Patent GB-1211951-A (GB)
Title: Colour television receiver
IPC: H04N9/24
Filed: 1969-03-04
Abstract: 1,211,951. Colour television receiver. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 4 March, 1969 [5 March, 1968], No. 11462/69. Heading H4F. A colour television receiver for displaying a received PAL signal on a picture tube of the indexing type comprises a converter 21 to which is applied at input 19 a signal of indexing frequency fi (as shown of frequency kfi due to limiting and multiplication stage 18), at input 23 a colour sub-carrier signal of frequency f s  quadrature-modulated by colour information (fs, chr) the phase of one of the quadrature components alternating from line to line, and at input 25 an unmodulated colour sub-carrier signal of frequency fs, the converter supplying at output 53 a signal of writing frequency fw modulated by colour information which is suitable for displaying on the indexing tube (fw, chr). Signal kfi is fed directly and via a frequency divider 33, mixer 37, filter 41 and mixer 47, to a phase compensation stage 29, i.e. a mixer stage. The signals (fs, chr) and fs are alternately applied to the inputs of mixers 37 and 47 (an adder and a subtractor) at line frequency by means of change-over switch 63 operated by a switching signal (input 69) at half line frequency. This inversion of the sequence in which the signals are applied from 23 and 25 to the branch 19-31 from line-toline, eliminates the line-to-line inversion of the phase of one of the two quadrature components in the received PAL signal. A phaseshifting network 71 is included in the connection to input 57 when the latter is connected to converter input 25. The converter also includes a synchronous demodulator 73 to which is applied the output of mixer 47 and via a phase correction network 78 the output of divider 33. The phase ratio between the signals applied to demodulator 73 are such that it supplies a luminance correction signal (M - Y) at output 79. Signals (M - Y), (fw, chr) and Y are applied to an adder 85, the output of which (M+fw, chr) is applied to the grid of the picture tube 15. Writing frequency fw is in a fixed frequency ratio with the frequency fi, and this ratio is made equal to the ratio between the number of indexing strips per unit length and the number of colour strips per unit length on the screen of tube 15. The colour hue of the displayed picture may be adjusted by adjusting the phase shift which occurs in phase-shifting network 71 and by adjusting the phase of fs at input 25. Although the mixer stages 37 and 47 are shown connected in series in the same branch of the phase compensation circuit, they may alternatively be incorporated in different branches of the phase compensation circuit, provided that one mixer stage adds the frequencies of the signals applied thereto and the other subtracts them. Frequency multiplication stage 18 may sometimes be omitted. Phaseshifting network 71. may be incorporated in another lead to switch 63. To remove colour errors due to differential phase errors in the PAL signal a modulator is connected between output 5 of unit 1 and the input 23 of the converter. This modulator is supplied with a signal of double the subcarrier angular frequency, i.e. 2# s , and of such an amplitude that the modulator produces a signal of the form 0À74 (B - Y) sin # s t Œ 0À89 (R - Y) cos # s t.