Dropout detecting circuitry for a frequency modulated carrier, particularly for a video recorder

摘要

The carrier is modulated over a definite frequency deviation range and is demodulated in an FM detector whose output voltage is used in an evaluation circuit to produce a switching pulse representing the dropout. The FM detector is constituted by a demodulator providing a digital output signal having a first value "1" within the deviation range and a second value "0" outside the deviation range. An evaluation circuit is connected to the demodulator and produces the switching pulse responsive to only the second value "O". The demodulator includes an retriggerable monoflop and an edge-triggered D flip-flop, and the FM carrier is supplied to the control input of the retriggerable monoflop and to a first control input of the edge-triggered D flip-flop. One output of the monoflop is connected to a second control input of the D flip-flop and the switching pulse is provided at one of the outputs of the D flip-flop. Two such demodulators may be used, and have unequal pulse durations at the outputs of the respective monoflops and have their outputs connected to the inputs of an AND stage whose output voltage serves as the switching pulse.