1-1 After decades of scientific development, television is no longer a laboratory curiosity, but a full-fledged industry des lined to contribute its share to man's welfare and standard of living. Few other developments have been so long predicted and awaited, and few other industries have had so many false starts before finally becoming practical realities.
What is television? Why has it been so long sought? Is it the motion picture brought into the home, or radio with vision? Perhaps the word itself will help us answer some of these and other questions. Literally, its translation from Greek or Latin means the art of seeing at a distance. These words are an exact description of television. Through the medium of television we are able to view distant places from the privacy of our homes.
The entertainment and educational possibilities of television are almost limitless. Spot news pick-ups, coverage of sporting events, the drama of the theater, musical concerts, are only a few of the programs already being transmitted. Television's ability to let you see as well as hear your favorite radio programs, comedians, artists and speakers enrich your leisure hours.
Full-time programs indicate the educational advantages of television. Children learn quickly from a picture, and many excellent programs, devoted entirely to children, appear regularly. They provide entertainment and at the same time; teach patriotism, tolerance, facts about distant peoples and countries, and other useful lessons.
Classroom demonstrations with television receivers have been tried in high schools and colleges, and mass education via television has been proven practical and continues to grow. Renowned lecturers reach large audiences without having to travel to them. An important use of such facilities has been demonstrated in medical schools and to the public. A television camera, suspended over a patient in a hospital operating room, sends close-up pictures of intricate operations on the human anatomy . If the viewers had actually been in the operating room itself, they would not have had as good a view as that obtained by television.
Scientist, lawyer, laborer . . . we shall all derive benefits from television. Television combines the sound of radio with the information of the newspaper, the entertainment of the theater and the motion picture. It is a unique development, borrowing from all fields of communication. It has developed its own specialists and techniques and evolved as an art in itself . Well over 500 commercial television stations have been authorized by the Federal Communications Commission. Applications for additional stations are high.
Estimates indicate that 5,000,000 television receivers were produced in 1954 bringing the total number in the country close to 35,000,000 . Production estimates for coming years indicate there will be 60 million television receivers in use by 1960. Many television networks are in operation, linking distant cities together. One means of connecting stations is being provided by the American Telephone and Telegraph Company, which has been laying coaxial lines between television areas. New York, Philadelphia, Baltimore, Washington, Richmond, Buffalo, Cleveland, Toledo, Detroit, Chicago, Milwaukee and St. Louis have been linked by cable and relay. This intercity network has been extended to the Pacific Coast, making possible nationwide television transmission. The extensive network of the A.T. & T. system is shown on the map of Figure 1.
With increases in the production of television receivers and the entrance of new manufacturers into the field, competition has become keener and prices lower. Approximately 100 manufacturers are now in the business of making television receivers and kits. Although the present price range of receivers extends from about $100 to $2500, the average retail price of receivers in volume production is around $200. This figure is expected to drop each year. It is not unlikely that large screen receivers will be available within two years for less than $150, thus bringing television into the price range of the lowest income groups.
Figure 1. Network routes of the American Telephone and Telegraph Company.
Television is one of the country's fastest growing industries. Leaders in the industry estimate that by 1960, five years from today, it should represent an over-all capital investment of $10,630,000,000 and give employment directly and indirectly to as many as 1,000,000 persons. Its effectiveness as an advertising medium has resulted in large increases in the sale of many products.
In considering television as an industry, it is necessary to break it down into its essential components. They are television broadcasting, the manufacture, distribution and servicing of television receivers, industrial television, and military television.
1-2 Television Broadcasting. The Federal Communications Commission has made provision for approximately 1,000 stations, in the band now in use, and in the band between 475 and 890 megacycles. Although the cost of television station installations varies greatly, $500,000 is a conservative estimate for a complete unit. To supply the equipment for 1,000 stations requires an industry in itself.
A station originating network programs may employ as many as 150 persons; a small station in a city under 50,000 population, perhaps as few as seven or ten. In addition, there are the artists, script writers, and advertising copy writers. The film industry, which supplies a number of programs for use by television outlets, will create further employment.
