| 摘要 |
An off-hook detector for use in subscriber line interface circuits to indicate status of the subscriber line, or loop, exhibits reduced sensitivity to common mode or longitudinal currents. A central signal from the exchange switches the transmission bridge for the subscriber line from a balanced to an unbalanced condition. In the balanced condition a difference detector indicates loop status. In the unbalanced condition, a comparator compares the voltage on one side of the transmission bridge to a reference voltage to indicate loop status.
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