| 摘要 |
PURPOSE:To set automatically sensitivity of a voice detector corresponding to the environment in which a caller exists, and to hold a calling state even when sensitivity of the detector is low, by controlling said detector so that the sensitivity of the voice detector rises. CONSTITUTION:A sound signal from a transmission line 3 is detected by a PCM coder 5, is converted to a digital code train, is subjected to a necessary delay by a delaying circuit 6, and also is converted to an analog sound signal by a PCM decoder 7. Also, a digital signal of the coder 5 is applied to a comparing circuit 9, is compared with a threshold level of a threshold level generating circuit 10, and when it exceeds a prescribed threshold level, a detecting signal is applied to a voice detecting circuit 13. When a signal of the circuit 9 is continued for a prescribed time or more in this circuit 13, it is decided that a voice exists, a transmission start signal is generated and is applied to a transmitter 8 and an antenna switch 14. In case when the circuit 13 does not decide that a voice exists, no start signal is generated, the switch 14 is switched to a receiver 16 side, a receiving time is monitored by a receiving time monitoring timer 17, and when it exceeds a prescribed time, it is applied to a threshold level controlling circuit 11. |
