| 摘要 |
PURPOSE:To vary a threshold value for detecting a break of a carrier in response to variation in the level of an input signal, by providing a muting detecting circuit which varies a threshold value for the transmission and inhibition of the signal of a gate circuit. CONSTITUTION:A gate circuit 16 is provided on the input side of a peak detector 11 to inhibit the transmission of such a carrier that the AM modulation rate of an intermediate-frequency amplified 1F2 exceeds about 80%. Consequently, a detection signal regarding a carrier having a > about 85% AM modulation rate as a break of the carrier appears at the output of the peak detector 11. Further, a burst noise is integrated by a low-pass filter 9, so recovery time is extended. Therefore, a carrier break detection signal generated by the peak detector 11 is passed through a delay circuit 12 which extends only the recovery time to put a muting circuit 13 in operation and only an OR signal is outputted temporarily from a matrix circuit 15 to perform automatic switching to monaural reproduction, eliminating the burst noise. |
