GB1351402A - Decoding systems for colour television receivers - Google Patents

Abstract

1351402 Colour television SONY CORP 26 Jan 1972 [29 Jan 1971] 3746/72 Heading H4F In a PAL demodulator system, Fig. 4, the chrominance signal, Fig. 3, is passed to demodulators 2 and 3 via a direct path comprising switched diode 12, or a delayed path via one line period delay 10 and switched diode 13. The operation of the diode switching and the associated control means 11, 14, 15 is such that every other line is 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-a typical line sequence to the demodulators being: 7, 7, 9, 9, 11, 11 ... The demodulators are thus fed with signals having a constant carrier phase, i.e. the "phase alternating line" is compensated for. The constant phase is + or - ve depending on the relative phase of the switching action of circuit 11 and it is the purpose of the invention to make said constant phase always + ve or - ve whenever and however the system is enabled, independent of the four frame cycle undergone by the burst blanking period B B in its wandering through the interfield spaces. The gated out bursts Fig. 5A, are rectified at 14 to give waveform 5B which, after differentiation at 15 gives waveform 5C. The positive spikes S 2 cause the high conduction of transistor 16 and the production of waveform 5D at its collector. A capacitor 19 charges up during the periods between the negative going pulses and also in the burst blanking interval to give waveform 5E. A transistor 22 conducts at a predetermined threshold of time t 1 and produces, until the end t 2 of the burst blanking interval, a switching pulse 5F for the bi-stable circuit 11 producing the diode switching pulses, the circuit 11 thereby always being in the correct state at the beginning of the ensuing field to give the bursts of the required phase.