GB1163753A - Improvements in or relating to Television Apparatus - Google Patents

Abstract

1,163,753. Colour television receivers; phase detectors. RADIO CORPORATION OF AMERICA. 28 Nov., 1966 [7 Dec., 1965], No. 51821/65. Headings H3A and H4F. The invention relates to a colour television receiver for use with P.A.L. type signals which include a colour synchronizing burst which is advanced and retarded in phase by 45 degrees with respect to the - (B-Y) modulation axis in alternate lines, Fig. 1, for the purpose of signalling the phase inversions which are imparted to R-Y modulation axis. In accordance with the invention the phase detector, Fig. 4, in the reference oscillator control loop provides in known manner a first output B<SP>1</SP> from resistor network R1, R2 via low-pass filter C1, C2, R3 for the purpose of providing a D.C. error signal to control the reactance stage for the oscillator and also provides a second unfiltered output A<SP>1</SP> from resistor network R5, R6 which is in the form of a square wave due to the phase changes of the bursts and which serves to control a diode switch, whereby the reference oscillator signal which is applied to the R-Y demodulator is inverted in phase with the inversion of the R-Y modulation axis. Resistor networks R1, R2 and R5, R6 function to isolate the two detector outputs so that the square wave does not interfere with the reactance stage control. Resistors R5 and R6 may be replaced by inductors. An RC or tuned filter operating at 7À8 kc./s. may be included in the square wave output to remove carrier frequency components. In an alternative arrangement, Fig. 3 (not shown), the resistor network R5, R6 is dispensed with and the square wave output is derived from the junction of resistors R1, R2, a high value resistor being inserted between the junction and low-pass filter C1, C2, R3 so as to isolate the square wave and D.C. signal outputs and prevent the filter from attenuating the amplitude of the square wave.