| 摘要 |
A generator of ringing signals at a subscriber station of a telecommunication system, designed to respond to electromechanically generated lower-frequency call signals of large amplitude as well as to electronically generated higher-frequency call signals of small amplitude emanating from a central office, comprises a signal identifier and a level discriminator connected in parallel to an incoming line. The signal identifier includes a local oscillator emitting a train of pulses at a cadence substantially higher than any call-signal frequency which, in the presence of an incoming signal, are fed to a pulse counter during alternate half-cycles of a square wave derived from that signal. The value registered in any of such half-cycle by the counter is digitally transmitted to two circuit branches each comprising a further pulse counter adapted to receive the same square wave if the digital value read out from the first counter indicates a call-signal frequency in a predetermined range which is different for the two branches. Only one branch is enabled at any time, depending on the signal amplitude detected by the level discriminator. The counter in the enabled branch emits an activating pulse, after a certain number of square-wave cycles, to a ringing circuit which converts the locally generated pulse train into an intermittent audio-frequency oscillation driving a ringer; the counter of the enabled branch is periodically reset by this circuit which thereby terminates its own operation if that counter does not send out another activating pulse. |
