GB1212710A - Circuit arrangement for synchronizing a pal line switch - Google Patents

Abstract

1,212,710. Colour television synchronizing. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. (Philips' Gloeilampenfabrieken N.V.). 12 Dec., 1967, No. 56403/67. Heading H4F. In a colour television receiver for the PAL system, the colour synchronizing " burst " Bn (shown in one only of its two quadrature phase positions) is synchronously demodulated at the G-Y axis so that when the line frequency switch included in the demodulation circuit is operating at the correct phase (G-Y), the output Bn1 in response to the " burst " is small, whilst when the switch is operating in the incorrect phase (G-Y) 2 the output Bn2 is substantially at the maximum. The demodulation may be effected by a separate demodulator operating at the positive or negative G-Y axis, or by means of the R-Y and B-Y demodulators, Fig. 1 (not shown), whose outputs are matrixed to produce a G-Y signal. The output due to the " burst " is separated from the output due to the chrominance signal by means of a transistor gate stage, Fig. 3 (not shown), actuated by delayed line fly-back pulses. The separated output is integrated over a number more than 5 or more than 10 of lines in an integrating circuit to produce a signal for a correction signal generator which functions to change the phase of operation of the line frequency switch when a maximum amplitude signal is produced in response to the " burst." The gate stage may be constructed so as to require the input signal to exceed a threshold value. The bandwidth of the coupling circuit to the gate stage is restricted to reduce high frequency noise components. The integrated output from the gate stage passes through a diode which transmits only signals of a polarity which arises when the phase of operation of the line frequency switch is incorrect. The line frequency switch may function to reverse the phase of either the chrominance or reference signal applied to the demodulator. The demodulator may include a delay line system for separating the quadrature phase modulation components of the chrominance signal.