Adaptive group delay equaliser telephone system - has converter supplied with video signals and test pulse generator coupled to changeover switch

摘要

The adaptive group delay equalisation circuit for video telephone channels in long-distance communications networks uses a test pulse which is transmitted in the blanking interval of the video frame scan along with the modulated carrier frequency. A converter (1) supplied with the video signal and a pulse generator have their outputs selected by a changeover switch (S) which is connected to the carrier frequency channel (3). The channel output is connected to the equaliser which contains a unit for reducing slow frequency drift and an adjustable transversal filter (5) for adaptive equalisation of the modulated signals. frequency drift is removed by comparing the received frequency with a reference in a control loop containing a voltage-controlled oscillator.