Television, the electrical transmission of moving images with accompanying sound, is designed to extend the senses of sight and hearing. The principles employed in television are based on human vision, that is, how the eye perceives the scene before it, including its structure, lights and shadows, details, and colors. GENERAL Television must deal with an important difference between the way in which radio communications are transmitted through space and the usual way in which humans receive visual images. The retina of the eye receives an image of the whole scene before it, and hundreds of thousands of fibers in the optic nerve transmit to the brain, individually and simultaneously, signals that together represent the whole scene. Human vision thus uses hundreds of thousands of "channels" at once. In television transmission by microwave or cable, by contrast, the entire content of the scene must be sent through a single channel (see \Tradio and television broadcasting\t). To accomplish this, the scene is broken down into many small pieces (called picture elements) that look like the half-tone dots used in printed pictures. In the television camera an electrical signal is formed to represent the brightness (and in color television, also the color) of each picture element. These signals are sent over the channel, one at a time, to the receiver. At the receiver the signals are transformed back into light, and the picture elements are assembled on the viewing screen in their proper relative positions. Persistence of Vision Essential to this process is visual memory: even though the picture elements are laid down on the screen one after the other, they all must be perceived at the same time. This requirement is met by persistence of vision, a property of the eye. When light entering the eye is shut off, the impression of light persists thereafter for about 0.1 second. Thus, if all the picture elements in the image are presented successively to the eye in a tenth of a second or less, the whole area of the screen appears to be illuminated, although in fact only one spot of light is present at any instant. In a television system a still picture is presented in less than a tenth of a second, so that a series of still pictures can be presented at a rate greater than ten pictures per second. Motion in the scene is represented, as in motion pictures (see \Tcinematography\t), by a series of still pictures, each differing slightly from those preceding and following it. Although ten still pictures per second is an adequate rate to convey the illusion of motion, for such motion to be depicted smoothly, a rate of at least 24 per second is necessary; this is the rate used in professional motion pictures. Scanning The process of breaking down the scene into picture elements and reassembling them on the screen of the television receiver is known as \Tscanning\t. It is similar to the eye's motion when a person reads a page of printed matter. In scanning, the scene as focused within the television camera is broken down into a series of horizontal lines. The camera "reads" the topmost line from left to right, producing a succession of electrical signals that corresponds to the lights and shadows along that line. It then passes back to the left end of the next line below and traverses it in the same way. In this fashion the camera reads the whole area of the scene, line by line, until the bottom of the picture is reached. Then the camera scans the next image, repeating the process continuously. In the United States, 30 (Europe, 25) slightly different pictures are scanned each second, line by line. The television camera produces a rapid succession of electrical impulses, called the video signal; these impulses correspond to the succession of picture elements scanned in every line of every image. At the television receiver this signal, which has been transmitted through space or by cable, is recovered and used to control the picture tube. The picture tube produces an image that is composed of horizontal lines precisely like those used in the camera. As the camera examines the topmost line, a spot of light produced by the picture tube moves across the screen and produces the topmost line of light on the screen. The video signal causes the spot of light to become brighter or darker as it moves, and thus the picture elements scanned by the camera are reproduced line by line at the receiver, until the whole area of the screen is covered, completing the image. Then the process is repeated. The scanning motions in the camera and those in the receiver must keep in precise step. Otherwise the picture elements would appear in the wrong positions on the receiver screen, and the pattern of the image would be distorted or broken up entirely. Interlaced Scanning To avoid flicker, each still picture is presented twice by a process known as interlaced scanning. After the topmost line is scanned, an empty line is formed immediately below it, and the next scanned line appears just below the empty space. As the scanning proceeds, therefore, alternate lines are scanned, with empty spaces between the scanned lines. This