| 摘要 |
<p>A circuit for receiving voice and data signals on a balanced line, the data signal being an amplitude modulated form of a carrier signal having a frequency at least twice as high as the highest frequency in the voice band. The circuit matches the input impedance thereof to a first nominal balanced line impedance with respect to voice signals and a second nominal balanced line impedance with respect to data signals, applies the data signals to an unbalanced data output terminal, and applies the voice signals to a terminal connected to a bidirectional unbalanced input/output lead of a PABX. The circuit also receives data signals from a terminal connected to an unbalanced data input lead from the PABX and voice signals from the terminal connected to the unbalanced bidirectional input/output lead, mixes the incoming data and voice signals and applies the mixed signals to the balanced line while preventing the mixed signals from being fed back to the unbalanced data output lead and the unbalanced bidirectional input/output lead. The circuit further provides a low resistance DC current path for applying sufficient operating battery to the balanced line for long subscriber loops. The carrier signal is substantially removed in frequency from the upper limit of the voice band, thus alleviating the requirement of multistage, sharp cut-off and critically adjusted filters. However, the carrier frequency is low enough so as not to be substantially radiated from the subscriber's line or be seriously deteriorated during transmission.</p> |
