Facsimile receiver with improved FM demodulator

摘要

FM signals within a predetermined bandwidth representing light-dark variations in a document are transmitted over a communications network to a facsimile receiver. The FM signals are applied to differentiating and frequency doubling means so as to generate trigger signals with substantially equal amplitude and width for application to a single shot multivibrator. Reset means coupled directly to the input of the single shot multivibrator are responsive to the trigger pulses to reset the single shot multivibrator to the astable state when the frequency of the trigger pulses is sufficiently high that trigger pulses occur while the multivibrator is in a previously initiated astable state. An average DC voltage detector is connected to the output of the multivibrator such that the average DC voltage represents the ratio of the astable state duration to the stable state duration.