| 摘要 |
A digital communications system including a plurality of transmission centers interconnected by transmission paths, in which each center has a local oscillator determining the digit times at that center and the frequencies of the oscillators are controlled in response to the adjacent oscillators by reference to incoming signals. In such a system it has been found that incorrect modes can be set up in which large phase differences between oscillators persist as a result of the presence of closed control rings in the system, and according to this invention the presence of an incorrect mode is detected by obtaining an indication whenever the phase of any oscillator departs by more than 90 DEG from that of the oscillator of a particular center designated as the reference center. When the indication has been produced for a predetermined period of time the closed control rings are disabled leaving only oscillator control paths radiating from the reference center, until the indication is removed. The information transmission paths are not affected by the presence of the indication.
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