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From the 1940 book, We Present Television.
THE clue to the technique of television lies in one very simple statement: television pictures must be sent on the installment plan. When we look at a scene directly, we see it all at once and we see it continuously. Not so in television.
The scenes in television must be sent piecemeal, bit by bit, and each piece of the scene must be reassembled before the eye at the receiver. Moreover, the scene must be reassembled very quickly, so that the eye is not aware of the process.
In addition, to allow scenes in motion to be televised, the scene must be “photographed” many times per second; that is, a great many separate and complete pictures must be sent, one after the other in each second.
In this respect television is similar to motion pictures, since both employ the artifice of presenting to the eye a rapid succession of still pictures, each differing slightly from the preceding and following ones. In this manner, the motion in the scene is broken down into a series of smaller motions which may be photographed without blurring.
When the succession of pictures is presented to the eye rapidly enough, the screen seems to be continuously illuminated by the scene, and the motion appears smooth and continuous.
The only essential difference between the movies and television is the rate at which the separate pictures are presented: 24 pictures per second in the movies and 30 pictures per second in television, according to present standard practice. This discrepancy, incidentally, makes it rather difficult (but still quite possible) to transmit standard motion picture film programs over the television system.
The movies have adopted the rate of 24 “frames” (as each picture is called) per second for the very good reason that this rate offers a good compromise between the cost of film on the one hand and the satisfactory presentation of the subject matter on the other.
When television came along, 30 frames per second was deemed an advisable figure because of the type of alternating current usually used to operate television receivers.
The difference between the two rates has been satisfactorily overcome by the special movie projectors used in television studios. Essentially, so far as picture repetition goes, motion pictures and television use the same technique.
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|Simplified diagram of complete television system.|
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But here the resemblance ends. In the movies each separate frame, or picture, is presented to the audience in its entirety, all at once.
In television, on the other hand, each separate picture must be broken up into several hundred thousand tiny points of light, and these points of light must be conveyed to the receiver one at a time. This is the installment plan with a vengeance.
The requirement of dissecting each picture into its essential elements, tiny points of light, is the reason why television is such a difficult art technically. It explains why television took so long in reaching a form suitable for the public, as well as the reasons why the new art faces so many restrictions.
The necessity for this piecemeal method of transmitting television pictures lies in a simple but inescapable property of electrical communication systems. A single electrical communication circuit can transmit but one item of information at a time—no more.
In the human eye—which is in itself an electrochemical communication system—this limitation is circumvented by providing many hundreds of thousands of separate fibers in the optic nerve, each of which operates simultaneously with all the others. The eye is thus enabled to see the whole contents of a scene at one glance.
But in television we cannot employ hundreds of thousands of communication circuits at once. In fact, we must be content with just one link, that existing between the broadcasting station and the receiver. This one link is called upon to handle the whole television process, 30 pictures per second, each picture dissected into roughly 200,000 tiny points of light.
These two figures multiply to the startling figure of 6,000,000 points of light to be conveyed, successively, over the system, each second. This extremely rapid rate of conveying information makes the modern television system by far the most comprehensive means of electrical communication yet in existence.
For comparison, consider the ordinary telephone circuits used to connect the stations of the national networks for sound broadcasting. These circuits are capable of handling approximately 5,000 items of information per second (here an item of information is an electrical vibration corresponding to the sound wave which is picked up by the microphone). Such a circuit can handle speech and music with a very considerable degree of realism.
The television system, in contrast, must work at a rate some 1,000 times as fast. The ancient Chinese have said that a picture is worth ten thousand words, which is not such a bad guess.
Before going on to examine how the television system performs its difficult task, let us examine in more detail what we mean by the tiny dots of light which the system must convey from transmitter to receiver. In the illustration below is shown an ordinary photoengraving (upper left corner) and a three-times enlargement which reveals the structure of the reproduction. The enlargement shows the picture to be made up of a great many printed dots.
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|Illustration to show the difference between a fine and coarse halftone screen.|
In the darker portions of the picture the dots are of large diameter, forming an almost solid black mass in the black regions. In the brighter portions of the picture the dots are of small diameter and in the high lights they may be missing altogether.
By this method of printing, the amount of light reflected from the page varies in accordance with the contents of the scene.
In television a very similar process is used. The tiny points of light previously mentioned correspond with the halftone dots in the engraving.
