| 摘要 |
PURPOSE:To decrease the effect of environmetal noise, by providing a nonlinear input/output characteristic for a transmitter and a frequency characteristic for the input/output characteristic. CONSTITUTION:Sound outputs of the transmitter 1 are amplified to a suitable level at an amplifier 2. The amplified output is inputted to a high pass filter 3 and since the cut-off frequency is selected to about 1kHz, the sound component passes through the high pass filter as it is and rectified and smoothed via a rectifier circuit 4 and a smoothing circuit 5. A reference voltage Ea set at a variable resistor RV of a voltage comparator 6 is set equal to an output voltage outputted from the cicuit 5 when an input sound pressure level Pi at transmission is Pa. When the Pi is larger than the Pa, the output of the voltage comparator 6 switches an analog switch 10 to the position of a feedback resistor Ra and when the Pi is lower than the Pa, the switch 10 is selected to the position of a feeback resistor Rb. When Ra>Rb, the gain difference of a sound feedback amplifier 7 is (Va-Vb) dB, allowing to improve noise. |
