AUDIO FREQUENCY SQUELCH SYSTEM

The audio frequency squelch system selects signals in the upper and lower portions of the voice frequency range and processes these signals separately. The signals are limited and then differentiated to produce pulses which trigger a monostable multivibrator, with variations in the duty cycle of the output pulses which occur at a voice syllabic rate (under 3.25 hertz) being selected. The selected signals from the two channels which are above a particular frequency (0.2 hertz) are summed by passing the signals from one channel through a phase shift circuit and applying the signals from the two channels to a difference amplifier. The difference signals which exceed a predetermined amplitude are detected and applied to a timing circuit which controls the squelch switch of a radio receiver. A hysteresis feedback circuit from the timing circuit to the detector reduces the threshold level of the detector when a signal has been detected. The utilization of responses from the two channels provides responses from both low frequency and high frequency voice signals so that a relatively large number of detections are obtained and the time constants of the squelch circuit can be relatively low for fast response.