| 摘要 |
PURPOSE:To enable a good telephone conversation even when the noise of surroundings is high by raising automatically the gain of a receiving circuit, and lowering automatically the gain of the transmission circuit when a noise detecting part detects the noise of more than a prescribed level. CONSTITUTION:When the noise of the surroundings is detected by a microphone 11, it is impressed to a logarithmic amplification and detection and smoothing circuit 9 through an amplifier 10, and here, a direct current voltage, which is logarithmically proportional to a noise level, is fetched. Then, a control circuit 8 raises the gain of a gain circuit 2 and makes a received sound volume be loud, and on the contrary, it lowers the gain of a gain control circuit 6 and makes the transmitted sound volume be low, when the output of the circuit 9 exceeds a certain prescribed level. Accordingly, since when the noise of the surroundings is large, the sound volume of the receiving circuit comes loud and the gain of the transmission circuit comes small, a characteristic, such that the sensibility of a human ear comes lower and the talking voice comes loud when the surrounding noise is large, can be corrected.
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