Television receiver

(electronics) A receiver that converts incoming television signals into the original scenes along with the associated sounds. Also known as television set.

The equipment used to receive the transmitted modulated radio-frequency signals and produce synchronized visual images and sound. The radio-frequency portion operates on the superheterodyne principle. See also Modulation; Radio receiver.

The first television receivers to be mass-produced were monochrome; that is, they provided pictures in black and white only. Later, color receivers, which produce pictures in full color as well as black and white, became available. Many television receivers now can receive stereophonic sound or alternate language in accordance with multichannel television sound standards. For basic discussion of a television system See also Television; Television standards; Stereophonic radio transmission.

Early television receivers used vacuum-tube technology. Present-day receivers use solid-state technology with many functions integrated on a few chips. The only function still primarily implemented by using vacuum-tube technology is the display by the cathode-ray tube (CRT). See also Cathode-ray tube; Integrated circuits; Vacuum tube.

Since most broadcast television transmissions in the United States are horizontally polarized, the most basic type of television-receiving antenna is the horizontally mounted half-wave dipole. More complex antennas combine several dipole elements of various lengths, and passive reflectors may be used to achieve some degree of horizontal directivity, which increases the amplitude of the receiver signal and reduces interference from other stations. See also Antenna (electromagnetism); Polarization of waves; Yagi-Uda antenna.

The tuner of a television receiver selects the desired channel and converts the frequencies received to lower frequencies within the passband of the intermediate-frequency amplifier. The output from the tuner is applied to the intermediate-frequency (i-f) amplifier. The output of the intermediate-frequency amplifier consists of modulated radio-frequency signals, which when detected provide signals corresponding to the transmitted picture and sound information. See also Amplitude modulation; Amplitude-modulation detector; Frequency modulation.

Picture synchronizing information is obtained from the video signal by means of sync separation circuits. In general, sync separation circuits perform the following functions: (1) separation of the sync information from the picture information; (2) separation of the desired horizontal and vertical timing information by means of frequency selection; and (3) rejection of noise signals. See also Electrical noise; Television scanning.

The display device for a monochrome television receiver is a cathode-ray tube, consisting of an evacuated bulb containing an electron gun and a phosphor screen, which emits light when excited by an electron beam. The intensity of the electron beam is controlled by the video signal, which is applied either to the grid or to the cathode of the electron gun. Television receivers designed to produce images in full color are necessarily more complex than those designed to produce monochrome images only. In monochrome systems, the video signal controls only the luminance of the various areas of the image. In color systems, it is necessary to control both the luminance and the chrominance of the picture elements. See also Picture tube.

The chrominance of a color refers to those attributes which cause it to differ from a neutral (white or gray) color of the same luminance. In qualitative terms, chrominance may be regarded as those properties of a color that control the psychological sensations of hue and saturation. For color television purposes, chrominance is most frequently expressed quantitatively in terms of the amounts of two hypothetical, zero-luminance primary colors (usually designated I and Q), which must be added to or subtracted from a neutral color of a given luminance to produce the color in question. As a practical matter, color television receivers produce full-color images as additive combinations of red, green, and blue primary-color images, and it is necessary to process the luminance and chrominance information in a color signal in such a way as to make it usable by a practical reproducing device. See also Color.

Color television broadcasts in the United States employ signal specifications that are fully compatible with those used for monochrome, making it possible for color programs to be received on monochrome receivers and monochrome programs to be received on color receivers. (Color pictures are produced, of course, only when color programs are viewed through color receivers.) Compatibility is achieved by encoding the color information at the transmitting end of a color television system in such a way that the transmitted signal consists essentially of a normal monochrome signal (conveying luminance information) supplemented by an additional modulated wave conveying chrominance information. Although it is added directly to the monochrome signal component before transmission, the color subcarrier signal does not cause objectionable interference, because of the use of the frequency interlace technique. Because the chrominance information involves two variables, the modulated subcarrier signal varies in both amplitude and phase, and it is necessary to employ synchronous detectors to recover the two variables. A phase reference for the special local oscillator, which provides the synchronized carriers in each color receiver, is transmitted in the form of so-called color synchronizing bursts. These are short samples of unmodulated subcarrier transmitted during the horizontal blanking periods after the horizontal sync pulses.

Special decoding circuits are necessary in a color receiver to process the luminance and chrominance information in a color signal so that it can be used for the control of a practical color cathode-ray tube utilizing red, green, and blue primary colors.

The great majority of color television receivers employ the shadow-mask color cathode-ray tube in which color images are produced in the form of closely intermingled red, green, and blue dots. The primary-color phosphor dots are excited by three separate electron beams, which are prevented from striking dots of the wrong color by the shadowing effect of an aperture mask located about ½ in. (1.25 cm) behind the special phosphor screen. The beams in such a cathode-ray tube are deflected simultaneously by the fields produced by a single deflection yoke placed conventionally around the neck of the tube. New cathode-ray-tube designs and deflection yokes are self-converging and do not require auxiliary convergence deflection.

In addition to the same controls required for monochrome receivers (such as brightness and contrast), color receivers normally have controls for convergence, hue, and saturation. The convergence controls, considered servicing adjustments only, adjust the relative amplitudes and phases of the signal components that are added together to form the proper waveforms for the convergence yoke. The hue control usually adjusts the phase of the burst-controlled oscillator and alters all the colors in the image in a systematic manner comparable to the effect achieved when a color circle diagram is rotated in one direction or the other. The proper setting for the hue control is normally determined by observing skin tones of persons on the television screen. The saturation control, frequently labeled chroma or simply color, adjusts the gain of the chrominance circuits relative to the monochrome channel and controls the saturation or vividness of the reproduced colors. When this control is set too low, the colors are all pale or pastel, and when it is reduced to zero, the picture is seen in black and white only.