| 摘要 |
<p>Apparatus and method for detecting the occurrence of a dial pulse signal in a telephone circuit. Transient signals within the frequency range of possible dial pulse signals are detected, and the amplitude of the detected signals are compared with a predetermined amplitude level. A first time period and a second time period are initiated in response to a first detected signal which exceeds a certain initial reference amplitude, with the duration of the first time period being less than the interval between the two consecutive transient pulse signals to be expected of a true dial pulse signal. The duration of the second time period is substantially equal to the period of a true dial pulse signal. A received signal resembling a dial pulse signal is detected during the first period only if the amplitude of such signal exceeds a second reference amplitude which is greater than the initial reference amplitude. A second apparent dial pulse transient signal condition, occurring after the termination of the first time period but before the termination of the second time period, is interpreted as the occurrence of a true dial pulse signal, in response to which a corresponding dial pulse output signal is produced. A second apparent dial pulse transient signal condition occurring in the first time period, however, merely extends the duration of the first time period and provides no dial pulse output signal. An amplitude comparison circuit automatically adjusts the reference amplitude in response to the signal level of the circuit providing input signals to the apparatus.</p> |
