| 摘要 |
<p>An input circuit for a telephone answering machine. The circuit includes a first detector which gives a full-wave rectified output representing an incoming ringing signal or an outgoing dialling signal on a telephone line connected to the machine. The first detector is coupled via a delay to a first input of a Schmitt Trigger circuit. A second detector is coupled to a high-pass filter which blocks an incoming ringing signal but passes high frequency components of an outgoing dialling signal. The second detector, which also gives a full-wave rectified output, is coupled via a pulse stretch circuit to an inhibit input to the Schmitt Trigger circuit. The Schmitt Trigger circuit gives an output signal if there is an output from the first detector but no output from the second detector ie, only if there is an incoming ringing signal. Each of the detectors includes a differencing amplifier having resistors connected to its first and second inputs so that the application of an a.c. voltage causes a greater current to flow to the second input then to the first input. A source of direct current is connected to the first input and the ringing or dialling signal is applied to the second input. When the alternating signal voltage makes an excursion in one sense, the current flow into the second input is greater than the sum of the current flows into the first, and the output voltage of the amplifier moves in one direction. When the alternating voltage signal makes an excursion in the opposite sense, the current flow into the first input is reduced to zero and the output of the amplifier again moves in the one direction.</p> |
