Television; present methods of picture transmission

Front matter

TELEV/S/ON

PRESENT METHODS OF PICTURE TRANSMISSION

BY

Preface

IN preparing a book on Television one is confronted with the possibility of a revision before the book is off the press. The kaleidoscopic manner in which advances are being made is, of course, responsible for this.

Nevertheless, since the authors feel that for the good of the subject a review of the field is at the moment imperative, they have undertaken this difficult task. At the outset, and during the preparation of the manuscript, they have seen clearly the reason why no one else in this country has ventured such a book to date. They have rushed in where angels fear to tread in spite of the condemnation which is likely to fall upon their heads. The mass of material which could be included in a book of this kind is enormous and we have been forced to use our best judgment as to what to leave out. There will be many to tell us that we have omitted material which ought to have been included and included other material which might well have been excluded. Our argument is that it depends upon the point of view. This, of course, necessitates an explanation of our own viewpoint. We have attempted to give a true picture of the state of television to-day and to give the reader the necessary background to enable him to begin a special study of any one feature. Our style, we hope, is also of a sufficiently popular nature to enable the layman, or the possible investor, to gain a proper perspective of the subject. The book is in no sense a compendium. We have tried to give credit where credit is due and to subdue the blatant claims of pretenders who fill the daily press with extravagant statements. If in

iii iv PREFACE

any case we have done an injustice, it has not been intentional.

We have finished the book without knowing what the future of television is to be. Technical difficulties make us somewhat skeptical yet past accomplishments make us, perhaps, over optimistic. We believe, we have fairly curbed our desires to prophesy considering the excellent opportunity afforded by the nature of the subject.

We hope that the book will be found useful and that we may later be able to improve it and keep it up-to-date. Suggestions from readers will be greatly appreciated.

We have had much helpful cooperation in its preparation from individuals and corporations. Without them its writing would have been impossible. We are particularly indebted to the following to whom we extend our sincere thanks: J. L. Baird, of London; Professor Arthur Korn, of Berlin; Jenkins Laboratories, Washington, D. C.; and also to the following corporations : Bell Telephone Laboratories, Case Research Laboratory, Inc., General Electric Co., Radio Corporation of America, Raytheon Manufacturing Co., Westinghouse Electric and Manufacturing Co.; and to the Institute of Radio Engineers.

With these few words on behalf of what is to follow; we leave the book to your tender mercies.

H. H. SHELDON E. N. GRISEWOOD

Contents

CHAPTER PAGE

INTRODUCTION i . . vii

I. ESSENTIAL ELEMENTS OF TELEVISION AND PICTURE

Introduction

Who wants television? Will television serve any useful purpose, become popular as a source of entertainment; or will it, having been perfected, serve a very narrow field?

Hardly has the talking movie made its appearance in almost every theatre, than we are told that it is a passing fancy, that it will soon complete its run. This is not a novel statement; we were told the same thing about the automobile, the airplane and the radio ; but they are still here.

Television is not like collecting postage stamps, an avocation which may appeal to a few or be in vogue for a few years and then subside. It is not like police dogs or short skirts which are for the moment fashionable. It is the realization of a desire that has existed in man from his earliest beginnings. The automobile, giving man the speed he has always desired, has stayed. The airplane, giving him the wings he has long sought, has stayed. Television, giving him the distant sight he has longed for ever since, in his ape-like form, he climbed to the topmost, swaying branches of the tallest tree to look afar, is here to stay.

Seeing and hearing things at a distance is as natural a desire as eating. The Indian put his ear to the ground to hear the approach of his enemies so as to be prepared. Lookouts from high hills were common. A great advance in vision was made by the invention of the telescope. This has been greatly improved up to the present time; but still we are not satisfied. We want to see more and further.

The modern newspaper is, after all, nothing more than an instrument to enable us to see and hear more and further.

Vll viii INTRODUCTION

We can visualize from its printed pages what has happened: but would we read the account as written in the newspaper, had we the time and facilities to be present at the event? Television brings us somewhat nearer to a perfect realization of an event; wherefore its stay will be permanent.

A stadium may seat from seventy to one-hundred thousand people, an auditorium is large if it seats four-thousand, a theatre is built to accommodate considerably less than the last named figure. Why? A theatre is built so that all may hear the spoken word, the auditorium is used by a symphony orchestra which may be heard at greater distances than the human voice and so may be housed in a larger building. One comes to a stadium, however, to see; hence it may be made very much larger. The attendance at stadiums is a good guarantee of the success of television. The advantage lies all with the latter when it is coupled with radio so that both eye and ear may be satisfied.