The following estimated figures give a clue to the possible scope (employees and investment) of the television broadcasting industry in 1960 (including color) . Employees Full-time employees in stations 45,000 Advertising agencies, artists, etc. . 24,000 Film supplies 2,000 Manufacturers of equipment 13, 000 Total 84,000 Investment Television transmitters $170,000,000 Manufacturing facilities 510,000,000 Total $680,000,000
1-3 Television Receiver Manufacturing. Nearly 40 million television receivers are in use. To place a receiver in a home requires many steps and many hands: among them, purchase of parts from specialized suppliers, assembly by the manufacturer, distribution through jobbers and dealers, installation and servicing, etc. A statistical breakdown of this phase of television activity based on an estimate for current operations appears below:
Employees
Receiver manufacturing
Distributors, dealers, installation and service
Parts suppliers
Investment
Television receivers
Manufacturing facilities
240,000 150,000 75,000
Total
465,000
$5,000,000,000
500,000,000
Total
$5,500,000,000
1-4 Television Networks. In order to supply the television stations all over the country, some 20,000 miles of coaxial cable are necessary to provide network-program service. Microwave radio relaying facilities have added another 10,000 miles of circuit connections.
The number of employees to install and service this network equipment is estimated at 10,000. The investment in cables and radio relay facilities is over $100,000,000.
1-5 Industrial Television. Television has many non-entertainment uses such as in department store advertising, mass education through private facilities, aircraft navigation, plant protection, and traffic control. The estimates of employment and investment in this field are: Employees needed for manufacture, installation and maintenance, 5,000. Investment required for equipment, $20,000,000.
1-6 Theater Television. Commercial application of theater television has not advanced to any great extent thus far, but, in order to compete against home receivers, new developments provide that half of the nation's film theaters may be equipped to show televised events on large theater screens. Theaters would need their own pick-up equipment and relay facilities, requiring additional employees. The possibilities in this field are: Employees for installation, maintenance and operation, 10,000. Investment for theater equipment and pick-up and relay facilities, $50,000,000.
1-7 Military Television. Military applications of television are veiled in secrecy, but we do know of the use of television to control robot planes and operate guided missiles. If world events continue to require military preparedness on a large scale, such applications of television will be constantly expanded.
1-8 Future Developments. The full potentialities of television have not yet been realized. With the improvements that have come in program quality as more and more advertisers look to this new medium as a means of selling, there have been technical advances to make program coverage even more exciting.
As radio now spans continents, so will television. The international Olympics, the coronation of a foreign king, travelogs in distant countries, all will make television a more entertaining and educational medium. "Two-way television" has made it possible for two entertainers located in distant cities to appear on the same screen, or for two political candidates to debate an issue though they are separated by a great distance . Political campaigns, conventions, congressional hearings are influenced by television, which will continue to make the public more acutely aware of its government's representatives and operation.
It is not beyond the realm of possibility that international television will create a better understanding among peoples of different lands and serve as a potent instrument of peace.
The use of television in airplanes will become a practical reality. Stations will have camera-equipped planes as part of their remote pick-up equipment. The airplanes will be able to take off at a moment's notice to give the viewing audience a birds-eye view of floods, forest fires, rescues at sea, volcanic eruptions, and other newsworthy happenings.
Further studies of the transmission and reception of signals at very high frequencies may make it possible to use built-in antennas and eliminate the need for outdoor installations.
An inevitable reality, no matter what the manifold applications of television are, is color television. All the beauties of art and nature take on more vivid reality when televised in color. Television is the long sought goal of man to span time and space. No other medium more adequately fulfills man's dream of bringing the world into his home.
1-9 Color Television. On duly 23, 1953 the National Television System Committee placed before the Federal Communications Commission a proposal of signal specifications as the technical transmission standards for commercial color television broadcasting in the United States. The Federal Communications Commission accepted and approved these specifications authorizing compatible color television to operate on a commercial basis starting January 1954.
The National Television System Committee has contributed a great deal to the progress of Television by this effort. Formulation of these signal specifications was an achievement requiring millions of dollar's worth of effort by engineers and scientists in the electronics and allied industries.
The NTSC color television Standards fulfill three basic conditions.
(a) They provide for the best color television service possible within the present standard television channels.
(b) They provide a "compatible" color television signal which will produce a high quality monochrome image on existing black and white television receivers without requiring any change whatever in such receivers.
