be desired, because no fine lines of the features and no 
expressive shadows are portrayed. The teeth are a 
single band of white, with no shadows to demark 
them. The reproduction is certainly markedly 
improved by scanning this kind of subject with 100 
lines. The 72-line system requires 40,000-cycle 
capacity circuits, attainable over only very short 
hauls of wire lines. Increasing the requirements to 
100 lines doubles the width of the channel necessary. 
A 72-line reproduction, however, is of a standard that 
constitutes a good visual supplement to a telephone 
conversation but, as the basis for an entertainment 
service, even a 100-line reproduction is not likely to 
be of permanent entertainment value. I doubt that 



the reproduction of such a picture signal, enabling the 
broadcast listener to see a close-up of the speaker or 
artist, would justify the cost of the reproducing 
installation necessary and the assignment of the 
frequency facilities essential to its distribution. 

The requirements of an entertainment service are 
considerably higher than those of a mere communica- 
tion service. Communication, whether of speech or 
vision, of a standard so low that some fatigue to the 

FIG. 68. An early Jenkins television reproducer or televisor, with magni- 
fying lens. 

eye or ear is involved in utilizing it, is, nevertheless, 
useful and serviceable. Ordinary telephone circuits 
transmit an audio signal confined to a band between 
250 and 3,500 cycles, but to hold the interest of a 
broadcast listener for sound-entertainment pur- 
poses requires faithful transmission and reproduction 
throughout a minimum band from 100 to 4,000 
cycles and good response from 50 to 5,000 cycles. 
Television for communication and television for 



entertainment are, in a similar way, of two quite 
different standards. The entertainment value of a 
72-line service limited to profile, three-quarter and 
full-face views of individuals will rapidly pass, in the 
hands of the average person, through the stages of an 

FIG. 69. A photograph of a 48-line television reproduction made at the 
General Electric Laboratories. 

intriguing novelty, an occasional entertainment and 
an utter bore. 

Minimum Requirements of an Entertainment Service. 

Two full-length figures in action have much greater 
possibilities as entertainment subjects than a single 
close-up. They can gesture, struggle, box and fence. 
Simple dramatic situations are readily portrayed 



by a system of that capacity. Both by costume and 
activity, they can indicate different pursuits and 
situations. This is a minimum standard for a 
pioneer commercial television service offering pro- 
gram variety and real entertainment value. 

In order to view two figures in action, the field of 
view must be fairly extensive. The actors must be 
given some space in which to move and there must 
be some, if limited, space for background. Perhaps a 
stage 8 to 10 feet square may be adequate for a 
starting point. The height of the space embraced 
by the scanning apparatus should be not less than 8 
feet and preferably 10. In such a stage, simple 
dramatic situations of considerable variety and scope 
could be successfully staged. Viewed as a plane, this 
requires the scanning of a field of view approximately 
100 square feet in area. For convenience in calcula- 
tion, we will base the figures on an area of 100 by 100 
inches in size, or a total of 10,000 square inches. 

This rather extensive field can be scanned to 
various degrees of detail. Suppose, for example, we 
set as a minimum requirement that the individual 
fingers of the hand of each of the actors may be seen, 
not an unreasonable requirement considering the 
accomplishments of the motion picture. The scan- 
ning apparatus, under the conditions, must respond 
to light changes each tenth of an inch, requiring a 
system of 1,000 lines, clearly beyond the capacity of 
any television system so far conceived. A glance at 
the table of frequency requirements shows that our 
present methods of television demand, to transmit a 
picture signal of that density, too extensive com- 
munication facilities to make a system of that 



capacity a reasonable expectancy, so long as we are 
limited to those methods. 

With 5 lines to the inch, there will still be consider- 
able detail in the scene. The pupil of the eye will be 
darker than the rest of the eyeball ; the buttons on the 
coat will show; a smile or a frown will be clearly 
portrayed. But 500 lines at 16 repetitions requires a 
2,000,000-cycle channel, and we are not yet justified 
in hoping for such liberal communication facilities for 
television. If we scan the scene to but 200 lines, any 
element smaller than % inch is blended as a single 
impression; the buttons of the coat merge with the 
rest of the coat; the eyes blur into mere dark shadows; 
the figures approach the character of silhouettes. 
But 200 lines will still be capable of portraying con- 
siderable detail if the subjects move closer to the 
scanning apparatus, thereby restricting the field of 
view, when facial expression and detail are of interest. 
Rapid action in the entire scene of 10,000 square 
inches will be not only discernible but entertaining. 
Experience has shown, however, that too much 
close-up is tiring to the observer. At least during 
the novelty stages, 200-line television will neverthe- 
less be adequate to enjoy extensive public interest. 
Television of this quality and detail is within range 
of the experimental channels assigned to the purpose. 
The actual accomplishment of 200-line television 
requires the solution of many difficult problems and 
it will herald a period of further development lead- 
ing to the establishment of standards worthy of 
permanent public support. 



The program possibilities of television, at first sight, 
appear to be limited only by the breadth of human 
imagination. In the field of news and sports broad- 
casts, any event sufficient to arouse general public 
interest is a subject for a television broadcast; wherever 
the news photographer rushes with his camera is a 
logical place for the television pick-up; indeed, 
wherever people congregate to satisfy their curiosity, 
the radio eye has its place. In the field of entertain- 
ment, the motion picture, the drama, the musical 
comedy and in every form of spectacle, there is a 
logical foundation for a television program. In 
politics and education, the inanimate loudspeaker will 
be given new life and new means of holding audiences, 
with the aid of information portrayed to the eye. 
Radio humor has hitherto lacked the visual element; 
that is now to be supplied by television. The range of 
educational subjects effectively presented through 
broadcasting will be enormously extended when the 
lecturer can supplement his discourse with illustra- 
tions, drawings and charts through a television 

Television Programs Must Regard Technical Limitations. 

It is manifestly impossible to discuss a subject of 

such broad range in a single chapter or a single volume; 



indeed a man would be of barren imagination if he 
could not, in a few hours, evolve a thousand sugges- 
tions for television programs. The question to be 
considered here is merely a very limited phase of this 
exceedingly broad subject: the program possibilities of 
television in the light of the limitations imposed by 
existing .methods of television. 

During the early stages of television-program 
presentations, limitations inherent in the performance 
of television systems will preclude certain types of 
programs, and impose restrictions on the form in which 
others may be presented. Natural progress in the 
science will eventually free the art of television pres- 
entation of these barriers, one by one, and will leave 
unrestricted opportunity to creative talent. But 
until that freedom from technical limitation is 
established, the success of the television-program 
director will be measured by his ability to appreciate 
and understand the limitations circumventing him, 
and by his ingenuity in sidestepping these technical 

Since the progress of the art depends upon public 
support, the more clearly the limitations are under- 
stood, the more can be made of the existing opportuni- 
ties for effective presentation. Only by utilizing to 
the fullest what facilities we have available during the 
early stages of the art can rapid progress be made in 
building up audiences, thereby financing the essential 
improvements which must continually manifest them- 
selves to merit continued public support. Therefore, 
although we may look forward to a progressive 
emancipation of creative talent in television-program 
evolution through technical progress, we must be 



prepared, from the first inception of television service 
to develop program interest. 

Evolution of Television-program Direction. 

The first television features have been evolved by 
the engineers who developed the apparatus used, as a 
part of the work of demonstrating the capabilities of 
their respective devices. So long as the objective is 
merely the demonstration of technical possibilities, 
showmanship is a secondary consideration. The next 
phase of program development is television as an 
experimental adjunct to the broadcasting station, and 
in this capacity it serves principally as an embellish- 
ment of the sound-broadcasting program. Until the 
technical quality of the reproduction is greatly 
improved, the television program will remain largely 
in the hands of the broadcast-station-program man- 
ager. This will be, or is, a critical period in television- 
program development, because the danger is ever 
present that television will be regarded only as a means 
of eavesdropping on the radio-broadcasting studio. 
Such a limited conception cannot long hold an 
audience, although it is sufficient to arouse initial 
interest. The absence of the visual element in sound 
broadcasting is realized by every program director 
and the facility of television will offer him a welcome 
opportunity to strengthen the attractiveness of his 
program efforts. 

Vision, the Missing Element of Broadcasting. 

The ability to describe the visual elements of a 
broadcast event in vivid detail is considered a primary 
qualification of a good news and sports announcer. 





His success depends largely on his ability to build a 
realistic image of the scene in the listener's imagina- 
tion. To test this capacity of candidates for announc- 
ing positions, the program director of a leading station 
in New York requires them to stand at the window of 
his office and give a running description of what they 
see taking place on busy Broadway below. Tele- 
vision's ability to present the visual aspects of an 
event directly to the eye represents a tremendous 
enrichment of home entertainment by radio. The 
public clamors for the availability of what it imagines 
practical television to be. 

Anyone familiar with the performance of available 
home-reproducing equipment, however, realizes that 
there is a considerable disparity between the attained 
reproduction and that which the public has been led to 
expect through the rosy publicity accorded to tele- 
vision. It would be much more conducive to public 
confidence if television news tended to have a closer 
relation to the facts, so that the curiosity it arouses 
would not be invariably dispelled by a disappointing 

Forty -eight-line Television as an Adjunct to Sound 

Even 48-line television, the pioneer service standard, 
has considerable program possibilities. But they can 
be realized only through intelligent showmanship and 
careful guidance of program policies within the existing 
limitations imposed by the pick-up and reproducing 

Because of lack of detail, 48- and 60-line tele- 
vision is necessarily only a supplement to a sound 



program. When the mind is supplied with a wealth 
of detail regarding a scene through the ear, the 
eye needs little more than a suggestion to produce a 
vivid visual impression. Up to this time, however, 
most television programs of this order of quality have 
been presented without sound accompaniment, leaving 
the imperfect image produced at the mercy of concen- 
trated attention. In those few instances where sound 
has accompanied such meager television, it has vastly 
improved the entertainment produced. We shall 
always remember Merle Trainer's pleasant humor 
before the pick-up during the first public demonstra- 
tion of projected television at Schenectady. Had he 
performed in ghostly silence, the newspaper reports of 
that historic demonstration would have been concerned 
with a critical examination of the quality of reproduc- 
tion rather than reflecting (as they did) the genuine 
pleasure experienced by the liberal representation of 
news and technical writers present. To be successful 
from the program standpoint, 48-line television 
features must be closely coordinated with sound and 
strong reliance placed on the entertainment value 
contributed by the sound element. 

Humor Aided by Crude Television. 

Humorous dialogue acts will receive a new lease of 
life with the aid of 48- or 60-line television as a supple- 
ment to sound broadcasting. Successful television 
comedians will learn the tricks of the television 
close-up and the highly contrasting make-up necessary 
to produce a visual effect. They will move up close to 
the scanning disc for a grin or a wink, so the maximum 
of detail will be concentrated in its reproduction. 