There are three distinct phases of any message delivered orally. There is the spoken word which may be written or printed on paper. The statement that such words may have several meanings requires no elaboration. Lawyers frequently take advantage of this to convey one idea to a jury; whereas a totally different conception might be gleaned from the statement appearing on the records. However, if we know the tone of voice, the accent, the pauses in such a statement, would we know the correct story? Thus authors of fiction write, ' 'Villian!' he hissed," for the purpose of conveying the idea of venom, even though you cannot hiss such a word if you tried. Or turning to the heroine, we might find, " 'JomV she sobbed." Though it is difficult to sob such a word, we get the idea. This helps, but for fullest realization we must see the expression on the heroine's face as well as hear how she sobbed the word. At this stage we double or triple the amount of information the single word "Jonn" can convey. It is obvious, then,

INTRODUCTION ix

that for full comprehension television is essential. Up to this point we can only fill in the details from past experience ; a fact which accounts for our greater enjoyment of a radio play when it is one that we have seen as compared to one that we have not seen.

Thinking of television in its broadest sense, that is including the transmission of pictures by wire, there is even here an advantage over the usual telegraph message. Where advertisements are to be duplicated in various cities, the usual telegraph message is not sufficient; only a picturegram can convey the exact form. Where one wishes to send a message in, let us say, Sanskrit, whose alphabet is not familiar to the telegraph operator, the advantage of being able to send a picture of one's handwriting is apparent. Such service has already become a part of the regular routine of our large telegraph companies. That it is destined to supplant the usual service with which we are familiar, may not at the moment be obvious. When one realizes, however, it is now possible to transmit a picture by wire in less than one minute, in the laboratory; there can be little doubt that putting such speed into commercial practice will greatly increase the number of words it is possible to send over a telegraph line in any given time. The tedious ticking off of the individual letters for each word will eventually appear as antiquated as an ox-cart alongside a powerful, electric locomotive.

If one makes an inventory, one will soon discover that there are a number of forms of entertainment from which he is barred without television in conjunction with his radio. Dancing, which forms such a large part of any musical revue, is entirely absent. This was realized when one broadcasting station tried to send over the air a series of Broadway musical shows, direct from the stage. We feel sure that no one will deny this was a failure. Without the stage setting, the display of color and dancing, it was lifeless.

x INTRODUCTION

We have long had movies without sound; today, they are being combined. In radio we now have sound without movies; the condition is rapidly being remedied. When this page reaches you, the combination may have been effected in your home; as it has been, for some time, in the laboratory.

The radio expert or amateur who says he is not interested in the development of television might as well say he is not interested in the three electrode vacuum tube. Both are part of his business.

In this book we have attempted to bring together, in a manner that can be understood by all, a summary of the achievements in the field of television together with a description of accessory equipment. We believe, that at the time of going to press, it presented fairly the accomplishments to date. We realize that the next few years will bring a multitude of changes that may at times leave us somewhat behind. It is our hope, however, that public acceptance of this book will make, it possible, through frequent revisions, to keep in step with the parade. We invite our readers to assist us with suggestions.

Chapter I Essential Elements Of Television And Picture Transmission

TELEVISION and picture transmission both require that the light and shade of an object or of a picture be translated into varying electrical impulses. These are transmitted by wire or radio to their destination; where they are changed from the electrical form back into the original light and shade, either to be viewed directly or to be recreated into a picture. To do this there are certain fundamental steps which are unavoidable and which differ only in their method of application. When we fully understand these, we have mastered the entire problem.

All modern television systems require, first of all, illumination of the object in order that the light and shade used to produce the picture may be present. In the early Baird experiments, in which the illumination covered the entire subject at one time, the heat and glare were so great that only dummies could be used. Increasingly sensitive apparatus now makes it necessary to use illumination no greater than strong sunlight. Most systems, however, do not light up the entire object at once; but use a narrow beam which rapidly traverses the object along successive strips.

Thus, if we were to place a series of parallel fine wires before the object, the light would follow the length of each of these successively. In effect this same thing happens when the entire object is illuminated; for, although the light itself is unchanged, the successive stripes are viewed in the same way by a moving lens system. We may say then, without reservation, that present-day scanning, as this process is called, is accomplished by viewing successive portions of an object along a straight line and viewing the consecutive lines in some convenient manner. This is not at all unlike reading the words on this page along any line and then reading the lines one after the other. In this manner the entire page is covered. In an analogous way every portion of the surface of this page would be covered in order, if being scanned for television transmission.

