Patent GB-1158714-A (GB)
Title: Improvements in or relating to Colour Television Equipments
IPC: H04N11/16
Filed: 1966-10-25
Abstract: 1,158,714. Colour television. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. 25 Oct., 1966 [2 Nov., 1965], No. 47919/66. Heading H4F. To correct phase errors produced in transmission in a PAL system a colour sub-carrier phase error correcting circuit is provided at the end of the transmission path before the receiver wherein the colour sub-carrier 1, Fig. 1, to be corrected is first mirrored about a mirror axis U<SP>1</SP> the phase of which, with respect to the non- switched modulation axis U, is shifted by the phase error angle #, the resultant mirrored colour sub-carrier 2 is then mirrored about the non-switched modulation axis U to produce a twice mirrored sub-carrier 3 which is then combined with the original sub-carrier 1 to form colour sub-carrier 4 of correct phase but double the amplitude of the sub-carrier 1, α being the correct modulation angle of the sub-carrier. In the correction circuit, Fig. 2, the colour sub-carrier to be corrected in phase is fed from input terminal 5 to modulator 6 and via filter 7 to another modulator 8 the output of which is fed via filter 9 to one end of resistance network 10 whose other end is connected back to terminal 5 through adjustable delay line 11. Terminal 5 is also connected to one line period delay line 13 which is connected to resistance networks 15 and 16 and coil 14, and the intermediate tappings of networks 15 and 16 are connected to frequency doubler 17 and 18 whose combined outputs are fed to oscillator inductance capacity 19 of twice the original sub-carrier frequency #. The output of oscillator 19 is fed to modulator 6. The tapping 12 on network 15, at which appears both the colour sub-carrier signal corresponding to modulation axis U and the unswitched component of colour burst is connected to crystal oscillator 21 of sub-carrier frequency Q via gate 20 which is opened during the colour burst. The output from oscillator 21 is fed via adjustable phase shifter 22 to a further frequency doubler 23 the output of which is fed to modulator 8 and via gate 24 opened during line blanking periods to oscillator 19. In operation delay line 13, coil 14 and resistance networks 15, 16 operate as a delay demodulator to add and subtract the colour sub-carriers of a pair of successive lines and two signals of sub-carrier frequency appear at the tappings of resistance networks 15 and 16, the phase of these signals being freed from the modulation phase α of the received sub-carrier and coinciding with the modulation axes U, V including the phase error #. These signals after being frequency doubled and brought into phase synchronize oscillator 19, the phase of the output of which is proportional to the phase error angle #. The lower side band of the output of modulator 6 is filtered out by filter 7 and the expression (# - #) indicates that the direction of phase modulation is reversed with respect to the sub-carrier at terminal 5 which corresponds to a mirroring about the axis U<SP>1</SP> of Fig. 1. The colour bursts taken from the U signal by means of gate 20 synchronize oscillator 21 and its output is brought into phase with axis U by adjustment of phase shifter 22, and after being frequency doubled is used to effect in modulator 8 a mirroring about axis U of the sub-carrier from filter 7. The lower side band (Q + #)t-#) is filtered out so that the sub-carrier 3 appears at the output of filter 9. The resistance network 10 combines the two-carriers 1 and 3 and also halves the sub-carrier 4 so that a phase correction colour sub-carrier of approximately the same amplitude as the receiver colour subcarrier appears at output terminal 25. To ensure that the phase of the colour burst is not distorted by mirroring the output of frequency doubler 23 is fed via gate 24 to oscillator 19.