GB1428973A - Circuit arrangements for colour television receivers - Google Patents

Abstract

1428973 Colour television EMI Ltd 16 July 1973 [20 July 1972] 33919/72 Heading H4F The phase of operation of a PAL-switch 14 is controlled by the output of a phase comparator 16 receiving one input from the reference oscillator 7 and one input which is derived from the received "swinging" bursts but is of the same phase for all lines, phase comparator 16 producing an error signal if incorrect phase of operation of switch 14 is detected. In the embodiment shown in Fig. 1 oscillator 7 provides outputs on lines 12, 13, 10 of phases +(R - Y), -(R - Y), + (B - Y) respectively, the signals at the input and output of a oneline delay line 2 being fed via burst gates 8, 17 to a switch 15 operated at half line frequency in synchronism with PAL-switch 14, switch 15 thus passing to phase comparator 16 bursts of only one phase. If the switches 15, 16 are positioned as shown on reception of a line including a burst of +135 degree phase relative to + (B - Y) phase the output of phase comparator 16 maintains gate 18 closed to pass switching pulses 9 to switch 14, but if the switches 15, 16 are positioned as shown on reception of a line including a burst of - 135 degree phase relative to + (B - Y) phase the output of phase comparator 15 opens gate 18 so stopping the switching action of switches 14, 15. Phase comparator 16 then receives from switch 15 each received burst and provides an output to close gate 18, this process being repeated if necessary until switch 14 restarts in the correct phase. In an alternative embodiment (Fig. 3, not shown) the reference oscillator receives bursts alternately from the input and output of the one line delay line and operates at the phase of these bursts, i.e. at + 135 degree phase relative to +(B - Y) phase if the PAL-switch and the synchronously operated switch feeding the oscillator are operating correctly, the oscillator output being passed via respective phase shifters to the PAL switch and (B - Y) demodulator. The phase comparator receives the reference input to the (B - Y) demodulator and the bursts (of - (B - Y) phase) from the chrominance signal adder (22) feeding the (B - Y) demodulator, detection of incorrect operation causing an extra switching pulse to be applied to the PAL-switch.