| 摘要 |
PURPOSE:To make the optimum balancing network (BNW) always automatically selectable and to attain calls of high speech quality, by using a BNW group and comparing balance return losses with each other by utilizing speech sending signals at the time of sending speech. CONSTITUTION:A speech sending signal detecting circuit 10 for detecting speech sending signals, balancing network (BNW) group 20 to be used for matching with the impedance of a main wire, plural speech receiving amplifiers 12-14 which are connected with the BNW group 20, plural effective value calculating circuits 15-17 for calculating the effective value of each output of the amplifiers 12-14, and a selection circuit 11 which detects 18 the smallest value among the outputs of the circuits 15-17 after performing comparison on them and selects only the output of the speech receiving amplifier outputting the smallest value, are provided. Therefore, this main-wire trunk circuit can always set the balance return loss optimum.
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