Defect detection and compensation

Abstract

In a video disc player, color picture information recorded in FM carrier form is recovered from the output of player's signal pickup apparatus by an FM demodulator. The FM demodulator includes zero crossing detector circuitry providing a pulse in response to each zero crossing of its signal input, a monostable multivibrator developing a pulse of a substantially fixed width when triggered by an output pulse of the zero crossing detector, and means for low pass filtering the output of the monostable multivibrator to develop signals for application to the player's video signal processing circuits. An output pulse of the zero crossing detector, and an output pulse of the monostable multivibrator subject to a selected delay comprise the inputs to a gate circuit, which develops an output pulse upon any instance of overlapping presence of the respective input pulses to indicate the presence of input signal conditions tending to produce a 'white' defect during picture display. A capacitor is subject to charging toward the turn-on threshold potential of a second gate having its inputs coupled thereto. Resetting of the capacitor charging circuit is effected in response to an output of the monostable multivibrator. The capacitor charging time constant is selected so that the threshold is reached if the recurrence rate of the multivibrator output pulses falls below a reference rate less than twice the lowest intended picture carrier frequency, whereby gate switching indicates the presence of input signal conditions tending to produce a 'black' defect during picture display. The respective defect indications are subject to summing and stretching, and are utilized to control substitution of stored picture information during defect occurrences.