represents the first showing of the still picture. The next image also consists of spaced lines, and its lines fall precisely in the empty spaces of the preceding image, so the whole screen is filled by the two sets of interlaced scanning lines. Picture Detail For fine details to be visible in television images, each picture element must be quite small. Experience with 16mm motion pictures has shown that the image should contain at least 100,000 picture elements, preferably 150,000 or more. This figure in turn determines how many scanning lines are required to cover the screen. In the United States the scanning pattern contains 525 lines from the beginning of one picture to the beginning of the next, and each line contains about 435 picture elements. Not all of the lines are actively used, and some details straddle two adjacent scanning lines and thus take two lines to be fully represented. With these effects taken into account, the total number of picture elements into which the scene is divided is about 150,000 elements. In Europe and many other parts of the world 625 lines are used, and the image contains about 200,000 picture elements. Rate of Transmitting Television Information The area of the scene is scanned in 1/30 of a second, and during that time about 150,000 picture elements must be covered. This number corresponds to scanning at the extremely high rate of 4,500,000 picture elements per second. The actual rate is about 8,000,000 elements per second, because two adjacent lines are required to depict some picture elements and time is needed to transmit other signals that synchronize the scanning processes at the camera and picture tube. The Television Channel The transmission of the video signal at this fast a rate requires a wide channel in the radio spectrum. Each television channel in the United States occupies a frequency range of 6 MHz (1 MHz = 1 megahertz = 1 million cycles per second). This is 600 times as wide a band of frequencies as an AM sound broadcast station uses. The major portion of the channel is used to transmit the video signal, which occupies a band of about 4 MHz (because a single cycle can transmit 2 picture elements, a 4-MHz band can transmit 8,000,000 picture elements per second, as required). Other portions of the channel are required for an additional component portion of the video signal and for the accompanying sound transmission. The 6-MHz channel used in the United States is so wide that the spectrum has room for only 82 channels. They are assigned among cities and towns at sufficient geographic and frequency separations so that interference between channels does not occur (see \Tfrequency allocation\t). BLACK-AND-WHITE TELEVISION Television Cameras The principal elements of a typical black-and-white television camera are the lens, the camera tube, and the scanning and focusing coils. The lens (which is often of the zoom type, particularly in sports telecasting) focuses the scene on the front end of the camera tube. The tube that was most widely used in the late 1970s was the vidicon, which is an evacuated glass cylinder. At the front end of the tube is a flat glass plate, the inside of which is coated with a photosensitive material, a sulfur compound of antimony. (Another widely used tube, the plumbicon, is similar to the vidicon in operation but uses a compound of lead.) Underneath the antimony coating is a thin, transparent coating of metal. The electrical resistance of the antimony compound is lowered when light falls on it. The optical image from the lens falling on the antimony coating causes its resistance to change in proportion to the amount of light reaching it at each point on its surface; that is, a pattern of electrical resistance is formed that matches the pattern of light in the image. The metallic coating beneath the antimony coating is maintained at a positive voltage, causing each point on the antimony coating to assume a positive charge, with the amount of charge depending on the amount of light falling on that point. Thus a pattern of positive electric charge is built up, and the charge elements corresponding to the picture elements pass through the antimony to its rear surface, where they are stored. At the opposite end of the camera tube is a structure known as an electron gun. This forms a narrow \Telectron\t BEAM that travels down the tube and encounters the charge pattern on the rear of the antimony coating. The focusing coils are arranged to keep the electron beam narrow (that is, sharply focused) so that the beam that strikes the coating has the size and shape of the picture element. The scanning coils are arranged to move the electron beam over the stored charge image in the interlaced scanning pattern previously described. In this manner, the electron beam reads the stored charge image, line by line. By the time the scanning of the image is completed, the charge pattern has been replenished and the next image is scanned. The electrons have negative electrical charge. When they hit a point on the antimony coating, they neutralize the positive charge stored at that point. This sudden change in charge is registered as a