GB1503806A - Defect detection and compensation system - Google Patents

Abstract

1503806 Drop out compensation RCA CORPORATION 7 May 1975 [6 June 1974] 19162/75 Heading H4F A video record playback system provides dropout compensation by detecting when the frequency of the carrier falls outside a predetermined range, and in that time substituting a delayed video signal. As described the video signal is demodulated by zero-crossing detector 15 and low pass filter 17 and is normally supplied via switch 21 to processing circuitry 23 and receiver 25. The frequency of the video carrier is indicated by the amplitude of a signal produced by filter 31, and comparators 33, 35 produce defect indication pulses when said amplitude lies respectively above or below predetermined thresholds. A control circuit 40 controls switch 21 to supply to the signal processing circuit either the undelayed video signal, or during a drop out, the video signal corresponding to the previous line as developed in circuit 50. Since filter 17 has a slower response time than filter 31, the defect indication pulses will terminate before the end of the drop out in filter 17 which would result in the undelayed video signal being switched in before the end of the drop-out. To prevent this, retriggerable one shot 41 provides a pulse long enough to lengthen the defect indication pulses to compensate for the slow response of filter 17. In the event of a series of defect pulses separated by less than the period of the one shot, mixer 43 produces a single long defect indication signal, and switch S supplies the delayed signal for the whole period of the drop outs. In the event of a single, long defect indication pulse, no lengthening occurs. As an alternative to the retriggerable one-shot, the defect pulses may be passed through an envelope detector comprising a diode and capacitor wherein the discharge time of the latter is chosen to sufficiently lengthen the defect pulses. Furthermore duplication of FM detectors (15, 17 and 55, 57) may be avoided by providing a 1 delay in the processing circuit from which the output is fed back to switch 21. The invention may be applied to the playback of video discs or any other form of video record.