| 摘要 |
PURPOSE:To control sound volume automatically to an optimum level according to ambient and in-use conditions by separating a noise, varying the level of the noise and deciding on the control amount of a received signal as a threshold value, and adjusting it through an attenuator. CONSTITUTION:When there is no noise on a reception side, a noise detection part 2 inputs the signal of a microphone 1. A resistance R16 is large and a resistance R17 is small. Therefore, noise and a voice signal are separated from each other and the electronic attenuator part 4 is adjusted by the level (threshold value) of an input to an operational amplifier OP2 in accordance with a value set through the variable resistance VR1 of an AGC circuit 3. If there is noise on the reception side, only continuous noise is outputted from an operational amplifier OP5 to charge a capacitor C6. Its output is inputted to an operational amplifier OP6 as a noise level. At the AGC circuit part 3, the output of an operational amplifier OP is determined by resistances VR1, R22, and R23 and its output is the input level (threshold value) of the operational amplifier OP2. By this threshold value, the output of the OP3 is decided on and the attenuator part 4 indicates the sound volume of reception. |