In television transmission, an electrical impulse for each one of these dots must be sent over the system, one after another, fast enough to allow the whole picture to be reassembled before the eye in 1/30th of a second.
The illustration shows the reason why television images must be divided into so many points of light. The enlargement is obviously too coarse to allow a satisfactory portrayal even of a close-up, and if a distant scene were in view, the coarse structure would obscure the contents of the scene altogether.
Suppose, however, that the whole area of the illustration were covered with detail as fine as that shown in the upper corner. Then a much more satisfactory reproduction is possible. The enlargement contains (count them if you wish) no less than 15,000 halftone dots.
If the whole picture contained detail as fine as that in the upper corner, the scene would contain about 130,000 dots, which is close to the 200,000 figure previously mentioned. In other words, the modern television system, working at its best, should be able to do somewhat better than the reproduction shown in the corner, when reproducing a picture the size of Figure 1.
It should be remarked that this degree of detail, while satisfactory for most purposes in television programs, compares very poorly with other means of pictorial reproduction. For example, a fine-grain photograph, printed by contact, contains some 50 million different points of light in the space of an 8-by-10-inch print.
The average 35-milimeter professional movie, as presented to the audience under average conditions, contains about 500,000 points of light, the average 16-milimeter home movie about 125,000.
This fact accounts for the oft-quoted assertion that the detail of television pictures is about the same as that of a good home movie taken on 16-milimeter film. The performance of the television system is definitely superior, so far as pictorial detail goes, to the 8-milimeter home movie.
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|One of the first commercial adaptations of television is its use in showing merchandise, modeled and displayed on receivers throughout large department stores for the convenience of shoppers. (Courtesy American Television Corporation.)|
: From Camera Tube to Picture Tube
So much for the questions of pictorial detail, and how much of it can be handled by the present television system. Let us turn now to a brief description of the equipment used to perform the television miracle. We begin with the camera, which views the scene to be televised in much the same manner as does the ordinary motion picture camera.
The television camera is essentially a closed box fitted with a lens which admits light to the interior and focuses the scene on the sensitive plate within. The sensitive plate is placed in the same position as the film in an ordinary camera, but it operates in quite a different way.
Ordinary photographic film is affected by the light in a photochemical way, that is, the image is brought out and reproduced by chemical means. In the television camera the image is developed photo-electrically, that is, an electrical process takes place on the sensitive plate.
The television camera differs from the photographic camera in another way: In ordinary photography the whole picture is taken at once. In the television camera the whole scene is received at once, but it must be dissected immediately into its constituent points of light.
Thus the camera must perform three functions:
- it must have a lens to focus the image
- it must be capable of transforming the light image into an “electrical” image
- it must dissect or “scan” the image.
The lens performs the first function. The second two functions are performed by the camera “tube,” which is enclosed within the cava housing.
There are several forms of camera tubes now available for television use, but the one most widely used, and the most significant in the modern history of the medium, is the type known as the iconoscope. This tube (see Figure 1) is a dipper-shaped glass structure from which the air has been exhausted and inside which is the sensitive plate previously mentioned.
The iconoscope is so placed in the camera that the sensitive plate lies opposite the lens, and thus receives the image.
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|FIGURE 1. Schematic diagram of the iconoscope.|
The sensitive plate itself is a thin sheet of mica on the front surface of which have been deposited several million tiny droplets or globules of silver treated with a surface layer of cesium and oxygen.
When light falls on these silver globules, the silver gives off minute particles of negative electricity (electrons). The electrons given off are attracted by a positive charge placed on another electrode in the tube, and are thus removed from the sensitive plate. Moreover, the amount of charge thus lost by each globule depends directly on the amount of light falling on that globule.
Thus, all over the surface of the sensitive plate, negative electricity is released by the action of the tiny points of light which make up the image of the scene. The result is that the sensitive plate acquires a deficiency of electric charge which corresponds, point for point, with the lights and shadows of the scene to be televised.
The longer the image of the scene continues to fall on the plate the greater the deficiency becomes, and in this way the iconoscope stores the light until it is ready for use.
The important function of the sensitive plate is, therefore, to transform the optical image into a corresponding electrical image and to preserve the electrical image until it can be dissected into its elemental points of electricity which correspond with the elemental points of light in the original scene.
The third function of the camera, dissection of the image, is performed by an “electron gun” located in the side arm of the tube.
