The Truth About Television

Radio Record, Volume II, Issue 12, 5 October 1928, Page 2

The Truth About Television

By

Edgar H.

Felix

(Consultant to the Radio Broadcasting Ca. of New Zealand Ltd.) N this article, Mr. Edgar H. Felix, one of the leading authorities on radio and radio developments, gives a complete analysis of the present position of television. This article is extracted from the columns of "Radio Retailing," a leading radio publication in the U.S.A. Although long, it is worthy of reproduction, because of the complete treatment it gives the subject in which there is growing interest because of repeated claims of success as made by cablegram. CEE SE

HE eagerness with which anything pertaining to the visual transmission and reception of images was examined by the radio trade during the R.M.A. show in Chicago is but a refiection of public interest in this new field. The public has already purchased tens of thousands of dollars’ worth of scanning discs and neon tubes without any assurance of practical results. The American urge to experiment and pioneer is readily capitalised, but there is enough shrewdness in the buyer to make him study the product before risking his dollar. The dealer must know what he is selling and what it will accomplish before he can face the customer successfully. That there are pitfalls as well as profits in the new field is clearly apparent from a review of the progress of television in England. Selfridge’s, a leading London department store, fired the opening gun by announcing a sale of "television" receivers at 32 dollars 50 cents. Buyers flocked, sought information, and awaited demonstrations. Information came to them in the form of a magazine, the first issue of which was eagerly purchased. Dealers, following the example of the London store, stocked scanning discs, selen- ium cells and neon lamps. Television made news and the Press supported it Hberally at the start. Within two months, however, a leading British trade paper reported "the television flurry is over." The public had examined and passed its judgment. Some dealers had lost friends by selling goods which did not give satisfaction. A trade publication, as a warning to dealers, had offered a 5,000-dollar prize for a satisfactory demonstration of the television devices, and the challenge had been unheeded. The progress of television has been set back seriously in England by the premature appearance of equipment unsupported by broadcasting and unable to sell itself by demonstration. Dealers had stocked on faith instead of upon actual results. In Boston, they are having a television flurry and another may be expected any time now in Chicago. A careful survey in Boston reveals that no dealer has been able to show bona fide television images to prospective customers. Unless dealers can do so soon, the television flurry will be over in Boston too. The experimenter market knows its radio and it doesn’t rent telescopes to look at the moon on cloudy nights. : LL kinds of radio image equipment are now being offered to the radio trade. A host of manufacturers is jumping into the field, getting all set to be in on the mushroom market when anything pertaining to television sells. Many of them are making meritorious products which do what is claimed for them. While there is a seller’s market and the public is clamouring for goods, no live dealer wants to overlook opportunities because that is when big profits are made. As long as one simple rule is followed-know your goods and repregent them accurately-the television mevkes is an opportunity, By observ-

ing that simple maxim, you may avoid the poisonous mushrooms, and profit from the sale of wholesome ones. The principal radio vision products now being offered are scanning dise¢s, neon lamps and still picture recorders. To sell a neon dise in any given territory, there must be available a television signal and a means of synchronising with it which can make a recognisable image with the particular dise being offered. It must be possible to

set up a demonstration in your store or laboratory so that you can show the experimenter just what kind of an image he may expect to reecive. A 24-hole scanning disc is useless in a territory where only a signal intended for a 36hole scanning disc is available. [zt must be remembered, also, that absolute synchronisation must be maintained between the transmitter and the receiver. This is accomplished by means, usually, of synchronous motors at each end, but unless both the transmitter motor and the receiver motor are on the same power line, absolute synchronisation is almost impossible to attain, due to the differences between frequencies of:the various power companies. In the majority of cases, of course, the television receiver will be on power lines far removed from the broadcasting station. In these _ instances, synchronisation is maintained