change in voltage on the metallic coating, which is connected to the camera terminal. As the electron beam scans the charge image, it thus produces a succession of voltage changes at this terminal, which constitute the video signal. The video signal at the camera terminal is weak, so it is amplified at once within the camera housing. After further processing, the amplified video signal is transmitted to the receiver, where it reaches the picture tube and re-creates the image. Picture Tube At its narrow end, a typical picture tube used in black-and-white receivers contains an electron gun similar to that in the camera tube. Within the gun is a control electrode, to which the video signal is connected. The control electrode strengthens and weakens the electron beam in order to re-create the brighter and darker portions of the image, respectively. For the electron beam to operate in this fashion, the tube must have a high vacuum, and it must be capable of withstanding the pressure of the outside air. The electron gun, with a focusing coil, keeps the electron beam narrow, so that when it strikes the wide end of the tube, it has the appropriate size and shape to produce a picture element. The scanning coils are at the base of the wide end of the tube. Rapidly reversing currents, produced by the receiver, are passed through these coils, producing magnetic fields that deflect the electron beam. One set of coils causes the beam to move from left to right and back again, while the other causes the beam to move from top to bottom and back. The horizontal (left-right-left) motion is rapid, occurring 15,750 times per second; the vertical (top-bottom-top) motion is much slower, occurring 60 times per second. The combined effect of the two motions, which are precisely synchronized, is that the electron beam moves over the wide end of the tube in two sets of spaced lines, which are interlaced as described above. After two such interlaced patterns have been produced (in 1/30 of a second), the full pattern of 525 lines has been formed. Inside the wide end of the tube is the viewing screen. It consists of two coatings. The first, deposited on the inner surface of the glass face, is a material known as a \Tphosphor\t. The second is a thin coating of aluminum on the phosphor. The phosphor is a complex compound, typically containing oxides of sulfur and zinc, that glows with a bluish white light when struck by high-energy electrons, the amount of light depending on the strength of the beam. To assure that the electrons in the beam have sufficient energy, a high voltage (typically 10,000 to 20,000 V) is applied to the picture tube between the electron gun and the aluminum coating. The positive high voltage at the screen end of the tube attracts the negative electrons; when these electrons hit the screen, they produce a spot of light of high (but adjustable) brilliance. To avoid the hazard of electric shock, the high-voltage supply of the receiver is designed to operate at a low current, a few thousandths of an ampere at most. Synchronization of Scanning To keep the camera scanning in step with the electron gun at the picture tube, horizontal and vertical synchronization signals are used; these signals are produced in equipment associated with the camera. The horizontal signals, impulses that occur 15,750 times per second, control the horizontal motion of the beam within the camera tube. The vertical signals, at 60 times per second, control the vertical motion. The two sets of signals are both derived from the same quartz-crystal generator, so that the two sets of lines fit precisely one within the other. The synchronization signals are sent over the air by the television station. Within the receiver the signals are recovered, the horizontal signals being used to control the horizontal scanning motions of the picture tube and the vertical signals controlling the vertical motions. Thus the scanning at each receiver proceeds in rigid step with the scanning at the camera. These timing signals are inserted in the video signal. The horizontal pulses occur between successive lines, the vertical ones just after the bottom of the picture is reached. Television Sound A separate signal is used to broadcast the sound portion of a television broadcast most commonly by means of \Tfrequency modulation\t, the same method used for \TFM radio\t. The microphone associated with the camera picks up the sound, producing a signal that is amplified and transmitted to the broadcast station, where it controls a separate transmitter. The signal transmitted has a frequency that varies with the sound pressure picked up by the microphone. Both picture and sound signals are broadcast over the same channel. The potential for high-quality sound that frequency modulation possesses is usually not realized in television because loudspeakers small enough to fit into television cabinets cannot reproduce the bass notes in proper proportion to the other registers. Transmission through Space For the video signal to be transmitted over the air, it must be carried by a broadcast signal (the carrier signal). The carrier signal is an alternating