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color television[¦kəl·ər ¦tel·ə‚vizh·ən]
television designed to transmit images in color. Color television allows the viewer to perceive images more completely by transmitting the wealth of colors in the surrounding world.
The transmission of color images by television is based on the theory of three-component color vision. A variety of natural colors can be reproduced optically by means of three primary colors. In accordance with this principle, a color television camera uses three light filters—red, green, and blue—to create three monochromatic images on the light-sensitive targets of the television camera tube. The optical images are then converted into three linear video signals ER, EG, and EB, which are proportional to the red (R), green (G), and blue (B) components, respectively, of the color registered during the image-scanning process. Special methods are used to encode the color information in the television signal and transmit the signal via a communications channel.
In a color television receiver the video signals are extracted (by decoding) from the television signal; when fed to the receiver’s kinescope (picture tube), they control the brightness of the luminescence from the phosphors. Thus, in the most common three-color, three-beam kinescope with a shadow mask, the video signals are fed simultaneously to the control electrodes of three electron guns. As a result the current of the electron beams varies with the amplitude of the video signals.
The phosphors on the kinescope screen are usually applied in the form of a mosaic of small circles (phosphor dots) grouped in triads (Figure 1). Each dot luminesces with its particular color when an electron beam strikes it, producing red (Rr), green (Gr), or blue (Br). Because of the shielding effect of the mask, the beams only excite their own colors in the phosphor dot triads. Thus, each beam separately produces a red, green, or blue color on the screen, and together the beams produce an image having a color that depends on the ratio of the brightness of the red, green, and blue colors of the luminescence. By additive color synthesis, any color can be produced within the limits of the receiver’s primary color triangle on the chromaticity diagram (Figure 2). When necessary for correct color reproduction, a matrix color corrector is used in the transmission channel to convert the linear video signals to video signals for the receiver’s primary colors.
In addition to the linear matrix correction, the linear video signals ER, EG, and EB undergo a nonlinear correction (gamma correction) to compensate for the nonlinearities of the camera tube and receiver kinescope. As a result, nonlinear video signals , and are produced in accordance with the formulas
where γ is an index for the degree of modulation in the kinescope characteristic. The signals and each cover a broad band of frequencies up to 6 megahertz (MHz).
Signal formation and transmission. The video signals , and can be transmitted to a receiver sequentially or simultaneously. One color television system transmits the color fields sequentially, with a field frequency of 150 hertz. The system has a number of inherent drawbacks. The principal ones are high cost because of the need for a communications channel with a pass-band three times greater than the frequency band of the standard black-and-white system, color fringing during rapid motion of the subjects during transmission, and color breakup when the viewer glances from one point to another on the screen. Because of such disadvantages, the system is not used for television broadcasting; because of its simplicity, however, it does find some application, for example, to transmit images of hollow body organs.
Color television systems that transmit simultaneously usually require three standard television channels or one broadband channel with a passband of 3 × 6 = 18 MHz. A three-channel color television system using simultaneous transmission is therefore incompatible with the standard black-and-white system of television. Inasmuch as compatibility is one of the principal technological and economic requirements for color broadcasting systems, various methods are used to multiplex the spectrum of the transmitted signal (see LINE MULTIPLEXING) so that the television signal of one color program may have a frequency spectrum up to 6 MHz. One such method, which is used in all the standard color television systems, is to use special encoding matrices (Figure 3,a) to replace the broadband signals , and with the following signals: (1) a luminance signal , equal to and having a bandwidth of 6 MHz, which carries only information about the brightness distribution of the transmitted scene (the factors α = 0.30, β = 0.59, and δ = 0.11 have been determined by coloiimetric calculations); and (2) color-difference signals and , which contain information about the chrominance of the transmitted scene; they have bandwidths from 0.5 to 1.5 MHz and are transmitted on subcarrier frequencies located in the spectrum of the luminance signal.