by hand manipulation of a variable rheostat connected with the receiving motor, keeping it in time with the impulses received. Advances in the art will undoubtedly correct some of these details. At present, however, they must be taken into consideration. The fundamental principles of all image transmission are simple, and, while the dealer is facing the technical buyer, he must be qualified by definite knowledge to answer the experimenter’s questions. The broadcasting of visual images is similar to tonal broadcasting in the means used for radiation and reception, but fundamentally different in the method by which the signals are collected at the transmitter and restored to their original form at the receiver. A device, consisting usually of a photoelectric system, responds to variations in intensity of light, converting them into electric currents. These variations are combined and radiated, received, and amplified, and then converted back into light impressions so that they may be seen by the eye. TPHE transmission of visual images is somewhat more complex than sending tonal impressions. The eye responds to an infinitely greater number of impresSions in a given time than does the eur. We cannot easily gather the impressions perceived by the eye into a single electrical current because of their vast number. Air wave impulses, sufficient to give us a complete musical reproduction, are a composite of many different frequencies, all of them compressible Within a band of 0 to 5000 cycles. This relatively narrow band covers from the lowest to the highest of the fundamentals, and all the necessary overtones to enable a listener to distinguish any musical instrument. The total number of sound impulse impressions responded to by the ear in a second numbers only in the thousands. The eye responds to millions of impressions every instant, and makes an impression of them. upon the _ brain through the telegraphic nerve system. Examining a 4 x 5 photograph, you look over its entire surface in an instant. If it is of 138 screen, such as is used in high grade magazines, a4x5 picture consists of 358,780 separate dots. A cheaper magazine uses a hundred sereen, requiring 200,000 dots for a4x 5 picture, while even the poorest of newspaper reproductions have a

screen of at least 45 and therefore Cea sist of 40,500 dots in a + x 5 size. For the transmission of such images by wire or’radio, a separate electrical impression of the intensity of light and shade on each spot must be transmitted and reproduced at the receiving end. The eye comprehends these numerous impressions at one glance, but:the eye of radio, the photoelectric cell, makes an impression of but one spot or area at a time. The transmission of the poorest kind of newspaper picture, 4 x 5 size, consists of sending 40,500 separate messages, each an electrical impression of the intensity of a single spot. To secure the impression of motion, o1 television, the complete picture must be repeated at least sixteen times per second, so that the lagging effect of the eye gives the subject continuity. Therefore, to reproduce the crudest 4 x 5 picture, an electrical impression of 645,000 dots must be sent each second. as compared with the requirement of sending 5000 impressions, the maximum necessary for tonal transmission. Consequently, assuming a 4 x 5 picture ot the crudest newspaper sereen as the minimum standard of an image having entertainment value, 125 times as much ether space is required for its transmission as is used for sending a complete musical programme. This required ether space amounts to double the entire present broadcasting band! -Obviously, an impossible procedure at this time. Any sacrifice from this standard, admittedly a low one, is attained by subtracting from the clarity or stability: of the image. For practical purposes, it is difficult to conceive of any widespread system of radio visual transmission which can be accommodated on a conventional broadeast channel. Short waves are satisfactory for experimental purposes, but the public at large cannot be served on short waves because, first, they are not available, being required for much more important national and international communication; second, they require a new and separate receiving set for reception; and third. fading and skip distance effects make their reliable reception over large service areas impossible. Assuming a2 maximum modulation: of 5000 cycles and considering that sixteen complete pictures must be sent each second for television, 312 is the maximum number of image areas of which a television picture sent on a broadcast channel may consist. If we reduce the size of such a picture to one square inch, that is to 1 x 1 inches in size, it would be equivalent to 17 screen, or about one-third the clarity of the poorest kind of newspaper picture. Furthermore, this assumes that no means of radio synchronisation is employed which would require additional ether space. Obviously, such an image leaves much to be desired from the standpoint of entertainment value. In spite of these difficulties, television is as certain as safe commer cial aviation. But television must await the discovery of a simple, inexpensive means of sending more than one © visual image impression at a time. So long as we must send impressions of the subject point by point in a series progression, television will remain only

an experimental art, conducted for the scientist and inventor rather than for the home user. ] How Television is Now Conducted.