(c) They provide for the reception of satisfactory black and white pictures on a color television receiver when it is tuned to a transmitter broadcasting black and white television signals.
These achievements were of high importance for the adoption of color television by the public. The quality of the color system encourages sponsors, broadcasters and the public to invest further in color television. The owner of the black and white set is assured against obsolescence, while the color set buyer can receive monochrome programs. In the same manner, the broadcaster gains an additional color picture audience without inconveniencing his black and white picture following. While the cost of color cameras and transmitter equipment will be high, the service cost for piping in network shows will be relatively little additional to black and white.
1-10 Television vs. Radio Broadcasting and Motion Pictures.
Although television is actually a specialized form of radio, it requires not only certain modifications of familiar radio circuits, but also various new circuits, techniques, and equipment.
Whereas standard radio transmission involves the conversion of sound into electrical energy at the transmitter and the reconversion of electrical energy into sound at the receiver, television requires that light be converted into electricity at the transmitter and back again into light at the receiver.
Television is similar to motion pictures in many respects since both media reproduce on a viewing screen a series of images which create the illusion of uninterrupted motion. The methods used to obtain the illusion of motion in television and in the motion picture are considerably different, but, since both media must satisfy the requirements of the human eye, there are certain salient features of each which are identical. An understanding of how motion pictures create the illusion of motion is an excellent stepping stone in learning the more complex system used in television. To understand either medium, some of the elementary properties and functions of the human eye must be known.
THE HUMAN EYE AND THE TELEVISION SYSTEM
1-11 How the Eye Sees. Just as radio achieves the duplication of sound and the properties of the ear, so must any television system possess the remarkable powers of the eye to see and reproduce an image. A study of the characteristics of the human eye and how it sees will unfold many of the mysteries of television systems, past and present, and explain the need for many of the features in these systems. With this understanding of the visual organs of the body, the reader will appreciate how closely present day television imitates the functions of the eye in order to accomplish its purpose.
A sketch of the structure of the human eye is shown at the top of Figure 2, and a cross-section of a simple camera is shown in the bottom half for purposes of comparison. When taking a picture with such a camera, the first step is the focusing of the light reflected from the subject so that it will form a sharp image on the sensitized film. This is accomplished by changing the position of the lens with respect to the subject. If after the focusing operation is completed, the subject is moved further away from the camera, it becomes necessary to repeat the focusing procedure. This is necessary because the lens is not capable of simultaneously focusing on objects at different distances from the camera.
The eye employs a very similar focusing system. Its lens is composed of an elastic, transparent material, whose curvature and focal length are controlled by a number of tiny muscles.
These muscles automatically adjust the focal length of the lens so that no matter what the distance is to the object, the image always falls perfectly in focus upon the back wall of the eye.
It is because of this automatic focusing property of the lens that we are able to look at objects close at hand and then almost instantly look at objects in the distance.
HUMAN EYE; OPTIC NERVE; TO BRAIN CAMERA
Figure 2. The action of the human eye is similar to that of a camera.
The back wall of the eye is known as the retina and like the film in the camera, receives the image. Whereas the camera film is composed of millions of tiny light-sensitive particles of silver which are acted upon in various degrees by the light received from the subject, the retina is coated with a material known as the visual purple, in which are embedded about 18,000,000 light-sensitive elements called rods and cones. The light which enters the camera strikes the film which is later chemically processed to form the image on its surface. In the eye, the information gathered by the millions of rods and cones must be conveyed in a systematic pattern to the brain through the fibers of the optic nerve, which is attached to the retina.
All of the rods and cones function independently of each other and send their "nerve currents" through separate fibers. Thus, what the eye actually sees is a picture composed of many stimulations from these minute elements of the retina. Each element contributes its share of information, depending upon how much light from the object falls on it. Finally, the brain coordinates these stimulations into what we call sight.
The extraordinary flexibility of the eye is further exemplified by the fact that as you read across this page, the light reflected from the printed words is constantly changing. Therefore, as soon as the rods and cones convey one message to the brain, they must be ready to send succeeding messages as you scan from word to word. The sense of sight thus consists of millions of successive impulses sent to the brain from the elements of the retina.