The amplitude or frequency of the subcarrier frequency oscillations is modulated by the color-difference signals in the color encoder so that a chrominance signal Uc is produced. The signals and Uc, the sync pulses Us, and the color-sync signals Ucs are combined to form the total color television signal eT (Figure 3,b). When transmitting the reference white color (which is taken to be the radiation from a standard source D6500, where the subscript 6500 designates the color temperature in degrees K), the video signals supplied to the input of the color encoder satisfy the condition ; for the reference white color and .
Producing the color image in the receiver. In a color television receiver the total signal eT from the output of the video detector is fed to a decoder consisting of an electrical bandpass filter, detectors for the subcarrier frequency oscillations, and a decoding matrix. The bandpass filter extracts the signal Uc + Ucs from the signal eT, and the former is fed to the input of the subcarrier detectors; the color-difference signals and are derived from the outputs of the subcarrier detectors. From these signals and the luminance signal , the video signals of the primary colors , and are formed for the receiver and are fed to the three-beam kinescope. In some cases the color-difference signals , and (the second is obtained by combining the first and third in specific proportions) are fed directly to the control electrodes of the kinescope, and the luminance signal is fed to the cathodes. Here, the matrixing is performed by the kinescope guns, and the electron beams are also modulated by the signals , and . When the reference white is to be reproduced, the standard color D6500 is produced on the kinescope screen.
Historical outline. In 1907–08 the Russian engineer I. A. Adamian proposed a method of simultaneous transmission for motion-picture frames, and in 1925 he proposed a system of three-color television that used sequential transmission of color fields obtained by means of the disk invented by P. Nipkow; the Scottish inventor J. Baird gave the first practical demonstration of the system in 1928. In 1929 a simultaneous color television system using mechanical scanning was demonstrated at the laboratory of the American Telephone and Telegraph Company (USA); three independent channels were used to transmit the signals. The Soviet engineer Iu. S. Volkov proposed the use of a cathode-ray tube with three screens for a color television receiver in 1929; the three color-separated images (for the primary colors R, G, and B) were combined by means of beam splitters (beam-splitting
Note on Figure 3. (a) Simplified block diagram of a compatible color television system with luminance and chrominance signals transmitted within a single (multiplexed) frequency spectrum, (b) nominal spectrum format of the total television signal formed; (TS) transmitted scene, (CSO) color-separating optical system, (TCT) television camera tubes, (GC) color gamma correctors, (EC) encoder, (DC) decoder, (K) kinescope, (ER), (EG), and (EB) video signals at the camera tube outputs, , , and video signals at the encoder and kinescope inputs, luminance signal, (Ue) chrominance signal, (f) frequency of oscillations
semireflecting mirrors). In the period 1938–50 an electronic, sequential color television system was developed in the USA by the Columbia Broadcasting System; it was used in the USA from 1951 to 1953 as the standard system of television broadcasting. A similar system was developed in the USSR between 1948 and 1953 and was used for experimental broadcasting in Moscow from 1954 to 1956.
Color television broadcasting was begun in 1953 in the USA with the NTSC system, which was adopted as the standard in the USA in 1954, in Canada in 1964, and in a number of other countries on the American continent, as well as in Japan in 1960. A new color television system was created in the USSR in 1958; it featured quadrature modulation of the color subcarrier, which made it compatible with the black-and-white television system, and was first used for experimental television broadcasting in 1959. By 1966 the Soviet-French SECAM III system had been developed, and it was put into service simultaneously in the USSR and France in October 1967 (seeSECAM). Color television broadcasting was begun in 1967 in the German Federal Republic, Great Britain, the Netherlands, and other countries in Western Europe as well as in Australia, which used the PAL system developed between 1962 and 1966 in the German Federal Republic.
Standard systems. As of 1978 there are three standard systems of color television: SECAM, NTSC, and PAL. They differ primarily in the methods used to form the television signal.
