INFORMATION TRANSFER SYSTEMS

1378292 Colour television; multiplex radio signalling MARCONI CO Ltd 19 April 1972 [12 June 1971] 27652/71 Headings H4F and H4L Simultaneous transmission of two signals U and V, e.g. the two chrominance signals in a colour television system or stereo radio signals, is effected by frequency modulating an oscillator output with instantaneous values of the expression arc tan V/U, and then amplitude modulating the frequency modulated output with instantaneous values of the expression #U<SP>2</SP> + V<SP>2</SP>. As shown in Fig. 1 the two signals U and V are fed to two balanced modulators 3 and 4 supplied with signals from a constant frequency oscillator 5 with 90 degree phase shift between them. The outputs U sin wt. and V cos wt. are combined at junction 14 to give #U<SP>2</SP> + V<SP>2</SP> sin (w + k#)t, where k is the modulator constant and # = arc tan V/U. Limiter 7 removes the amplitude term #U<SP>2</SP> + V<SP>2</SP> and discriminator 9 provides an output the magnitude and polarity of which is representative of the divergence of the applied frequency (w+k#) from the frequency w. The output of discriminator 9 is integrated 10 and applied to a VCO 11 to frequency modulate its output. The output from oscillator 11 has the form sin (w s +k#)t where w s is the centre frequency, and this output is then amplitude modulated 12 by #U<SP>2</SP>+V<SP>2</SP> which is passed by envelope detector 13. A decoder for this signal comprises, Fig. 3, two synchronous demodulators 33 and 34 to which the amplified 31 encoded signal #U<SP>2</SP>+V<SP>2</SP> sin (w s +k#)t is applied. The two demodulators are also supplied with signals having a 90 degree phase difference (phase shifter 36) which are derived from the. amplified encoded signal after it has been amplitude limited 32 and delayed 35. The two signals U and V are derived from the two demodulators 33 and 34 via low pass filters 37 and 38. It is arranged that the delay period # of delay 35 is such that k#=1, and w s is chosen to be equal to 2#nk where n is an integer. The amplifier 31 preferably included an AGC cicuit which is operated during the flyback periods of the television signal.