Another element of the eye which will enable us to appreciate its corresponding part in the television system is the iris. The iris is the ring-shaped, colored matter in the eye which we refer to when we speak of the color of someone's eyes. The opening in the center of the iris is the pupil. The light rays which enter the eye must pass through the pupil. The iris regulates the size of this opening automatically so that the amount of light which passes into the eyeball remains approximately the same.
In bright sunshine, the iris closes down the pupil to a very tiny pinpoint so that excessive light does not enter the eye. At night, the iris opens fully to let in all available light. The control over the amount of light is likewise important when taking a picture with a camera in order not to "overexpose" or "underexpose" the film. Similarly, television cameras are equipped with an iris or adjustable diaphragm which the operator sets according to the brightness of the scene.
1-12 Persistence of Vision. The foregoing description of how the eye transforms light rays from a "still" object into sight does not explain how we are able to perceive continuous motion.
The ability to perceive motion is based upon another characteristic of the rods and cones; that is, they do not instantly respond to changes in light intensity, but have a lag of about one-tenth of a second. Likewise, the eye must be stimulated for a definite period of time, depending upon the intensity of light, before an impression will register on the retina. In order to observe a moving object, the eye must register each successive motion at least 1/500,000th of a second in bright light. Once this impression has been registered, the lag characteristic of the eye will hold it until the next impression is made. If these impressions occur at intervals of less than one-tenth of a second, the eye will blend them into a sensation of continuous motion.
This lag characteristic of the eye is called persistence of vision and makes possible the motion picture and television.
1-13 The Repetition Rate of Images.
Motion picture film offers an excellent example of how the eye sees moving objects by virtue of its persistence characteristic. A typical film is shown in Figure 3 and is seen to consist of a series of still pictures, each of which differs slightly from the next. When film is run through a projector, each still picture or "frame" is held in front of the lens for a definite period of time while the light is projected through it onto the screen. The shutter then cuts off the light while the still picture is removed and the next one moved before the lens. The shutter then opens and the next image is projected. If the successive still pictures are projected rapidly enough, the image formed in the eye by one frame will persist through the dark interval between frames and blend into the next image.
Figure 3. A strip of motion picture film.
In early motion pictures 16 frames were flashed on the screen during each second, but at the low light levels used in theaters a definite flicker was noticeable. In other words, even though the average one-tenth second lag of the eye would be expected to eliminate flicker effects when more than ten frames are projected per second, still, the illumination of each scene was not sufficiently high to register a persisting impression from as many as 16 frames per second.
Present day motion pictures overcome the effects of flicker by projecting 24 frames per second and, by the action of the shutter, project each frame twice. There are thus produced 48 separate projected pictures per second and no interrupted motion is visible. As will be explained later, the modern all electronic television system provides 30 frames per second, each of which is broken into half-frames or "fields", thereby giving, in effect, 60 pictures per second.
Figure 4. Greatly enlarged portion of a photographic negative. (Courtesy Dastman
Kodak Co.)
1-14 Breaking the Picture into Elements. We have already shown how it is possible to break down a moving scene into a succession of complete pictures and still retain the illusion of continuous motion. This technique is satisfactory for motion pictures, but falls short of the requirements of television. No television system has yet been developed which enables us to show a complete picture in even a single frame. The "dissecting" of the continuous motion has to be carried further; successive frames must be broken into elements. To appreciate how a picture maybe divided into small elements and still appear uniformly solid, we can examine the methods employed in photography and photoengraving to reproduce a scene in a newspaper or book.
The scene is first photographed with a camera and transferred to film. If the developed image on the film is examined under a microscope, it will be found to consist of many tiny grains of silver, as shown in Figure 4. These silver elements are so deposited that in the darker portions of the picture they are closely bunched together, while in the lighter shades, they are more finely dispersed or do not exist at all. Because the grains are so fine, they appear as a solid picture to the naked eye.
To print the picture on paper with ink, the photograph is now etched onto a copper or zinc plate. Examination under a microscope of a picture reproduced from such a photoengraving reveals an even coarser distribution of black and white dots than appeared on the photographic film. In the photoengraving process, the film is projected onto a sensitized metallic plate through a transparent screen which is ruled with tiny squares.
