Patent GB-758346-A (GB)
Title: Improvements relating to colour television
IPC: H04N9/24
Filed: 1952-11-14
Abstract: 758,346. Television. PHILCO CORPORATION. Nov. 14, 1952 [Nov. 15, 1951], No. 28782/52. Class 40 (3). In a colour television receiver synchronism between the colour signals applied to the picture reproducing tube and the scanning of the colour reproducing stripes thereof is maintained under the control of signals derived by scanning " indexing stripes associated with the colour reproducing stripes. Fig. 1 shows one embodiment employing a single gun tube 10 the control grid 16 of which, in addition to being modulated by colour and brightness signals, is also modulated by a pilot wave of colour sub-carrier frequency which co-operates with the scanning of the " indexing " stripes in the screen 22 to generate the " indexing " signal. The pilot wave is generated by an oscillator 80 (synchronized at colour subcarrier frequency, 38.5 Mc/s., via " burst" separator 78) and fed to control grid 16 via an adding circuit 75 and the lower side-band (31.5 Mc/s.) of the signal produced across load resistor 48 associated with the " indexing" stripes (determined by the frequency of the pilot wave and the rate of scanning the " indexing " stripes-7 million/sec.) is selected by a filter/amplifier 52 and fed to a mixer 54 to which the colour modulated sub-carrier derived from I.F. amplifier 76 is also supplied. The output of mixer 54, the central frequency of which is equal to the average rate of scanning the " indexing " stripes, is then effective to modify the brightness signal (derived from detector 68 via L.P. filter 74 and also fed to adder 75) in synchronism with the scanning of successive colour triplets of the screen 22. In this embodiment the signal outputs of 74 and 76 are assumed to be in terms of the ICI primaries " X," "Y," " Z,'' 74 supplying Y and 76 supplying (X-Y) and (Z-Y) and to make these signals conform to the particular primaries R.G.B. of the screen phosphor stripes a colour matrixing component is produced by synchronously detecting the colour sub-carrier at a particular phase in mixer 84 and supplying the detection products in proper relative amounts to the adder 75. For this purpose, mixer 84 includes phase-shifting means for the input signals (colour sub-carrier from 76 and demodulating wave from oscillator 80) and amplifying means to establish the output signal amplitude at a proper value relative to the signals supplied to the adder 75 from filter 74 and mixer 54. Undesired signals (cross-modulation of the applied signals) caused by non- linearities of the beam-current control-voltage characteristic of the picture tube may be cancelled by a signal derived from cathode resistor 82 and combined in correct phase and amplitude in amplifier 85 with the signal developed across load resistor 48. In an alternative embodiment (Fig. 3, not shown) the picture tube has a separate electron beam to which the pilot wave is fed the cancellation arrangement of Fig. 1 being unnecessary and the embodiment also assumes the use of primary colours of a specific primary colour system M 1 (N - M) and (O - M) (M = R+G+B/3) so that the matrixing component (from synchronous detector 84, Fig. 1) is also unnecessary. In the embodiment of Fig. 1 the sound carrier is derived directly from detector 68 whereas in the arrangement of Fig. 3 (not shown) it is derived by synchronous detection in detector 120. Specifications 715,506, 745,353 and 753,451 are referred to.