Since some of the most popular broadcasting features 
of the day lend themselves to such television portrayal, 
a wealth of program material is at once available to the 
broadcasting station with a television transmitter. 
Broadcasting Studio Scenes as Television Material. 

The natural tendency of program directors of tele- 
vision broadcasters will be to extend their pick-ups to 
broadcasting studios. Indeed, without portable scan- 
ning systems, television programs will be strictly 
limited to such scenes. Bands and orchestras do not 
lend themselves to satisfactory reproduction with only 
48-line definition. Consequently, close-ups of solo 
artists are likely to predominate in early programs. 
So logical is this concentration of the television pick-up 
on the faces of baritones in pain and sopranos stretch- 
ing toward the top notes, that it constitutes something 
of a danger to the commercial success of television. 
A face presented for television transmission must 
possess qualities which render it worthy of that 
distinction, or else television reception will be quickly 
branded as monotonous entertainment. Since this 
crude standard of television does not admit of fine 
graduations of detail, the grotesque features of a 
clown will be more impressive than the features of an 
outstanding beauty. 

Television as an Aid to the Lecturer. 

The successful lecturer and distinguished speaker, 
expounding before the television pick-up, will realize 
that he must hold the attention of the eye as well as 
of the ear. He will, therefore, learn to take the fullest 
advantage of the limited area of pick-up available to 
him. He will realize that he cannot hold his audience 


by a mere exercise of his jaws. Therefore, he will 
turn his head from side to side slowly, presenting his 
face from every angle, wherever the context of his 
address makes that gesture natural. He will avoid 
quick motion as carefully as he avoids rigidity. He 
will appreciate the fact that the major motions, such 
as turning the head from side to side and tilting the 
chin upward, will make far more impression on those 
watching him than the modes of expression he has 
learned to use before audiences in the lecture hall. 
He will find that concentration of expression by 
closely focusing the eyes and hardening the lines of the 
face, which he has learned to assume when driving 
home a major point, will be totally lost on his tele- 
vision audience, equipped only with 48-line repro- 
ducers. Special lighting equipment to accentuate 
contrast in the face will be utilized to make the tele- 
vision reproduction more expressive. 

Lecturers and speakers will take advantage of every 
possible opportunity to present charts and models of a 
character which are suited to television reproduction 
to relieve the observer of the monotony of watching 
the mechanics of a mouth in motion, a sight perhaps all 
too familiar in the average home to require the intro- 
duction of complex radio devices. Whatever is used 
as subject matter along these lines will be carefully 
tested before the television pick-up to assure that it is 
held in the best possible position to secure the greatest 
detail in reproduction and that every element to which 
the speaker refers in his talk is actually reproduced. 
This is a most important point, likely to be overlooked 
frequently by lax program directors. For instance, if 
the speaker, in describing the route of a trip, holds up 


before the television pick-up a map of such detail that 
it is a meaningless blank in the reproducer, the 
audience will be annoyed by the lack of consideration 
so evidenced. If, instead, a crude crayon map, 
preferably drawn before the television pick-up, is 
offered, the action of drawing will hold the eye and the 
simplicity of the map will not exceed the definitive 
capacity of the 48-line reproduction. Likewise a 
conventional graph with a line too fine for the tele- 
vision pick-up to discriminate is an imposition on the 
audience, but the same curve reproduced in solid 
black from the base of the graph against a white 
background will show up clearly. Every attempt to 
show the television observer detail beyond the capacity 
of the television pick-up and reproducing system will 
be an annoyance which will merely emphasize the 
shortcomings, while careful observance of the limita- 
tions will make possible informative and appreciated 

Subjects beyond the Scope of 48-line Television. 

For the present at least, 48-line television is inade- 
quate to handle extensive outdoor scenes, to pick up 
programs from the stage of the conventional theater 
or to reproduce athletic events. All of these possibili- 
ties have been promised by ambitious television 
publicists and the failure to perform has accounted for 
the lethargy of experimenters in supporting the 
industry. Statements have been made to the press 
that there are 15,000 to 20,000 television reproducers 
in the metropolitan area of New York City at this 
writing, but actually, there are not 1,000 in maintained 
operation. The number can be readily increased to 



50,000 or 100,000, even with the crude 60-line repro- 
duction now available, by merely utilizing intelligent 
program direction and avoiding promises which 
amount to misrepresentations. 

Possibilities of 100-line Television. 

Assuming that we are confined to present methods 
and principles, the availability of 100-line television 
represents the beginnings of general service to the 
public. This standard offers genuine entertainment 
and educational value without relying entirely on the 
experimental or pioneer appeal, or leaning too heavily 
on sound. The area which can be comprehended 
simultaneously is still limited, if reasonable detail is to 
be provided, but the area limitation is no longer of a 
magnitude to test the ingenuity of a good stage 
director. For the broadcasting of prize fights, the 
observer of 100-line reproduction has the equivalent 
of a seventy-fifth row seat, but coupled with vivid and 
accurate announcing, that is far from a minor service. 
One-hundred-line television can stand on its own feet 
and its inauguration on a practical basis will be a 
substantial bid for public favor. 

The capacity of 100-line television system permits 
reproduction of full-length figures in considerable 
detail appearing on a stage of sufficient capacity to 
portray any type of room setting. Furniture, win- 
dows and doors are clearly distinguishable and can 
serve significant parts in the dramatic portrayal. 
There is ample room for two or three actors in major 
parts, and their gestures and motions are easily 
discernible, although, of course, close-up views of 
each actor individually must be relied on for details 



of facial expression. Nevertheless, any simple dra- 
matic presentation, involving a limited number of char- 
acters, can be run off smoothly without undue emphasis 
on the shortcomings of television by clever use of 
close-up and action. 

Athletic events which do not require an extensive 
area, such as boxing and wrestling, are not beyond the 
capacity of television of 100-line quality. But out- 
door scenes must be more carefully chosen, lest the 
field comprehended be too extensive to be portrayed in 
detail. By skillful restriction to essential close-ups, 
however, almost any event can be made interesting to 
the television audience, particularly when supported 
by capable announcing. Indeed, the advent of 
100-line television introduces such a wide latitude to 
the scope of the television program director, that it is 
needless to consider more than certain possible 
developments which may increase the number of types 
of program which may be successfully presented. 

Variable Repetition Rate and Detail. 

The transmission of 100 lines at 20 repetitions 
requires the transmission of 200,000 picture elements 
each second. The possibility of lower repetition rates 
for the presentation of even more comprehensive 
scenes suggests itself forcibly. If we had some form 
of projector which maintained its illumination for a 
second after the originating light impulse was removed, 
scenes of fine texture and detail having 200,000 picture 
elements per frame could be unfolded before the 
observer's eye. A view from a plane in flight, for 
example, requires detail or wealth of picture elements, 
rather than a high repetition rate. If slow unfolding 



of a scene over a period longer than is accommodated 
by persistence of vision is relied upon, the scanning 
progression must be carried out so that the scene 
appears logically before the eye either beginning at 
the center of the field of reproduction and expanding 
outward in an ever increasing circle, or building up 
from the bottom, simulating the rising of a curtain. 

If the operator at the transmitting point could have 
complete control over the speed of operation of the 
receiving system over a wide range, great program 
possibilities would be unfolded which could not be 
handled by a single-speed system. In view of the 
possibility of television systems of variable repetition 
rate and picture-element density per repetition, the 
premature establishment of a fixed standard for the 
television repetition rate and scanning texture appears 
undesirable. The number of picture elements per 
repetition and the repetition rate may be altered by a 
skilled operator at will according to the requirements 
of the scene. He may use high-speed television for 
portraying rapid motion in a limited field of view and 
slow-speed television, equivalent to a rapid suc- 
cession of still pictures, for detail, realism and 
comprehensiveness . 

Program Possibilities of Flexible Detail and Speed 


As an example of how a specific event might be 
handled with such facilities, consider the start of the 
first transatlantic dirigible as viewed through a system 
which can be arbitrarily altered through a range from a 
100-line, 20-repetition rate to a 450-line image at the 
rate of 1 repetition per second. Such transmissions 



can all be conducted through communication channels 
capable of handling a 100,000-cycle band. At this 
outstanding news event, the television audience is 
first shown a close-up view of an express elevator in the 
lobby of New York's tallest skyscraper, where, a 
quarter of a mile above is moored the new trans- 
atlantic air liner. The announcer describes the busy 
scene and retails gossip about the notables who are 
making this the point of embarkation for the aerial 
transatlantic voyage. The field of view here can be 
restricted to a small area so that high-speed television 
is used to advantage. The audience is then given a 
view up the elevator shaft as the elevator travels its 
way skyward. This scene would, of course, also be 
transmitted as high-speed television. Any distortion 
which might be introduced by the rapid motion of the 
elevator would only increase the illusion of speed. 

The next view might be of the dirigible itself as seen 
from the mooring mast with the city below as a 
background. The television pick-up would be care- 
fully placed with no rapidly moving objects near it, so 
that the maximum detail could be scanned, producing 
an image of 200,000 picture elements renewed once 
each second and maintained by means of a fluorescent 
screen. The reproduction is sufficiently good to 
enable the announcer to describe the craft of the air in 
considerable detail to his listening and observing 

The next point of pick-up might be a close-up view 
showing the crew loading mail and baggage with 
feverish activity, which, very probably, continues at an 
accelerated pace only as long as it is under scrutiny of 
the television pick-up. The next view may again be a 



still picture taken from the dirigible and looking 
directly downward at the city below, making the 1,200- 
foot skyscraper look as if it were 20 miles high. The 
next scene is again high-speed reproduction, this time 
of the officers of the dirigible saying farewell to terra 
firma, followed by a view of the dirigible casting off. 
At first high-speed television will be necessary, but 
as the ship moves further away, the repetition rate is 
progressively reduced, the detail correspondingly 
increased, until the ship finally stands out as a speck 
in the sky, with the panorama of the cheering city 
waving farewell. 

The point I am attempting to bring out by this 
example is the fact that so long as communication 
channels remain a primary limit, the use of widely 
ranging repetition rates makes possible a great 
improvement in the program values of television 

Television of Motion-picture Quality. 

To assure to the television impresario as complete 
freedom of technical limitations as that enjoyed by 
the motion-picture director requires a considerably 
greater number of picture elements per frame than is 
offered by 100-line television. Remarkable as 100-line 
quality would appear in comparison with existing 
systems, it is, after all, only the definition embodied in 
a single square inch of magazine half-tone. Because 
television systems shade the entire field of reproduction 
while half-tones have only half their areas shaded 
according to the subject, a direct comparison between 
television picture elements and half-tone screen is 



invalid. A careful series of tests 1 made by Julius 
Weinberger at the time that he was in charge of the 
Research Laboratories of the Radio Corporation of 
America, established a number of values for half-tones 
which portray about the same information to the eye 
as television of various standards. 