This device sprays a stream (or “beam”) of electrons, much like water from the nozzle of a hose, at the sensitive plate. The stream of electrons is very sharply defined, and it may be directed at any point of the plate by a pair of magnetic coils arranged around the arm of the tube.
The current passed through these coils is used to direct the electron stream over the surface of the plate in a series of horizontal lines, back and forth, until the plate has been “scanned” from top to bottom. The motion of the electron stream is very similar to that of the eye in reading a page of printed matter.
The beam moves across the top of the picture, then quickly reverses its motion and begins again at the next line, and so on until it has traversed the plate over its entire surface.*
* Actually the beam traverses the plate in two series of lines alternately, one set of lines filling in the spaces between the set previously traced. This technique, known as “interlacing,” is employed to avoid flicker in the reproduced pictures.
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|Television camera uses “interlacing” to avoid flicker in reproduced pictures.|
The electron stream, in other words, explores the sensitive plate systematically, to determine the information contained on it. Whenever the electron stream hits one of the silver globules, it restores the charge deficiency on the globule to equilibrium, and the amount of charge restored is, of course, determined by the amount previously lost because of the light falling on the plate.
Thus, as the electron beam moves successively over the silver globules, it restores to each an amount of charge corresponding to the amount of light falling on that globule. The charge restorations in turn give rise to voltage impulses which are conveyed from the camera tube to the television transmission circuit.
Thus the camera tube generates a succession of voltage impulses which correspond to the values of light and shade successively passed over by the electron stream. The two-dimensional picture is thereby transformed to a succession of electrical impulses which contain all the information inherent in the scene.
This dissection process, as we have seen, must take place at a very rapid rate. The picture must be dissected or scanned in 1/30th of a second. Furthermore, in order to convey sufficient detail, the picture must be divided into about 400 lines (the present standard is 441 lines from the beginning of one picture to the beginning of the next).
This means that some 13,230 lines must be covered by the electron stream, in 30 successive pictures, during each second of the performance. Each line in the picture contains about 500 points of light and shade, so the 400 lines contain in all some 200,000 pictorial elements.
More astonishing is the rate at which the electron stream must move. The sensitive plate in the iconoscope is about 4 inches wide, and the electron stream covers this distance from left to right in roughly 1/15,000th of a second, that is, at a rate of about one mile per second.
The stream returns from right to left at a speed about 7 times as fast, or 7 miles per second. Such a rate of movement is well-nigh impossible in any mechanical system. It is easily possible, however, when the agile electron is used as the agent for dissecting the picture.
The succession of electrical impulses generated by the camera tube must then be conveyed to all the receivers within range of the transmitter. This is done, first, by amplifying the minute electrical impulses, after they leave the camera, by roughly 1 million times, and then imposing the strengthened impulses on a radio carrier wave which is radiated from the antenna.
When the radio wave encounters your television’s receiving aerial, it has been greatly weakened by its flight through space.
Hence, after it is conducted to your receiver, it must be amplified several thousand times before the electrical impulses representing the picture may be separated from the radio wave.
Once separated, the impulses are again amplified, this time only 10 to 20 times, and they are then ready to control the picture tube in your television.
The picture tube, as shown in Figure 2, is a funnel-shaped glass structure, pumped free of air and closed at the wide end. In the narrow end is an electron gun, very similar to that used in the iconoscope.
This electron gun generates a stream of electrons which travels through the tube to the wide end.
There the stream encounters a chemical substance which is coated on the inside of the glass face. This substance (usually a compound of zinc, oxygen, and silicon or sulphur) has the peculiar property of fluorescing, or producing light, when bombarded by the electrons in the stream.
In consequence, when the stream hits the screen, a spot of light is formed. The color of the light in modern television tubes is white, although in some tubes it may have a greenish or bluish hue.
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|FIGURE 2. Schematic diagram of the kinescope.|
The electron stream in the picture tube, as in the camera tube, may be directed at any point on the fluorescent screen, by means of electromagnetic coils (or by means of electrostatic metal plates sealed within the tube).
By means of the electric current applied to these coils, the electron stream is caused to traverse the screen in a series of adjacent horizontal lines which match exactly the lines traced out by the electron stream in the camera tube. In this way the surface of the fluorescent material is caused to glow successively along each of these lines, and the motion of the electron stream is so rapid that the screen appears to be illuminated continuously over the rectangular area (see Figure 2) on the screen.
