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 These were soon superseded, however, by electronic scanning methods; a television system employing electronic scanning was patented by V. K. Zworykin in 1928.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.
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.
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.
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 in the United States 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 broadcasting.
The next great advance in television will be the adoption of a high-definition television (HDTV) system. Non-experimental analog HDTV broadcasting began in Japan in 1991. In 1994 the FCC approved a U.S. standard for an all-digital system, to be used by all commercial broadcast stations by mid-2002. Although it was hoped that the transition to digital broadcasting would be largely completed by 2006, less than a third of all stations had begun transmitting digital signals by the mid-2002 deadline.
The most noticeable difference between the current system and the HDTV system is the aspect ratio of the picture. While the ratio of the width of a current TV picture to its height is 4:3, the HDTV system has a ratio of 16:9, about the same as the screen used in a typical motion-picture theater. HDTV also provides higher picture resolution and high quality audio. Each frame of video consists of 720 or 1,125 horizontally scanned lines instead of the current 525. Furthermore, the lines are scanned sequentially, not interlaced as they are now.
The wide availability of television has raised concerns about the amount of time children spend watching TV, as well as the increasingly violent and graphic sexual content of TV programming. Starting in 1999 the FCC required TV set manufacturers to install "V-Chip" technology that allows parents to block the viewing of specific programs; that same year the television industry adopted a voluntary ratings system to indicate the content of each program.
Various interactive television systems have been tested or proposed. An interactive system could be used for instant public-opinion polls or for home shopping. Many cable television systems use an interactive system for instant ordering of "pay-per-view" programming. Others systems poll their subscribers' equipment to compile information on program preferences. Several competing commercial systems have connected televisions to the Internet.
See D. G. Fink and D. M. Lutyens, The Physics of Television (1960); M. S. Kiver, Television Simplified (7th ed. 1973); R. Armes, On Video (1988); K. B. Benson and J. C. Whitaker, Television and Audio Handbook (1990); K. B. Benson, Television Engineering Handbook (1992); D. E. Fisher and M. J. Fisher, Tube (1996).
During the 1980s and early 90s, the growing number of cable networks, improved programming, increased channel capacity (which reached 150 in some systems by 1992), and greater freedom in terms of programming content greatly expanded the industry. There are 10,828 operating cable systems in the United States serving 28,798 communities and 62 million subscribers; this comprises about 64% of all households. Viewers pay a monthly fee for a package of cable television programming, known as basic cable, and additional monthly fees for networks such as HBO, which are known as pay TV services. Cable television offers a wide variety of specialized programming, including channels devoted to specific interests, such as news, sports, movies, business information, weather, cooking, home shopping, and family viewing. It can also transmit programs from foreign cities, such as the proceedings of the British House of Commons. The industry finances its programming from subscriber fees and advertising revenue. New technologies, such as fiber optics, digital compression, and interactive television, allow cable operators to offer more programming choices and services. The cable lines installed by cable operators are also to use to provide broadband Internet access to the homes of subscribers.
See G. Mair, Inside HBO (1984); T. Baldwin, Cable Communications (1988).
Electronic system for transmitting still or moving images and sound to receivers that project a view of the images on a picture tube or screen and recreate the sound. Early versions (1900–20) of the cathode-ray (picture) tube, methods of amplifying an electronic signal, and theoretical formulation of the electronic-scanning principle later became the basis of modern TV. RCA demonstrated the first all-electronic TV in 1932. Cable TV systems (introduced in the late 1940s), colour TV (in the 1950s), and recording or playback machines (in the 1980s; see VCR) followed. Digital high-definition (HDTV) systems (1990s) provide sharper, clearer pictures and sound with little interference or other imperfections and have the potential to merge TV functions with those of computers.
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Broadcast serial drama, characterized by a permanent cast of actors, a continuing story, tangled interpersonal situations, and a melodramatic or sentimental style. Its name derived from the soap and detergent manufacturers who originally often sponsored such programs on radio. Soap operas began in the early 1930s as 15-minute radio episodes and continued on television from the early 1950s as 30-minute and later hour-long episodes. Usually broadcast during the day and aimed at housewives, they initially focused on middle-class family life, but by the 1970s their content had expanded to include a wider variety of characters and situations and a greater degree of sexual explicitness. In the 1980s similar series began to be aired in prime-time evening hours (e.g., Dallas and Dynasty). Seealso Carlton E. Morse; Irna Phillips.
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Private, nonprofit U.S. corporation of public television stations. PBS provides its member stations, which are supported by public funds and private contributions rather than by commercials, with educational, cultural, news, and children's programs that are produced by its members and by independent producers worldwide. Its popular programs have included Sesame Street, Masterpiece Theatre, Great Performances, NewsHour with Jim Lehrer, and Nova. PBS was founded in 1969 to coordinate and provide services to its member stations, which now number about 350. Funding is provided mainly by viewers' contributions, state governments, and grants from businesses and private foundations; the U.S. government, through the Corporation for Public Broadcasting, supplies about 15percnt.
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System that distributes television signals by means of coaxial or fibre-optic cables. Cable television systems originated in the U.S. in the late 1940s to improve reception in remote and hilly areas, where broadcast signals were weak. In the 1960s they were introduced in large metropolitan areas where reception is sometimes degraded by reflection of signals from tall buildings. Since the mid-1970s there has been a proliferation of cable systems that offer special services and which generally charge a monthly fee. Besides providing high-quality signals, some systems can deliver hundreds of channels. Another feature increasingly offered by cable operators is two-way, interactive communication by which viewers can, for example, participate in public-opinion polls as well as connect to the Internet. Cable operators are also involved in the development of video compression, digital transmission, and high-definition television.
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