Videophone system synchronizer

Abstract

In a videophone system sound, video, synchronizing and signalling signals are to be transmitted through a single pair of telephone cables of existing telephone communications. In that case sound, synchronizing and signalling signals are transmitted in a digital form and the video signals are transmitted in an analog form. It is then possible to achieve optimum picture quality at the given bandwidth of the existing telephone communications. If for the given bandwidth the video signal were also digitalized, this would be at the expense of the picture quality. However, it is then necessary to transmit information regarding clock pulses for decoding at the receiver end. To be able to synchronize at the receiver end only on two frequencies instead of on clock pulse frequency, line frequency and field frequency, the information regarding the clock pulses is cotransmitted in such a manner that a clock pulse generator at the receiver end can be synchronized therewith which generator supplies clock pulses. A field code word is also transmitted during each field flyback period. In the receiver the regenerated clock pulses are divided in frequency in a divider stage and the divided signal is compared with the detected code word in a synchronizing verification circuit. As long as there is no phase equality between component signals and detected code word, an output signal from the verification circuit resets the divider stage.