GB1521296A - Circuit arrangements for demodulating chrominance television signals - Google Patents

Abstract

1521296 Television receivers INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA SpA 16 July 1975 [25 Oct 1974] 12903/78 Divided out of 1521295 Heading H4F A receiver for a PAL or S.E.C.A.M. television signal includes a single synchronous demodulator which is fed with the chrominance signal. When a PAL signal is received the demodulator is also fed with a reference subcarrier switched in phase at line frequency. When a S.E.C.A.M. signal is received the demodulator is fed with a modified form of the chrominance signal. In both cases the demodulator output consists of the two colour difference signals, alternately at line frequency. In the embodiment illustrated (Fig. 4) when a PAL signal is present at C' colour bursts separated by gate 54 cause generator 58 to provide a subcarrier in phase with the -(B-Y) signal. The subcarrier is fed via a half line frequency switch 59 direct or via 90 degree shifter 61 to demodulator 55, which provides R-Y and -(B-Y) signals alternatively. These signals pass, via resistor 63, switch 62, and amplifier 67, both directly and via a oneline delay 76 to a switch 75 which outputs the R-Y and -(B-Y) signals simultaneously to a PAL matrix. When a S.E.C.A.M. signal is received the bursts fed to gate 58 are not of the frequency necessary to operate the generator 58. The lack of an output from 58 is detected (60), causing a switch 53 to close. This allows the received signal to pass to unit 56 whose output phase-varies linearly with frequency and is fed to the demodulator 55, which operates now as a frequency discriminator. The demodulator output passes to a de-emphasis circuit 64-66, slected by switch 62, and D.C. clamp 68, 70, 73, and on to the amplifier 67, &c. Switch 62 is controlled in the same way as switch 53. To ensure that the ratio of the amplitudes of the signals at the output of the circuit is correct the signal fed to the demodulator 55 passes via a limiter 52 whose threshold is switched at half line frequency between limiting values in the ratio 1:1À16. The limiter has no effect upon the PAL signal since the a.g.c. operative when PAL is received maintains the signal below the lower limiting value. Whichever type of signal is received a frequency selective amplifier 79 selects the half line frequency component resulting from the demodulated bursts. This component controls the phase of bi-stable 72 which, in response to line frequency pulses H, generates a half line frequency signal. If the component disappears unit 80 generates a colour killer signal to control amplifier 67.