The equivalent of motion-picture clarity in tele- 
vision appears to require 1,000,000 or 2,000,000 
picture elements per frame, a number which involves 
excessively heavy communication burdens, regardless 
of the channels used for the purpose. The probabili- 
ties are that before such standards are attained or 
even approached, television will have been attacked 
from new angles, such that the required number of 
picture elements can be transmitted without the 
necessity for 15 to 20 communication impulses for each 
picture element each second. The progressive scan- 
ning method is not the only one conceivable, although 
it has been adhered to so religiously by all the workers 
in the field that we are inclined to accept it as the only 
possible avenue of development. Just to indicate one 
alternative, a combination of still-picture-reproduction 
methods and television would permit the portrayal of 
subjects in motion without such an extensive quota 
of channel facilities. The transmission of a frame of 
motion picture as a facsimile or phototelegraph might 
be conducted at the comparatively slow rate of one 
frame each 2 seconds. When a suitable amount of 
film has been made in this way to complete a subject, 
it can then be exposed through a motion-picture 

1 WEINBERGER, JULIUS, T. A. SMITH and G. RODWIN, Standards for 
Commercial Television, Proc. Institute of Radio Engineers, Vol. XVII, 
No. 9, September, 1929. 


projector. It would take 30 times as long to make the 
film as to expose it, but since the transmission of each 
frame is extended over 2 seconds, only one-thirtieth 
the channel facilities necessary to deliver the same 
quality by the conventional methods would be used. 




The advertiser and the user of radio broadcasting 
as a goodwill advertising medium have eagerly 
watched the development of television, impatiently 
awaiting the day when it will give them their oppor- 
tunity to exhibit their trade-marks and products in the 
homes of a vast army of potential buyers. The 
inauguration of television as a practical service will 
win widespread attention through the aid of the 
newspaper and the broadcast station. The first 
satisfactory programs will command a position in the 
limelight, of immense advertising value. We may 
expect then that the first evidence of public enthusiasm 
will witness a feverish rush on the part of advertising 
organizations to utilize the new facilities, differing only 
in its greater magnitude from that which accompanied 
the discovery that the public would listen to radio 
telephone broadcasting in 1922 and 1923. 

But only in this single respect is the real start of 
television likely to resemble the inception of broad- 
casting service. Sound broadcasting started without a 
plan for its economic support. The biggest question 
in the minds of station operators was how long the 
radio fad would survive. At first each advertiser 
erected his own station until overcrowding of channels 



became a serious problem. The American Telephone 
and Telegraph Company, with station WEAF, began a 
large scale experiment in selling time, which eventually 
resulted in the development of sponsored programs 
and chain distribution. Several years of intensive 
selling effort were required to establish broadcasting 
as a legitimate medium for advertising and several 
more years before agencies generally were equipped 
with personnel suited to handling radio-broadcasting 
problems for clients. All of this evolution took con- 
siderable time, with the result that radio advertising 
progressively and gradually reached the status of 

The Beginnings of Commercial Television. 

Television will find a complete structure ready to 
commercialize it. Broadcasting stations have organ- 
ized personnel and established contacts in the adver- 
tising field, the advertising agencies have specialists in 
handling radio problems for their clients, and the 
advertiser is already accustomed to radio as a medium 
of approach to the public. Consequently there will 
be no long period of adjustment and development. 
Advertising will be ready for the visual medium long 
before the medium is ready for advertising. While 
television reproducers in the home are still one of the 
seven wonders of the world, the first television 
advertiser will intrude his billboard in the home. The 
embryo days of radio broadcasting suffered no such 
handicap; direct advertising, as it is practiced today 
by all stations, did not begin until news of the studios 
had long been relegated to the inside pages of the 
newspapers, and the number of columns devoted to it 



carefully regulated by the amount of paid advertising 
from radio manufacturers. 

Economic Structure for Support of Television Broad- 
casting Established. 

Television must go forward suspected if not con- 
spicuously branded as an advertising medium. That 
assures it only perfunctory publicity support in the 
press, because the newspapers are no more interested 
in assisting competition than any other kind of business 
enterprise. On the other hand, while broadcasting 
had to rely on the press to become known, television 
can go forward no matter how successfully the news- 
papers may contrive to shun it. Television is 
associated with radio broadcasting and it therefore 
is quite independent of the press so far as publicity is 
concerned. Its association with the broadcasting 
medium is so close that television advertising tech- 
nique will be largely imitative of the broadcast 
advertising technique as to method and standard. 

The prospect that television will be supported by 
means other than advertising programs appears 
exceedingly remote. The precedents established by 
sound broadcasting apply so logically to television that 
it will be next to impossible to establish the newer 
field on a different basis. The public prefers to pay 
for broadcasting indirectly, either by purchasing 
products advertised by radio in larger quantities so 
that production costs are reduced, or, if that is 
impossible, paying more for the goods themselves. It 
cannot be expected to reverse itself with television and 
pay for the service rendered directly through taxation 
or subscription, although it would, in that way, be 



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FIG. 71. The first advertisement of a television performance on the 
stage of a theater appeared in the Schenectady Union-Star, May 22, 
1930. (Courtesy of General Electric Co.) 


relieved of the necessity of submitting to visual 

Association of Television with Broadcasting. 

The same corporate structures which dominate the 
radio and broadcasting industries are carrying out the 
major researches in television. Consequently, tele- 
vision is inevitably associated through every element 
of its fabric research, manufacture, communication 
facilities and advertising with the existing sound- 
broadcasting structure. 

Television is a welcome addition to sound broad- 
casting, the program development of which has slowed 
up decidedly in the last few years from the standpoint 
of news and novelty values; television promises a 
revival of public interest in every element of the radio 
field. The manufacturers of radio receivers suffer 
from vastly excess production facilities and hopefully 
await the stimulation of television. The broadcast 
advertiser appears to have come to the end of his 
ingenuity in devising features which captivate the 
listener. All the frantic efforts of broadcasting 
organizations to arouse the listener from the attitude 
of complaisant acceptance, such as international 
rebroadcasting and special pick-ups of news events 
from airplanes and destroyers, have produced hardly 
more than a ripple of response. The public simply 
accepts broadcasting as a service which is acceptable 
if well performed. The blatant direct advertising is 
accepted as a necessary and unimportant evil, no more 
troublesome or conspicuous and no less effective than 
billboards on the highways, car cards on trolleys and 



buses, and advertising columns and pages in news- 
papers and magazines. 

From every standpoint, therefore, we may expect 
that television, once it takes hold with the public, will 
be intensively exploited under the direction of the 
personnel or organizations experienced with sound 
broadcasting. So logical is this conclusion that we 
may predict that the same program and advertising 
standards will apply to the mediums which have been 
evolved for sound broadcasting. Probably only slight 
changes will be made to adapt programs to television 
opportunities, because long training in the field 
inevitably molds the imagination to established 
formulas. The television pick-up will simply take its 
place at the side of the microphone, and television will 
be little more than an extension of sound broadcasting. 
When the quartet sings, you will be able to see it as 
well as hear it; when the announcer talks about throat 
irritation, he will point significantly to his Adam's 
apple ; and when he describes the spotless factory, that 
intensely interesting sample of standardized architec- 
ture will appear in the television reproducer. 

Influence of Television on Program Types. 

As the standards of visual portrayal improve, 
television will tend to influence the character of 
programs which predominate in the schedules. The 
script program, which rose in a relatively short time 
from insignificance to a predominant position, will be 
greatly strengthened by good television portrayal. 
In fact, its possibilities will be so enhanced that the 
script is likely to be overdone. The average listener 
cannot devote much more than half an hour or an hour 



an evening to the concentrated attention required by a 
script feature and, therefore, too many script programs 
will gradually tend to reduce audiences, despite their 
prospective effectiveness. But any such influence will 
be counterbalanced by the added attention given to 
the feature by its following as a result of the tremen- 
dously increased vividness and entertainment value 
contributed by effective visual presentation of the 
dramatic performance. 

The musical-comedy type program will encounter 
some difficulties because of the limitations of television 
reproduction. Large choral and orchestral groups for 
adequate portrayal require a television system of 
considerable capacity, and it is likely, during the initial 
stages at least, that our view of such features will be 
limited to a few solo and specialty artists. Dancing, 
the stand-by of the musical comedy, requires an 
extensive field of reproduction for effective presenta- 
tion, but the skillful director will devise specialties 
which can be successfully reproduced in close-up 

Advertising by Television. 

The broadcast advertiser relies on opening and 
closing announcements for the capitalization of his 
goodwill program. The mere extension of television 
to such announcements can materially add to their 
influence. The trade-marked package can be shown 
to the audience instead of being merely described, and 
the cigar advertiser who appeals to the young man can 
actually demonstrate that cigar smoking will make 
any man look like a major executive. In fact, since 
television reproduction of tobacco smoke is undeniably 



realistic even with the crudest system, television can 
count on liberal support from the tobacco companies. 
The food advertiser, too, will find television a powerful 
adjunct to his broadcast advertising. A reproduction 
of a luscious strawberry shortcake is much more 
effective in creating 'an appetite than any word of 
mouth description, The real estate advertiser can 
show his model home, and the motor-car maker his 
latest body styles. In every field, the effectiveness of 
the radio medium will be enormously enhanced by 
television of a satisfactory quality. 

We are promised, for the first time, a medium of 
dual appeal, both to the eye and to the ear, reaching 
directly into the home. If television progresses 
technically as rapidly as did sound broadcasting in 
ten years, a decade of general service will make it the 
greatest medium of expression for the advertiser. 
The printed page has held its own against sound 
broadcasting because of the superior information 
capacity of presentation to the eye. But radio 
communication of both sound and sight will eventually 
make that combination the most powerful medium for 
sales stimulation. 

Adapting Television to Testimonial Advertising. 

Testimonial advertising has been extensively utilized 
both on the printed page and through the radio 
medium. The routine technique for presenting testi- 
monials may be followed when television supplements 
sound broadcasting. The announcer may read the 
prepared statement while the television system scans 
a photograph of the endorser. But bringing the 
endorser before the microphone and television pick-up 



will so greatly increase the effectiveness of the testi- 
monial presentation that it is likely to become the 
customary way of utilizing that advertising device. 
It involves a process somewhat more difficult and 
expensive than the established method. An agency 
copywriter prepares a written statement of endorse- 
ment designed to fit the product in hand and the 
scruples of the intended endorser. Then a high- 
powered salesman visits the executive or society 
leader whose signature is required and with the 
assistance of a check, the size of which varies according 
to the standing of the person involved, overcomes his 
natural reluctance to submitting to this undignified 
procedure. The whole thing is over in a minute, so far 
as the endorser is concerned. 

