| 摘要 |
PROBLEM TO BE SOLVED: To remove the cause of electric hawking irrespective of the circuit balance of a transmission/reception circuit by transmitting a transmission signal at timing when it is not overlapped with a received pulse width modulation signal at pulse width modulation and time-divisionally transmitting a pulse width modulation signal to a transmission path. SOLUTION: At the time of calling, an input signal from an oscillation circuit 17 is converted into a pulse whose frequency is F and whose pulse width is T/4 in a T/4 pulse width generation circuit 16 through a calling/called change-over switch 19. Then, it is converted into a saw-tooth-wave in a saw-tooth-wave generation circuit 13. The converted saw-tooth-wave and reference voltage changed by the sound signal of a micro computer 11 are inputted to a comparator 14 through a capacitor 22. A pulse width modulation signal whose pulse width changes by sound is outputted to a transmission path 18. A called side PLL circuit 35 receives the signal and outputs a signal whose phase is synchronized with the output of the oscillation circuit 17 and outputs a pulse signal whose phase differs from the output of the transmission circuit 11 by 90 degrees is outputted from a T/4 pulse width generation circuit 36. When pulse width similar to a calling side is modulated, it is outputted to the transmission path 18 without hourly overlapping it with the calling side signal.
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