current of very high frequency. On channel 2, for example, the picture carrier frequency is 55.25 MHz (55,250,000 cycles per second). This signal is generated initially by a quartz-crystal oscillator at a lower frequency, which is then multiplied and amplified until it reaches a power level of many kilowatts. The video signal controls one of the amplifiers, changing its power output. The modulated carrier current is directed through the transmitting antenna, where it creates an electromagnetic wave that radiates through space (see \Telectromagnetic radiation\t). The antenna is designed to radiate waves in the horizontal direction toward the surrounding audience, with little or no power wasted in the upward direction. The amplitude of the radiated wave continually changes in response to the video signal it carries, more power being radiated during the dark portions of the picture, less power during the bright portions, with maximum power output during the synchronization pulses. When the carrier signal is modulated, two bands of signal frequencies are produced. These sidebands carry identical information. One of these, occupying a space of about 4 MHz, is transmitted in full, but only a portion of the other is radiated. This technique, called vestigial sideband transmission, saves a substantial amount of valuable space in the radio spectrum. The radio waves used in television broadcasting travel in essentially straight lines, are intercepted when they strike any massive object, and are greatly weakened when they encounter the horizon. To reach the largest possible number of viewers, therefore, the transmitting antenna must be located as high above the local terrain as possible. The primary service area of a television station thus seldom extends beyond 80 km (50 mi), although marginal reception is often possible at 160 km (100 mi) if a highly sensitive receiving antenna is used. Television Ghosts When the television signal is intercepted by a nearby structure, such as a building or water tower, it is reflected in all directions, including back toward the transmitter. A receiver located between the transmitter and the reflecting structure, then, receives two signals, one directly from the transmitter as intended, the other by reflection from the structure. The reflected signal, having traveled a greater distance, arrives later than the direct signal. Radio waves cover about 300 m (1,000 ft) in a millionth of a second. Hence, if the reflected path is longer than the direct path by, for example, 3 km (1.86 mi), the reflected signal arrives 10 millionths of a second later than the direct signal. As noted above, the scanning of a line takes about 60 millionths of a second, so during the scanning of each line, both the direct and the reflected signals produce images, the reflected signal producing a "ghost" of the intended image. (In this example, the ghost image would be to the right of the intended image by about one-sixth of the width of the picture). Conditions in which a reflected signal exists, called multipath reception, are common in built-up city areas having many tall buildings. To minimize the effect, the receiving antenna must be as high as possible and oriented so that it discriminates against the reflected signal. One method of avoiding reflections is to feed many receivers by coaxial cable from a single antenna (community antenna) located high above surrounding structures, where it is free from reflected signals. Receiving Antenna The typical outdoor receiving antenna is constructed of several parallel horizontal metal rods of different lengths spaced one behind the other. Such an array has directional properties, displaying maximum sensitivity on the line at right angles to the metal rods. For local reception, a less elaborate antenna suffices, such as the extendable telescoping rod provided in portable television receivers. If the distance to the transmitter is great or if for any other reason the received radio wave is weak, the image displays "snow." In the antenna rods random currents are produced by motions of free electrons, and these currents produce in the image the speckled effect known as noise or snow. By using an antenna having many metal members and orienting the antenna so that it is pointed directly at the station, the signal current can be made stronger than the noise current, and the snow effect can be reduced. Useful reception can be obtained from weak signals. At the typical limiting distance of reception of 80 km (30 mi), images can be picked up and recognized clearly through the snow when the power intercepted from the incoming radio wave is less than one-trillionth of a watt. Because the picture tube requires a power of about one watt to operate, the receiver must then amplify the signal power a trillion times. Television Receivers In a typical black-and-white television \Treceiver\t, the signal from the antenna is fed to the \Ttuner\t. Two channel selector switches--one for the VHF (very-high-frequency) channels 2-13 and the other for the UHF (ultra-high-frequency) channels 14-83--connect circuits that are tuned to the desired channels and, at the same