Figure 5. Coarse and fine half-tone engravings.
The image which is formed on the metallic surface consists of many small dots. In light portions of the picture they are extremely tiny and little ink is deposited from them, whereas in the darker portions they are larger and even run together. Figure 5 shows the same picture made from photo-engravings of different screen fineness. At the left, the dots are easily visible to the naked eye, while in the engraving to the right the dots are hardly discernible.
The silver grains and printed dots show how a picture can be divided into a pattern of separate elements, each element contributing a portion of light or darkness to the combined image.
Since there are about 18 million rods and cones on the retina, the eye can see this many detailed elements in a photograph or photoengraving. The finest screens used for photo engravings contain about 14,400 elements per square inch. The full resolving capabilities of the eye are thus hardly taxed by the details in the best photoengraving when viewed from a distance of several inches.
Figure 6. The Carey television system.
1-15 Proper Viewing Distance. Not only does the number of elements in a picture determine how pleasing the overall effect will be to the eye, but the distance from the eye to the picture must be considered as well. For example, if the coarse engraving shown to the left in Figure 5 is viewed from several feet, the large dots will no longer be distinguishable and the picture will appear as uniform as the one shown in the right of Figure 5, but which is viewed at a distance of several inches.
The proper viewing distance is extremely important in television, for it determines the picture size that is most comfortable for viewing. For example, it is entirely possible for a television picture to be too large for the size of a room. The average person watches a moving picture (in the theater or on a television set) most comfortably from a distance roughly 10 to 12 times the height of the picture. Thus, an 8" x 10" image will be viewed most satisfactorily by the average eye at a distance of approximately eight feet. Sitting nearer only increases eye strain and makes the viewer conscious of the picture elements and coarseness of detail. Moving further away (as was done in comparing the coarse and fine engravings of Figure 5) improves the picture by blending the details together-but the picture now becomes too small for comfortable viewing. A picture of any given size, then, must be viewed from a definite pre-determinable distance.
Any screen size viewed at the proper viewing distance provides about the same pictorial detail if the screens used contain the same number of picture elements and are of equal brightness. The midget television receiver, with its 3-1/2" x 4-1/2" image, which must be viewed from a short distance, produces just as sharp an image as the projected image receiver which projects images as large as several feet and has to be viewed from greater distances. The only advantage large television screens offer is the ability to accommodate a larger audience more comfortably around the receiver.
1-16 A Simple Television System. It is apparent from the foregoing discussion of picture frames and elements that it is not necessary to reproduce a scene so that it will appear as one continuous mass to the eye.
An early attempt to devise a television system was made by G. R. Carey of Boston in 1875. Carey knew of the properties of the eye and tried to take advantage of this knowledge. He attempted to simulate the action of the retina and optic nerve of the human eye by substituting an electro-mechanical system consisting of photocells and light bulbs. Carey's system also illustrates the basic principle of the conversion of light energy into electrical energy. See Figure 6. In this simple illustration, light from the subject is focused by a lens onto a bank or mosaic of photocells. Each photocell generates an electrical current in proportion to the amount of light which falls on it. This action is very much like the rods and cones in the retina of the eye which produce "nerve currents" when stimulated by light.
If light does not fall on a photocell, it does not produce a current. Thus the photocells receiving the light reflected from the letter X develop currents while the others which do not receive light remain inactive. To convey the electrical information held by the cells, Carey connected each cell through a pair of wires to a lamp located in a bank of lamps. The lamps which receive currents from the active cells light up and reproduce the original image.
This system is impractical because it requires at least 250,000 photocells, pairs of wires, and light bulbs to produce a picture of acceptable detail. Carey's television set-up also failed because the small currents from the photocells could not light the lamps to a sufficient brightness. It remained for Lee de Forest to invent the amplifier tube which could magnify tiny currents before photocells could be put to such use.
The difficulty of running two wires from each photocell to a light bulb might be overcome by switching a single pair of wires consecutively to each photocell and its corresponding lamp. If this switching is done at a rate which is faster than the persistence characteristic of the eye, the eye will be deceived into seeing a continuous image on the bank of electric light bulbs, rather than a single element at a time. If we also use an amplifier to increase the strength of the photocell signals, Carey's basic television system assumes the more practical configuration shown in Figure 7. Herein we have the fundamental principle of all television systems which have since followed-the dividing of the picture into elements before transmission, sending an electrical current from each element successively over a single pair of wires, and the reconversion of these currents into light at the receiver.
