Patent GB-1508031-A (GB)
Title: Circuit arrangement for the processing of chromatic television signals
IPC: H04N11/16
Filed: 1975-04-01
Abstract: 1508031 Colour television INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA SpA 1 April 1975 [9 April 1974] 13308/75 Heading H4F A circuit, for deriving chrominance signals from chroma signals of the type in which the phase of one chrominance component is reversed in adjacent lines and in which a colour burst reference signal accompanies each video line signal or accompanies only alternate line signals, includes means for introducing a phase shift in the burst signal relative to the chrominance components prior to passing the burst signal and the chrominance components to a synchronous demodulator. The demodulator provides one of the chrominance signals and also an identification signal which controls a switch so that the phase of a sub-carrier is reversed periodically with the correct phase. In a first embodiment, Fig. 1, which can demodulate both standard PAL signals and ISA signals in which the colour burst is present only during alternate lines and is in anti-phase with the B-Y component, the B-Y and R-Y components of the chroma signal at the input C pass via switch 1 and amplifier 6 to respective demodulators 12, 11. The colour burst is passed to the demodulators via +45‹ phase shifter 7. It is also passed to phaselocked loop 9, 15, 16 to provide a sub-carrier in phase with the R-Y component. This sub-carrier passes via - 90 degree phase shifter 18 to demodulator 12 which consequently provides the B-Y chrominance signal and, in response to the colour burst in alternate lines, negative pulses at half line frequency, i.e. identification signals. The pulses pass via a half line filter 28 to control the phase of a switching pulse generator 25 which reverses switch 22 during successive lines so that the sub-carrier from the phase locked loop is passed either directly or via phase inverter 19 to demodulator 11, which therefore provides the R-Y chrominance component together with positive pulses at half line frequency. The generator 25, switch 1, and burst gate switch 27 are controlled by line frequency clock pulses at H synchronized with the colour burst. Phase shifter 7 may be Œ 45 degrees or Œ 135 degrees, the operation being generally similar to that just described. A second embodiment, Fig. 2, for demodulating PAL signals only, can tolerate substantial phase error in the generated sub-carrier. The colour burst passes via - 45 degree phase shifter 40 to switch 42 which, during alternate lines, passes it via -90 degree phase shifter 45 to a sub-carrier oscillator 47. The sub-carrier passes to demodulator 12 which provides the B-Y signal and also, during the other alternate lines, negative identification pulses at half line frequency. These pulses control switching pulse generator 25 as in Fig. 1. Phase error merely reduces the amplitude of the identification pulses. If switching generator 25 operates in the wrong phase positive pulses appear with the B-Y signal and these are used to reverse the phase of the generator. The shift introduced by phase shifters 40, 45 may alternatively be -90 degrees, -45 degrees, respectively. The operation is generally similar but a reduced amplitude identification signal appears at both demodulators 11, 12. These signals may be added to give a signal of greater amplitude than in the first alternative. Half line frequency filter 28 may be replaced by a low pass filter or a peak detector. In a modification of Fig. 2 (Fig. 3, not shown) switch 42 feeds unit 10 via a -90 degree phase shifter (50), phase shifters 45, 48 are omitted, a -90 degree phase shifter (51) is placed between sub-carrier generator 47 and demodulator 12, and burst gate 27 is fed by demodulator 11.