To deliver the same testimonial by radio and 
television may require that the endorser be persuaded 
to visit the broadcasting studio in person. He must 
then read the prepared speech with more than a show 
of sincerity; his voice must carry conviction and his 
face must show no embarrassment. The use of radio 
in political campaigns has already shown how effec- 
tively the voice reveals the character and the convic- 
tion behind it. Perhaps television may eventually 
restrict the advertiser to bona fide testimonials. 

On the other hand, an endorsement delivered 
sincerely by a qualified authority will undoubtedly 
prove tremendously effective. An eminent physician 
presenting a bona fide recommendation will win the 
respect he deserves. But the tittering society leader 
who does not know what she is talking about may no 
longer find the mere possession of a name an avenue 
to easy money. Television will confer a great boon to 



advertising by tending to limit the testimonial to its 
proper sphere and increasing the effectiveness of the 
genuine endorsement. 

New Types of Advertising Presentation. 

Combined sound and visual radio presentation will 
offer opportunities to make direct advertising appeals 
without losing audience. Because we have the atten- 
tion of the reader of the printed page for only a 
moment, the most effective appeals are generally 
superficial. The story must be comprehended at 
a glance and the copy must, therefore, be brief. 
Attempts to present technical points, on which the 
relative superiority of products really depends, have 
not been considered successful because they call for 
too much effort on the part of the reader. 

With sound broadcasting alone, direct advertising 
is conspicuously blatant and annoying. But the high 
cost of sound broadcasting makes the direct advertise- 
ment unavoidable. The conception that the adver- 
tiser relies on capitalization of goodwill has been 
displaced, and the purpose of the program now is to 
win listeners so that they may have their receivers 
tuned to the station used when the advertising is 
broadcast. What is needed in broadcasting is a 
means of delivering the advertising message without 
the accompanying reaction. Television offers that 
opportunity. It permits the extension of human 
interest to the advertising announcement and the use 
of subject matter far beyond the scope of sound alone. 

An interesting speaker, aided by models, photo- 
graphs, diagrams and the product itself, can present 
even a technical story in two minutes which can be 



grasped by the average mind without conscious effort. 
He can get over more information in that time than 
could safely be embodied in ten advertising pages. A 
story which presents real information is vastly more 
interesting than the unmitigated bunk which consti- 
tutes the average radio advertisement of today. This 
superficial character of advertising substance is very 
largely due to inherent weaknesses of the medium used 
and rarely to any real lack of fundamental points 
which might be presented. So long as the listener's 
interest in a product is merely casual, the advertiser 
receives only a moment of attention requiring no 
conscious effort on the part of the prospect. Tele- 
vision with sound makes it possible to remain within 
these limitations and still to present a point of real 
substance and significance. The utilization of this 
opportunity is surrounded with difficulties, but skill in 
preparation of copy and exhibits and selection of a 
suitable voice and personality to present them will 
make possible the use of the radio medium for effective 
and interesting direct selling. 




The sweep of enthusiasm which accompanied the 
general recognition of the possibilities of radio 
broadcasting several years ago has naturally led to 
predictions of a similar wave of public interest upon 
the inauguration of television broadcasting services. 
Indeed, the program possibilities of television and 
the superior perception of the eye make logical the 
anticipation of an even more enthusiastic reception 
for practical television. It is no wonder, then, that a 
ready public and the radio industry have listened with 
eager ears to rumors of the availability of television. 
Indeed, television has been so long expected that 
we are inclined to complain of its slow progress. 

However, broadcasting was many years in the 
making before its spectacular entry to commercial 
acceptance in 1922 and 1923. Many years before 
KDKA broadcast its first program, numerous experi- 
menters broadcast entertainment programs by radio 
telephony on regular and frequent schedules for the 
benefit of experimentally inclined amateurs, serving 
audiences more numerous than the family of television 
experimenters of today. Thousands of people heard 
these broadcasts and marveled at the skill of the 
young wizards who assembled crude receivers capable 



of intercepting them. A considerable number of 
companies, some of them the forerunners of the great 
radio manufacturing organizations of today, were 
engaged in making components for a brisk amateur 
trade. The general public did not anticipate a 
broadcast structure as it exists today because there 
was no example of a similar service, but it nevertheless 
marveled and waited. With the example of broad- 
casting before us, it has been easy to visualize the 
possibilities of television and this has stimulated 
expectancy all the more. 

Essentials to Public Acceptance. 

Broadcasting had to await the accomplishment of 
three requirements to win public acceptance: (1) 
reliable and satisfactory programs sufficient to justify 
sustained interest; (2) simple automatic home equip- 
ment requiring little or no special skill in operation; 
and (3) inexpensive receivers assuring an adequate 
return in entertainment value for the investment. 

When KDKA began its service, receivers were far 
below present-day standards, but they possessed to a 
substantial degree the elements necessary to public 
acceptance. The first purchasers of radio receivers 
were so-called experimenters who desired to bask in 
the reflected glory of the wizards who had made radio 
possible. Their patronage made possible intensive 
commercial development which brought radio rapidly 
to the point of general acceptance on the basis of its 
service alone. 

Television is in the status of radio-telephone broad- 
casting prior to the general availability of low-cost 
vacuum tubes. Experimenters with some technical 



skill and patience can, in various parts of the country, 
enjoy crude reproduction of visual broadcasts. But 
neither the character of the programs, nor the cost, 
performance and quality of reproduction attainable 
with such equipment as yet justifies general public 

Requirements of the Experimenter Element. 

Even for an appeal limited to the experimenter 
market, service requirements must be met. Although 
the pioneer television enthusiast will be no more 
exacting in his requirements for fidelity of reproduction 
than his predecessor in the early days of broadcasting, 
he will not support a budding television industry 
unless he is certain of a recognizable reproduction after 
only a moderate effort. His kit of television parts 
must be assembled in a few hours with only such 
simple tools as a screw driver, a pair of pliers and a 
soldering iron. Perhaps the general availability of 
reliable 60-line television service and kits which can 
be built up and made to operate in not more than 6 
hours of home labor and at a cost not exceeding $50 
will bring the beginnings of a television industry. 
But even experimenter support cannot be long retained 
unless there is rapid improvement in the art, just as 
there was in broadcasting in its early days. 

It may be held that the requirements set forth in 
respect to both reproducers and program services of a 
standard sufficient to interest the experimenter have 
already been met. Some 30 stations have been 
licensed for experimental television by the Federal 
Radio Commission, of which a small proportion are 
maintaining regular schedules, including one at Jersey 



City, serving the New York metropolitan area, 
another at Washington, D. C., and a third at Chicago. 
One manufacturer is offering the public three types of 
television reproducing equipment for 60-line, 20- 
repetition service. The reproducer chassis includes a 
760-cycle eddy-current motor to effect synchronizing 
from the inital impulse of each line, scanning disc, 
viewing lens and necessary mountings, and it sells for 
$75 assembled. The same equipment mounted in a 
cabinet completely light shielded for efficient observa- 
tion in daylight costs $395. A special radio receiver, 
with power supply, covering from 100 to 150 meters 
and responding to from 15,000 to 30,000 cycles is 
offered for a list price of $175. But it appears that the 
experimentally inclined buyer of radio equipment does 
not yet feel that the three requirements (1) entertain- 
ing programs, (2) foolproof and reliable equipment and 
(3) low-cost reproducers representing a fair return on 
the investment have yet been produced. 

Television of 1930 Compared with Broadcasting of 

Television, as it is today, is receiving far less support 
from the experimentally inclined than did broadcasting 
of 1922 and 1923. Yet, if the status of the two arts at 
these periods can be fairly compared, they will not be 
found to vary greatly in standard of program material 
offered or in fidelity of reproduction attained. The 
first sound broadcasts were practically all limited *b 
presentation of individual artists. The pick-up of 
orchestras with the microphones then available was 
wholly inadequate and unsatisfactory. Reproduction, 
likewise, was so limited in range that it could not claim 
musical value. At the most, recognizability was the 



standard offered. The reproduction attained by a 
1922 radio receiver through the loudspeakers then 
available, if compared with a modern receiver, would 
appeal to any listener as truly ludicrous. The first 
loudspeaker on the market was a small tin spiral 
affair, reminiscent of the old bulb-type automobile 
horn. We hardly appreciate how rapidly reproduction 
quality has advanced in ten years. From the stand- 
point both of range of pick-up and of fidelity of repro- 
duction obtained, television of 1930 is very nearly on a 
parity with broadcasting of 1922. 

More than half a million people built their own radio 
receivers during the first three seasons that this low 
standard of broadcasting was generally available. 
The public overlooked the shortcomings of radio 
reception and hailed it as a marvel and a mystery. 
The new industry rose rapidly to prosperity in a period 
of industrial depression. The rapid technical progress 
made in its halcyon days would have been impossible 
had it not been for the liberal public support which it 
enjoyed from the first. 

But television, with probably as much to offer the 
experimenter in 1930 as broadcasting in 1922, and 
perhaps even more, has failed to attract a following in 
any way comparable to the enthusiastic group of 
experimenters from whom were recruited many of the 
leaders of the radio industry which they helped to create. 

'^ae failure to develop a large group of interested 
experimenters in the television field can be largely 
ascribed to loss of confidence occasioned by unwise 
publicity and to failure to take advantage of the 
program possibilities of 60-line television. Limited as 
these possibilities are, they offer as much opportunity 



to make news and arouse human interest as was at the 
disposal of the pioneer broadcasters. 

But performance must take the place of the kind of 
promises with which the television industry has fed a 
gullible and willing press. The continued appearance 
of such romantic stories as "television broadcasts from 
airplanes in flight,"' "presidential inaugural ceremo- 
nies" and "high-quality reproducers before Christ- 
mas," before the technical facilities to make them 
possible are developed, has caused the public to 
classify television publicity with that originating from 
makers of magnetic belts for curing rheumatism and 
snake oil for sharpening razors. 

The tendency to unrestrained exaggeration in 
television springs from sources on both sides of the 
Atlantic. British newspapers publish excited stories 
about television feats in America, while American 
newspapers make themselves ludicrous by describing 
as remarkable such stunts as synchronization of speech 
with action accomplished in a British demonstration 
of television drama. If speech and vision picked up 
simultaneously by microphone and photocell and 
directly radiated could be transmitted out of synchrony, 
it would be news because that, at least, would involve 
an electrical delay system of widespread application 
in long-distance telephony and in overcoming distor- 
tion with synchronized broadcasting. Were the effort 
expended to secure publicity diverted to producing 
features within the capacity of the 60-line systems now 
operating, it would help materially to develop a 
following for television which the industry badly needs. 