The lights and shadows, or variations of light, which are noticed as we pass over the picture in this or some similar fashion, must next be changed into corresponding variations of electrical current. This has presented the greatest difficulty in the development of television apparatus. In the first experiments, which promised success, the selenium cell was used. Such cells, to be described later, have the peculiarity of decreasing their resistance to the flow of electrical current, in proportion to the intensity of illumination. They are, however, temperamental devices having a fatigue effect which causes them to fall off in effectiveness after a short time of continuous use. To recuperate, they must be left in the dark. Also the change in resistance does not coincide with the illumination changes but follows somewhat later. This peculiarity is suitably known as a "lag." For these reasons, the selenium cell did not satisfy the requirements in its original form, although there are indications that these faults may yet be overcome.

Television was really born with the advent of the photoelectric cell. This cell consists of two electrodes, one of which is coated with an oxide or a hydride of an alkali metal — for example, caesium oxide or potassium hydride. Such coatings, when illuminated, give off copious supplies of elec-

ESSENTIAL ELEMENTS OF TELEVISION 3

trons, the number of which is a direct function of the light intensity for any given color. These cells have no apparent lag nor fatigue although their sensitivity is not yet all that could be desired. When inserted as part of a circuit they give an apparent change in resistance under illumination; although the effect should not be interpreted as a strict resistance change.

SENDING STATION

(Courtesy of Bell Telephone Laboratories.) FlG. i. — A typical television system.

The variations in current given by either of the above devices are extremely small and must be greatly magnified to be transmitted over any distance. This is accomplished by vacuum tube amplifiers, particularly well constructed to avoid distortion. The amplified output may be sent over wires or on modified carrier waves, as is done in radio transmission. In the former case it is received as changing current,— in the latter, by the usual radio mechanism; so that if put through a loud speaker it would produce actual changing sounds. The picture would be "heard," so to speak.

In either case the next stage in the procedure would be to reconvert the electrical variations into light variations similar to the original. In the first types of apparatus this was done by again amplifying the currents at the receiving end and causing them to actuate an electromagnet. The latter would vary a slit width or in some other manner control the amount of light falling upon a sensitive film on which the picture was to be produced. This method was adapted only to picture transmission. There was no hint of television; although if it had been physically possible to have carried on the operation with sufficient speed the picture could have been viewed through a translucent screen.

To procure a speed sufficient for television, it is necessary to obtain some method of reproduction free from the inertia which accompanies a moving mass; a difficulty to which all of the mechanical schemes were subject. The introduction of the neon glow-lamp solved this problem. This lamp glows under a high voltage placed across its terminals and has so little lag that it can easily follow thousands of fluctuations per second.

It is, of course, apparent that the lines drawn by such fluctuating lights must always be exactly in step with the scanning operation, at the sending end. If there is a slight lag of the one behind the other, the picture will be askew. This synchronization of the scanning with the reproduction on the receiving drum, or in the receiving frame, has been one of the principal experimental difficulties in television.

For the transmission of a moving object, if the image is to appear continuous, the original must be scanned completely at least ten, preferably sixteen, times per second. This likewise is a difficult technical problem.

Here we have the important features both of picture transmission and of television. The means of accomplishing each particular step are as varied as the number of experimenters. The purpose of the following chapters will be to consider some of the best known methods in greater detail and to discuss the accessory apparatus.

Chapter II Historical Background Of The Development Of Television

ALTHOUGH television is still in its infancy, perhaps not yet well out of the embryo, we must go back over eighty years to find the earliest apparatus for picture transmission by electric currents. Nor should we feel that this early work was unimportant in the development which has made possible the systems in use today.

In 1847 Bakewell designed a method for the transmission of writing or sketches over wires; a system not very different from one suggested some four years earlier by Bain, but not developed by him at that time. The apparatus is illustrated diagrammatically in Fig. 2.

FIG. 2. — Bakewell system.

The metal cylinders (i) and (2), one at each end of a telegraph line, are revolved synchronously. Over each cylinder a metal stylus (Si and S2) describes a spiral path, similar to that of the needle in the early Edison phonograph, which used cylindrical records. Note that our problem in timing would be exactly the same as requiring that two such phonographs, located at a distance from each other, and playing the same record, should sound a given note at identically the same instant. This apparently could be accomplished by having the speed of rotation of both cylinders identical and starting the needle (or stylus) at exactly the same time.

If now a design be drawn with an insulating material, such as shellac, on the sending cylinder ( i ) ; the telegraph circuit will be broken when the stylus passes over this insulation, but closed again when the stylus strikes the uncoated metal. That is, the current in the line will be intermittent; only flowing while stylus (Si) is passing over a portion of its spiral path which is not covered by the design to be transmitted. The problem then is to make a record on cylinder 2 which will show when current is flowing in the line as distinct from when it is not.

