| 摘要 |
Disclosed are hands-free land-line and radio telephones comprising a public telecommunication network connection part for transmitting a signal from the transmission signal input terminal to the public telecommunication network connection terminal and a reception signal from the public telecommunication network connection terminal to the reception signal output terminal, a first voltage controlled attenuator for controlling an output of the microphone by a given loss corresponding to a first control voltage level, a second voltage controlled attenuator for controlling a reception signal by a given loss corresponding to a second control voltage input level, a first detection circuit for generating a rectified voltage proportional to a magnitude of an input audio signal by rectifying an output signal of the microphone, a second detection circuit for generating a rectified voltage proportional to a magnitude of an input audio signal by rectifying an output signal of the reception signal output terminal, a control voltage output means for generating a gradient voltage signal corresponding to the difference of a transmission signal and a reception signal, and an inverting amplifier for inverting an output of the control voltage output means, wherein attenuation rates of the first and the second voltage controlled attenuators are controlled automatically in a complementary fashion, by the difference between a transmission signal and a reception signal.
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