| 摘要 |
PURPOSE:To facilitate handling by detecting the leakage of reply message leaked through a cancelling circuit inputted to a control means identifying a DTMF signal and applying prescribed control and switching automatically the constant of the calcelling circuit so as to minimize the leakage thereby minimizing the manual adjustment. CONSTITUTION:A reply message signal fed to an input terminal 12 is sent to a telepone line via a cancelling circuit 11, a capacitor C2, a resistor R8, a line transformer 13 and connecting terminals 14,15. Further, the DTMF signal sent from a caller telephone set to the telephone line is fed via the connecton terminals 14,15,the line transformer 13, the resistor R8, the capacitor C2 and the cancelling circuit 11 to a microprocessor 18, where the signal is identified. The microprocessor 18, in response to the detected leakage, controls each terminal connected with resistors R1 R14 to an H or an L level thereby changing the potential at a point A in 16 ways, and thereby changing the on-resistance of a transistor Ql so as to minimize the leakage of the reply message detected by the microprocessor 18.
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