This rectangle of light is the illuminated screen on which the reproduced picture appears. It should be borne in mind that, while the screen appears to be illuminated continuously, actually all the light present at any instant is the light present in the rapidly moving spot of light. It is only the persistence of vision which gives the impression of continuous illumination over the whole picture area.
How, then, is the picture reproduced? Here the chain of amplified electrical impulses comes into play. These electrical impulses are conducted to the electron gun, and there they control the number of electrons which enter into the stream.
As the number of electrons changes, so does the brilliance of the fluorescent spot. In fact the spot of light may be entirely extinguished, or made to have any value of brilliance up to the maximum of which the tube is capable, simply by varying the control voltage applied to the electron gun.
Thus, as the electron stream moves across each line in the scanning motion, the brilliance of the light it produces is varied in accordance with the brilliance of the light in the corresponding line scanned in the camera tube. In this way the picture is reproduced, line by line.
It is necessary, of course, that the motion of the electron stream in the picture tube at the receiver correspond exactly with the motion of the electron stream in the camera tube at the broadcast station. Otherwise the camera tube might be “scanning” a line at the bottom of the picture while the picture tube was scanning a line at the top, and the reproduced picture would have its top where the bottom should be.
Or, even if the same lines were scanned at the same time, unless the positions of the two electron streams were identical at every instant, the points of light in the reproduced image would be out of position. So it is essential that the two scanning motions be exactly synchronized.
In practice this requirement is met by sending special “synchronizing” signals from the transmitter to the receiver. These signals are generated in equipment maintained at each broadcast station. The signals are used to control the camera tube electron stream, and at the same time they are sent over the air, on the same wave which carries the picture information, so that they are available for controlling the electron stream at the picture tube.
Adjustment of the receiver to make proper use of the synchronizing signals is of course highly essential. Most modern receivers have control knobs especially intended to restore synchronization in case the picture reproduction falls out of step with the picture dissection process at the transmitter.
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|A symbolic arrangement of the iconoscope, the eye of television, the microphone, the ear of television, and the kinescope, television’s reproducing tube.|
No mention has been made thus far of the synchronized sound system which provides the sound portion of a television program. In general, the sound transmission system is similar to that used in standard sound broadcasting, except that the wave length of the radio waves employed is much shorter. Usually an entirely separate sound transmission system is employed, including the microphone, telephone circuits, and radio transmitter.
At the receiver, the radio waves carrying the picture and the sound are commonly received on a single antenna and may be handled together in one or two amplifying stages. But thereafter they are separated, the sight portion being amplified further before controlling the picture tube, and the sound portion being similarly amplified before controlling the loud-speaker.
Synchronism between the sight and sound aspects of the program is maintained without any special precautions, because the sight and sound transmission processes are substantially instantaneous. Moreover, whatever small delay (usually only a few millionths of a second) does occur, applies equally to the sight and sound transmissions.
In this respect television has a decided advantage over the motion pictures, in which elaborate steps must be taken to synchronize the sound track with the projection of the picture.
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|No elaborate steps have to be taken to synchronize Bill Allen’s descriptions of a football game with the projection of the picture.|
Why Has Television Taken So Long in Attaining a Practical Form?
The modern television system we have just examined is a truly remarkable achievement in science and engineering. In fact, the system makes more comprehensive use of electrical technology than any other branch of electrical engineering.
Equally remarkable, however, is the length of time which has been required to bring the system to its present state of utility. As J. C. Wilson has pointed out, television as an idea dates from the inception of the telephone, in about 1875, and practically all of the knowledge necessary for the fruition of the idea has been available since the days prior to the World War of 1914-18.
But the particular amalgamation of theory and practice required to produce the practical television system of today has required a very steady effort over the past twenty-five years.
The fact that television systems began to appear, as ideas, so early in electrical history resulted from the discovery in 1873 by a British telegrapher named May that the chemical element selenium possessed the property of transforming changes in light to corresponding changes in electricity.
This is obviously a completely essential property to any system of television, and its discovery at once started a great many inventors devising means of applying it in a practical television system.
One of the first was that of the Frenchman Senlecq, who proposed in 1877 that a mosaic be built up of selenium cells, each cell to be connected by a separate circuit to a shutter which would drop down when its cell was illuminated. Behind the shutters was a source of light which shone through when the shutters were operated.
