| 摘要 |
PURPOSE:To attain sure communication connection without mis-detection of noise by connecting a switch circuit in series with an output terminal of a band pass filter detecting a level of an answer tone and conducting a switch circuit when no network control signal is detected within a prescribed time after the answer tone is detected. CONSTITUTION:A detection signal is outputted from a 1st level detector 10, a one-shot multivibrator 13 is triggered and an output signal rises from a Q output terminal Q1 of the one-shot multivibrator 13 without an output of the detection signal from a 2nd level detector 11 within a prescribed time, 20msec, then a D type flip-flop 14 is latched and an 'H' level is outputted from a Q output terminal Q2 as a carrier detection signal of an 'L' level from the output terminal. Thus, he frequency component of the answer tone 2.1kHz is included and the carrier detection signal is outputted to a reception signal not including only the frequency component in 400Hz of the network control signal.
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