| 摘要 |
PURPOSE:To attain a calling even at a place where ambient noises are loud, by inhibiting transmission and making receiving voices louder when the ambient noises are loud. CONSTITUTION:A handset 1 is provided with a receiver REC, unidirectional microphone M1 for transmitter, and unidirectional microphone M2 mounted in an opposite direction to the microphone M1. Circumferential sounds are inputted to the microphone M2. The output of the microphone M2 is converted into a DC level by a rectifying and smoothing circuit DET2 and an 'H' level is outputted from a comparator COP2 whose value becomes higher than a threshold voltage E. An analog switch SW1 disconnects a transmission amplifier A2 from the transmitter M1 and, as a result, the output of the transmission is inhibited. Moreover, since another analog switch SW2 switches the feed back quantity of a reception amplifier A1, the gain of the amplifier A1 becomes larger.
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