The SECAM system has been adopted in the USSR and most of the socialist countries, as well as in France and a number of African countries. The signal Uc is formed by alternate frequency modulation of the subcarrier oscillations with the signals and . Thus, on some lines (for example, the even lines) of a television frame the modulating signal is (the center frequency f0B of the sub-carrier oscillations in this case is 4.406250 MHz), and on the other lines it is (the center frequency f0B = 4.250000 MHz). As a result, for each line in the transmission channel there is a luminance signal and one of the color signals or . In order to form the color-difference signals in the receiver, it is necessary to eliminate time differences in the signals and . An ultrasonic delay line is used to make the signals coincide in time; it creates a delay equal to the scanning time for one line (64 microseconds). Because of the use of frequency modulation, the chrominance signal Uc in the SECAM system is not overly susceptible to amplitude-frequency and phase distortions.
The NTSC (National Television Systems Committee) system forms the Uc signal by the balanced amplitude modulation of two subcarrier oscillations having the same frequency f0 = 3.579545 MHz with the video signals (or with the video signals . The subcarrier oscillations being modulated are shifted in phase by 90° with respect to each other (that is, they are in quadrature). The sum of the oscillations at the output of the color encoder gives the signal Uc, which has no oscillations of the subcarrier frequency in its spectrum by virtue of the balanced modulation (only the side bands are present). The signal Uc is modulated both with respect to amplitude and phase (such modulation is called quadrature modulation), with the amplitude controlling the saturation of the transmitted color and the phase controlling the hue. The signal Uc is detected in the receiver with two synchronous detectors, to which the signal Ucs and the oscillations of the subcarrier frequency from a local oscillator are fed, the latter being controlled with respect to phase and frequency by the color sync signals Ucs. The color sync signals are transmitted in the total television signal in the form of trains of color bursts applied to the back porch of each horizontal blanking pulse. The advantages of the NTSC system include high noise immunity, relatively simple color encoding and decoding, and good color definition; the principal disadvantage is the high susceptibility of the Ucs signal to amplitude-frequency and phase distortions.
The PAL (phase alternation line) system is similar to the NTSC system; the principal difference is that in PAL the oscillations of the subcarrier frequency, which are modulated by the signal , change phase by 180° from line to line. In order to separate the chrominance signal into its quadrature components, the receiver uses an ultrasonic delay line of 64 microseconds and an electronic commutator. The PAL system is not overly susceptible to phase distortion, which gives it an advantage over the NTSC system.
Use and future development. Color television is replacing black-and-white for broadcasting, and work is in progress on systems of stereoscopic color television. Color television technology is being used more widely for nonbroadcast television in practically all fields of application. It has been used in space research to monitor the condition of astronauts and the docking of spacecraft (for example, in July 1975 during the docking of the Soviet and American spacecraft Soyuz and Apollo) and to transmit from space color images of the earth’s surface and other objects in space. Color television is also used in medicine, for example, for endoscopy and for demonstrations of surgical procedures. Applications of color television in metallurgy, physics, and chemistry are also promising. Both professional and amateur color video recording on magnetic carriers (tapes, disks, and cards) is becoming more and more common; arrangements are being made to produce large editions of color video recordings on polyvinyl chloride disks and to manufacture relatively inexpensive attachments that enable television receivers to reproduce such recordings.
Soviet television is moving in the direction of a complete transition to color. To this end, arrangements are being made to produce on an ever greater scale the studio and remote equipment needed for transmitting color programs. The territory served by color broadcasting is being expanded through the use of satellites with stationary orbits in the Ekran communications system and a network of ground repeaters. A color television complex capable of transmitting 20 programs is being constructed in Moscow. Currently under development is a system for transmitting a wide range of reference data in the form of pages that are reproduced on a television screen.
The most important problems in color television today include the transition to single-tube television cameras in conjunction with single receiver kinescopes. In the field of stereo color television, methods are being sought for compressing the frequency band, systems are being developed for the transmission of images from several positions, and research and development of holographic television methods is continuing.
REFERENCESTelevidenie, 3rd ed. Edited by P. V. Shmakov. Moscow, 1970.
Novakovskii, S. V. Tsvetnoe televidenie. Moscow, 1975.
Novakovskii, S. V. Standartnye sistemy tsvenogo televideniia. Moscow, 1976.
Tekhnika tsvetnogo televideniia. Edited by S. V. Novakovskii. Moscow, 1976.
S. V. NOVAKOVSKII