Patent GB-1311745-A (GB)
Title: Quadrature-modulated carrier decoder arrangement
IPC: H04N9/45
Filed: 1970-06-05
Abstract: 1311745 Television PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 5 June 1970 [10 June 1969] 27236/70 Heading H4F In a television system wherein two components QC 1 , QC 2  of a signal 3 are modulated on first and second carriers in quadrature, and wherein the two carriers may tend to depart from the ideal 90 degree phase relationship, it is necessary to demodulate the signal in first and second synchronous demodulators 9, 11, using signals SC 1  and SC 2  (17, 23 in Fig. 1) at 90 degrees to the two carriers to prevent crosstalk between the components. If the signal is of the type containing periodically occurring first and second carrier sync. signals which are modulated on the first and second carriers, respectively, at different times, e.g. during two intervals in each horizontal blanking period, as shown in Fig. 2, or alternately in successive blanking periods, the correct phase of each demodulating signal SC 1 , SC 2  may be obtained by gating the output of the corresponding demodulator during the interval when the sync. signal due to the other quadrature carrier component is present, and using the amplitude of the gated signal (which should be zero for the required 90 degree relationship) to control variable phase shifting circuits 29, 61 interposed between a demodulating signal generator 37, 43 and the inputs of the demodulators. Thus, in Fig. 1, a conventional arrangement comprising an oscillator 37 and a phase detector 43 in a phase locked loop acting on the first sync. signal, a first gate 49 operated by pulses 283 and a 90 degrees phase shifter 33 provide demodulating signals of approximately correct angle which are then subject to fine correction in the phase shifters 29, 61. The output of demodulator 9 is passed via an adding or subtracting circuit 73, where it is combined with a D.C. signal 199, to a switch 85 operated by pulses 281. During the pulse 281, when differential amplifiers 113, 115 have inputs 105, 107, respectively, at earth and a level related to a reference level occurring in the signal between the two bursts, switches 153 and 225 are operated by pulses 287 and 289, respectively. The outputs of the switches are connected to second inputs of the differential amplifiers via circuits 189, 261 which convert the resulting pulsatory voltage into a D.C. control voltage so that the inputs 105 and 207, and 107 and 275 tend to become equal. Subsequent to the opening of switch 225, a switch 223 is closed by a pulse 291 during the second burst 299 and the output of a further A.C.-D.C. converter 261 is used to control the variable phase shifter 29 so as to make the differential amplifier input 107 substantially equal to the input 275, whereupon the amplitude of the demodulated burst 299 will be minimized. Subsequent to the opening of switches 153 and 85, a circuit comprising a switch operated by a pulse 293, a further D.C.-A.C. converter 187 and the combining circuit 73 acts to clamp the correctly demodulated signal DQC 1  to a reference level in the signal occurring after the second burst. The circuitry for the signal DQC 2  is similar, the switch 227 being operated by a pulse 285. In modifications the generator 37 may be replaced by a passive regeneration (filter) circuit responsive to the colour burst and the coarse phase control omitted, or either of the circuits 29, 61 may be constructed so that it also performs the function of frequency and phase coarse control. The other demodulating signal is then derived from this output via the other fine control circuit.