| 摘要 |
PURPOSE:To realize identification with high reliability by detecting that the frequency and the generating timing of two frequency signals are within the standards and identifying the frequency signal in compliance with the standards as a DTMF signal when a reply message is muted and the frequency signal in compliance with the standards is obtained regardless of the muting. CONSTITUTION:The reply message is fed from an input terminal 23 and sent to a telephone line from connection terminals 27,28 via a switch circuit 24. A DTMF signal is fed to band pass filter circuits 29-31 via the connection terminals 27,28, a line transformer 26 and a cancelling circuit 25, only signals (a),(b),(c) of each frequency component are extracted and fed to a microprocessor 32. When the frequency and the generating timing of the signals (a),(b) are detected by the microprocessor 32 and they are in accordance with the standards, the switch circuit 24 is turned off for a prescribed period to mute the reply message. When the signals (a),(b) in compliance with the standards are obtained for the muting period and the same signals (a),(b) are obtained afterward, the signals (a),(b) are decided to be DTMF signals.
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