1. Distant; at a distance; over a distance
2. Telegraph; television; telecommunication
1. A communication that transmits images of objects (stationary or moving) between distant points; SYN. television system.
2. Broadcasting visual images of stationary or moving objects; SYN. telecasting, TV, video.
(TV) Reproduction at a distance by radio waves of visual images. For transmission, a television camera converts the pattern of light it takes in into a pattern of electrical charges. This is scanned line by line by a beam of electrons from an electron gun, resulting in variable electrical signals that represent the visual picture. These vision signals are combined with a radio carrier wave and broadcast as magnetic waves. The TV antenna picks up the wave and feeds it to the receiver (TV set). This separates out the vision signals, which pass to a cathode-ray tube. The vision signals control the strength of a beam of electrons from an electron gun, aimed at the screen and making it glow more or less brightly. At the same time the beam is made to scan across the screen line by line, mirroring the action of the gun in the TV camera. The result is a recreation of the pattern of light that entered the camera. Thirty pictures are built up each second with interlaced scanning in North America (25 in Europe), with a
total of 525 lines in North America and Japan (625 lines in Europe).
In addition to transmissions received by all viewers, the 1970s and 1980s saw the growth of pay-television cable networks, which are received only by subscribers, and of devices, such as those used in the Qube system in the US, which allow the viewers' opinions to be transmitted instantaneously to the studio via a response button, so that, for example, a home viewing audience can vote in a talent competition. The number of program channels continues to increase, following the introduction of satellite-beamed TV signals.
Further use of TV sets has been brought about by videotext and the use of videocassette recorders to tape programs for playback later or to play prerecorded videocassettes, and by their use as computer screens and for security systems. Extended-definition television gives a clear enlargement from a microscopic camera and was first used 1989 in neurosurgery to enable medical students to watch brain operations.
In 1873 it was realized that, since the electrical properties of the nonmetallic chemical element selenium vary according to the amount of light to which it is exposed, light could be converted into electrical impulses, making it possible to transmit such impulses over a distance and then reconvert them into light. The chief difficulty was seen to be the “splitting of the picture” so that the infinite variety of light and shade values might be transmitted and reproduced. In 1908 it was found that cathode-ray tubes would best effect transmission and reception. Mechanical devices were used at the first practical demonstration of television, given by J L Baird in London Jan 27, 1926, and cathode-ray tubes were used experimentally in the UK from 1934.
Baird gave a demonstration of color TV in London 1928, but it was not until Dec 1953 that the first successful system was adopted for broadcasting, in the US. This is called the NTSC system, since it was developed by the National Television System Committee, and variations of it have been developed in Europe; for example, SECAM (sequential and memory) in France and PAL (phase alternation by line) in West Germany. The three differ only in the way color signals are prepared for transmission. When there was no agreement on a universal European system 1964, in 1967 the UK, West Germany, the Netherlands, and Switzerland adopted PAL while France and the USSR adopted SECAM. In 1989 the European Community (now the European Union) agreed to harmonize TV channels from 1991, allowing any station to show programs anywhere in the EC.
The method of color reproduction is related to that used in color photography and printing. It uses the principle that any colors can be made by mixing the primary colors red, green, and blue in appropriate proportions. In color television the receiver reproduces only three basic colors: red, green, and blue. The effect of yellow, for example, is reproduced by combining equal amounts of red and green light, while white is formed by a mixture of all three basic colors.
Signals indicate the amounts of red, green, and blue light to be generated at the receiver.
To transmit each of these three signals in the same way as the single brightness signal in black and white television would need three times the normal band width and reduce the number of possible stations and programs to one-third of that possible with monochrome television. The three signals are therefore coded into one complex signal, which is transmitted as a more or less normal black and white signal and produces a satisfactory —or compatible—picture on black and white receivers. A fraction of each primary red, green, and blue signal is added together to produce the normal brightness, or luminance, signal. The minimum of extra coloring information is then sent by a special subcarrier signal, which is superimposed on the brightness signal. This extra coloring information corresponds to the hue and saturation of the transmitted color, but without any of the fine detail of the picture. The impression of sharpness is conveyed only by the brightness signal, the coloring being added as a broad color wash. The
various color systems differ only in the way in which the coloring information is sent on the subcarrier signal.
The color receiver has to amplify the complex signal and decode it back to the basic red, green, and blue signals; these primary signals are then applied to a color cathode-ray tube. The color display tube is the heart of any color receiver. Many designs of color picture tubes have been invented; the most successful of these is known as the “shadow mask tube”. It operates on similar electronic principles to the black and white television picture tube, but the screen is composed of a fine mosaic of over one million dots arranged in an orderly fashion. One-third of the dots glow red when bombarded by electrons, one-third glow green, and one-third blue. There are three sources of electrons, respectively modulated by the red, green, and blue signals. The tube is arranged so that the shadow mask allows only the red signals to hit red dots, the green signals to hit green dots, and the blue signals to hit blue dots. The glowing dots are so small that from a normal viewing distance the colors merge into one another a
nd a picture with a full range of colors is seen.
High-definition television (HDTV) offers a significantly greater number of scanning lines, and therefore a clearer picture, than the 525/625 lines of established television systems. In 1989 the Japanese broadcasting station NHK and a consortium of manufacturers launched the Hi-Vision HDTV system, with 1,125 lines and a wide-screen format. The Eureka research project has gathered together 30 European electronics companies, research laboratories, and broadcasting authorities to provide a common 1,250-line system for Europe by 1993.
Digital television is a system of transmitting television programs in digital codes. Until the late 1980s it was considered impossible to convert a TV signal into digital code because of the sheer amount of information needed to represent a visual image. However, data compression techniques have been developed to reduce the number of bits that need to be transmitted each second. As a result, digital technology is being developed that will offer sharper pictures on wider screens, and HDTV with image quality comparable to a cinema. A fully digital system has been demonstrated in the US. A common world standard for digital TV, the MPEG–2, was agreed in April 1993 at a meeting of engineers representing manufacturers and broadcasters from 18 countries.
TV technology was pioneered in the US by David Sarnoff and Lee De Forest and sets became available in the 1930s, but few performances were televised until the late 1940s, when local and network shows were scheduled in major cities and, by coaxial cable, across the nation. Live performances gave way to videotaped shows by the late 1950s, and color sets became popular from the 1960s.