Figure 7. An improved television system using a single pair of wires and an
amplifier.
Animated cartoon signs, such as those in Time Square, New York City, are an application of the crude light bulb television system. Figure 8 illustrates their operation. To transmit the simple object, it is first projected from a film onto the photocell bank. The pair of wires is then switched to the photocell in the upper left-hand corner. Simultaneously, the corresponding lamp in the receiving bank lights up to produce the white portion of the image. The switching continues along the top from left to right, illuminating the lamps in succession, and then drops down to the second row and starts across again from left to right. The uniform white background causes equal currents to be given off from the photocells, and so the lamps in the first few rows light up with the same intensity. As the switching proceeds across the third row, the photocells which received no light from the black object do not produce currents to cause their corresponding lamps to glow. By continuing to switch from left to right and down the bank of photocells, we trace out all the black and white details.
This process of switching from left to right and top to bottom is called scanning and is much like the pattern followed by the eye in reading the lines on this page. This method of scanning has been carried over into all mechanical and electronic television systems.
Figure 8. A practical light bulb television system.
1-17 Nipkow's Mechanical Disc. In 1884 Paul Nipkow announced the first practical mechanical device which overcame the limitations of Carey's method of televising an image. Nipkow devised a rotating metal disc, perforated with small holes arranged in the form of a spiral. The Nipkow disc offered a suitable scanning method for dissecting the object into elements at the transmitter and was also used to reconstruct the image at the receiver. It also introduced the principle of scanning by successive lines rather than individual elements.
The method of linear scanning is illustrated in Figure 9. Consider yourself an observer looking at the picture of the woman through a tiny window which is focused on the upper left-hand corner Now move the window to the right across the top of the picture, and then quickly return to the left, but drop the aperture down slightly to line number 2. Continue this operation until the entire picture has been scanned into, say 20 lines. Suppose it were now possible to scan all these lines in less than one tenth of a second. The observer would not then see the individual lines because the persistence characteristic of the eye would enable him to retain the impressions, from the first to last line, and blend them into a unit.
Figure 9. How linear scanning is accomplished.
In the case of a moving scene, if we scan the scene a second time in the next one-tenth of a second, and so on, so that we view the entire scene ten times per second, we will just be on the 1?orderline of the eye's persistence of vision for blending the successive frames into continuous motion. Nipkow used this principle with his rotating disc technique by producing 60 scanning lines per inch in each frame, while presenting 20 frames per second. Refer now to Figure 10 for a description of Nipkow's television system.
The circular disc contains a series of round holes arranged in a spiral. In practice these holes were made either rectangular or square and were covered with lenses in order to gather the maximum amount of light. The distance between successive holes determines the width W of the picture to be reproduced at the receiving end while the radial distance 'from the first (hole 1) to the last hole (20) in the spiral determines the height H of the picture. Each hole is closer to the center of the disc by an amount equal to the diameter of the holes.
The object to be televised is focused onto the shaded area shown in Figure 10. As the disc rotates, each hole sweeps across the image focused on the shaded area, allowing light to pass through the disc. If we were to examine the elements of light, passing through the hole in the disc, on a screen behind the disc, they would appear in the fashion shown at the right of Figure 10. The scanning lines are shown only for illustrative purposes. Actually they would blend together if the disc rotated faster than 10 times per second.
Figure 10. Rotating disc used in Nipkow's television system.
Figure 11. Transmitter and receiver apparatus for a mechanical television
system.
1-18 Converting the Scanning Lines into Electrical Signals.
Suppose that instead of a screen, a single photoelectric cell is placed behind the disc. The light falling on the cell from the first hole would cause an equivalent amount of current to flow from the cell. The variations in photoelectric current for one of the scanned lines would appear as shown in Figure 10. Then, as the second hole moved across the photocell to produce the second scanned line, the chain of current variations would continue. In this manner, all the light variations in the picture are transformed into a continuous series of electrical signals. The apparatus which was employed in mechanical television transmitters to pick up scenes by this method is shown in Figure 11.
