GB1496495A - Processing circuit for television chromatic signals - Google Patents

Abstract

1496495 Colour television receivers INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA SpA 17 Dec 1974 [19 Dec 1973 9 April 1974] 54345/74 Heading H4F A PAL colour television receiver has a switching voltage generator operating at half line frequency and controlled in phase by the output of a phase comparator. The comparator receives a reference burst extracted from the received PAL signal for comparison with a locally-generated sub-carrier. The sub-carrier generator is excited by alternate reference bursts which are passed to it via a gate controlled by the switching voltage generator. The switching voltage controls the phase of the subcarrier signal fed to a synchronous demodulator in known manner. In one embodiment, Fig. 1, the chrominance signal 1 quadrature-modulated on a sub-carrier is passed to a.g.c. 2 and gate 8. Gate 8 is enabled at line frequency by input 9 to pass the reference burst to gate 11 and phase comparator 16, e.g. a synchronous demodulator. Gate 11 is enabled at half line frequency by the output of switching voltage generator 10 and passes alternate bursts to sub-carrier regenerator 12, e.g. a crystal filter. The continuous sub-carrier from regenerator 12 passes via 90 degree phase shifter 13 to comparator 16. Units 17, 18, e.g. a gate and a low pass filter, receive the comparator output and provide pulses at half line frequency if the inputs to the comparator are out of phase. The pulses are passed to a frequency selective circuit 19 whose output changes the phase of switch 10 as necessary, switch 10 being switched at line frequency by input 9 to provide the half line frequency switching signal. The sub-carrier from unit 12 controls a.g.c. 2 and colour killer 3 via, for example, an amplitude detector, and also passes to 45 degree phase shifter 14. The output of shifter 14 is additionally shifted by 90 degrees and applied to demodulator 5 to provide the B - Ysignal. It is also passed direct and via inverter 22 to switch 23 which is controlled by the switching voltage from unit 10 to apply the subcarrier with the required phase to demodulator 4 which provides the R - Y signal. Modifications of Fig. 1 involve the removal of units 17 and 19 or of 17 and 18, the use of a second phase comparator receiving the outputs of units 19 and 10 to synchronize unit 10, or the replacement of unit 19 by a pulse generator. Further, gate 11 may be replaced by a switch directing the reference bursts alternately to units 12 and 16. A second embodiment (Fig. 3, not shown) is generally similar to that of Fig. I but there is no phase shifter between units 12 and 16, the required phase shift being introduced between units 8 and 16. Further, the comparator output passes to switch 10 only via a diode (31). The phase shifter (30) may alternatively follow units 8 or 11.