GB1065616A - Improvements in or relating to colour television apparatus - Google Patents

Abstract

1,065,616. Colour television. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. Sept. 30, 1963, [Sept. 29, 1962]. No 38377/63. Heading H4F. In a colour television system of the N.T.S.C. type colour falsification due to differential phase errors is reduced by simultaneous switching of the transmitter and receiver in such a manner that the direction of phase rotation of the chrominance signal is reversed in alternate lines, i.e. PAL type system. According to the present Specification the phase of some of the colour burst signals is changed, the phase changes being used to indicate the direction of phase rotation of the chrominance signal and control a switching operation at receivers so that the colour signal is demodulated in correct phase. As shown in Fig. 1, the colour bursts 1, 2 and 4 are transmitted with oscillations of the phase position B<SP>1</SP>-Y<SP>1</SP> while the oscillations of the colour burst 3 are rotated by 180 degrees and transmitted in the phase position - (B<SP>1</SP>- Y<SP>1</SP>). So that the phase change does not affect the normal synchronization of the demodulator reference oscillator of the receiver the response time of the demodulator reference oscillator is made long compared to the duration of a single burst. Preferably a burst of different phase is only transmitted after every second field scan. As shown in Fig. 3, a high frequency signal passes from the antenna 10 of a receiver to a high frequency amplifier 11 and to a mixing stage 12 in which the signal is mixed with the oscillation generated by the local oscillator 13. The intermediate frequency so produced passes to an amplifier 14 and the audio carrier is passed to a sound demodulator 16, audio amplifier 17 and loud-speaker 18. From demodulator 19 the video frequency brightness signal is obtained from the amplitude modulated picture carrier and passed through amplifier 20 to the cathode of a three-gun type colour reproducing cathode-ray tube 21. The colour subcarrier is selected by filter 22, amplified at 23 and passed simultaneously and in equal phase to the two synchronous demodulators 24 and 25. From separating stage 26 the synchronizing pulses are passed to line deflection circuit 27, and field deflection circuit 29 and the colour synchronizing bursts are passed to a datum carrier oscillator 31 so that it generates an oscillation of constant frequency and phase. This datum oscillation passes through a 90 degrees phase rotating network 33 to the synchronous demodulator 24 and to the grid of valve 34, appearing across resistor 36 unchanged in phase or across resistor 35 changed in phase by 180 degrees and by means of switch 37 the two oscillations are alternatively supplied to the synchronous demodulator 25. Switch 37 may be an electronic switch controlled by a square wave oscillation 39 produced by a generator 40, the frequency being controlled by line synchronizing pulses supplied along line 5. The phase of the generator 40 is controlled by a phase indicating circuit 41 in which the phase position of the colour synchronizing oscillations obtained from separating stage 26 are compared with the phase position of the datum oscillation and from these results the position of switch 37 is determined. One of the synchronous demodulators 24 or 25 may be used for the phaseindicating circuit because the comparison of phases is effected during line gaps. Instead of changing the phase of the colour bursts by 180 degrees the phase may be changed by 90 degrees with the result that the colour bursts used to identify a particular sense of rotation of the chrominance signal appear in the output of the (R<SP>1</SP> - Y<SP>1</SP>) demodulator which otherwise has no synchronizing output. The colour difference signals from demodulators 24 and 25 are passed to matrix 42 which produces with luminance signal Y the three colour difference signals. U.S.A. Specification 2,754,356 is referred to in which the phase of the colour sub-carrier wave signal in addition to the colour burst signal is changed with each change in the sequence of the colour components of the chrominance signal.