GB1519822A - Demodulator cicuit for colour television signals - Google Patents

Abstract

1519822 Television receivers INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA S.p.A. 30 June 1975 [5 July 1974 17 Jan 1975] 27500/75 Heading H4F An arrangement for demodulating a PAL chroma signal includes means for separating the two colour components which are then passed to respective synchronous demodulators. The demodulators are also fed with respective sub-carriers generated by means controlled by a half line frequency switching means so arranged that the sub-carriers have the required phase irrespective of the phase of the switching means and without the need for phase identification. means. In one embodiment, Fig. 1, the chroma signal is fed direct and via an n-line delay 2 (n odd) to an adder 4 and subtractor 5 which respectively provide the B-Y and R-Y components, the R-Y component inverting at half line frequency. Demodulators 6, 7 generate the colour difference signals. The chroma signal also passes to a burst gate 10 which passes the bursts to switch 11 switched at half line frequency to deliver successive bursts to alternate oscillators 12, 13. The oscillators generate respective constant phase sub-carriers in phase with the bursts applied to them. The sub-carriers are applied to adder 14, which delivers a subcarrier phased along the B-Y axis to demodulator 6, and to subtractor 15, which delivers a sub-carrier phased along the R-Y axis. An inverter 16 inverts the R-Y sub-carrier during alternate lines. Because the inverter is controlled by the half line frequency signal that controls switch 11 the sub-carrier fed to demodulator 7 is inverted at half line frequency and has the phase required for the R-Y component to be demodulated. In a second embodiment, Fig. 2, the chroma signal is fed directly and via a delay 2 to a reversing switch 30 switched at half line frequency. The switch directs alternate line signals to one input of an adder 4 and a subtractor 5, and the remaining line signals to the other input. The adder provides the B-Y component to a demodulator 6 and the subtractor the R-Y component to a second demodulator 7. The R-Y component has constant phase. Constant phase demodulating sub-carriers are derived from generators 22, 23, phase-controlled by constant phase bursts separated by gates 20, 21.