If the curtain rose suddenly on a perfected television 
service offering as regular program features all the 



great outstanding news events, performances by stars 
of the stage and screen, major athletic events and 
occasional glimpses of remote parts of the world, the 
public would hasten to spend a billion dollars as fast as 
a busy television industry could deliver the apparatus. 
The realization of these opportunities has spurred 
scientific research in the field and opened newspaper 
columns to television. It has encouraged the invest- 
ment of capital and the initiation of manufacturing. 
But at the present writing television can hardly be 
styled a busy industry. 

Reasons for Lack of Support to Television. 

The lack of public support to the existing television 
services is in a large measure due, as has already been 
said, to premature promises and to failure to offer 
programs for the eye which could arouse the slightest 
public interest. The program possibilities of 60-line 
television are perhaps greater than the technicians in 
charge of producing them seem to realize. Further- 
more, while recognizable images are easily obtained, 
we are far from the final refinement of 60-line repro- 
ducers. Although 60-line television is limited to a 
small field of view, it nevertheless provides ample 
entertainment for the experimentally inclined, whose 
interest lies principally in securing a recognizable 
reproduction and then improving its quality. It 
offers a practical opportunity to become acquainted 
with the fundamental design and service problems 
which are likely to exist, in less exaggerated form, it is 
true, when the art reaches a commercial standard. 
Those who have grappled with the difficulties as they 



exist today will be fully equipped to progress with an 
industry destined to become one of major importance. 

The real problem of the industry, then, at the 
present time, is the utilization of facilities that already 
are available. It must arrange for proper association 
of sound with visual programs, and the design of radio 
receivers which automatically reproduce both elements 
of entertainment. The sound and visual elements 
of the program must be profitably combined for 
radiation by the same carrier and filtered in the 
receiver to actuate sound and visual reproducing 
systems. Reproduction must be removed from the 
peephole category and developed to the point where 
even the simplest of home reproducers can entertain a 
family group simultaneously. Synchronization and 
framing must be taken out of the classification of a 
sporting proposition and made automatic and reliable. 
The program policies of television broadcasters must 
evolve rapidly beyond the present state in which the 
mere radiation of an image is all that is attempted. 
Programs must offer entertainment and educational 
material unavailable by other means and of a character 
making some discernible appeal to the intelligence. 
Finally safeguards must be set up against the extensive 
advertising uses of television so that it does not 
promptly create an unfavorable public reaction, just 
at the moment when audiences are beginning to be 
built up. 

All of these measures do not call for any technical 
or policy developments which offer any very serious 
problems. They require only a deliberate cooperative 
effort of broadcaster, manufacturer and research 



engineer under competent and understanding execu- 
tive guidance. 

Taking Television Out of the Experimental Classifica- 

Having taken the steps necessary for a sound 
foundation for an appeal to the experimenter class, the 
immediate concern of a television industry is rapid and 
continued development in television transmission and 
reception. The brilliancy and contrast of the repro- 
duction must be steadily improved to the point where 
no eye fatigue is entailed in observation over relatively 
long periods. The receiver must be automatic in its 
operation, and all manipulation concerned with syn- 
chronization and average shading values must be 
eliminated. If continued public support is to be 
maintained, the reproduction must rise quite rapidly 
from 60 to 100 lines, so that programs do not remain 
long restricted to crude reproduction of individual 
faces. But it is not necessary to jump from 60-line 
portrayal to a value sufficient to handle extensive 
outdoor sports events in a few months. So long as 
improvement is steadily maintained, the scope of the 
field of view within range progressively increased and 
fidelity of reproduction improved step by step, public 
interest will be sufficient to maintain a successful 
industry. Even if we should never arrive at the point 
where extensive fields of view, like that seen from an 
airplane in flight, or a comprehensive view of a football 
field in its entirety, will be within the capacity of 
television reproducers, the development of general 
service is by no means precluded. We may even be 
limited to a stage 20 feet wide for another half century, 



but a limitation of that character would not prove a 
tremendous obstacle to the development of a sub- 
stantial television industry. 

Handling Sporting Events with Systems of Limited 

The public is particularly interested in the possi- 
bility of enjoying sporting events through television 
reproducers. The technical problems involved in 
picking up such an extensive scene as a football field 
in sufficient detail to observe the plays appear to be, 
with our present knowledge of the art, wellnigh 
insuperable. But even if we remain limited to a field 
of view 20 feet wide for a long time to come, it will be 
possible to give an entertaining representation of such 
an event. Portable scanning equipment in charge of 
skilled operators accompanied by competent announc- 
ers can maintain a continued series of close-up views 
portraying all the important high lights of the game. 
The observing audience will be in the position of a 
person watching the game at a considerable distance 
through a pair of binoculars. While they will not be 
able to view the entire field at any time, they will 
nevertheless enjoy a reproduction of that small portion 
of it where intensive action is taking place. 

The development of sound broadcasting received its 
greatest impetus from major prize fights. The Demp- 
sey-Carpentier fight from Boyle's Thirty Acres, 
broadcast with Major J. Andrew White at the micro- 
phone in 1923, made WJZ famous overnight. The 
Dempsey-Firpo fight, a few months later, proved such 
an exciting radio event that it added tens of thousands 
to the growing army of pioneer radio enthusiasts. 



Because prize fights take place in a field of view of 
limited scope, they offer television its greatest oppor- 
tunity to spring to the forefront of public attention. 
It requires programs of this character, rather than 
programs taken from a silhouette film of a child 
bouncing a ball, to build up initial audiences. Limited 
as is the scope of 60-line television, the program value 
of the offerings so far presented has tended to depre- 
ciate to a minimum the service attainable with that 
quality of television. 

Necessity for Close Association of Television Broad- 
casting with Reproducer Manufacture. 

The problem of receiver design may tend to fall 
into hands somewhat disassociated from the interests 
having charge of the broadcasting of television pro- 
grams and the design of pick-up apparatus for the 
purpose. With sound broadcasting the disassociation 
of the engineers in the two phases of the science, 
transmission and reproduction, did not lead to particu- 
larly harmful results because channel spacing, carrier 
frequencies used and audio-frequency ranges employed 
were naturally standardized by the allocations made 
to broadcasting stations. Each group could develop 
its equipment independently, without requiring radical 
changes in the course of the other. New microphones 
and studio amplifiers could be developed, new studio 
acoustics and pick-up systems devised, extensive wire 
lines arid methods put into operation, without requir- 
ing corresponding alteration in receiver design. 

In the television field, the two phases of develop- 
ment must be somewhat more closely coordinated. 
To secure the most effective results, progress in one 



field should be closely reflected by complementary 
changes in the other, and the inevitable compromise 
between the conflicting requirements of the two made 
in a manner which secures the best overall result. The 
specifications of radio receivers must reflect every 
improvement in television transmission, such as 
changes in scanning method, number of lines, contrast 
embodied in the picture signal, size of reproduction, 
scanning motor speed and, indeed, every detail which 
might be specified in a set of standards. 

Premature Standardization Will Impede Progress. 

We have had one attempt to fix standards for the 
television industry by a group of radio manufacturers. 
These standards specified the number of frames per 
second, direction of rotation of scanning disc, scanning- 
disc speed and several other details. The stand- 
ards were adopted in order that there might be no 
unnecessary confusion and that all experimenters in 
the field might work along the same lines. But from 
the moment that these standards were adopted, there 
was opposition to them. Progress is certain to be 
impeded by any fixed standards, at least until we 
arrive at the point where the attained reproduction in 
home television is sufficiently good to justify public 
support purely on the ground of education and 
entertainment values. 

An instance of the kind of thing which may upset 
any premature effort at standardization is the possible 
alteration of frequency assignments made to tele- 
vision by the Federal Radio Commission. Five 
channels, one of which is limited by preferential rights 
to foreign countries in this hemisphere, have been 



devoted to experimental television services. These 
five channels have been squeezed out of a part of the 
radio spectrum which is intensively utilized for many 
different kinds of commercial services. The prospect 
of enlarging this band by extensions in its immediate 
vicinity is exceedingly remote. Congestion is, there- 
fore, bound to set in as soon as a practical service is 
rendered which cannot as easily be disposed of by 
synchronization of carriers, the hope of solution for 
congestion in the broadcast band. 

In broadcasting, it is a relatively simple matter, 
from the technical standpoint at least, to distribute pro- 
grams and controlling frequencies over wire networks 
so that the same channel may be used simultane- 
ously in different parts of the country. But wire net- 
works have not yet been adopted to the distribution of 
such broad bands of signals as are required for high- 
quality television. Consequently, numerous independ- 
ent transmitters are necessary to national coverage 
and a great number of transmitters cannot be accom- 
modated on five channels. Either the signal band 
requirements of television of commercial quality must 
be reduced or else a new band of wavelengths for 
television purposes must be unearthed. In absence of 
a radical change in transmission methods, therefore, 
channels for numerous transmitters can be found only 
in the ultra-high frequencies not now utilized for other 
services. Any move in that direction would render 
obsolete receivers made with only the existing experi- 
mental channels in mind. 

In view of these and many other considerations, it 
appears that any effort in the direction of standardiza- 
tion is premature, necessary as standardization is from 



the standpoint of commercial development. Even if 
we go too far in indicating common trends, we are 
likely to deflect the course of technical development of 
television along the lines which will restrict its prog- 
ress. The technical future of television must be 
somewhat more clearly demarked before it is safe to 
attempt to crystallize such design details as number of 
scanning lines per frame and number of frames per 
second. Any standards so fixed accommodate the 
needs of appliances which are likely to be displaced by 
instrumentalities of greater flexibility and capacity. 

Any attempt to predict the date of the emergence 
of television from the laboratory to general service 
must take into consideration not only the technical 
status of the science but the leadership developed by 
the industry. While the burden remaining on the 
shoulders of the laboratory technician are considerable, 
the greatest opportunities for service to the industry 
lie in its intelligent direction, to the end that the 
utmost use be made of the facilities which it already 
has available to it. The development of that execu- 
tive talent has as much to do with determining the 
date when the television era will unfold as have the 
tasks remaining unfinished in the laboratory. 






