| 摘要 |
A telephone line interface and side-tone attenuation circuit includes two opto-electric converters. The first has an input interconnected to receive a signal proportional to a near-end signal, and an output interconnected to modulate a current proportional to the near-end signal onto the subscriber loop. The second has its input interconnected to provide a signal proportional to a current on the subscriber loop, including the current proportional to the near-end signal, at its output. A summing amplifier receives signals proportional to the output voltage of the second opto-electric converter and a signal proportional to the near-end signal. The output voltage of the second opto-electric amplifier includes a component that is substantially 180~ out of phase with the near-end signal, allowing the near-end signal to be cancelled at the summing amplifier. |