This may be accomplished if the receiving cylinder be covered with a chemically prepared paper such that the passage of an electric current through the paper will change its color. For this purpose we might saturate a piece of porous paper with an aqueous solution of potassium ferricyanide and ammonium nitrate. In this case we will obtain a dark blue color in portions through which the current has passed.

Using such a paper, it is clear that if the synchronism be exact, a negative of the drawing on the transmitter will be traced on the receiving cylinder. That is to say, the paper will be turned blue except when the transmitting stylus is passing over the insulating material. Thus white portions will correspond to the lines of the drawing or writing to be transmitted. The received diagram would then appear like a blueprint made from a line drawing on tracing cloth.

The movement of each stylus actually describes a helical path, like the thread of a screw, on the cylinder above which it moves. When the paper is removed from the cylinder and opened out flat, however, this path will appear

It

FIG. 3. a — Shows the letter as it appears on

the sending cylinder; b — as it is received in the Bakewell system.

as a series of parallel lines — their spacing depending on the lead of the stylus control. (See Fig. 3.)

As might be expected, the difficulty in obtaining synchronism in the two cylinders militated against the success of the Bakewell system. However, the telectograph used commercially by T. Thorne Baker for transmission of pictures between London and Paris in 1908 was fundamentally the same as the method described above. In the scheme employed by Ferree as recently as 1924, for radio and wire transmission of photographs, the principles of the early development described can be readily recognized.

Another system which was tried by early investigators, notably the French Post-office Engineer, Charbonnelle, is that of making the depth of silver deposit on a photographic film act as the means of varying the current in our telegraph circuit. In order to do this, the gelatine containing the active silver salt must be laid on a metal sheet instead of on the usual celluloid or glass backing. On exposure, the reduction to metallic silver occurs at those portions where the greatest amount of light is received. Now place the negative on a cylinder so that the metal backing forms one terminal of the circuit and the stylus, pressing lightly on the surface, the other. As the unreduced silver salt and gelatine are poor conductors, whereas the silver image is good, it is natural to expect the resistance of the circuit to vary, being least where the stylus is over those portions in which the "printing-out" or deposition of silver is a maximum.

The fluctuating current thus obtained might be employed, as in the instance previously described, to act on a chemically prepared paper at the receiving end. Unfortunately, however, experimenters using this scheme seem to strike serious difficulties. Chief among these difficulties is the tendency for the flow of current to follow the path of least electrical resistance rather than the shortest geometric path between the stylus and the metal sheet. Hence the current in the circuit at any instant will not be simply a function of the amount of silver deposit; as would be necessary for an accurate reproduction.

Failing to obtain satisfactory results in transmission directly from photographic negatives, it was not surprising that early experimenters should turn to the half-tone or process-screen reproductions used in newspaper work. Examination of such a picture under a magnifying glass will reveal that it is composed of large numbers of tiny dots of various shapes and sizes. In the light portion of the picture these dots will be far apart; in the dark portions so close together as to merge into one another. The general effect produced by the ensemble will depend on how fine grained a structure is used. A picture composed of some seventeen-thousand dots to the square inch leaves little to be desired; one with only four-hundred, will be barely passable even when viewed at arms' length; the usual newspaper production contains four-thousand two-hundred and twenty-five dots per square inch.

Suppose that we should imagine our photograph placed on a sheet of fine cross-section paper; so that each square may be designated by a letter and number as is done in describing a chess-board; or by two numbers, an abscissa and ordinate, as is done in plotting charts. We then split our picture into a number of small parts, each one of which may be transmitted separately and the whole reassembled according to the designation of the squares. Such a system makes possible the simultaneous transmission of different portions of a picture over separate wires; thus enabling the entire reproduction to be made more rapidly than would be the case if only a single wire were used. It then remains to find some method for describing the appearance of these squares. We might use a letter of the alphabet to indicate a certain size and shape of dot as used in the half-tone. In

FIG. 4. — The detail of the reproduction depends upon the number of picture elements used per unit area, (a) Made through a 6o-screen; e.g., 3600 elements per square inch, (b) through a ico-screen ; e.g., 10,000 elements per square inch, (c) through a i5O-screen; e.g., 22,500 elements per square inch.

this fashion the square might be coded and transmitted like a written message.

