| 摘要 |
A plural channel amplitude modulated station carrier telephone system and method wherein two a.c. signals are applied to modulate a called subscriber's channel carrier signal at a central office to signal an incoming call for the subscriber, wherein a phase locked loop is used at the subscriber terminal equipment to lock with one of the two a.c. signals following recovery of the two a.c. signals from the called subscriber's carrier signal, wherein the output of the phase locked loop is utilized to provide for the synchronous detection of the other of the two a.c. signals, and wherein a ring circuit responds to the synchronous detection of the other of the two a.c. signals to ring the called subscriber's telephone. Central office ringing signal frequency information is contained in the above-mentioned a.c. signal to develop a local ringing signal voltage having the same frequency as the central office ringing frequency for ringing the subscriber's telephone. Other features of this invention include a ring circuit with either a special transistor switching or relay switching arrangement for operating the ringer in the subscriber's telephone. Also disclosed is a synchronous detection circuit employing a special phase locked loop for synchronously detecting an a.c. signal.
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