time, discriminate against signals from undesired channels. These circuits also form part of an amplifier, which is designed to add as little snow to the signal as is feasible. The amplified signals from the desired channel are then passed to the mixer, which transposes all the signal frequencies in the channel to different values, which are called intermediate frequencies. The output of the tuner consists of all the signals in the desired channel, but the intermediate channel is fixed in the frequency band from 41 to 47 MHz, no matter what channel is tuned in. From the tuner the 41-47 MHz channel, with all picture and sound information present, is passed successively through several additional amplifiers (from two to four intermediate-frequency--or IF--amplifiers), which provide the major portion of the amplification in the receiver. Their amplification is automatically adjusted, being maximum on a weak signal and less on a strong signal. Thus far the receiver handles the signals in the channel just as they are received from the transmitter, except for the shift to intermediate frequencies and the amplification. The next stage is video detector, which removes the high-frequency carrier signal and recovers the video signal. The detector also reproduces (at a lower frequency) the sound carrier and its frequency variations. The sound signal is then separated from the picture signal and passes through a frequency detector, which recovers the audio signal, that is, the signal equivalent to the microphone signal at the transmitter. This signal is amplified further and fed to the \Tloudspeaker\t, where it re-creates the accompanying sound. The picture signal (video signal) issuing from the video detector is used for two purposes: to control the picture tube scanning and to control the strength of the electron beam in that tube. For the former purpose, a circuit is used that responds only to the upper portions of the video signal, the synchronization signals. Additional circuits separate the horizontal signals from the vertical ones. The horizontal scanning coils surrounding the picture tube receive the rapidly reversing currents previously mentioned. These are produced by the horizontal scanning generator, the timing of which, in turn, is controlled by the horizontal synchronization signals. A similar generator that produces the more slowly reversing currents for the vertical motion is controlled by the vertical synchronization signals. The video signal is also passed through an additional amplifier before being connected to the control electrode of the picture tube. It is connected in such a way that the portions of the video signal representing the darker picture elements reduce the strength of the electron beam and those portions representing the brighter picture elements increase the beam strength. The synchronizing signals of the video signal are also present at this point, but because this signal is of a lower amplitude than the signals representing black, they have no effect on the image. In fact, the electron beam is extinguished entirely after it has completed scanning a line, and the beam returns to the beginning of the next line while extinguished. A similar extinction occurs while the beam moves from bottom to top of the image. Receiver Controls Various controls are available to the viewer. Channel switches select the desired channel, and a fine-tuning control adjusts the tuner to be centered on the desired channel. (In some receivers this function is performed by an automatic frequency-adjusting circuit.) The contrast control adjusts the amplification of the video signal prior to its application to the electron gun of the picture tube. By advancing this control, the video signal range is increased; that is, the black portions are made blacker and the white portions whiter. The brightness control applies a steady voltage to the control electrode, thus setting the average value of screen brightness, on which the variations of brightness caused by the video signal are imposed. Advancing this control brightens the picture as a whole. In some receivers, particularly color sets, the average value of brightness is maintained at its proper value automatically, and the screen fades to black during the intervals between scenes. The horizontal and vertical scanning controls adjust the timing of the respective scanning current generators so that their timing is precisely controlled by the respective synchronization signals. If the vertical-hold control is misadjusted, the picture as a whole rolls upward or downward on the screen. Misadjustment of the horizontal-hold control causes the image to break up into horizontal strips. Other controls, intended for servicing the receiver, are the height and width controls, which adjust the values of the scanning currents and thus the vertical and horizontal dimensions of the image. Linearity controls are used to keep the scanning motions across and down the screen precisely uniform. COLOR TELEVISION Color television employs the basic principles of black-and-white television. The essential difference is that a color broadcast is in reality three