This system was quite capable of reproducing crude silhouettes, but there is no evidence that it was ever actually built and made to operate. This is, so far as is known, the first proposal for a television system, and for it Senlecq deserves the title “Father of Television.”
Following Senlecq’s suggestion, a great many others were advanced from 1877 to 1884, all very similar in principle.
In 1884 came the invention by the German Nipkow of the rotating scanning disk, which until 1930 was very widely used in television development.
Nipkow’s disk made use of the very significant technique, previously suggested by several others, of examining the scene to be transmitted and dissecting it into points of light which could be conveyed successively over a single electrical circuit. Nipkow’s work ranks high in the history of the medium, principally because he realized so early a system which was not improved upon, basically, for nearly fifty years.
Shortly thereafter, in 1890, the Englishman Sutton proposed a system for a television receiver which ranks in importance with Nipkow’s system for the transmitter. Sutton’s apparatus used a scanning disk and an electrically controlled light source known as a “Kerr Cell.” This method of reassembling the image was likewise remarkable in that it was used widely in practical television systems for nearly forty years.
Neither of these systems is used to any extent in modern television work, but they were necessary antecedents to more modern practice.
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|Patent drawings for Nipkow’s mechanical rotating disk system.|
Most of the early work was based on the electrical sensitivity of selenium to light, and this effect was not ideally suited to the purpose. In the first place, selenium cells are rather slow in action and hence incapable of following rapid changes in light which occur in images in motion.
In the second place, the selenium cells were highly variable in their sensitivity and far from permanent.
So it remained for the early television workers to discover the utility of the true photoelectric effect, which is the basis of the light-sensitive action of the silver globules in the iconoscope.
The photoelectric effect was discovered by Hertz, the “Father of Radio,” in 1887, when he noticed that a spark could be made to jump over a gap much more readily if one of the electrodes were illuminated with strong light than if the event occurred in darkness.
The German Hallwachs later studied the effect systematically and came to the conclusion that the light set free electrical particles from the surface of the electrode. Sir J. J. Thompson then identified the particles as electrons, and Albert Einstein came forth with his theory of the photoelectric effect, on which his fame as a physicist rests as much as on his theory of relativity.
The practical side of the photoelectric effect was advanced by Elster and Geitel, who as early as 1890 built practical photoelectric cells which would pass electrical current when light fell upon them, but not when they were in darkness. These cells, the precursors of the modern “electric eye,” were very important tools in the hands of the television worker, and have since proved, in one form or another, to be the backbone of the television camera.
The specifications for the modern electronic system of television described in the beginning of this chapter were laid down, surprisingly enough, in 1907 by two workers independently.
The Russian Boris Rosing and the Englishman A. A. Campbell-Swinton came forth in that year with the proposal for a light-storage camera tube very similar in principle to the modern iconoscope, and suggested at the same time that the picture-reproducing apparatus be a “cathode-ray tube” very similar to the electronic picture tube of the present day.
The cathode-ray tube had previously been developed by Braun as a means of studying the behavior of electron streams. It has since been highly developed as an electrical tool, but its practical use in a television receiver was to wait until 1928.
The development of the light-storage picture tube was equally long in completion. In fact, the first successful attempt to put Rosing’s and Campbell-Swinton’s ideas into practice came in 1928, when V. K. Zworykin applied for his patent on the iconoscope.
The iconoscope has been vastly improved since that time, but credit for the first camera tube capable of transmitting high-definition television images surely belongs to Zworykin. This important invention is one of the first American contributions of basic importance.
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|V.K. Zworykin demonstrates a working television.|
Another American worker of great importance in television is Philo T. Farnsworth, who has invented several forms of camera tubes, of which the “image-dissector” is perhaps the most widely used (for film transmissions primarily, since it does not have sufficient sensitivity to pick up studio or outdoor scenes unless the lighting is very intense).
Farnsworth and his associates also have contributed much to the problem of deflecting the electron streams used in electronic camera tubes and picture tubes.
On the practical front of television system development must also be mentioned the American C. F. Jenkins and the Englishman John L. Baird. Baird is given credit for transmitting the first half-tone television images in motion, in 1925, although Jenkins achieved the same result within a few months.
These images were extremely crude, but they were improved to form the basis of the early systems of mechanical television which were shortly thereafter made available to the public.