Various types of apparatus, using elements developed 
for television transmission and reception, present 
opportunities for profitable research and manufacture 
to the forthcoming television industry. Considering 
the seasonal character of radio-receiver production, 
which depends upon similar buying impulses, television 
manufacture is quite certain to exhibit the same profit- 
absorbing seasonal characteristics. Large research 
and royalty costs are likely to be a heavy burden, at 
least during the earlier years of operations. The 
contribution of by-product fields to building up a 
stabilized industry is, therefore, a matter of the utmost 
practical importance to the manufacturer and engineer 

Relation of By-product Applications to the Television 


In using the term by-product, I am assuming the 
existence of an established television industry with 
general public support. Prior to the attainment of 
such commercial status, these by-products are the 
established field and television is the experiment, just 
as ship-to-shore and transoceanic radio telegraphy was 



once the commercial reliance of the radio interests 
while radio telephony was an experiment. Today the 
once all-important radio-telegraph business is con- 
sidered so foreign to the main occupation of the radio 
interests that only a clause of the White Act prevents 
its sale to other communication interests. Eventually 
television manufacture will consider facsimile and 
industrial applications of devices used in television 
only as by-products, which were once useful in main- 
taining manufacturing schedules during seasons dull 
for television and in bearing a part of the research 

The television industry itself will at first be largely a 
by-product of the radio industry. The electrical 
elements of television reproducers are, from the pro- 
duction standpoint, similar to radio receivers, although 
electrically there is a vast difference between amplifiers 
for handling such comparatively narrow bands of 
signals as are involved in musical reproduction and 
the wide bands required for commercial quality tele- 
vision. Furthermore, television reproducers require 
association with radio receivers of special character- 
istics which, electrically, differ widely from broadcast 
receivers, but, mechanically and structurally, are 
similar to them. 

When television grows to commercial manhood, it 
will merely advance from the by-product stage to full- 
fledged partnership with the radio industry. From 
the standpoint of this combined radio and television 
industry with its many amusement ramifications, the 
industrial uses are obviously destined to fall into the 
classification of allied industries not logically adapted 
to direct association. But in the pioneer stages, these 



by-products of television may be the avenue to 
commercial and economic growth. The development 
work undertaken to maintain a position in these 
associated fields will, no doubt, lead to many improve- 
ments which may be embodied in television equipment. 

Sound Motion Pictures as a Field for Television Manu- 

Television owes the availability of practical com- 
mercial photoelectric cells largely to research and 
development work undertaken for talking pictures and 
facsimile systems. Talking motion-picture recording 
on film involves a system somewhat paralleling the 
functioning of a television scanning apparatus. Sound 
waves are impressed on a microphone associated with a 
vacuum-tube amplifier system, the output of which 
controls a light source directed upon a narrow section 
of the film known as the sound track. One system of 
recording known as the variable-density method 
employs a variable aperture magnetically controlled by 
the audio currents originating with the microphone, 
which increases or decreases the intensity of the light 
projected on the film. Another method of securing 
the variable light required for recording with the 
variable-density method is closely similar to that 
employed for securing a variable light for television 
reproduction. The amplified sound currents are fed 
to a glow lamp, corresponding to the familiar neon 
tube, known as an Aeo lamp. It depends upon a 
luminous charge taking place in actinic gas for its 
response to fluctuations in applied voltage. Another 
type of sound recording on film uses a vibrating mirror 
like that of an oscillograph, which makes a relatively 



wide band of light on the sound track in response to a 
sound impulse of maximum intensity and a narrower 
band for smaller intensities. This is known as the 
variable-area system. 

The range of frequencies employed in sound is 
narrow as compared to that required by television. 
The problem in connection with talking motion 
pictures is not so much to extend the range of response 
as it is to develop rugged equipment responding faith- 
fully to the required range. The range of frequency 
handled by the reproducer on the stage loudspeaker 
limits the useful recorded range to perhaps eight 
thousand cycles. Consequently, it has been possible 
to concentrate development work on improved fidelity 
and reliability without considering increased range, 
the primary problem of the television engineer. In 
working along these lines, contributions have naturally 
been made toward reliable television apparatus. 

The heart of the equipment used for the reproduc- 
tion of talking motion pictures is the photoelectric cell. 
Regardless of the method used in recording on film, 
the reproduction process "is similar. A fixed light 
source is projected through the sound track on the 
film and a variable light produced, fluctuating to 
correspond to the original speech and music recorded. 
The light is focused on a photoelectric cell so that its 
current output is a counterpart of the originating 
sound waves. The current produced is amplified by 
vacuum tube to the point where sufficient power is 
available to actuate the bank of loudspeakers behind 
the screen. 

The quantity requirements for photoelectric tubes 
for use in the motion-picture theaters are fairly con- 



siderable. While it is quite probable that eventually 
photoelectric tubes of much higher quality than is 
required for sound production will be developed for 
television purposes, the fact there is a market already 
existing for photoelectric tubes can be useful in 
stabilizing production and justifying research expense 
in tubes especially adapted to television. 

Facsimile Telegraph Equipment as a By-product of 

Facsimile telegraphy, phototelegraphy or trans- 
mission of photographs over wire lines in a similar 
way involves the technique of the television engineer. 
Fundamentally, a facsimile is television without the 
maintenance of the elements of motion. Facsimile 
has the same relation to television that the stereoptican 
has to motion pictures. 

Research in phototelegraphy followed closely upon 
the invention of the telegraph itself. Numerous 
methods have been devised depending upon mechanical 
contactors working from the face of a copper etching. 
The more modern systems closely parallel the methods 
of television. 

While there are numerous methods and systems, the 
same fundamental principles apply to most modern 
facsimile devices. A negative transparency of the 
photograph to be transmitted is mounted on a glass 
cylinder on a fine spiral thread. A fixed pencil of light 
is projected through the cylinder which, because of its 
spiral motion, progressively explores the entire trans- 
parency. The light passing through the negative 
controls the output of the photoelectric cell producing 
the picture signal. 



This process is closely analogous to a television 
system except that we have the field of view in the 
exceedingly convenient form of a photographic nega- 
tive and that we are faced with no time limitation to 
complete scanning. We do not depend upon light 
reflected for varying distances as with television, 
because the field of view is first conveniently reduced 
to a single layer of varying density by photographic 

Since we may take as long as we please to complete a 
single scanning of the field of view, the picture signal 
may embody a vast number of picture elements. A 
4 by 5 photograph may be readily broken down into 
over 100,000 picture elements, yet only a narrow 
communication channel is required for its transmission. 
One to three minutes can be used in the process of 
transmitting the picture signal representative of 
100,000 picture elements instead of J^ second as with 
television, because, in the former case, the collation of 
these elements is accomplished in permanent form by 
photography, and in the latter it must be accomplished 
by the eye itself. 

Reproduction of Facsimile Pictures. 

Reproduction of the still picture at the distant 
point involves a magnetically controlled light valve 
which controls the intensity of a fixed light ray, 
directed to a cylinder of the same proportions and 
rotating in the same manner as the one upon which 
the transparency is mounted for transmission pur- 
poses. Mounted on this cylinder is photographic 
paper which is exposed by the fluctuating light source. 



After progressive exposure, the photograph is finished 
in the usual way. 

Many elements of the television system are em- 
ployed in the typical facsimile process. The subject is 
scanned by a photoelectric cell and the resulting 
picture signal converted into corresponding light. 
Transmitting and receiving motors are maintained in 
accurate synchrony. The elimination of the limita- 
tion that scanning must be completed within a fixed 
period, such as a twentieth of a second, makes possible 
facsimile reproductions of practically perfect texture 
that can be sent over telephone wires of no broader 
range than are used for broadcast pick-up work. 
Such photographs are used interchangeably with 
original photographs in newspaper work without any 
discernible sacrifice of clarity. 

Special Features of Various Systems. 

Pictures sent across the Atlantic by radio are not 
of such good quality because of long distance trans- 
mission through the unstable radio medium. In fact, 
until Captain R. H. Ranger invented and developed an 
ingenious system in which the light intensities traced 
on the reproduction are controlled not by signal 
intensity but by signal duration, the effects of fading 
and other disturbances made long distance trans- 
mission of pictures of commercial quality impossible. 

There are also other variations from the typical 
facsimile transmission system first described. In 
transmission, for example, a positive print may be 
used on a cylindrical drum. The light is then reflected 
from the photograph to the photoelectric cell instead 
of being projected through it. This method intro- 



duces problems of spectral diffusion, but it is one of 
great practical convenience. At the reproducing end, 
likewise, various methods may be used to trace the 
reproduction on the desired surface. One system 
developed by Austin G. Cooley employs the brilliant 
corona discharge for exposing the photographic paper. 
Corona is rich in ultra-violet rays and the frequency 
of the light produced is at the end of the spectrum to 
which photographic paper is the most responsive. 
Consequently, papers of slow exposure can be used, 
which is not the case when feebler sources of illumina- 
tion are employed. Therefore, elaborate precautions, 
such as operating the reproducing equipment only in 
feeble red light, necessary with systems in commercial 
use, are obviated. In fact corona reproduction has 
developed to the point where it can be carried out in 
broad daylight. 

Light-sensitive Devices for Industrial Control. 

Another class of device of great commercial promise 
is the application of light-sensitive devices to industrial 
control. Light source, photoelectric cell amplifier and 
control relay constitute a combination having a 
seemingly endless number of applications in industry. 
Devices of this character may be divided into two 
classes, first, those depending upon interception or 
mere presence or absence of light, and, second, those 
which are controlled by accurate response to vary- 
ing light intensity. The interception classification 
includes such devices as burglar alarms, sound devices, 
safety and warning units. A light beam may be 
directed to the combination of a safe. If any person 
or object intercepts that beam of light, a photoelectric 



cell with associated amplifier immediately closes a 
relay which may be used to control any electrical 
operation. The light ray to be intercepted may be of 
infra-red, which is invisible. A similar system may 
count containers passing on a moving belt, to start off 
machinery when material for fabrication appears, or 
may actuate a warning device when a person or object 
steps into the ray of light marking the limits of safety. 

Photoelectric devices of the second class may be 
designed to respond to total reflection or only to 
reflection of one color or frequency. Total reflection 
is involved in photometers, smoke recorders and 
paper-grading machines. Devices measuring inten- 
sity for each frequency of the visual spectrum are 
called spectrophotometers. Color measuring by pho- 
toelectric analysis is involved in applications such as 
cigar sorters, bean sorters, coffee-roasting control- 
lers, dye measuring indicators and automatic color 

The reason why such devices are likely to have 
extensive employment in industry is that they are far 
more rapid and accurate in response than the human 
eye. A trained operator may be able to sort two or 
three objects of a certain kind per second, but a photo- 
electric device can sort hundreds. In fact, the limit is 
in the mechanical systems to handle the product and 
not in the light-sensitive system. Unlike the human 
operator, moreover, the photoelectric device is not 
subject to fluctuations due to fatigue or changing 
conditions of illumination. 

While these applications are not so closely related 
to television as it might appear, both the engineer and 
production facilities necessary to produce them are 



similar to those needed in television manufacture. It 
is obvious, therefore, that the television industry has in 

FIG. 72. A photoelectric smoke-density recording and warning device. 
(Courtesy of Westinghouse E. & M. Co.) 

the manufacture of parts for talking motion picture, 
facsimile and industrial control devices, a means of 



attaining production stability and freedom from 
excessive seasonal characteristics. The radio-receiver 
business has found this the pitfall that has wrecked 
many of its pioneers who once enjoyed prosperity. 

