GB1336698A - System for identification of burst signal phase in pal television system - Google Patents

Abstract

1336698 Colour television SONY CORP 12 May 1972 [14 May 1971] 22422/72 Heading H4F A PAL colour television receiver includes a one (or 3 or 5...) line delay line 19 and a switch 20 operated at the end of each line period and controlled so as to pass only received lines which have a positive (R-Y) modulation axis and the corresponding bursts, which are fed to demodulators 22 and burst gate 27, incorrect mode of operation of the switch 20 at the start of reception being corrected by a circuit 26. Circuit 26 uses the fact that for periods of nine lines, which include the vertical synchronizing intervals, the bursts in a PAL transmission are missing, and the bursts immediately preceding and following each such interval are at +135 degrees phase. Thus there is a ten-line interval between bursts of + 135 degrees phase, but a twelve line interval between bursts of - 135 degrees phase. Due to the action of one-line delay line 19 these intervals in the signal from burst gate 27 are nine and eleven lines respectively. Circuit 26 compares the length of a square wave pulse whose leading and trailing edges coincide with the start and finish of each such interval, with a square wave pulse of ten lines duration from a multivibrator triggered simultaneously with the other square wave pulse. If the pulse from the multivibrator is the shorter, flip-flop 25 is caused to change switch 20 over to its other (desired) mode. The subcarrier oscillator is caused to operate at the phase of the bursts from burst gate 27, phase-shifters of 45 and 90 degrees being included in series between the oscillator and demodulators. A colour killer 32, automatic chroma control 31 and colour control between switch 20 and amplifier 21 are included.