GB1345295A - Colour television receiver - Google Patents

Abstract

1345295 Colour television GENERAL CORP 10 March 1972 [18 March 1971] 11187/72 Heading H4F A demodulating circuit for a PAL colour television signal operates to reduce relative phase errors between the reference subcarriers by altering the phase of one of the reference subcarriers if the reference subcarriers are not in quadrature. A B-Y oscillator 6 and an offset R-Y oscillator 8 are provided, phase modulator 9 being fed with a phase correcting voltage from the junction of resistors 17, 18 and a saw-tooth voltage from 10. The signal from 9 is passed to demodulators 13, 14 to which are also fed signals from B-Y oscillator 6 via +45 degrees phase shifter 11 and - 45 degrees phase shifter 12. If the signals from 6 and 9 are in quadrature the outputs from 13 and 14 are square waves of equal amplitude and opposite phase, which run to zero, but if the signals from 6 and 9 are not in quadrature the outputs from 13 and 14 are not equal in amplitude and their sum is passed via an integrator to 9. Alternatively the correction signal may control the signal passed from the input to oscillator 8. In an alternative embodiment (Fig. 5, not shown) the signal from 9 is demodulated using two demodulators operating in antiphase fed by a signal from 6 via a 90 degrees phase shifter, the signal from 9 also being demodulated using the signal from 6 directly, the three resulting outputs being matrixed before being passed as the correction signal to 9.