Tye television signals, now being ra‘diated, are obtained by setting a subject before a bank of flood lights. A scanning dise is used at the transmission point which reflects light from the subject point by point in a regular predetermined order to a photoelectric cell. The photoelectric cell may first "look" at the upper right hand corner of the subject as impression number one. Light is reflected on a photoelectrie cell through a tiny aperture from that point in the image. As a result, an electric current, proportionate to the intensity of light reflected from that point, flows through the photoelectric cell, The dise scanning hole then moves slightly to the right, making a second impression, and so on, point by point, an electrical impression is made of the top line of the picture. Where the subject is black, little light is reflected, and little photoelectric current flows; where it is white, such as in a white background, much photoelectric eurrent flows, producing maximum modulation. Having swept across the upper line of the subject, the second hole of the scanning dise falls into line and makes a series of impressions immediately underneath the first. This is continued for as many "sweeps" of the subject as the scanning system contemplates. For example, WGY is radiating signals for a 36-hole disc so that there are 36 sweeps of the subject for one complete impression: the Jenkins system, using short waves, is making 48 lines. Fundamental Limitation to the Entery tainment Vale of Television. Inasmuch as all of these systems (there are no exceptions to this rule) depend upon reflecting a beam of light from the subject to a photoelectric cell, the range of the "eye" of television is necessarily limited. The subject must be sufficiently close so that a beam of light reflected from it will cause a change of electric current through the photoelectric cell. For a standard potassium photo cell, this range is limited to about one foot, and this fact is the reason that so far only faces have been "televised" with its aid. The public imagines football games and _ prize ‘dights coming before its eyes through television scanning dises, but the most we ean hope for, at the present time, ’ is profiles and full front views of single faces. If we attempt to crowd two faces before a television scanning disc, the number of images gathered is so few that the faces become unrecognisable. A FEW weeks ago, the Bell Labora- *~* tories showed an improved photoelectrie cell which permitted, for the first time, the scanning of a full-size human figure. This was used in connection with the same television trans--mitter and receiver which had been shown with such acclaim to the world nearly a year and a half previously. Among the elements of the reproducer : synchronising devices, a neon tube with 2500 pairs of elements and a room full of control instruments. At the re‘cent demonstration, the statement of Walter S. Gifford, president of the American Telephone and Telegraph Company, which was originally made nearly two years before, was again given out stressing the fact that tele-

vision is far from the point where it may be introduced into homes. "The elaborateness of the equipment required," says Mr. Gifford, "by the very nature of the undertaking, precludes any present possibility of television being available in homes and offices generally." All of the limitations of chan-. nel shortage and lack of detail, more fully described in subsequent paragraphs, apply to television transmission with the more sensitive cell. One by one these problems may be overcome, but to the engineer who understands them it looks like a matter of many years. At the receiving end, we obtain an electric current similar to that flowing through the photoelectric cell at the transmitting point, through the usual transmission and reception proeesses. When these currents are sufficiently amplified, they are applied to a neon tube. The intensity of the light of the neon tube then varies exactly as the light reflected on the photoelectric cell through the scanning system. Considerable amplification is required to cause the neon tube’s light output to fluctuate visibly in this manner, and no system has yet been demonstrated which does not need at least a five-stage audio am- plifier to make even a powerful television signal cause the neon tube to fluctuate sufficiently to make a visible image reproduction. But this is not the most important problem. Experimenters can make five-stage audio amplifiers work. The image is reconstructed at the receiving point by watching the neon tube through a series of pinhole apertures in a revolving scanning disc. The receiving scanning disc must he precisely similar to that used at the transmitter to set up the image. If an attempt is being made to reproduce the face at the transmitting end in 1x 1 size at the receiving end, the seanning dise consists of a spiral of holes an inch apart. The neon tube at the receiving end should have a plate of at least 1 x 1 size so that the image can be reproduced. " . . for the present, universal television consists merely of moving shadows, at best. However, backed up by ‘sufficient broadeasting, even moving shadows can be merchandised . . . provided they are merchandised as such

At the precise instant that the upper left-hand corner of the subject at the transmitter is being "examined" through the hole in the scanning dise by the photoelectric cell, the scanning dise at the receiving end must also be "looking at" the upper left-hand corner of the plate of the neon tube. 30th discs must then sweep across the top line of the picture in exact synchrony, the receiving dise completing its one-inch trip across the plate of the neon tube at the same rate that the scanning dise at the transmitting end makes its sweep of the top subject. The neon tube at the receiving end fiuctuates in intensity with the shading of the picture. This perfection of synchrony must obtain while 24, 36, or 48 apertures pass over the subject at the transmitting end and over the plate of the neon tube at the receiving end each ~ sixteenth of a second. HE importance of perfect synchronisation cannot be over-estimated. The most advanced public demonstra-

