GB1323434A - Dropout compensator for pal colour television - Google Patents

Abstract

1323434 Colour television MINNESOTA MINING & MFG CO 21 July 1970 35361/70 Heading H4F A dropout compensator for a PAL colour video signal having quarter-line offset comprises a dropout detector 10 which controls a switch 14 to insert compensating signals when a dropout occurs. The compensating chrominance signal is produced by passing the output from the switch 14 through a trimmer delay 104, high-pass filter 106, two line delay 108 and phase inverter 110 to produce a signal which has the same phase relationship as the input video signal. The luminance signal is passed through a one or two line delay 102, and may be amplitude modulated on to an oscillator frequency before being passed to the delay line, being demodulated before being added to the chrominance signal. In an alternative embodiment (Fig. 6, not shown) the delays 104, 108, 110 are replaced by two approximately one-line delays connected together by two summing networks. One of the summing networks adds together the output of the first delay line and its input to give an averaged blue chrominance signal and the other summing network adds together the output of the first delay line and a signal derived from a phase-inverter connected to the input of the first delay line, so giving an averaged red chrominance signal. The outputs from the summing networks are added to give an averaged chrominance signal which is passed to the second delay line. In a further embodiment (Fig. 7, not shown) a single approximately one line delay line is used, its output and input being added to give an averaged blue chrominance signal which is used to lock an oscillator operating at twice the subcarrier frequency. The output from the delay line and the oscillator are fed to a balanced modulator, the output from which is passed through a low pass filter to isolate the desired chrominance signal. The oscillator may be locked by a signal formed by the addition of two signals, which are the averaged red and blue chrominance signals which have each been frequency doubled, the red signal having additionally been phase inverted so as to be in phase with the blue signal. In all the embodiments the input, which may be from a magnetic type, may be fed directly to the switch 14 and to the delay circuit without passing through the switch 14. All of the embodiments may be operated using modulated or demodulated video signals.