telecasts in one. The screen of a color receiver actually displays three images superimposed on each other; these images present, respectively, the red, green, and blue components of the colors in the scene. This use of three primary colors in the television follows the method used in color photography and color printing, in which three layers of colored dyes (in photography) or three interspersed sets of fine colored dots (in printing) give to the eye the impression of all the natural colors. Primary Color Images Color television achieves reproduction of the wide range of natural colors by adjusting the relative brightness of the red, green, and blue images. If two images are suppressed (for example, red and green), only the remaining color (blue) is seen. If one image is suppressed (for example, blue), the other two (green and red) can cover the range of colors from green to red, including the intermediate colors orange and yellow, by making the green image brighter or dimmer than the red one. When all three colors are present in the proper proportions, white light is produced. By adjusting portions of the scene to be brighter than the others, the whole range of grays from black to white can be produced. Finally, by allowing one or two of the three colors to predominate, the white light can be given the tint of the stronger colors, and thus pastel shades of all the natural colors can be reproduced (see \Tcolor perception\t). Color-Picture Tubes When the screen of a color receiver is viewed through a powerful magnifying glass, it is seen to consist of many tiny dots (or line segments in the more recent tubes) that glow with red, green, and blue light. At the rear of the picture tube is the electron gun, which produces three separate beams of electrons. These three beams hit the colored dots, and the tube is so designed that each beam can hit dots of only one color; a mask prevents each beam from striking the others. Because the colored dots are so small that they cannot be seen separately by the viewer, the effect is three superimposed images in the primary colors. By adjusting the strength of the respective beams of electrons, the relative brightness of the image produced by each can be changed. Such changes can be made to occur not only over the screen as a whole but individually in the separate parts of the scene. Color-Television Cameras The three electrical signals that control the respective beams in the picture tube are produced in the color-television camera by three camera tubes (typically vidicons). The camera has only a single lens, behind which a set of mirrors produces three identical images of the scene, which are focused on the three camera tubes. In front of each camera tube is a color filter; the filters pass respectively only the red, green, or blue components of the light in the scene viewed by the camera. Thus one camera tube forms a signal representative of the red image, the next forms a signal representative of the blue, and the third conveys an image in green. The three signals produced by the camera are transmitted to the respective electron guns in the picture tube, where they re-create the scene. Connecting the Camera to the Picture Tube One way of connecting the camera to the picture tube, actually used in some cases within the studio, is to use three separate cables, one for each of the primary color signals. To broadcast color programs by this method, however, would require each station to use three channels. The number of channels available for television is so limited that this method would be wholly impractical. Moreover, if a black-and-white receiver were to tune in on such a color telecast, it could receive only one of the three channels, and the tonal values reproduced would be unnatural because they would be confined to a monochrome (single-color) rendition of the scene. An alternative approach devised in the late 1940s used electronic circuits to recast the red, green, and blue signals from the color camera into two quite different signals, which can be accommodated in a single channel and which produce correct monochrome rendition of colorcasts (broadcasts in color) received on black-and-white sets. These are known as the luminance signal and the chrominance signal. Luminance Signal The luminance (brightness) signal is produced by adding together, electronically, the three signals from the color camera, in the approximate ratios 30% red, 60% green, and 10% blue. This combination produces white light on the screen of the color receiver. During a color telecast, this is what is seen when the color-intensity control of the receiver is turned off. Moreover this combined white-light signal is just what a black-and-white receiver is designed to receive from a black-and-white broadcast, so the receiver gives a correct monochrome rendition of the color broadcast. The luminance signal is broadcast by the station, in fact, as if the scene were picked up by a monochrome camera. It carries the whole content of the scene, including its finest details, but in tones of black and white only. Chrominance Signal The chrominance (color) signal is also derived