The current variations from the photocell are amplified and transmitted on a radio frequency carrier to the receiver.
1-19 Reconstructing the Image at the Receiver. At the receiver, as shown in Figure 11, the current variations are fed to a neon tube whose light output instantly changes in intensity with changes in the current flowing through it. When more light falls on the photocell at the transmitter, the neon tube becomes brighter; when the light fades, it becomes dimmer. The observer looks at the neon light through a scanning disc, identical in size and shape to the one at the transmitter. Both discs run at exactly the same speed, and are synchronized so that when a given hole in the transmitting disc is in front of the photocell, the corresponding hole in the receiving disc is in front of the neon lamp.
The observer actually sees a series of flashes through each hole in the receiving disc. Because of his persistence of vision, these flashes blend into a picture which is a reproduction of the one at the transmitter.
Several variations of the Nipkow disc method were used in mechanical systems which were developed after its introduction, but all of them depended upon the principle of linear scanning and used a single photocell and lamp. The improvement over Carey's system which required a multitude of cells and lamps is obvious. The Nipkow mechanical system represented the furthest advance in the art of television until the early 1930's when it was made obsolete by the introduction of the present day cathode ray electronic scanning system. It is well to list the shortcomings of the rotating disc method of television so that the reader may appreciate how they have been overcome when he begins to study the electronic system in the next chapter.
1. Although the Nipkow disc increased the detail in the picture and eliminated the need for a multiplicity of photocells and lamps, its 60 line picture did not possess the detail required for high quality television. Only 1800 impulses of light were produced during a single scanning of the entire picture. It will be recalled that the high quality reproduction of a photograph by the photoengraving process utilizes as many as 14,400 elements in one square inch of picture. The picture detail produced by the mechanical system was therefore far below the standards achieved in printing.
2. It was extremely difficult to synchronize the transmitter disc with the receiver disc. Most types of motor drives developed for mechanical systems were affected by variations in the power line voltage and frequency. These effects were particularly bad when the transmitter and receiver were on different power lines.
3. The light from the neon lamp was insufficient for comfortable viewing over any length of time. Much of the light was lost in a magnifying lens that had to be used to "blow up" the approximately one inch square picture that was obtained with most scanning discs. Some scanning discs were constructed which were as large as several feet in diameter, but even these could only produce a picture several square inches in area.
4. Finally, the mechanical system was cumbersome and difficult to maintain and could not achieve the portability that was required to make television really practical.
1-20 The Transmission Frequency Band Required for Television. Before turning our attention to the modern electronic television system, we can use our knowledge of the characteristics of the human eye and the factors that govern a high quality, detailed picture to set up certain standards. The first consideration is the number of elements desirable in the television image. In order to approach the standards of the 120 line per inch half-tone engraving which has 14,400 elements per square inch, and 8" x 10" television picture must have 960 lines (8 inches times 120 lines per inch) and 1,120,000 elements per frame (80 square inches times 14,400). At the 30 frames per second repetition rate which has been chosen for television, over 30 million elements must be scanned in one second. This is turn requires an equivalent electrical signal varying at a frequency of about 15 million cycles per second. A television channel capable of transmitting this band of frequencies occupies too extensive a portion of the frequency spectrum to be practical. A compromise must be made in the maximum detail of the television image in order to reduce the bandwidth requirements.
In practice, the television channel is limited to a bandwidth of six megacycles, part of which is occupied by the sound frequencies that accompany the picture. Only four megacycles of the channel are devoted to the television signal, thereby limiting the picture to less than 200,000 elements. This figure may appear inadequate in comparison to the more than 1,000,000 elements found in a fine half-tone. It should be realized, though, that a television picture is viewed from a greater distance than that used when reading a newspaper or book. The television image will therefore be almost as good as the half-tone under actual viewing conditions.
1-21 Frame Frequency. It was mentioned above that a picture repetition rate of 30 per second has been standardized for the modern television system. This figure was arrived at after careful consideration of flicker effects and power line frequencies. At first, a rate of 24 per second was considered because it would coincide with that already established for motion pictures. Since it was to be expected that many television programs would consist of the televising of motion pictures, the problems of synchronizing the two systems could be minimized if both used the same repetition rate.