tion of television, so far given, was that made by the Bell System. two years ago.’ This made a picture of 50 sereen, one inch square, or a total of 2500 image points per picture. The impression was enlarged to motionpicture screen size by means of a neon tube consisting of 2500 pairs of elements. Hach of these was mechanically switched in, one at a time, sixteen times per second, by a rotary contact switch. This amounted to a total of 40,000 contacts per second, and each contact had to be accurate within a forty-thousandth of a second so far as time is concerned. This remarkable result was obtained by using two separate synchronising signals sent on short wave channels. The difficulties of manual synchronisation which is being attempted by television systems having no specific means of synchronisation, can best be appreciated by imagining what the result would be if the motor used at the Bell System demonstrations were slightly off speed. At correct cynchrony, let us suppose, the motor operating the 2500 contacts revolves at 2000 revolutions per minute. It makes 2,400,000 contacts per minute, each at the correct instant. Suppose the motor ran off speed five parts in ten thousand, which would make the motor turn 2001 r.p.m. instead of 2000. Every sixteenth of a second, then, 2512 contacts would be closed instead of 2500, and the second picture would already be 383 1-3 per cent. off synchrony, so that the image would not be recognisable for more than one sixteenth of a second. Those now experiencing difficulty in the hand operation of a d.c. motor by means of a rheostat, must appreciate they are attempting manually to stabilise the speed of a motor within ten thousondths of 9 ner cent.

ployment of sixty cycle alternating current with power lines as the means of synchronising. This may be satisfactory when the listener is on the same power line as the broadcasting station radiating the television signal. In that case, both the transmitter and. receiver use synchronous motors, operating from the same power source. Where there are rural and d.c. districts involved, or non-interconnected and non-synchronised power lines, synchronisation by this method is uncertain. The claim is made by some, however, that current from independent power systems is sufficiently close to rated frequency to permit the synchronisation of television from any sixty-cycle line. Electric clocks are simply small synchronous motors operating from sixtycycle a.c. It is the practice of power houses to check the time with Western Union each hour and to speed up or slow down the alternators so as to make up for the loss or gain in cycles experienced. Since we require accuracy of one part in 10,000 to hold a reasonable image for a fraction of a second, it is obvious that there is considerable variation in "60" cycles. Only if special arrangements were made among alternating current power systems to maintain absolute synchrony, a condition not yet obtaining, can there be any widespread use of a.c. synchronization for television. In the New York area, for example, there are, within twenty miles of the metropolitan district, at least six unsynchronized alternating power SySstems and two important direct current areas, each of which would re-

quire special broadcast transmission which would not be interchangeable with the other districts. Practical and widespread television is not attainable until synchronizing signals are radiated with the television transmissions or erystal oscillators of sufficient stability to be accurate to one part in a million are available at low cost. HE next point to consider is the availability of channels for teievision reception. The ideal would. be to transmit television occasionally through ordinary broadcasting stations so that the ordinary receiver could be used and so that the television programme could be associated with musical entertainment. Our broadcasting structure has been designed for a2 maximum modulation of 5,000 cycles, making possible the arof a spectrum with ten kilocycle separation. Most of the television promised in the broadcast band does not fit within these channel limitations.

It is easy to calculate the frequency band required by a television transmission using the usual scanning disc having a single spiral of apertures. These dises usually rotate at 960 r.p.m., that is, one revolution each sixteenth of a second. The maximum number of impressions made by a Single sweep of the subject is usually equal to the total number of holes in the disc. Thus, with a 24 hole scanning dise, which is the fewest number of sweeps of the subject to which even the simplest profile can be reduced, each swee}) of the subject makes 24 image impressions on the photo-electric cell and the entire subject therefore consists of 24 x 24, or 576 impressions. With the meagre illumination afforded in the five hundredth of a second or less that the subject is illuminated at the receiving point, eighteen or twenty images per second should be used rather than the usual sixteen used in motion picture practice, where every detail of the reproduction remains illuminated on the screen for at least one thirtieth of a second. The total number of impressions per second is the product of the number of holes on the disc and the number of revolutions per second. In the case of a 24 hole disc making sixteen revolutions, 9216 images per second are sent. Since there are upper and lower side bands in transmission, a frequency space of twenty kilocycles ig required for modulation, infringing upon at least three broadcast channels. With a 48 hole dise, revolving at 16 r.p.s., about seven broadcasting channels are used.

