| 摘要 |
PURPOSE:To obtain a device which has a high response speed and to prevent howling surely, by combining a circuit which sends out the output signal when an audio signal having a prescribed level or higher is detected, plural analog switches controlled by this output signal, and logic circuits. CONSTITUTION:A telephone line L is connected to a hybrid circuit 1, and analog switches 4 and 5 are connected in series to the receiving system connecting the circuit 1 and a power amplifier 3 of a speaker 2, and a series circuit of a BPF8 and analog switches 9 and 10 is connected to the transmitting system connecting the circuit 1 and a power amplifier 7 of a microphone 6. A comparing circuit 11 having a reference voltage Vref is connected to the output side of the BPF8, and the output of the circuit 11 is applied to a timing timer 12. The output of an audio amplifier connected to the circuit 1 is connected to a comparator 11A which compares this output with a reference voltage V'ref, and a timing timer 12A is connected to the circuit 11A. Timers 12 and 12A control swtiches 4, 5, 9, and 10 through NAND gates 13 and 15 to raise the response speed, and howling is prevented surely. |
