Patent GB-1360522-A (GB)
Title: Circuit for demodulating a colour difference signal in the pal  system colour television signal
IPC: H04N9/455
Filed: 1972-03-06
Abstract: 1360522 Colour television NIPPON VICTOR KK 6 March 1972 [9 March 1971] 10281/72 Heading H4F A PAL colour television decoder includes two reference subcarrier oscillators each operating at a different constant phase determined by the bursts fed to it, the resulting subcarriers being used, after addition and subtraction, if necessary to demodulate the PAL chrominance signal, a switch being provided to ensure correct demodulation independent of the relative phase of the half-line frequency square wave switching signal controlling the switch and the received PAL signal. In one embodiment, Fig. 4, the subcarrier oscillators 14, 16 are each synchronized by alternate bursts received from switch 11, adder 19 providing a signal of - U axis phase and subtractor 20 providing a signal of + V or - V axis phase which coincides with the modulation axis of the V signal in the line of the PAL signal received when switch 11 is in the position c-a shown. A second switch 37 and a phase inverter 39 are provided to ensure correct operation of demodulator 33. Switch 37 and phase inverter 39 may alternatively be provided between input 30 and demodulator 33 (Fig. 5, not shown); or chrominance and reference signals may be fed directly to demodulator 33, the output from the demodulator being passed through switch 37 and phase inverter 39 (Fig. 6, not shown). Alternatively switch 37 and phase inverter 39 may be provided to receive the input signals from 30 and provide an output to burst gate 32 and demodulator 33 (Fig. 7, not shown). Alternatively burst gate 32 in Fig. 4, may be omitted, each oscillator being provided with a separate burst gate and demodulator 33 receiving its chrominance input from a subtractor receiving the inputs from contacts a and b of switch 11, switch 37 then not being necessary (Fig. 10, not shown); or demodulators 33 and 34 may receive every second line of chrominance directly and repeated via a one-line delay line (Fig. 24, not shown). In a further embodiment, Fig. 13, one subcarrier oscillator 51 provides an output of the average phase of the bursts, i.e. - U phase, and the other subcarrier oscillator 50 provides an output of the average phase of one burst phase and a signal opposite in phase to the other burst phase, i.e. +V or - V phase as in Fig. 2, switch 37 and phase inverter 39 again being provided in the reference or chrominance input to demodulator 33 or in the output of demodulator 33, or they may be dispensed with, separate burst gates being provided for the two oscillators 50, 51, the chrominance input to demodulator 33 being from the equivalent of position 53 in Fig. 13 (Fig. 16, not shown). In a further embodiment (Fig. 20, not shown) an oscillator equivalent to 51 in Fig. 13, is provided, the other oscillator receiving every second burst, the resulting outputs being matrixed to provide the ŒV reference signal, a switch and phase inverter again being provided in one of a number of positions, or demodulator 33 receiving the subtracted outputs of switch contacts 11a and 11b (Fig. 23, not shown).