EVERAL attempts to circumvent the carrier channel difficulties have been made by ingenious inventors. Senabria, co-operating with WCFL of Chicago, uses a scanning disc with three sets of spiral apertures so that his dise revolves at one-third the usual speed. He makes a fifteen line picture, each picture consisting of a seanning, of only one-third of the subject, but, by slightly displacing each image, covers the area of a 45 line picture. The same effect would be secured with a 45 hole dise operating as follows :-During the first rotation of the disc, the first, fourth, seventh, tenth, ete, holes would sweep the disc, the others being for the time closed; during the next revolution, the second, fifth, eighth, eleventh, ete., would sweep the disc; and in the third revolution, the third, sixth, ninth, and

twelfth. In that way, the received picture is made to appear like a forty-five line picture, although it uses the channel space of a fifteen-line picture. Whether this is a real gain is questionable because the flicker has been increased threefold, and, to secure a quality and freedom from flicker equal to that attained by a forty-five line disc wouid require the making of 48 reproductions per second with Senabria’s disc. On the other hand, this ingenious expedient, has made it possible to experiment with television, transmission, and reception at a minimum use of frequency space, and, while clarity and fidelity of reproduction are not yet a consideration in the field, it atfords the gateway to useful experimentation. WRNY, New York, has announced that it will soon begin broadeasting television images consisting of ten images per second and 386 sweeps of the subject. Reducing the number of images to ten makes it doubtful whether this ean be ealled television because any normal motion would result in a hopeless blur. The transmissions have not been started at this writing, hence no results can be reported. WLEX has been broadcasting in Boston for some weeks with 2 36-hole dise, but no public demonstrat#n of radio reception after several weeks of transmission has been successful. WGY is sending images requiring a 24-hole dise and synchronisation is left to the experimented’s ingenuity or luck. The Alexanderson system is not yet in commercial form. A recent demonstration used 40,000 cycle modulation. The Jenkins system is also in the laboratory stage and is not yet commercialised.

N England, the Baird system has been exploited. No regular radio transmissions have been reported, although they have been widely promised. An American company, to exploit the Baird system, has been formed with much preliminary announcement but no public demonstrations. The apparatus, marketed in England, consisted of a scanning dise and a selenium cell. With these, the experimenter Was expected to build a transmitter. Later, by purchasing and assembling more piurts, he would have the privilege of viewing the image transmitted on a scanning dise mounted on the same shaft as the transmitter. The range of the system is thus the length of a shaft on a motor. The same plan has been used in various demonstrations on this side of the water, where the subject has been scanned on one side of the dise and the neon tube mounted on the other. This merely shows -what kind of an image could be received if transmission and reception were synchronised. If this fact is not clearly explained at such demonstrations, they come mighty near to being misrepresentation. A new system is coming forward in Kngland, known as the Fuller system, Which makes still pictures. It is rumoured that the apparatus will cost 150 dollars, The advantage of still picture transmission is that there is no limitation as to the time required in transmitting a complete picture. With the Fuller system, a gelatine etching is made from which photograph prints can be made. This is a rather complex process which probably offers ser?ous sales resistance, but, on the other hand, it is possible to make a_ high grade picture, synchronising with a

signal entirely within the broadcasting band. In the United States, the Rayfoto system makes a 4 x 5 picture consisting of 110,000 image points. It produces positives and therefore no films or prints need be made. A finished picture is secured by a simple finishing process, requiring but a few seconds. 110,000 image points are equivalent to about 80 screen, nearly double the sereen of a newspaper picture. Phonograph records of radio transmissions are available for test and demonstration purposes and broadcasting at this writing is actually going on in New York, St. Louis, Milwaukee, Winnipeg, and has been arranged for in Philadelphia, Detroit and Toronto. But this is not television.

THE proponents of still-picture transmission hold that they will soon be able to send high grade pictures, properly synchronised, in the form of motion picture film, which can then be projected on a home projector. This makes it possible to avoid that stringent limitation imposed by direct television that the complete subject must be reproduced each sixteenth of a second. Telephoto reproduction, furthermore, makes a permanent record which can be examined as often as desired, instead of a fleeting image which can be held only a fraction of a second. Radio transmission of still photographs is the furthest present commercial development of radio vision. "Television" itself, or the reception of satisfactory moving images by radio in American homes, is still something to be looked forward to in the very indefinite future. Tor the present, universal television consists of merely moving shadows, at best. UWowever, backed up by enough stations broadcasting moving images, aud the public informed of just how little they will receive and how crude the images are, even moving shadows can be merchandised, provided they are sold as such. It is, undoubtedly, a market for experimenters only, and must continue to be for several years to come.