| 摘要 |
PURPOSE:To control the switching between the transmission and reception state with high accuracy by comparing each level extracted from surrounding noise mixed in a reception circuit and a transmission circuit with a level obtained by the addition of each level to a transmission level and a reception level respectively. CONSTITUTION:A transmission signal from a microphone 1 is amplified (9) and rectified (10), and its output is inputted to a charge/discharge circuit 11 whose charging time constant is smaller then the discharge time constant and a charge/discharge circuit 16 whose charging time constant is larger than the discharge time constant, from which the 1st and 1nd transmission signal levels are outputted respectively. The reception signal inputted via a side tone proof circuit 4 is amplified (12) and rectified (13), its output is inputted to a charge/discharge circuit 14 whose charging time constant is smaller than the discharge time constant and a charge/discharge circuit 17 whose charging time 2nd reception signal levels are outputted respectively. The addition between the transmission level of the circuit 11 and the surrounding noise reception level of the circuit 17 is compared (15) with the addition (19) between the surrounding noise transmission level of the circuit 16 and the reception level from the circuit 14, and its comparision output controls variable attenuators 3, 6 inserted respectively to the transmission circuit and reception circuit.
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