GB1395491A - Circuit arrangements for colour television receivers - Google Patents

Abstract

1395491 Colour television EMI Ltd 30 May 1972 [11 June 1971] 27399/71 Heading H4F In a PAL colour television receiver which includes a switch 2, Fig. 1, which selects only alternate lines of chrominance information (or routes alternate lines through different channels), the phase of the demodulating oscillations from oscillator 12 applied to the demodulator 11 for the R-Y component which alternates in phase from line to line is controlled 21, 19 in dependence upon the phase of operation of switch 2. Switch 2 and burst gates 15 and 20 are operated in response to line flyback pulses at terminal 16, and the colour bursts from gate 15 (which alternate in phase Œ45 degrees about the -(B-Y) modulation axis at line frequency), control oscillator 12 which is connected in a phase-locked loop with discriminator 13 and reactance stage 14. The output of oscillator 12 which is in phase with the + (R-Y) modulation axis is compared with the separated bursts from gate 20 in phase discriminator 21, the output of which is used to control the state of switch 19 which passes the oscillations from oscillator 12 either directly (for selected even lines), or via a 180 degree phase shifter 18 (for selected odd lines), to the R-Y demodulator 11. In addition the output of discriminator 21 may be used to control a true adjustment circuit 9 in dependence upon whether the odd or even lines of chrominance information are selected (delay line 7 repeats each selected line for the subsequent line period). If the switch 2 selects even lines then switches 19 and 26 are held in the positions shown, but if switch 2 trips and selects instead the odd lines then the waveform from discriminator 21 changes and throws both switches 19 and 26 to their other positions. Phase locked loop 12-14 may be replaced by a passive oscillating circuit. A fixed phase shift is preferably included in one of the input lines to discriminator 21 so that the compared signals are in phase or in phase quadrature. Burst gate 20 may be connected to the other end 8 of the delay line 7, or to the other terminal 3 of switch 2, and it may be controlled to pass only every nth burst. In a second embodiment, Fig. 2, the demodulating oscillations from oscillator 33 (i.e. oscillator 12 in Fig. 1), are fed via phase shifting circuits 35 ( -45 degrees) and 36 (+45 degrees) to demodulators 32 and 31 respectively or vice versa in dependence upon the phase of operation of switch 28, i.e. the phase discriminator 44 holds open AND gates 40 and 42 or does not inhibit gates 41 and 43. A hue adjustment circuit of the type shown in Fig. 1, may be included in this embodiment.