| 摘要 |
An audio frequency squelch circuit responsive to a signal including voice signals in the presence of noise includes a limiter for receiving the signal and a monostable multivibrator coupled to the limiter and triggered by negative (or positive) transitions in the limiter output. The multivibrator produces pulses of fixed amplitude and time duration to provide an output wave having a duty cycle which varies with frequency. The pulse wave is passed through a low pass filter having a bandwidth sufficiently wide to allow coherent variations in frequency at a voice syllabic rate, and which removes random noise inputs, with opposite phases being applied to high pass filters which reject signals varying slowly in frequency. The opposite phase signals are applied separately to comparators operating against a fixed voltage reference, one of which provides an output in response to voice frequency peaks which are sufficiently above the mean frequency and the other provides an output in response to voice frequency peaks which are sufficiently below the mean frequency. The comparator outputs are applied to an OR gate which controls an integrator and timer to actuate the squelch. |