Several broadcast stations here and in Europe broadcast regular programs to the public from 1928 on, and simple receivers were available during the ensuing five years. However, the pictures possible in the mechanical system of scanning did not have sufficient pictorial detail to support an entertainment service, and public interest lagged.
Thereafter it became clear that electronic methods were essential, and they were developed with vigor by several commercial engineering concerns.
One of the first organizations to offer a public program service based on electronic methods was W6XAO of the Don Lee Broadcasting System in Los Angeles. This station began transmitting scheduled programs in 1933 and has been on a regular schedule ever since.
In 1936, the National Broadcasting Company and the Radio Corporation of America began a series of field tests in New York City, using the all-electronic system. These transmissions were not intended for the public, but were viewed by members of the technical and executive personnel of the two companies.
On April 30, 1939, these transmissions were extended on a regular schedule for reception by the public. Coincidentally, several manufacturers of television receivers offered models for public sale.
This event was the first co-ordinated effort to establish television service for the nontechnical public (the Don Lee transmissions on the West Coast had been received for the most part on receivers built by amateurs, since few commercially manufactured receivers were available).
For this reason the opening of the New York World’s Fair, on April 30, 1939, is commonly viewed as the first major telecast for the public in the United States.
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|Television is demonstrated at the World’s Fair. (Courtesy, RCA.)|
The Boundaries of Television—What Can Be Done and What Cannot
The modern television system has great capabilities, and equally great limitations. We have already discussed one phase of the area within which television can operate, namely, the pictorial detail of its reproductions. The present television system is capable of reproducing a picture about the equal of 16-milimeter home movies (although not every broadcast, nor every receiver, attains this level in the present state of the technique).
This degree of detail is an achievement, but it is also a limitation which the program producers must keep in mind in the programs they offer. The detail available is perfectly adequate for close-up pictures, but it becomes less so as the figures on the screen become smaller, that is, as the camera is removed further from the object.
Suffice it here to say that by the proper use of cameras with interchangeable lenses, it is possible to cover an area as large as a football field without causing the audience to be aware of the detail limitations, but it can be done only with expert use of the cameras.
Another limitation of less pressing importance is the fact that the picture is reproduced in black and white, rather than in colors. The universal experience with motion pictures has so accustomed us to this type of reproduction that we rarely question it, but it is important nevertheless.
Scenes in nature, viewed in full color, display contrasts of color which are partially, if not wholly, lost in the black and white reproduction. Part of this lost contrast may be restored, but to do so it is important that all the colors in the rainbow receive equal treatment from the television camera.
The early iconoscopes displayed a sharp preference for the red and infrared—so much so, in fact, that the silk lapels of dinner jackets often televised white or gray because they reflected so much infrared light.
Recent improvements in the iconoscope (as well as in other types of camera tube) have removed this over-sensitiveness to red. In fact, the modern iconoscope is capable of dealing with colored subjects substantially as well as the panchromatic films now used in motion picture production.
From this point of view, the color performance of the modern television system may be said to be completely satisfactory.
A third item of importance in the operation of television cameras is their sensitivity to light. How bright must a scene be before it can be properly televised? The best answer to this question can be stated in terms of the sensitivity of an equivalent photographic film. At present the most sensitive camera tube is the orthiconoscope (“orthicon” for short), an improved version of the iconoscope.
The iconoscope, previously described, produces undesired signals, or “dark spots,” in the picture, by the process of scanning. The orthicon (shown in Figure 3), however, does not develop these surplus signals, and the operations needed to remove them in the iconoscope are not necessary.
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|FIGURE 3. Schematic diagram of the orthicon.|
The orthicon has a sensitivity which approaches that of the films used in motion pictures, which means that it can televise a scene in ordinary room illumination. For the best results, however, more light is desirable, and in studio practice it is customary to employ several hundred foot-candles of light.
This is a large amount of artificial illumination, and it is accompanied by a considerable amount of radiated heat. The heat is one of the commonest complaints of television performers. Although the light is no brighter than that employed in motion picture studios, the duration of each scene in television is often longer than in the movies, and hence the over-all effect of the heat is greater.
In contrast to such high illumination, however, can be cited the experience of the NBC crews in televising the end of a football game on a cloudy day in the fall of 1939. On one such occasion the light was so dim that it produced no deflection on the standard Weston photographic exposure meter, but nevertheless a recognizable television image could be transmitted.
