| 摘要 |
PURPOSE:To apply optimum automatic adjustment to a side tone attenuation of a telephone set by matching a balanced circuit network to a line impedance connected with the telephone set while a push button switch is being depressed. CONSTITUTION:Since the side tone attenuation is maximized with a specific line impedance because the balanced circuit system is employed for the side tone preventing circuit 7, a signal whose side tone is attenuated from an oscillator 2 is extracted at a terminal of a receiver 1b of a handset 1 and converted into a DC by an AC-DC conversion circuit 4. The DC of a DC voltage/resistance conversion circuit 6 is varied to mimimize the output DC of the AC-DC conversion circuit 4. Thus, the voltage is changed while the push button switch 3 is being depressed and automatic adjustment is applied by a balancing circuit network so as to minimize the signal level of the signal outputted from the oscillator 2. Thus, the balanced circuit network of the telephone set 9 is matched with the line impedance connected with the telephone set to maximize the side tone attenuation.
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