This invention is intended to assure the quality of transmitting signals, when the signals are transmitted through the main channel and the electronic quality of the digital signals is becoming worse, they will then be switched over automatically to transmit through the spare channel. Due to the adoption of detection and compensation technique for phase difference of the signals between the main and spare channels, and effective switching-over of the signal-channel is assured. Thus, the faulty result of asynchronization (out of control) which appears in the picture synchronization of the system with the existing technique due to the problem of phase difference after switching over of the channel is eliminated. |