GB1267267A - - Google Patents

Abstract

1,267,267. Colour television. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 27 Jan., 1970 130 Jan., 1969; 8 Aug., 1969; 18 Dec., 1969], No. 3874/70. Heading H4F. In a PAL colour television chrominance signal component demodulator, -to reduce the influence of differential phase error between the subcarrier generated from the separated 'burst signals and the colour information in the chrominance signal, the generated subcarrier is phase corrected in response to a signal derived from the output of the demodulator circuit which contains components which are a measure of the magnitude and direction of the differential phase error. As shown, Fig. 1, the outputs of the chrominance signal synchronous demodulators 37 and 43 are fed via time selection circuits 57 and 59 (see later) and low-pass filters 63, 65 and 67, 69 to a non-linear signal handling circuit 75 (see later), the output of which is fed via a low-pass filter 79, 81 to a phase correction circuit 29 which corrects the phase of the subcarrier from the chrominance subcarrier generator 13, the frequency and phase of which is fixed relative to the mean frequency and phase of the swinging burst signal by means of a control loop including synchronous detector 9 and a D.C./A.C. component separator circuit 17. The outputs of the demodulators 37 and 43 each have a component of constant sign and a component of alternating sign, i.e. (B-Y) cos #Œ(R-Y) sin # and Œ(R-Y) cos # - (B-Y) sin # The sin # component in each signal is proportional to the differential phase error. The components of constant sign are fed to the non- linear handling circuit 75 which either forms the product or the quotient thereof which is used after filtering as the control signal. Time selection circuits 57 and 59 may be provided to ensure that the mean value of the signal is determined only over a portion, e.g. 5-20%, of each line period. These circuits being rendered conducting only during a portion of each line period by means of a pulse derived from a line flyback pulse and delayed by a delay circuit 83 which may -be controlled by a sine or sawtooth generator having a low frequency. A control signal dependent on (R-Y) and not (B-Y) may be derived-by passing both the demodulator outputs first through 180 degrees phase shifters operated by half-line-frequency switching voltages before they -are applied to the non-linear handling circuit. Both control signals may be used simultaneously in which case even when one of (R-Y) or (B-Y) is zero a control signal is formed. A non-linear handling circuit is described with reference to Figs. 2 and 3 (not shown), in which the effect of the polarity of the colour difference signals is nullified, the control signal always comprising - (B-Y sin #. A quotient forming non-linear handling circuit is described with reference to Fig. 4 (not shown) which comprises two logarithmic transfer circuits, the outputs of which are subtracted and passed to an anti-logarithmic transfer circuit and a sign correction circuit.