| 摘要 |
PURPOSE:To eliminate upleasant noise in listening sense by turning on/off a PB mute signal through the operation of a bias circuit corresponding to the presence of a PB signal to mask the switching noise of a transmission/reception circuit at the confirmation time of the PB signal. CONSTITUTION:When dial information is inputted from a pad 1, a dial circuit 2 sends a PB signal of 'H' level to a talking circuit 3. In this case, a transistor (TR) 5 is turned on to give a PB mute signal 200 to a the circuit 3, which switches the transmission/reception circuit thereby sending the PB signal 100 to telephone lines L1, L2 and sending it to a receiver R as a confirming tone. When the PB signal 100 is turned off, the TR 5 is turned off and then a PB mute signal 200 is turned off. When a of signal 100 reset in turned off, the circuit 3 switches the transmission/reception circuit to the talking side and switching noise is generated in succession to the signal 100, which masks the noise.
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