| 摘要 |
PROBLEM TO BE SOLVED: To prevent howling in a door phone system including a master device and a slave device connected with a wired transmission line for transmitting and receiving a voice signal. SOLUTION: A master device processing circuit 12 frequency-modulates a voice signal transmitted from a microphone 14 of the master device with a first carrier signal of a first frequency. A slave device processing circuit 22 frequency-modulates a voice signal transmitted from a microphone of the slave device with a second carrier signal of a second frequency. The master device processing circuit 12 traps a signal of the first carrier frequency on the transmitting line 40 connecting the master device and the slave device, extracts the signal of the second carrier frequency, frequency-demodulates the voice signal from the signal extracted, and supplies the demodulated voice signal to a master device speaker 16. The slave device processing circuit is connected to the transmission line 40, traps the signal of the first carrier frequency, extracts the signal of the first carrier frequency, frequency-demodulates the voice signal of the master device from the signal extracted, and supplies the demodulated voice signal to a slave device speaker 26. COPYRIGHT: (C)2011,JPO&INPIT
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