There are, however, a great many objections to be overcome. To obtain a good reproduction, a great many sizes and shapes of dots must be used, making coding a long and difficult process. H. G. Bartholomew and M. L. D. Mc- Farlane, in England, have removed the necessity of a human observer having to assign code letters to the various parts of the picture. Their device not only automatically codes the picture; but also perforates a tape which may be run through the ordinary telegraph or wireless transmitter. Here the problem is somewhat simplified by using only six color variations from white to black to describe a given io TELEVISION

portion of the original. This method, which is known as the Bartlane process, will be described in more detail in a later chapter.

L. J. Leishman also has devised a method of transmitting a half-tone without the necessity of coding. The system is not so very different from that described as unsatisfactory when applied to the usual latent image in silver. Here, however, the picture is first photographed through a process screen, the function of which is to split the original into dots of varying sizes. A positive is then made from this negative. The positive is formed on a copper or zinc plate covered with a mixture of gelatine and ammonium dichromate. The dichromate is rendered insoluble by the action of light; so that after washing, only those portions of the copper plate corresponding to the dark parts of the original will be covered with the gelatine and dichromate. After heating, these portions become an excellent thin insulation over the surface of the plate. The reproduction in Leishman's system is accomplished by a stylus actuated by electromagnets. Both transmitter and receiver employ cylinders above which a needle moves much as described in the Bakewell method. The reproduction, however, may be made by the mechanical movement of the recording needle without the medium of a chemically treated paper. One way in which this can be done is to allow the stylus to strike a carbon sheet placed over the paper on which the drawing or photograph is to be received.

The work of Edouard Belin, a French inventor, illustrates still another treatment of the problem, which has been successfully used by not a few experimenters. He made use of a picture formed in relief on the sending cylinder. The displacement of a needle passing in a close spiral path over this irregular surface is made to produce fluctuations in the line current. In an early form, demonstrated over a Paris-Lyons telephone line in 1907, this was

HISTORICAL BACKGROUND OF THE DEVELOPMENT n

done by amplifying the movement of the stylus by a lever, the far end of which moved over a device for varying the resistance in the circuit. A serious objection to such an arrangement lies in the fact that to produce such an amplified movement a considerable force must be exerted by the stylus; this causes cutting of the gelatine relief used for transmission. Later Belin improved the transmission by connecting the stylus to a microphone in the primary circuit of a transformer, the secondary of which was in the transmission line. This is readily recognized as similar to the ordinary telephone transmitter, except that the movements of the stylus, as it passes over the relief image, have replaced the condensations and rarefactions of sound waves.

The receiving portion of the Belin apparatus introduces a method markedly different from any hitherto mentioned. The varying currents are received on a Blondel oscillograph, an instrument in which the variation of current is measured by the displacement of a beam of light reflected from a mirror rigidly fixed to the suspension. The reflected light then passes through a wedge-shape aperture, called by Belin a "scale of tints" ; thence through a condensing lens onto a sensitized film which is carried by a rotating cylinder synchronized with the sending cylinder. It will be noted that here we are using the current in our circuit to vary the amount of light from a constant source which reaches a given portion of the film. Thus, the inertia of the receiving mechanism hastbeen greatly reduced, a major achievement since speed of reproduction is all-important.

Turning now to the method of Professor Arthur Korn of the Berlin Technical High School, we find the selenium cell used at the transmitter. The photographic film is placed on a glass cylinder which rotates and at the same time moves along parallel to its axis. Hence by keeping a small point of light in a fixed position, it may be made to traverse every portion of the film. The light, having passed through

II the translucent film, is allowed to fall upon a selenium cell, connected in the usual telegraph circuit. Selenium possesses the property of becoming more conductive for electricity under the influence of light. Therefore the resistance of the circuit at any time will be proportional to the darkness of that part of the film then under illumination. Clearly, in order to obtain the desired current variation, the source of light and the electromotive force acting in the line must both be constant. At the receiving end, a modification of the galvanometer scheme employed by Belin is used.

At first glance, it would appear that in this system the inertia effect has been reduced to a minimum since neither

TIME.

FIG. 5. — Comparison of the fluctuation of light intensity (solid line) and

conductivity of a typical selenium cell (dotted line). Note time lag and

rounding of conductivity curve.

transmitter nor receiver depend upon the movement of a stylus having mass. Unfortunately, however, the selenium cell is subject to a distinct lag — it does not respond instantly to light fluctuation. The accompanying diagram shows this effect quite clearly. Professor Korn has very ingeniously corrected this difficulty by the use of two cells arranged to compensate for each other. Nevertheless, considerable trouble still exists in the selection of suitable cells. It is for this reason that the selenium cell has been almost completely replaced by the photoelectric cell, as we shall consider later.