GB1531248A - Demodulator circuit for colour television signals - Google Patents

Abstract

1531248 Television receivers INDESIT-INDUSTRIA ELETTRODOMESTICI ITALIANA SpA 4 Nov 1975 [8 Nov 1974] 45736/75 Addition to 1499990 Heading H4F A colour killer is responsive to the output of a phase comparator fed with the two sub-carrier demodulating signals that are passed to respective demodulators for demodulation of a chroma signal, the colour killer serving to allow or prevent the passage of the chroma signal to the demodulators. The chroma signal is of the swinging burst PAL type. The chroma signal 1 passes via colour killer 2 to demodulators 8, 9. Burst gate 3 enabled by a line frequency signal 12 passed the burst via half-line frequency switch 14 to synchronize oscillators 17, 18. One oscillator provides a sub-carrier in phase with the PAL line bursts and the other a sub-carrier in phase with the NTSC line bursts. Switches 15, 16 which are synchronized with switch 14 ensure that demodulator 8 always receives a subcarrier having the phase of the burst in the line now being received and that demodulator 9 always receives a subcarrier having the phase of the burst in the preceding line. Both sub-carriers are also fed to a phase comparator 22, one being subject to a +90 degree shift at 21. During reception of the chroma signal the inputs to the comparator are alternately in phase and in anti-phase so the comparator output is a signal at half-line frequency. This signal passes via a half-line frequency filter 23 to a threshold unit 24 which maintains the colour killer 2 inoperative. In the absence of bursts neither oscillator 17, 18 oscillates, so the comparator output is zero and the colour killer is energized. Should a spurious burst-frequency signal be received during monochrome transmission both oscillators will oscillate with the same phase and the inputs to the comparator will therefore be in phase quadrature so that again the comparator output will be zero and the colour killer will be energized.