Television - transmission and reception of still or moving images by means of electrical signals, especially by means of electromagnetic radiation using the techniques of radio and by fiberoptic and coaxial cables. Television has become a major industry, especially in the industrialized nations, and a major medium of communication and source of home entertainment. Television is put to varied use in industry, e.g., for surveillance in places inaccessible to or dangerous for human beings; in science, e.g., in tissue microscopy (see microscope); and in education.

Evolution of the Scanning Process

The idea of "seeing by telegraph" engrossed many inventors after the discovery in 1873 of variation in the electrical conductivity of selenium when exposed to light. Selenium cells were used in early television devices; the results were unsatisfactory, however, chiefly because the response of selenium to light-intensity variations was not rapid enough. Moreover, until the development of the electron tube there was no way of sufficiently amplifying the weak output signals. These limitations precluded the success of a television method for which Paul Nipkow in Germany received (1884) a patent.

His system employed a selenium photocell and a scanning disk; it embodied the essential features of later successful devices. A scanning disk has a single row of holes arranged so that they spiral inward toward the center from a point near the edge. The disk revolves in front of a light-sensitive plate on which a lens forms an image; each hole passes across, or "scans," a narrow, ring-shaped sector of the image. Thus the holes trace contiguous concentric sectors, so that in one revolution of the disk the entire image is scanned. When the light-sensitive cell is connected in an electric circuit, the variations in light cause corresponding fluctuations in the electric current. The image can be reproduced by a receiver whose luminous area is scanned by a similar disk synchronized with the disk of the transmitter.

Although selenium cells proved inadequate, the development of the phototube (see photoelectric cell) made the mechanical disk-scanning method practicable. In 1926, J. L. Baird in England and C. F. Jenkins in the United States successfully demonstrated television systems using mechanical scanning disks. While research remained at producing pictures made up of 60 to 100 scanned lines, mechanical systems were competitive. These were soon superseded, however, by electronic scanning methods; a television system employing electronic scanning was patented by V. K. Zworykin in 1928. The 1930s saw the laboratory perfection of television equipment that began to reach the market in 1945 after World War II.

The modern scanning process, which is the essence of television accomplishment, operates as do the eyes in reading a page of printed matter, i.e., line by line. A complex circuit of horizontal and vertical deflection coils controls this movement and causes the electronic beam to scan the back of a mosaic of photoelectric cells in a 525-line zigzag 30 times each second. (The 525-line 30-frame-per-second system is used in the United States, Japan, and elsewhere; many other countries use similar but incompatible systems.) Because of persistence of vision only about 30 pictures need be transmitted each second to give the effect of motion. The development of interlaced scanning results in alternate lines being scanned each 1/60 sec, the remaining lines being covered in the next 1/60 sec.

Development of the Television Camera and Receiver

V. K. Zworykin's iconoscope (1923) was the first successful camera tube in wide use. Its functioning involved many fundamental principles common to all television image pickup devices. The face of the iconoscope consisted of a thin sheet of mica upon which thousands of microscopic globules of a photosensitive silver-cesium compound had been deposited. Backed with a metallic conductor, this expanse of mica became a mosaic of tiny photoelectric cells and capacitors. The differing light intensities of various points of a scene caused the cells of the mosaic to emit varying quantities of electrons, leaving the cells with positive charges proportionate to the number of electrons lost. An electron gun, or "scanner," passed its beam across the cells. As it did so, the charge was released, causing an electrical signal to appear on the back of the mosaic, which was connected externally to an amplifier. The strength of the signal was proportional to the amount of charge released. The iconoscope provided good resolution, but required very high light levels and needed constant manual correction.

The orthicon and image-orthicon camera tubes improved on the iconoscope. They used light-sensitive granules deposited on an insulator and low-velocity scanning. These could be used with lower light levels than required by the iconoscope, and did not require the constant manual manipulation. The vidicon was the first successful television camera tube to use a photoconductive surface to derive a video signal.

Solid state imaging devices were first demonstrated in the 1960s. Today's solid-state television cameras use semiconductor charge-coupled devices or CCDs. Each element in a CCD stores a charge that is determined by the illumination incident on it. At the end of the exposure interval, the charge is transferred to a storage register and the CCD is freed up for the next exposure. The charges in the storage register are transferred to the output stage serially during that time. Although almost all consumer video cameras and camcorders use CCD imagers, camera tubes are still common in professional applications.

In the television receiver, the original image is reconstructed essentially by reversing the operation of the video camera. The final image is typically displayed on the face of a cathode-ray tube, where an electron beam scans the fluorescent face, called the "screen," line for line with the pickup scanning. The fluorescent deposit on the tube's inside face glows when hit by the electrons, and the visual image is reproduced. Liquid crystal displays have also been used, mainly on small, portable sets; they are also finding increasing use as light valves on large-screen projectors. Although LCD technology is advancing rapidly, video projectors that use electron tubes can still produce better pictures. Other devices in the receiver extract the crucial synchronization information from the signal and demodulate (separate the information signal from the carrier wave) it.

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