GB1351403A - Decoding systems for colour television receiver - Google Patents

Abstract

1351403 Colour television SONY CORP 26 Jan 1972 [29 Jan 1971 (2)] 3747/72 Heading H4F In a PAL demodulation system the chrominance signal, Fig. 3, is passed to demodulators 2 and 3, Fig. 4, via a direct path including switched diode 12 or a delayed path including one line period delay 10 and switched diode 13. The operation of the diode switching the associated control means 11, 14-20, is such that alternate lines are fed twice in succession to the demodulators, i.e. a line, e.g. 7th is fed via diode 12 to the demodulators and is then fed again in the next line interval via the delay 10 and diode 13. The demodulators are thus fed with signals having a constant carrier phase, i.e. the "phase alternating line" is compensated for. The colour burst signals B have + on - signs (phases) depending as the phase of the chrominances signal carrier. The frame or burst blanking period B B moves its position in the line structure in a four frame cycle, Fig. 3, and the described embodiments select the earliest occurring first B + burst, i.e. that following the third burst blanking period. The burst signals Fig. 5A, are separated out by filter 4 and after amplification in AGC amplifier 18, 19 and detection by diode 20 are passed to an AND circuit 16. A monostable circuit 14 produces a signal S1, Fig. 5B, from the vertical sync. pulse Vs, lasting 5H and ending just before the selected burst is to occur. Signal S 1 in turn produces a gate signal S2 from generator 15 which selects and gates through AND circuit 16, the selected burst. After detection, said burst sets the state of bi-stable control circuit 11 such that the diodes switching circuit 9 is always controlled to produce a modified chrominance signal of the correct constant carrier phase. In the embodiment of Fig. 7 (not shown) a quartz crystal tuned coil, tuned circuit produces a continuous oscillation from the burst signals having a phase midway between the alternate chrominance signal phases. This exhibits exponention decay between bursts with the largest increase in amplitude occurring at the first burst signal after the blanking period. This increase is detected and used to produce a special pulse which is gated through AND circuit 17 as described above.