| 摘要 |
PURPOSE:To apply a circuit to a desable-tone whose phase is inverted on the way, by receiving a prescribed single frequency signal, whose phase is changed on the way of transmission in the beginning of data transmission, to obtain the third detection signal as a disable-signal. CONSTITUTION:The input signal from a terminal 8 is inputted to a single frequency signal detecting circuit 10 and a phase change detecting circuit 11 to output the first detection signal indicating presence of the single frequency signal and the second detection signal indicating presence of phase change of the single frequency signal. A control circuit 13 receives the first and the second detection signals and transmits the third detection signal, which is continued from the start point of the second detection signal to the end point of the first detection signal, to an output line 14. An echo canceller is controlled to be disabled or enabled by the third detection signal.
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