Even more important than the 24 frame scanning rate of motion pictures was the affect of power line frequency upon the synchronizing circuits. Most of the receivers in this country will be operatedfrom60 cycle lines. Unless the receiver rectifier circuits which convert the alternating current to direct current are perfectly filtered, some 60 cps or 120 cps ripple will get into the synchronizing circuits. The ripple voltage frequencies would not be a multiple of the picture frequency if the repetition rate were set at 24 per second. Synchronization would be unstable as the power line frequency opposed the frame frequency. Since the repetition rate of the present day television system is 30 cps, which is a multiple of the power line frequency, the problems of synchronization are greatly simplified.
It is realized that some areas in this country are served by 25 cycle lines, but these are few in number when compared to.
the preponderance of 60 cycle supplied homes which benefit by a frame frequency of 30 per second. In areas which do not have 60 cycle alternating current, additional filtering may be necessary in the receiver power supply circuits to avoid trouble.
1-22 Consideration of the Number of Lines in the Television Picture. It was pointed out in paragraph 1-14 that an 8" x 10" television picture with detail as good as that of a 120 line per square inch photo engraving would have to have 960 lines. This results in more picture elements than can be transmitted in a 6 megacycle channel. Hence, the present system uses 525 lines.
This number of lines produces the maximum number of elements that can be transmitted in the four megacycle portion of the six megacycle television channel allocated to the picture signal. The reader may wonder whether or not television systems have been built which use more than 525 lines. The answer is yes.
In this country and in Europe, television pictures with as many as 1000 lines have been produced. The cost of constructing receivers and transmitters to handle the additional band width required does not result in a proportional improvement in picture quality. It is unlikely that the 525 line standard will be superseded by any other in the near future.
1-23 Allocation of the Television Frequencies. After the width of the standard television channel had been fixed at six megacycles, the problem of finding a sufficient amount of space in the frequency spectrum arose. The space occupied by a television channel can be appreciated by comparing the AM broadcast band with a single television channel as shown in Figure 13.
Notice that there are 106, ten kilocycle, channels in the AM band which extends from 540 to 1600 kilocycles. And yet all 106 stations occupy less frequency space than a single television station.
THE AM BROADCAST BAND CONSISTING OF 106 CHANNELS EACH 10-kHZ WIDE. 54 TO 1600 khz
6 mhz TELEVISION CHANNEL
Figure 12. The space occupied in the frequency spectrum by all the AM stations
is less than that allotted to a single television station.
The FCC has therefore allocated a much higher frequency range to television stations than that used for radio broadcasting so that there maybe a reasonable number of channels available. These allocations are split into two bands; one extending from 50 to 72 mhz, and the other from 174 to 216 mhz. Twelve channels, each six megacycles wide, are available in these two bands. (The FCC originally assigned 13 channels to television in the bands from 44 to 72 mhz. and 174 to 216 mhz. However, later studies indicated that interference effects which appeared in the television picture were caused by signals from other services In the frequency spectrum. These signals combine with the television signal and produce a beat frequency that falls in the frequency range for which most intermediate frequency amplifiers of television receiver s are tuned. To meet the demands of other services for more frequency space and to solve the interference problem, the FCC took channel one (44-50 mhz.) away from television. This move will enable receiver manufacturers to design the IF amplifiers to tune to an IF carrier frequency of about 41 mhz., which frequency tests have indicated will be less troubled by signal interference from other services.) The future of television has been carefully planned by the FCC in order to minimize signal interferences between channels.
Channel assignments are staggered so that adjacent channels will not be used in the same area. This allows the use of a max mum of seven channels in any one area of the country. The FCC has determined the number of channels which will be available in the various areas of the country and the frequencies which will be used in each area. The reader may refer to Section 12 and readily ascertain how many television stations he will eventually be able to receive in his area. In addition to the stations assigned in the television band below 300 mhz., space has been allocated in the region from 475 to 890 mhz. to allow for further expansion.
Now that we know what to expect of the electronic television system in picture detail and number of commercial stations, let us begin the study of modern television techniques.