| 摘要 |
A plurality of telephone lines which convey frequency-shift signals and a corresponding plurality of data machines, which send and receive DC baseband data, are interconnected via a time-shared digital (filter) circuit receiver and digital circuit modulator under the supervision of a common central processor. The processor follows the progress of each call and determines when supervisory functions are required. These functions includes placing the telephone line in an on-hook or an off-hook condition, returning answer back signals, monitoring the line for incoming calls by detecting incoming ringing signals and, after the call is answered, monitoring the line for a continuous connection by detecting incoming carrier signals. A low-pass digital filter is arranged to filter the incoming signals to detect ringing or, alternatively, to filter the sum of the signaling frequencies to detect carrier. To detect the low-frequency ringing, the response of the filter is modified by limiting the rate at which the input signals are applied thereto and by "holding" the processed signals in the filter for the prolonged interval between the input signal applications. |
