GB1419694A - - Google Patents

Abstract

1419694 Colour television SHARP KK 9 March 1973 [10 March 1972] 11610/73 Heading H4F Automatic hue control in a PAL receiver is effected by gating 1, Fig. 1, the swinging bursts from a received chroma signal and producing in APC circuit 2 a continuous subcarrier synchronous with the average phase of successive bursts i.e. at f s <SP>1</SP> in Fig. 3. This subearrier is passed via a phase control circuit 3 and a 90 degree phase shifter 4 to produce a second subcarrier f so which is used to demodulate 20 along the fixed B-Y modulation axis. In addition this second subcarrier is fed to a polarity decision circuit 5 which senses the polarity of the chroma input signal and produces a positive or negative polarity D.C. decision signal in dependence upon whether the colour carriers f c1 and f c2 in the chroma signal are in the first and fourth quadrants of the phase diagram of Fig. 3 or in the second and third quadrants. In dependence upon this decision signal the phase of the subcarrier from circuit 3 is inverted in circuit 6 and its resulting phase is detected relative to the chroma signal in phase discriminator 7, integrated 8 and fed to phase control circuit 3 to control the phase of f s <SP>1</SP> to its correct phase of f s and thus effect automatic hue control. To demodulate the swinging R-Y component a subcarrier is produced by gating 9 alternate bursts and detecting 10 their phase relative to f s at the output of circuit 3, the resulting output being integrated 11 and used to control phase inverter 12 so that the appropriate sub-carrier f s <SP>1</SP> or f s2 is derived which is then line switched through 180 degrees in switch 13 in response to the output of a bi-stable circuit 16 controlled by line sync. pulses via delay 14 and shaper 15. In a second embodiment Fig. 2 the polarity decision circuit receives f s <SP>1</SP> and the chroma signal and its output determines whether the signal f s0 is inverted in circuit 6. The phase of the output from circuit 6 is detected relative to the received chroma signal and the resulting sequential outputs subtracted and integrated 8 and fed to phase control circuit 3.