FIG. 73. A photoelectric spectrophotometer developed by the Amer- 
ican Photoelectric Company for accurate color analysis throughout the 
visible spectrum. 

With the example of the radio industry before us, 
it is possible for producers of television apparatus to 
avoid the economic suicide involved in the concentra- 
tion of facilities in production which is pushed only 
during a small part of the year and which cannot be 
carried over successfully from season to season. 



The progress of every widely used invention from 
original conception to commercial acceptance has been 
slow and wearying. Only the pioneers remember the 
struggles against technical obstacles, financial dis- 
couragement and public inertia, which have charac- 
terized the early history of every major innovation. 
The motion picture, the automobile, the airplane and 
radio telephony, each struggled ten to fifteen years 
between the first practical demonstration and general 

Public Demands Perfected Television. 

From the technical standpoint, the road which 
television must travel is rendered doubly difficult 
because the public has already set high standards of 
performance which must be met before general accept- 
ance can be expected. The motion picture, on the one 
hand, has developed a definite conception of the 
attainable quality of visual reproduction, and sound 
broadcasting, on the other, has demonstrated how 
successfully a radio-communication system can convey 
all the information requirements of the sense of hearing. 
By simple but fallacious logic, acceptable television is, 
to the general public, the delivery of images of motion- 
picture quality into the home by radio. Anything 



less than that standard is considered merely as an 
experiment. Apparently television faces a tremen- 
dous handicap which, to be overcome, implies the 
successful solution of every major technical problem 
which today marks the boundaries of progress in the 

Once sufficient technical and program progress has 
been made, however, the attainment of public recogni- 
tion and acceptance will be easier in the case of tele- 
vision than for any previous invention of as broad a 
scope. Television is eagerly awaited. The commer- 
cial structure to carry out the essential broadcasting 
services is fully organized and operating under the 
direction of experienced executives. The facilities 
and personnel for the manufacture and distribution of 
reproducers for the home user are likewise fully estab- 
lished. Indeed, the acceptance for television is so 
highly developed that failure to deliver a product of 
commercial standards in the relatively near future will 
actually tend to weaken public confidence and will 
make more difficult the introduction of television when 
it is finally ready to make its bow as a regular program 

Broadcasting's Long Struggle from Conception to Accept- 

The short memory of the public in respect to the 
struggles of the pioneer is no better illustrated than by 
the history of sound broadcasting. The date con- 
sidered the most important landmark in the progress 
of broadcasting is November 2, 1920, when KDKA 
broadcast the Harding election returns. In an amaz- 
ingly short space of years from that date, a nationwide 



broadcasting system was established, a manufacturing 
industry organized, and a national retail distribution 
structure reaching to every hamlet evolved. 

But the only thing that makes November 2, 1920, 
significant in radio broadcasting is that it happens to 
be the particular incident that caught the public fancy 
and marked the beginnings of public acceptance. 
KDKA is no more the first broadcasting station than 
was the first automobile equipped with a self-starter 
the first automobile. There were literally scores of 
broadcasting stations preceding KDKA, some offering 
spasmodic and others maintaining regular service. 
Indeed, even Enrico Caruso can be numbered as a 
broadcast artist, his voice having been broadcast by 
Dr. Lee DeForest in 1909 through his radio telephone 
station. Stock promotion of radio telephone com- 
panies flourished in 1906. R. A. Fessenden demon- 
strated radio-telephone communication for a distance 
of 16 miles from Brant Rock to Plymouth, Mass., on 
December 11 of the same year. A few months later 
voices and music were heard from Brant Rock at 
Jamaica, Long Island, a distance of 180 miles. The 
period from 1906 to 1920 was an intensive struggle to 
overcome major technical problems in radio telephone. 

The first important demonstration of television 
was staged by C. Francis Jenkins in Washington, D. 
C., on June 13, 1925, before Secretary of the Navy 
Wilbur, Dr. George M. Burgess, director of the Bureau 
of Standards, and other notables. The transmitter 
was installed at Naval Radio Station NOF at Bellevue, 
D. C., and the reproduction apparatus at the Jenkins 
laboratory in Connecticut Avenue. The subject of 
the transmission was a strip of motion-picture film. 



Since that demonstration, progress in the art has been 
steady and the research effort expended unstinting. 
Television is entitled to a fair quota of time for develop- 
ment in the laboratory and, in the light of experience 
with previous inventions of similar magnitude, it has 
not lagged unduly. 

Electronic Control of Reproduction Light Essential. 

Many engineers who have worked intensively with 
television are frank to admit that there are stumbling 
blocks to progress which cannot be scaled but which 
must be circumvented. The two hurdles of the most 
imposing character to be overcome are the discovery 
of a practical non-mechanical scanning means and of a 
method of sidestepping the apparent channel limita- 
tions. I am very doubtful that high-quality television 
will ever be accomplished by a mechanical method of 
distributing a fluctuating ray of light on the field of 
reproduction. It is essential that we control the 
direction and intensity of the light source by electro- 
static or electromagnetic fields having the facility of 
acting as rapidly as light itself. The Karolus cell, 
which has been described, 1 is one such means of 
controlling the intensity of a light beam by effecting 
minute changes in the phase of doubly refracted 
polarized rays. The cathode-ray tube accomplishes a 
control over both direction and intensity of the light 
beam non-mechanically. In one of these, or in some 
similar method, lies the solution of the problem of the 
distribution of light in the reproduction. 

The fact that the cathode-ray tube, as we know it 
today, has a life of but a few hours should be no cause 

1 See pages 116-124. 



for discouragement. Dr. Alexander Meissner demon- 
strated radio telephony between Berlin and Nauen, a 
distance of 23 miles, in June, 1913. Contemporary 
engineers pointed out the futility of this experiment 
because the vacuum tubes used under the conditions 
lasted only 10 minutes on account of disintegration of 
the filament by positive ionic bombardment. The 
probabilities are that we already have with us today 
the beginnings of the device which will come to the 
forefront as the means of making high-quality home 
reproduction possible but, because of apparently 
serious deficiencies, it is regarded as unpromising. 

Channel Limitations Imposed by Progressive and Con- 
tinuous Scanning. 

The widest band of modulation which has ever 
been used for television is from 10 to 36,000 cycles. 
This is the band width required for transmitting 60- 
line images at the rate of 20 repetitions per second. 
Bell Laboratories engineers used from 10 to 40,000 
cycles for the 72-line wire television in 1930. With 
progressive scanning involving an impulse for every 
picture element in the reproduction within the time 
that the eye can collate these elements as a con- 
tinuous image, 100-kilocycle channels seem likely 
to prove inadequate. But the capacity of 100-kilo- 
cycle channels has never been tested, nor will we know 
exactly what entertainment value can be offered 
through them until we have seen 100-kilocycle tele- 
vision in practice. 

The remarks of Dr. Frank B. Jewett, president of 
the Bell Telephone Laboratories, at the public demon- 
stration of television reproduction at New York with 


subjects in Washington, are significant and have lost 
none of their force and applicability : 

While research and development work for the perfection 
of television will go on for years, enough has already been 
accomplished to indicate that it is likely to have a real 
place in the world's work of distant communication. 
Today (1927) we are relatively farther along in our work 
on television than we were on transoceanic telephony in 
1915 when the American Telephone and Telegraph Com- 
pany conducted the first successful test from Washington 
to Paris and Honolulu. Just what the ultimate field of 
television is to be can, as Mr. Gifford has said, be left to 
your imagination. The one thing that seems clear is that 
it will be a use closely associated with the telephone. 

In attempting to form a picture as to the future develop- 
ment of television, there is one inherent limitation of any 
television method which we should keep clearly in mind, 
however. This is the fact that it requires the use of a 
large group of frequencies and the transmission of these 
frequencies requires as great capacity as a considerable 
number of ordinary telephone circuits. It is this fact 
which puts television economically into a class quite 
different from that of ordinary telephony or telegraphy. 1 

Improved Brilliance and Contrast Reduces Channel 


We are still far from leaping the channel limit hurdle, 
although practice may prove somewhat more generous 
than theory in respect to the image embodied in a 
signal using the whole available channel. After all, 
the calculations made for picture-element requirements 
are based on the assumption that the resolving power 

1 JEWETT, DR. FRANK B., Bell Laboratories Record, Vol. IV. No. 3, 
May, 1927. 



of the eye is of a definite value. The value, however, 
as I have pointed out, is altered by the brilliance of the 
image viewed and its contrast. The values chosen for 
the calculations are based on the brilliance and contrast 
available for home motion-picture reproduction. If a 
method is developed for securing greater brilliance and 
contrast in television reproduction, the picture element 
requirements for a given clarity are reduced. The 
calculations made are justified only by the fact that 
there is no method in sight of projecting a more 
powerful image than we secure in the motion-picture 

But there is no valid reason for believing that the 
science of television, realizing its necessity, will not 
develop a controlled light source of greater intensity. 
Therefore, to assume that the 100-kilocycle limitation 
erects a definite barrier to the amount of detail in a 
television image is not justified because, first, we have 
never tested the capacity of 100-kilocycle channels 
and, second, we have undertaken no development 
work looking toward the employment of greater 
brilliance and contrast than is used in motion-picture 

Circumventing the Channel Limitation. 

The most fundamental assumption which is made in 
defining the limitations of television is that a com- 
munication impulse must be transmitted for each 
picture element at least 20 times a second. But there 
are no grounds for such an assumption. Suppose, for 
example, that someone developed an effective phos- 
phorescent screen such that an impulse of 1/1,000 
second set up illumination lasting a full second. 



Suppose further that there is no motion in the field of 
view. In that case, to maintain that scene before the 
eye, it would have to be scanned only once a second 
instead of 20 times a second. Obviously, the scene 
could consist of 200,000 picture elements and still be 
transmitted within the limitations of 100-kilocycle 

Another expedient to escape the channel limitation 
would be the use of photography instead of the eye for 
collation of the image. Supposing the television 
reproducer directs a ray of light onto a strip of unex- 
posed motion-picture film. If the scanning process is 
conducted at the rate of one frame per second, it would 
take 20 times as long to make the film as to reproduce 
it through a home projector. But it would be possible 
to offer a news-reel service of the air for reproduction 
by the standard 16-millimeter reproducer. The tele- 
vision reproducer would be tuned to the station sending 
the news reel, comedy or drama and allowed to operate 
automatically making film. The device might also 
have automatic means of finishing the negative. 
Although it would require an hour to make an amount 
of news reel which is exposed in 5 minutes, this fact 
would be of no special annoyance to the observer, since 
the operation of making up film is presumed to be 

There are obvious practical mechanical difficulties 
which stand in the way of rendering a general service 
by the method described. The main point is merely 
to show that there are conceivable methods of circum- 
venting the channel limitation. The limitations are 
not in the channels; they are imposed by the methods 
employed in the scanning and reproducing systems and 



the religious adherence to a relatively ancient concep- 
tion of television scanning. 