The sensitivity of the television camera is being rapidly improved at present, and it is not too much to suppose that eventually it will be able to work under any light conditions in which a movie camera may be used, if indeed it does not surpass the motion picture camera.
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|The orthicon, new television camera tube, which possesses many advantages over the older iconoscope. (Courtesy, RCA.)|
One of the most widely heard questions about television reception is that concerning the small size of the picture. Receivers commercially available at present produce pictures ranging from 3 by 4 inches to about 8 by 10½ inches. These picture sizes are small compared with the screens used in home movies, for example, but they are nevertheless quite capable of giving satisfaction, provided the audience sits close enough to the screen.
The important factor, so far as picture size is concerned, is the distance at which the picture is viewed, relative to the picture height. The usual preference of audiences, whether the picture is large or small, is to sit at a distance about 5 times as great as the picture is high.
For a picture 8 inches high (the maximum in commercial receivers) the corresponding viewing distance is somewhat more than 3 feet. At such a viewing distance the picture does not seem small, but obviously only a few people can sit as close to the screen as this.
Television pictures as large as 18 by 24 inches, with a viewing distance of 7½ feet, are desirable for the usual family audience in a living room. That such pictures will become available in the future cannot be doubted. In fact, television pictures as large as 12 by 15 feet have been successfully demonstrated in theaters, but the necessary equipment is not suitable for the home because of its high cost and technical complexity.
Cathode-ray picture tubes used in home television receivers have been built to accommodate pictures as large as 12 by 16 inches, but this is the upper practical limit at present because of the expense of the tube and its associated circuits.
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|A typical home television receiver. (Courtesy General Electric Co.)|
Finally, one of the most important boundaries of present-day television is the distance over which the programs may be transmitted. Because of the tremendous rate at which information must be sent over the television system, a very large amount of space must be used in the ether by each television broadcast station.
Actually the space occupied by one television station could accommodate 600 standard sound broadcast stations. Accordingly, it has been necessary to assign television stations to portions of the ether spectrum where space for them could be found, that is, in the short wave length region. Even in this region, room has been found for only 19 television station channels, and only 7 of these are considered useful at present for public service.
There will be available, therefore, only 7 choices of program in any given locality, when a sufficient number of stations have taken the air. This does not mean that 7 stations can be assigned in any one city, but they can be assigned within any area of, say, 200-mile radius, beyond which stations do not ordinarily cause interference with other stations.
By far the most important result of the use of very short waves for television transmissions is the limitation of distance. It is a good working rule (but not without exceptions) that the distance over which such ultra-short waves may be depended upon to give a reasonable quality of television reception, is limited to the horizon as viewed from the transmitting antenna.
The distance to the horizon increases with height, so it is desirable to erect the transmitting aerial on as lofty a pinnacle as possible. In New York City the two tallest skyscrapers, the Empire State Building and the Chrysler Building, have already been pre-empted for television stations. In the case of the Empire State Building the horizon is located about 45 miles away, and consistent reception seems to be limited to about 50 miles except in favorable receiving locations.
For the more usual case in a smaller city, the aerial may be raised no higher than about 250 feet, and in this case the horizon is 25 miles. Fortunately, the relation between the coverage of a station and the size of the community is such that most urban populations can be covered by stations within the city limits.
But rural areas, as well as some suburban areas, cannot depend on television reception, if indeed it can be made available to them at all.
So for the present, at least, television is an affair for the cities and the clusters of communities around cities. Fortunately for television, the bulk of the nation’s population resides in these areas.
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|Transmitting antenna array atop the Empire State Building for NBC’s New York television station; the upper section transmits sound and the lower, sight.|
At the receiving end, the location of the antenna is equally important. The antenna should be mounted as high in the air as possible and free of obstructions. Moreover an ordinary piece of wire, which serves well for most sound radio installations, is not suitable for television.
Rather a specialized structure, known as a “dipole aerial,” is desirable because such an aerial is sensitive and because it is comparatively free from signal reflections which cause “ghost” images in the received pictures. The dipole is simple: it consists of two rods, each four to five feet long, mounted horizontally on a wooden or metal mast, so that the whole structure resembles a “T.”
The two rods are placed at right angles to the direction from which the broadcast comes. The lead-in wire is composed of two conductors, one connected to each of the two rods.
