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television transmitter[′tel·ə‚vizh·ən tranz‚mid·ər]
a device or set of devices used to convert a television video signal and the accompanying audio signal into a composite radio-frequency signal for transmission from an antenna; it is part of a television transmitting station. A television transmitter includes one channel each for the video and the audio. The two channels may be handled by a single piece of equipment with a common power supply, control unit, cooling, and, in some transmitters, amplifier channel for the modulated oscillations; in older transmitter designs, the video and audio are handled by two separate radio transmitters.
The video channel has the following primary characteristics: unipolar modulation of the oscillations (in the USSR the polarity is negative), a wide band for the modulating frequencies (in the USSR, up to 6 megahertz), a very low-frequency signal (from direct current to 1–2 hertz) in the modulating signal, which corresponds to variations in the average illumination of the image, and suppression of a considerable portion of one sideband (in the USSR, the lower sideband) in the spectrum of the output radio signal (seeSINGLE-SIDEBAND MODULATION). The average frequency of the audio channel is in strict relationship to the carrier frequency of the video channel (in the USSR it is 6.5 megahertz higher).
The broadcast television transmitters produced in the USSR and other countries have peak power ratings from several tens of watts to several hundred kilowatts for the video channel; power ratings for the audio channel are correspondingly lower by a factor ranging from 3 to 20. The transmitters operate in the very-high-frequency (VHF) and ultrahigh-frequency (UHF) bands (in the USSR, at frequencies of 48.5–100, 174–230, and 470–638 megahertz). For VHF transmissions, the output stages of high-power television transmitters use vacuum tubes, primarily beam power tubes; for UHF transmissions they use both beam power tubes and multicavity floating-drift klystrons, which are preferred. The preliminary stages usually use transistors. Amplitude modulation is used for the video channel, and frequency modulation for the audio channel. In modern television transmitters, modulation of the video channel most often takes place in low-power stages at an intermediate frequency (several tens of megahertz), with subsequent frequency conversion to the operating range.
The first television transmitter in the USSR was completed in 1937. It has been used since July 1938 at the Experimental Leningrad Television Center to transmit television programs.
REFERENCESRadioperedaiushchie ustroistva. Edited by G. A. Zeitlenk. Moscow, 1969.
Oborudovanie radioperedaiushchikh televizionnykh i UKV ChM veshchatel’nykh stantsii. Moscow, 1974.
A. I. LEBEDEV-KARMANOV