Saturday, October 8, 2011

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.