| 摘要 |
PROBLEM TO BE SOLVED: To suppress the transmission of a noise from a transmitter of a telephone set, even if a single frequency(SF) signal is a signal obtained by DSP processing using a voice CODEC by processing a dial tone(DT) detection signal, a loop detection signal and an SF detection signal, controlling an output from a notch filter for an SF signal based on respective processed results and sending the controlled output to the telephone set. SOLUTION: When the existence of a loop is determined, a loop detector 6 in the in-band ringer receiving function circuit outputs a loop detection signal of a high level (H). When the transmission of an SF signal from a home TCU (terminal control unit) to a four-wire system subscriber circuit (4WLC) is stopped, the 4WLC transmits a DT signal to the TCU together with the SF signal. A route from the TCU to the telephone set is interrupted in the TCU, based on the loop detection signal and the DT signal and the SF signal transmitted from the 4WLC to the TCU so as not to transmit a signal to the telephone set.
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