Patent GB-1220550-A (GB)
Title: Pal identification circuit
IPC: H04N9/465
Filed: 1969-10-06
Abstract: 1,220,550. Colour television. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 6 Oct., 1969 [5 Oct., 1968], No. 48999/69. Heading H4F. Identification circuit arrangement for a PAL colour television receiver comprises a PAL decoder 1 the chrominance subcarrier signal component of alternating phase appearing at output 5 and the component of constant phase at output 7. The swinging burst is gated-out 65 and applied to a synchronous demodulator .37 and the constant burst is gated-out 17 and applied to a subcarrier regenerator 25 (passive integrator) the output of which is applied via a line switchable phase-inverter 31 to an input of the synchronous demodulator 37 and to the demodulator and matrix circuit 13 from which the colour difference signals are derived. Switchable phase-inverter 31 is controlled by a PAL phase-switching signal generator 53 which comprises a divide-by-two circuit operated by line flyback pulses the switching state of which is corrected by an identification signal from the synchronous demodulator 37. A control signal output of demodulator 37 is connected to a phase control circuit which controls regenerator 25, the output of which besides being applied to the demodulator 13 is applied to a coincidence circuit 75 to which an output of the synchronous demodulator 37 is also applied. This coincidence circuit effects the colour killer action in the absence of a received chrominance signal. In this arrangement the phase of regenerator 25 is adjusted to substantially 90 degrees relative to the desired phase when the PAL phase-switching signal generator 53 is in an erroneous switching state. Maximum output is then produced by the synchronous demodulator 37 and this voltage serves as an identification signal for restoring the correct switching state of generator 53, and for effecting colour killer action. In the correct switching state the voltage produced by the synchronous demodulator, due to the fact that it is in the phase control loop of the subcarrier regenerator 25, tends to zero. In a second embodiment, Fig. 4, whose operation is similar to that of Fig. 1, the phase inverter 31 is arranged to invert the complete chrominance signal component at output 5 of decoder 1, and changeover switches 91 and 93 are provided instead of gates 65 and 17. The output of the subcarrier regenerator 25 is connected through a 90 degree phase-shifter to a reference input 97 of the demodulator 13 and to the input of a phase inverter and distributer circuit 103 to which the output of the synchronous demodulator 37 is applied. Circuit 103 controls the phase regenerator 25. A further difference from Fig. 1 is that the input of the PAL phase inversion signal generator 53 and of the coincidence circuit 75 for the colour killer operation are connected to the output of a polarity correction circuit 109 to which is applied output voltages from circuit 103 which are a measure of the phase deviation between the colour burst signal and the subcarrier signal at the inputs of the synchronous demodulator 37. The other input of the coincidence circuit 75 comprises the output of the regenerator 25 via an amplitude detector 118.