GB1316109A - Signal demodulator circuit for pal/secam decoders - Google Patents

Abstract

1316109 Colour television RCA CORP 4 Sept 1970 [5 Sept 1969] 44076/69 Heading H4F A colour demodulator 2 for demodulating the red colour difference signal from a PAL or NTSC signal is modified for demodulating the red colour difference signal of a frequencymodulated " SECAM " (Registered Trade Mark) signal by including between the signal input 9 and one of the demodulator terminals 8 a variable phase shifter 11 which alters the frequency modulation of the incoming signal to phase modulation. The phase shifter 11 provides a phase shift of 90 degrees for signals at the centre frequency, the phase shift deviating linearly from 90 degrees for frequencies deviating from the centre frequency, and the resulting signal is demodulated with respect to the input red colour difference signal fed from input 9 to demodulator input 1. For use with PAL or NTSC switches S 1 and S 3 are opened and switches S 2 and S 4 are closed, as shown in Fig. 3, and a similar demodulator is provided for the blue colour difference signals. The demodulator output 6 is fed to a low pass filter (25, Fig. 5, not shown), an amplifier (26) and low level clamping stage (28) before being passed through a final amplifier (33A) to the picture tube. Before the final amplifier (33A) there is included a deemphasis stage (29) for the SECAM signal, this being switched off for PAL/NTSC by a signal which reverse-biases a diode which otherwise connects the de-emphasise stage to earth. Switched diodes may be used in place of the switches S 3 , S 4 shown in Fig. 3, the input SECAM signal may initially be amplitude limited, and the D.C. clamping level is derived from the colour bursts at the beginning of each line, these bursts being alternately at the red and blue centre subcarrier frequencies.