Videotelephone system with anti-crosstalk means

Abstract

The outlying stations of a videotelephone system, adapted to communicate with one another through a central office, include each a video-reception section and a video-transmission section as well as an audio section provided with telephone equipment. A sync-pulse generator in the video-transmission section, comprising a crystal-controlled oscillator working into a pair of frequency dividers, controls the line-scan and frame-scan generators thereof as well as those of a remote station to which it is connected via an outgoing video line. A local sync-pulse extractor connected to an incoming video line controls the line-scan and frame-scan generators of the associated video-reception section. A switch enables replacement of the crystal-controlled oscillator with a variable-frequency oscillator locked in step with the sync-pulse generator of the remote station by the output of the local sync-pulse extractor whereby interference phenomena due to pick-up of incoming sync pulses, in random phase relationship with the locally generated sync pulses, are eliminated. The switchover to the variable-frequency oscillator may be carried out manually or, at a called station, by a detector circuit responsive to incoming audio or video signals, preferably to ringing current; the detector circuit includes a relay which locks for the duration of the established connection under the control of a hook switch.