Fresh Viewpoint Needed to Produce Improved Television . 
The most important needs to produce a practical 
television service are minds and imaginations which 
approach its technical problems with a fresh point of 
view, and which do not merely accept the orthodox 
method already exhaustively investigated by numerous 
inventors. The problems of television are not insuper- 
able, nor are they more difficult than those which 
faced the inventors of any generally accepted enter- 
tainment of communication device. Their solution 
will have the inherent simplicity characteristic of 
every practical invention. We are familiar with so 
many elements of the ultimate television system that it 
is no strain to prognostic powers to predict that 
television is "just around the corner." The probabili- 
ties are, however, that most of us are congregated 
hopefully on the wrong corner. 



Advertising by television, 222-232 
effectiveness, 231-232 
programs, 222-227 
technique, 228-232 
testimonial, 229-231 
Alexanderson, Dr. E. F. W., analy- 
sis of projection, 134 
Karolus projector, 120-124 
observations on echo effects, 88- 


Appliances, television, industrial 
and commercial applications, 
Audience, television, experimental 

element, 235-236 
performance standards neces- 
sary to establish, 26-27, 


Band selector, 96-98 

Bell System, color television, 107, 

50-line television, neon grid 

screen, 110-112 
picture signal, maximum fre- 
quency of, 77 

progressive illumination scan- 
ning, 38-39 
superheterodyne receiver, 98- 

synchronizing, 153-155 

Bell System, 72-line television, neon 
water-cooled tube, 104-106 
progressive illumination scan- 
ning, 39-41 

synchronizing, 155-157 
transformer coupled amplifier, 

neon glow tube, concentrated 

ray, 106, 129 
selective fading investigation, 


Broadcast receivers, usefulness in 
television reception, 74-75, 

Broadcasting and television, early 
histories compared, 259-261 

Cathode ray tube, 112-115 
Channel requirements, methods of 

reducing, 262-266 
tabulations for detail and repeti- 
tion rates, 199-200 
(See also Picture signal.) 
Channels, transmission, assigned to 

experimental television, 94 
discriminating between neigh- 
boring, 92-93 
Color, elimination of, effect on 

reproduction, 13-14 
Color response, eye, 167-168 
photoelectric tubes, 62-66 
Color television, Bell System 
demonstration of, 107, 135-136 



Commercial possibilities of televi- 
sion (see advertising, televi- 
sion, performance standards). 
Cooley, A. G., facsimile system, 254 
Communication, nature of, 7 
Crystal oscillators for synchroniza- 
tion, 158-159 
Cuprous oxide cell, 59 


Detail, brain, information require- 
ments of, 181-184 
channel requirements imposed 

by, 83, 199-200 
definition of, 181 
distortion to, introduced by 

coarse scanning, 36-38 
by inadequate transmission 

system, 83-84 

eye requirements for, 183-191 
factors determining require- 
ments, 184 

half-tone screen, values of, 14-18 
picture signal frequencies, higher, 

contribution to, 78-79 
requirements in television repro- 
duction, 179-205 
selective fading, effect on repro- 
duction of, 87-88 
square aperture disc, effect on, 36 
uniform, necessity for, in entire 

scene, 174-175 
(See also Picture signal.) 
Developments in television, nature 

of recent, 2 
Disc scanning (see Scanning disc). 

Enlargement of reproduced image, 

134-135, 189-190 

Entertainment service, experimen- 
tal television, program possi- 
bilities of, 239-240 
prospects for development of, 


requirements to secure public 
acceptance, 26-27, 203-205, 
234-235, 258-259 
time required to develop, com- 
pared with broadcasting, 
Entertainment value, analysis of, 

early broadcasting compared 

with television, 236-237 
48- and 60-line television, 210-215 
100-line television, 215-219 
Eye, adjustments of, 165-166 

for television reproduction, 


to darkness, 169 
color response of, 167-168 
field of view of, 166 

exploration of, 173-174 
information to, motion picture 
and television compared, 
interfering illumination, effect on, 


persistence of vision, 176-179 
resolving power of, 185 
response to motion, 175-176 

to shading, 170-171 
sensitivity, 169-170 


Economic structure for support of Facsimile telegraphy, 251-254 

television, 224-226 Facsimile transmission, definition 

Educational uses for television, of, 6 

212-215 Field of view, definition of, 6 



Field of view, reconstruction, 24-25 
Framing, definition of, 139 

means for accomplishing, 161-162 
synchronization, distinction be- 
tween, 139-140 

Fundamental processes of televi- 
sion, 26 

Fundamental television system, 
description of, 27-29 

General Electric Company, 88-89, 


"Hunting" in synchronization sys- 
tem, 151 

Jenkins, C. Francis, drum scanner, 

projector, 135 
television system receivers, 99 

Karolus projector, 120-124 
Kerr Principle, 116-117 

(See also Karolus projector.) 

Light-sensitive element, 56-72 
definition, 56 

requirements in television, 57 
(See also Photoelectric cell.) 

Light source for television reproduc- 
tion (see Cathode ray tube; 
Neon glow tube; Karolus 

Light valve and variable light 
source, distinction between, 

Lines, definition of, 30 


Motion picture, picture elements in, 


film, scanning of, 50-52 
Motion pictures and television, 

comparison of, 52-55 


National Broadcasting Company, 

86, 89-90 
Neon glow tube, 102-112 

concentrated ray for projection, 

106, 129 

Jenkins multi-element, 109 
Neon grid screen, 110-112 
Nicol prism, 119 


Performance standards, means of 

evaluation of, 12-13, 49-50 
necessary to public acceptance, 

26-27, 234-235' 
Persistence of vision, 25, 132, 176- 


Photoelectric phenomenon, 56-59 
Photoelectric tube, characteristics 

of, 62-68 

color response, 62-66 
coupling to amplifier, 69-72 
description of, 59-61 
gas type, 61-62 

industrial applications, 254-257 
principle of, 59 
sensitivity of, 66-68 
television service requirements 

of, 71-72 
Wein cell, 59 

Phototelegraphy, definition of, 6 
Picture elements, brilliance, rela- 
tion to number required, 131 
definition of, 30 

increase of, for enlarged reproduc- 
tion, 190-192 



Picture elements, motion pictures, 

number for scene of perfect 

texture, 186-189, 194 
Picture signal, channel require- 
ments, analysis of, 81-83 
calculation of, 75 
table of, 199-200 
converting to light, 102-125 
definition of, 23 
higher frequencies, contribution 

to detail, 78-79 
information embodied in, 102 
low frequency requirements, 80 
maximum frequency in, 76-78 
synchronization, relation to, 137 
transmission of, 73, 91 
Picture transmission, definition of, 


Polarized light, 117-119 
Predictions, practical television 

service, 2-4, 147, 263 
Programs, television, and broad- 
casting, 223-228 
motion picture quality stand- 
ards, 219-221 
possibilities of, 206-221 

for variable repetition and 

detail, 216-219 
sporting events, 242-243 
technical limitations, 206-208, 

Projection, advantages gained by, 

172-173, 198 
concentrated neon glow tube, 

106, 129-130 
development, prospective, 133- 


Karolus method, 120-124 
Publicity, television, 238 


Quality, definition of, 181 


Radio-frequency amplifier, tele- 
vision receivers, 95-101 
Random voltages in coupling re- 
sistors, 34-35 
Ranger, Capt. R. II., facsimile 

system, 253-254 
Raytheon glow tube, 103-104 
Receivers, broadcast, for television, 

74-75, 92-95 
radio-frequency amplifiers of, 


requirements for television, 92-96 
superheterodyne, 98-99 
television design, 243-244 
Reception of television signals, 


Repetition rate, definition of, 19 
minimum, 127-128 
varying, possibilities of, 196-197, 

Reproduction, brilliance, relation 

to detail, 177-179 
enlargement of, 134-135, 189-192 
entertainment value, analysis of, 


faithful, 137-139 
forming image in, 126-136 
future development of, 261-263 
ideal subject for, 79-80 
illumination, effect on response, 


selective fading, effect of, 87-88 
Resistance-coupled amplifiers, 69- 
72, 99-101 

Scanning, definition of, 21 
Jenkins' drum, 108 
motion picture film, 50-52 
necessity for, 31 



Scanning, progressive illumination, 

advantages of, 41-43 
description of, 32-33 
progressive observation, de - 

scnption of, 32-34 
and progressive illumination of 

compared, 32 
purpose of, 30 
rating, method for, 49-50 
Scanning disc, development of, 


effect of large apertures, 35 
lens scanning type, 46 
limitations of, 44, 128-129 
moving mirrors for, 46 
prismatic, 45 
staggered spiral type, 47 
Selective fading, 85-88 
description of, 85 

effect on television reproduc- 
tion, 87-88 

methods of overcoming, 86-87 
Selectivity, band selector, 96-98 
requirements for television recep- 
tion, 92-96 
Selenium, 57 

Shading, eye response to, 170-171 
Short-wave synchronization signal, 


Sound accompaniment for tele- 
vision, 94-95, 208-216 
Sound communication, compared 

with visual, 7-11 
Sound motion pictures, 249-251 
Standardization, television, 243-246 
Synchronization, 137-160 
accuracy required, 141-142 
Bell System, 50-line demonstra- 
tion method used in, 153-155 
72-line demonstration method 

used in, 155-157 
clutch method, 143 
definition of, 24, 139 

Synchronization, early attempts, 


effect of inaccurate, 140-141 
framing, distinction between, 

independent crystal oscillator for, 


manual, 144-148 
photoelectric tube, use in, 150- 


power line, 148-150 
short-wave signal, 157-158 
tuning-fork method, 160-161 

Technical developments, future 

direction of, 261-266 
Television, broadcasting as supple- 
ment to, 208-216 
definition of, 5 
fundamental processes of, 26 
typical system, 27-29 
Transmission channel, calculation 

of requirements, 75-76 
capacity of 100 kilocycle chan- 
nels, 76 
Transmission, television, channels 

assigned to, 94 
echo effect, 88-91 
radio necessary to conclusive 

demonstration, 84 
reflections in, 88-91 
requirements, channel, 75-84 
(See also Picture signal; Selec- 
tive fading.) 

Visual communication, nature of, 


Wave length of radiations, table, 60 
Wein cell, 59 



Wire television, definition of, 5-6 Z 

significance of demonstration of, 

84 Zworykin, Dr. Y. K., cathode-ray 

tube developed by, 114-115 

Wired radio, prospective use for, tuning-fork synchronizing sys- 

90 tern, 160-161