Television

 

Development of Color Television

Several systems of color television have been developed. In the first color system approved by the Federal Communications Commission (FCC), a motor-driven disk with segments in three primary colors—red, blue, and green—rotated behind the camera lens, filtering the light from the subject so that the colors could pass through in succession. The receiving unit of this system formed monochrome (black-and-white) images through the usual cathode-ray tube, but a color wheel, identical with that affixed to the camera and synchronized with it, transformed the images back to their original appearance. This method is said to be "field-sequential" because the monochrome image is "painted" first in one color, then another, and finally in the third, in rapid enough succession so that the individual colors are blended by the retentive capacities of the eye, giving the viewer the impression of a full colored image. This system, developed by the Columbia Broadcasting System (CBS), was established in 1950 as standard for the United States by the FCC. However, it was not "compatible," i.e., from the same signal a good picture could not be obtained on standard black-and-white sets, so it found scant public acceptance.
Another system, a simultaneous compatible system, was developed by the Radio Corporation of America (RCA). In 1953 the FCC reversed its 1950 ruling and revised the standards for acceptable color television systems. The RCA system met the new standards (the CBS system did not) and was well received by the public. This system is based on an "element-sequential" system. Light from the subject is broken up into its three color components, which are simultaneously scanned by three pickups. However, the signals corresponding to the red, green, and blue portions of the scanned elements are combined electronically so that the required 4.1-MHz bandwidth can be used. In the receiver the three color signals are separated for display. The elements, or dots, on the picture tube screen are each subdivided into areas of red, green, and blue phosphor. Beams from three electron guns, modulated by the three color signals, scan the elements together in such a way that the beam from the gun using a given color signal strikes the phosphor of the same color. Provision is made electronically for forming proper gray tones in black-and-white receivers. The FCC allowed stereo audio for television in 1984.

Broadcast, Cable, and Satellite Television Transmission

Television programs may be transmitted either "live" or from a recording. The principle means of recording television programs for future use is videotape recording. Videotape recording is similar to conventional tape recording (see tape recorder) except that, because of the wide frequency range—4.2 megahertz (MHz)—occupied by a video signal, the effective speed at which the tape passes the head is kept very high. The sound is recorded along with the video signal on the same tape.
When a television program is broadcast, the varying electrical signals are then amplified and used to modulate a carrier wave (see modulation); the modulated carrier is usually fed to an antenna, where it is converted to electromagnetic waves and broadcast over a large region. The waves are sensed by antennas connected to television receivers. The range of waves suitable for radio and television transmission is divided into channels, which are assigned to broadcast companies or services. In the United States the Federal Communications Commission (FCC) has assigned 12 television channels between 54 and 216 MHz in the very-high-frequency (VHF) range and 56 channels between 470 and 806 MHz in the ultra-high-frequency (UHF) range (see radio frequency).
Most television viewers no longer receive signals by using antennas; instead, they receive programming via cable television. Cable delivery of television started as a way to improve reception. A single, well-placed community antenna received the broadcast signals and distributed them over coaxial or fiber-optic cables to areas that otherwise would not be able to receive them. Today, cable television is popular because of the wide variety of programming it can deliver. Many systems now provide more than 100 channels of programming. Typically, a cable television company receives signals relayed from a communications satellite and sends those signals to its subscribers. The first transatlantic television broadcast was accomplished by such a satellite, called Telstar, on July 10, 1962. Some television viewers use small satellite dishes to receive signals directly from satellites. Most satellite-delivered signals are scrambled and require a special decoder to receive them clearly.
See also broadcast.
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