Television

Reproduction of visual images at a distance using radio waves. 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 picture. These signals are combined with a radio carrier wave and broadcast as electromagnetic waves. The receiving ‘aerial’ (antenna) or satellite dish picks up the wave and feeds it to the receiver (TV set). This separates out the vision signals, which pass to the display device. This is usually a cathode-ray tube) in which a beam of electrons is made to scan across the screen line by line, mirroring the action of the electron 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).

Receiving antennas Because the wavelength of terrestrial-broadcast television signals is short, a resonant antenna becomes possible, and this usually consists of a half-wave antenna made of light alloy or steel tube, fed at the centre with low-impedance coaxial or balanced cable. Greater gain is obtained if a reflector element is added, and quite complicated arrays are used in areas of weak signal strength.

These antennas are mounted either vertically or horizontally to conform with the polarization of the transmitting antennas. Tubing is used for the elements, since an antenna made of wire would be too sharply resonant, with resulting loss of bandwidth, and therefore poor picture definition.

Signals from TV satellites are collected by a fixed ‘dish’ pointed toward the satellite, which is in geostationary orbit.

Television channels 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 systems that allow the viewers' opinions to be transmitted instantaneously to the studio, so that, for example, a home viewing audience can vote in a talent competition. The number of programme channels continues to increase, following the introduction of satellite-beamed TV signals and digital television.

Further use of TV sets has been brought about by videotext and the use of video 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.

History In 1873 it was realized that, since the electrical properties of the non-metallic 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 John Logie Baird in London on January 27, 1926, and cathode-ray tubes were used experimentally in the UK from 1934.

The world's first public television service was started from the BBC station at Alexandra Palace in North London, on November 2, 1936. In the USA, TV technology was pioneered by David Sarnoff and Philo Taylor Farnsworth (1906–1971) 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.

Color television Baird gave a demonstration of color TV in London in 1928, but it was not until December 1953 that the first successful system was adopted for broadcasting, in the USA. 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 in 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 uses the principle that any colors can be made by mixing the primary colors red, green, and blue in appropriate proportions. (This is different from the mixing of paints, where the primary colors are red, yellow, and blue.) 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 1 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 and 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 gathered together 30 European electronics companies, research laboratories, and broadcasting authorities to provide a common 1,250-line system for Europe by 1993. The first high-definition television sets in the USA went on sale in August 1998 in San Diego, California.

Digital television (DTV) is a system of producing, transmitting, storing, and displaying television programmes in digital codes. Until the late 1980s it was considered impossible to convert a TV signal into digital code because of the amount of information needed to represent a visual image. However, the development of data compression techniques made it possible to develop a digital technology in the 1990s that offered sharper and wider pictures with superior image quality. A common world standard for DTV, the MPEG-2, was agreed in April 1993 at a meeting of engineers representing manufacturers and broadcasters from 18 countries.

The advent of DTV constitutes a revolutionary overhaul of television, making a wider variety of programming available and improving the quality of image and sound. It is expected that DTV will have superseded the analog television system by 2006. By this time, industry experts hope, ‘multiplexing’ (where more than one television program can share the same bandwidth) of both HDTV and SDTV (standard definition) programs will have been widely adopted, as will the broadcast of nontelevision data, such as Web content and stock-market information.