from the three color signals produced by the camera, but by more elaborate electronic circuits. In effect, these circuits produce a signal that represents the difference between the luminance signal and the individual color signals from the camera. The chrominance signal is designed to be virtually invisible when received on a black-and-white set, but it carries the missing color information to color receivers, which have electronic circuits that reconstruct the red, green, and blue signals required by the picture tube. The chrominance signal is broadcast by the color broadcast station by a separate signal (called the chrominance subcarrier), which is modulated in two ways. By phase modulation (a variant of the frequency-modulation system used in high-fidelity-sound broadcasting), it conveys variations in the hue (the redness, greenness, blueness, and so on) of the colors of each area of the picture screen. The chrominance signal is also subject to amplitude modulation (the system used in AM sound broadcasting), by which it conveys variations in the saturation of the colors (their vivid versus pastel character). These aspects may be controlled to suit the viewer's taste. Separation of Luminance and Chrominance Signals Because the luminance and chrominance signals are transmitted simultaneously within the broadcast channel, special means must be taken to prevent one signal from interfering with the other. This is done by a technique known as frequency interleaving. The luminance signal does not occupy the entire channel; rather, frequencies of high-signal intensity are separated by gaps of low-signal intensity. The chrominance signal spectrum also has regions of high and low intensity. The chrominance signal is shifted slightly in frequency, so that its regions of high intensity fall precisely within the gaps of low intensity on the luminance signal. Thus both can be broadcast on the same channel, without interfering with each other. The result is that the three separate color signals required for color reproduction, after conversion into the luminance and chrominance signals, are broadcast over the channel that was originally designed to accommodate only the single signals needed for black-and-white broadcasting. APPLICATIONS OF TELEVISION Television broadcasting is the largest but by no means the only application of video technology (see \Tradio and television broadcasting\t). For example, television systems are used to bring live telecasts, particularly of sporting events, to audiences in theaters. In one type of projector used in theaters, the scanning pattern is produced by an electron beam on a thin film of oil, which serves as a light-valve through which light from a powerful lamp is passed to the screen. Three such projectors, superimposed, are used. Similar systems are used to bring specialized programs to business audiences and to pupils in classrooms. Specialized video systems are used to monitor industrial processes such as the handling of nuclear wastes and the operation of automated production systems in factories. Hospitals similarly use television to monitor patients in wards and to survey medical procedures in operating rooms and laboratories. Business systems, such as AT&T's Picturephone Meeting Service, enable executives to hold long-distance conferences. Television is also widely used in surveillance, as in banks, stores, and military establishments, and television systems have been adapted for piloting aircraft and for guiding weapons. Television cameras are also important components on many satellites and probes used in \Tspace exploration\t and have provided close-up views of distant planets. The rapid growth of \Tcomputer\t technology has also greatly widened the application of television. Video display terminals are an integral part of many computers, and properly adapted home television sets in many areas can receive computer data-base information by means of various \Tteletext\t and \Tvideotext\t systems. Home sets may also be used to VIDEOTAPE programs or to play prerecorded \Lvideodisk\ls, and \Tvideo games\t can be displayed on home screens. HISTORY The first proposals for television were made long before the electronic techniques of the present day were developed. In the 1880s the first proposal to transmit images by scanning was made by W. E. Sawyer, an American, and by Maurice LeBlanc of France. Black-and-White Television A few years earlier, in 1873, the fact that the electrical resistance of selenium (later used in early versions of the vidicon camera tube) was lowered when the material was illuminated was discovered by Louis May, an English telegrapher. In 1884 the German Paul Nipkow invented a mechanical system of scanning an image through holes in a rotating disk, but not until 1926 did the Englishman John Logie Baird and the American Charles F. \TJenkins\t actually demonstrate the transmission of images in halftone using the Nipkow disk. The development of electronic methods began in 1897 when the German Ferdinand \TBraun\t produced the first \Tcathode-ray tube\t, the ancestor of the picture tube. In 1907 the Russian Boris Rosing suggested the