1410984 Colour television SONY CORP 1 Dec 1972 [7 Dec 1971] 55549/72 Heading H4F A PAL colour television receiver uses a one (or three, or five ... ) line delay line 3 and a switch 2 operated at the end of each line period to produce a chrominance signal having the same modulation axes for all line periods, which chrominance signal is passed to demodulators 6, 7; the phase of operation of switch 2 is controlled so as to pass to the demodulators 6, 7 a chrominance signal having desired modulation axes (in the embodiment shown having the R-Y modulation axis at + 90 degrees relative to the B-Y modulation axis) by the use of further demodulators 13, 14 receiving the received chrominance signal from band-pass amplifier 1 and the delayed chrominance signal from delay line 3, respectively, which, in combination with adder 17, subtractor 20, envelope detectors 18, 21 and differential amplifier 19 produce an output from control circuit 22 if the phase of operation of switch 2 is wrong, thus producing a change in the state of a flip-flop 5 which controls switch 2. A subcarrier reference signal of phase equal to that of the bursts in the chrominance signal fed to demodulators 6,7 is produced by an oscillator 10, the correct reference signals for demodulators 6, 7 being produced by 45 degree phase retarder 11 and 90 degree phase retarder 12. The reference signals for demodulators 13, 14 are derived from 90 degrees phase retarders 15, 16. If the switch 2 is operating correctly, no output is derived from control circuit 22 and the operation of flip-flop 5 is not altered. Demodulators 13, 14 may be supplied with only the bursts contained in the chrominance signals derived from 1, 3 respectively, and these demodulators may be supplied with reference signals of other phases, e.g. phase retarder 15 may be omitted (Fig. 13, not shown). Demodulator 7 may receive the received chrominance signal from amplifier 1 rather than the chrominance signal from switch 2.
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