| 摘要 |
PURPOSE:To prevent a click sound from being heard from the receiver of a speaking circuit by making the mute terminal of a dial IC be at a high level, and making the transistor (TR) of a muting circuit come to ON during the transmission of a pulse. CONSTITUTION:When a hand set is off-hooked and a hook switch HS is closed, the TR Q2 comes to ON, and simultaneously the TR Q4 comes to ON, and the speaking circuit is connected to a trunk line. Here, in response to a key input from a key board, the output terminal DP of the dial IC repeats the high level and the low level 'H', 'L', and the TR Q1 comes to ON. According to this, the TRs Q2, Q4 come to ON, OFF, and a dial pulse is transmitted to the trunk line through a rectifier circuit 20. At this time, the mute terminal of the dial IC comes to the high level 'H', and the TR Q3 comes to ON. Thus, since both the ends of the speaking circuit is short circuited, the click sound is prevented from being heard from the receiver of the speaking circuit.
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