use of Braun's tube to reproduce television images. Using a rotating mirror drum for scanning, Rosing actually transmitted crude geometrical shapes but was unable to reproduce halftone images. In 1908 the Scotsman A. A. Campbell-Swinton proposed that the image be stored in the form of electric charge in a camera tube and reproduced on a picture tube--the essential features of today's system. The crude techniques of that day did not permit the system to be realized in practice, and not until 1923 did the American Vladimir R. \TZworykin\t, who had studied in Russia under Boris Rosing, apply for a patent for a camera tube (iconoscope) that used a stored-charge image. Meanwhile, the techniques of signal amplification using vacuum tubes had advanced to the point that, wholly by electronic methods, signals from Zworykin's iconoscope could be transmitted by wire to a picture tube. The transistor, invented in 1948, later replaced the vacuum tube and in turn led to integrated circuits. By the early 1930s experimental broadcasts of black-and-white halftone images, composed of 343 scanning lines, were conducted by engineers of the Radio Corporation of America (now \TRCA\t). By 1935, English engineers had developed a broadcast system using 405 scanning lines. The first telecasts regularly scheduled for the public began in London in 1936 using this system. (A mechanical system using 240 lines was also broadcast but soon abandoned because the electronic system was clearly superior.) Experimental broadcasts using 441-line images began in New York that same year, but not until 1941 did the Federal Communications Commission authorize public broadcasting in the United States using 525-line images. This effort was held in abeyance during World War II. After the war black-and-white broadcasting developed rapidly in the United States, England, France, and Germany. One million receivers were in use in America by 1949, 10,000,000 by 1951, and more than 100,000,000 by 1975. Color Television The development of color television took place in the United States during the late 1940s. In 1938 the Frenchman George Valensi proposed the general principle of a color system that would serve black-and-white receivers. In 1930 an American, Frank Gray, invented the technique of frequency interleaving, but not until 1949 was the concept of transmitting a luminance signal and a chrominance signal interleaved in the channel first publicly proposed. Work on such a system had taken place in the laboratories of several U.S. companies. To coordinate this work, in 1950 the National Television System Committee (NTSC) was convened. The committee clearly recognized that the new color service would have to be compatible with the existing black-and-white service so that the color telecasts would be receivable by the 25 million black-and-white receivers then in use. A noncompatible color system had been used by CBS in 1951 but was abandoned after 5 months because of lack of public support. By 1953 the NTSC had drawn up the detailed specifications of what has since been known as the NTSC compatible color system. Authority to broadcast color programs using this system was given by the Federal Communications Commission in 1954. For the ensuing decade the public showed only limited interest in the color service, and color receivers were not purchased in substantial numbers until 1964. Minor variations have subsequently been introduced in the color-television systems serving Europe and rest of the world, but the basic principles are those of the NTSC system. Current Developments One of the more notable advances in television technology since the advent of color television is the imminent availability of digital television systems. Digital electronics, derived from the computer industry, has already been applied to sound recording and reproduction and is currently being developed for the home television market. Digital television sets will convert incoming broadcast signals into the form of digital pulses, enabling the visual and audio information to be processed more precisely before being reconverted to analog form for display on the screen. Such sets will eventually also be able to display several channels at once and to freeze a desired picture frame for close-up inspection, as well as providing clearer images and eliminating various reception problems such as double-image ghosts. Also in the research and development stage in the early 1980s are flat-screen television sets that could be hung on the wall. Very small flat-screen black-and-white sets employing \Tliquid crystal\t technology appeared in the early 1980s, and a hand-held color set of this nature was introduced in 1984. Donald G. Fink Bibliography: Easton, Anthony T., The Home Satellite TV Book (1981); Grantham, Donald J., Basic Radio and Television Systems (1979); Grob, Bernard, Basic Television, 4th ed. (1975); Hartman, Bernard, Fundamentals of Television: Theory and Practice (1975); Kiver, Milton S., and Kaufman, Milton, Television Simplified, 7th ed. (1973); McNitt, Jim, ed., The Home Video Sourcebook (1982); Sambul, Nathan J., The Handbook of Private Television (1982).