Signals of the Television System

Special signals are sent out by the television transmitter in addition to the picture impulses. These special signals have the purpose of synchronizing the picture at the receiver with the picture that is picked up by the camera.

At the television receiver, the picture and audio signals are picked up simultaneously by a single antenna. The voltages induced in the receiving antenna are fed into the radio frequency stage of the receiver; and the picture carrier and the sound carrier are converted by superheterodyne conversion method into two separate intermediate frequency signals. One signal corresponds to the sound carrier and the other — to the video or picture carrier. Two separate intermediate frequency ampli­fier channels are employed, one for the picture signal and the other for the sound signal. The sound intermediate frequency signal is demodulated by a detector. After sufficient amplifi­cation by the audio amplifier, the sound signal is reproduced by the loudspeaker in the usual way.

The picture intermediate frequency signal is amplified by several stages having wide-band frequency characteristics and is then fed into the video (picture) detector, where the interme­diate frequency signal is then demodulated in the same manner as in an ordinary sound receiver. The video (picture) signal which appears in the output of the detector is then amplified in a video amplifier, which corresponds to the audio amplifier in a sound receiver, except that it must pass a much wider range of frequencies.

In place of the loudspeaker used in the sound system, a de­vice is used which converts the varying amplitude of the video signals into corresponding variations of light.

УРОК 3