When the receiving location is far from the transmitter and the received signals are weak, a more involved structure may be necessary. In this case several sets of rods may be used to increase the sensitivity of the antenna.
In cities, signal reflections from near-by buildings may be very troublesome in producing ghost images, and in this case also, a complex antenna structure may be necessary, in order to discriminate against the reflected waves. In any event the installation of a television receiving aerial is much more of a problem than that of a conventional radio aerial.
It is usually wise to have the installation made by a competent service man.
What About Improvements in the Future?
Many prospective purchasers of television receivers have hesitated to make the plunge because they fear that the equipment may very quickly go out of date, if not become completely obsolete. This fear has been fed by widely publicized statements to the effect that any change in the manner of transmission at the broadcast station will render all receivers useless, that is, until the receivers are modified to accommodate the change.
This is not strictly true, because very considerable improvements have been made in transmission since the inauguration of public television service, and all receivers have benefited.
|Automatic radio relay tower for transmitting television programs over intercity networks. (Courtesy RCA Communications.)|
But it is true that if the number of lines in the scanning pattern, or the rate of repeating the pictures, is changed, most commercial receivers are incapable of following the changes and major modifications must be made in the receiver circuits.
Moreover, the amount of pictorial detail which a receiver can accommodate is definitely fixed, and if the broadcast station increases the amount of detail beyond this limit, no improvement is noted in the received pictures.
These facts have made necessary the establishment of standards of transmission which will serve as the basis of all television broadcasting for a long period of time. The standards used in this country are those drawn up by the Television Committee of the Radio Manufacturers Association. At present all public program services are transmitted according to these standards, and all receivers for sale are designed for such transmissions.
However, the R.M.A. Television Standards have not been endorsed (or condemned) by the Federal Communications Commission, and lacking this official sanction there has been some fear that the standards of transmission might change. Several proposals for changing the standards have already been brought forward.
The F.C.C. has stated that it is too early to freeze the art by standardization. At the same time the government body has urged television receiver manufacturers to engineer just as much flexibility into their product as is economically feasible so that the future changes, if any, may be readily accommodated without extensive alteration of the receiver circuits.
Under these circumstances, the owners of the several thousand television receivers now in the hands of the public must take their chances regarding possible changes in the future.
From a practical standpoint, however, there seems little doubt that the present R.M.A. Standards will endure for a time long enough to justify the expenditure of several hundred dollars for a receiver. Although the government has taken no action regarding standards, the industry has done so, and the present broadcast stations show no disposition to change the standards.
The fact remains that standardization does have the effect of preventing large-scale improvements in the pictorial detail of the pictures.
In the future there will no doubt arise, a demand for more detail, and the industry will then be faced with the problem of making progress without violating the standards. The most practical suggestion for overcoming this difficulty is the proposal to open up a new portion of the ether spectrum for a totally new service, while maintaining the old service on the old standards until public demand for it no longer exists.
The F.C.C. has already laid the groundwork for such a wholesale expansion by assigning channels for television research which are not now useful for public program service, and by urging research organizations to develop their new ideas on these as-yet-unused channels. By some such scheme the problem of standards vs. progress can be circumvented without loss to the public.
In the meantime the industry must, in theory, warn purchasers of television receivers that the standards on which they are based do not yet have government sanction.
From the preceding discussion it may appear that television consists mostly of problems with partial solutions. This is to some extent true; certainly many compromises have been made.
But sight must not be lost of the central facts in the case:
- that a program can be sent which has the quality of a 16-milimeter home movie (and a sound accompaniment considerably superior to that of most professional movies);
- that the television camera can pick up any subject on which it can be focused, if the subject can be photographed by the motion picture camera;
- that ultimately about 55 per cent of the nation’s population (living in or near cities) may have such a service from at least one station, and a smaller percentage may have a choice of programs;
- and that all this is possible under conditions of transmission which were considered impossible even so short a time as ten years ago.
The progress made since that time is, of course, a good indication of the progress to be expected in the next decade. In the meantime, the system is available and is being expanded both geographically and economically so that more and more people may enjoy its unique power to bring the outside world directly into the home.
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|Television broadcasting station W2XB erected by the General Electric Company atop 1,500 foot hill in Helderberg Mountains adjacent to Schenectady, Alban, And Troy, New York, to cover area of 500,000 people. Many of NBC’s New York programs will be rebroadcast from this transmitter. (Courtesy General Electric. Co.)|