Improvements relating to colour television receivers

摘要

914,605. Colour television. SYLVANIA-THORN COLOUR TELEVISION LABORATORIES Ltd. May 20, 1960 [June 15, 1959], No. 20500/59. Class 40 (3). In an arrangement for transferring the modulation from a chrominance sub-carrier signal to an index-frequency carrier derived from the traversal of colour stripes on a cathode-ray tube screen, the sub-carrier is demodulated at three different phase angles by means of demodulators V1(a V2(a) and V3(a) and the resulting modulation signals remodulated on to the index-frequency carrier by means of modulators V1(b), V2(b) and V3(b). Each demodulator is connected to the associated modulator to form a cascade stage, a filter F1 and a sub-carrier trap circuit 77 being included between the stages to ensure that only the modulation signals are transferred. Earthed-base transistor stages may replace the valve stages shown, the collectors of the demodulators being connected to the emitters of the modulators. The chrominance sub-carrier is applied to the anodes of the demodulators by transformer LL, L1, L2, L3 and sub-carrier reference signals are applied in appropriate phases to the grids. In a modified arrangement, Fig. 3 (not shown), the sub-carrier is applied to the grids and the reference signals to the cathodes. The invention is described as applied to the N.T.S.C. system. The demodulators are operated at phase angles of 121‹, 219‹ and 353‹ to give the signals 0.90(R-M), 0.83(G-M) and 0.67(B-M), where M = 1/ 3 (R+G+B). In the modulators the signals are modulated on to the index-frequency carrier at phase angles of 0‹, 120‹ and 240‹. The signals are made equal in amplitude by making the gains of demodulators or modulator different or by selecting appropriate proportions of the signals as they appear across resistors R1 in the couplings between the stages. The gains of the demodulators may be made different by choice of ratios in transformer LL, L1, L2, L3. The modulators receive the index frequency carrier on the grids and may be arranged to operate with signals which are either at 120‹ phase intervals or all in phase. In the latter case delay lines are associated with the combining circuit EF in the anode circuits to bring the modulator outputs into the desired 120‹ phase relationship. A signal of the form Y-M may be derived from the signals R-M, B-M and G-M by means not shown. Reference is made to an arrangement in which the demodulators are operated at more convenient phase-angles (e.g. 0‹, 120‹ and 240‹) and the resulting signals are matrixed to give suitable signals to apply to the modulators. Specification 895,772 is referred to.