GB1319452A - Combined phase-detector and discriminator for pal-secam decoders - Google Patents

Abstract

1319452 Colour television receiver RCA CORPORATION 23 Sept 1970 [1 Oct 1969] 48175/69 Heading H4F A PAL/SECAM decoder includes a PAL or SECAM (R.T.M.) identification signal demodulator which detects the colour bursts of alternating phase (PAL) or alternating frequency (SECAM) and in response thereto synchronizes the phase of the 0/180 degrees PAL switch 5, Fig. 1 or the SECAM switch 6. The colour bursts in the received signal are fed via gate 2 to the identification signal demodulator 4. Demodulator 4 controls the phase and frequency of PAL oscillator 7 which in turn controls demodulator 4 for synchronous phase demodulation. The identification signals from demodulator 4 are fed to a phase comparator 19 for comparison with the output of a switching flip-flop 20 (bi-stable multivibrator driven by switch 21 under control of comparator 19) which delivers a square-wave for operating switches 5 and 6. A colour killer 24 may be controlled by a comparator 19. Identification signal demodulator 4 is shown in Fig. 2 in which the colour bursts, selected by gate 2 by the line flyback pulses fed to terminal 3, are fed to terminal T3 centre-tapped on the primary L 1 of a partly auto-transformer 29 having a secondary winding L 2 . Windings L 1 and L 2 are tuned by shunt capacitors. The ends T 1 and T 4 of the primary L 1 are connected across a first chain of diodes D 1 and D 2 having an intermediate point 30 for SECAM output, and across a second chain of diodes D 3 and D 4 having an intermediate point 31 for PAL output. Point 31 receives the PAL reference oscillations over line 9 (see also Fig. 1) for synchronous demodulation, and point 30 is connected to one end T 5 of secondary L 2 . Points 30 and 31 are both connected to an amplifier transistor 34 for common amplification of either PAL or SECAM demodulated identification signals at one half of the line frequency for feeding over output line 19a to the phase comparator 19 (see also Fig.1) and over line 8 to the PAL oscillator 7. Between PAL and SECAM operation nothing is switched except the energizing supply of the PAL oscillator 7, a signal being only derived at output 30 when SECAM signals are received and at output 31 when PAL signals are received.