| 摘要 |
PURPOSE:To prevent a malfunction occurring due to peripheral noise in a regular time by detecting the average level of the output of the gain variable amplifier of a transmitting signal and controlling the amplification degree of the variable amplifier. CONSTITUTION:A transmitting input from a microphone 1 is inputted to a rectifying/smoothing circuit 6 and to an average level detection circuit 20 after an amplification by a gain variable amplifier 5a and the amplification degree of the amplifier 5a is controlled by a control signal Vg that is outputted from the circuit 20. In this case, when a transmitting input becomes larger by regular peripheral noise, the amplification degree of the amplifier 5a is lowered by that the average level which is detected at the circuit 20 is gradually increased and a transmitting level that is inputted to a comparator 13 is lowered by that a signal Vo which is inputted to the circuit 6 becomes smaller. Thus, the malfunction that keeps, for a long time, a transmitting state which is generated regularly by the peripheral noise is prevented.
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