Patent GB-772791-A (GB)
Title: Color-television receiver
IPC: H04N11/14
Filed: 1954-02-02
Abstract: 772,791. Colour television. HAZELTINE CORPORATION. Feb. 2,1954 [Feb. 26, 1953], No. 3080/54. Class 40 (3). The invention relates to television systems utilizing constant luminance signals of the type described in Specification 689,356 and is concerned with the application of such signals to a single-gun, shadow-mask, cathode-ray tube of the type described in the June, 1950, issue of the R.C.A. Review, at pages 329-232. The direct application of constant luminance signals to such a tube results in a brightness distortion due to brightness contributed by the chrominance sub-carrier, and the invention involves the generation from one phase of the sub-carrier of a correction signal which is added to the luminance signal to remove the distortion. A chrominance error remains however in that the original signals employ asymmetrical modulation of the sub-carrier whereas the cathode-ray tube requires a symmetrically modulated signal, and in certain embodiments a correction for this is also effected as described in Specification 772,792, by remoulding the sub-carrier into symmetrical form.. In the first embodiment, Fig. 3, the detected video signals are received at terminals 27 and applied through circuits to be described to colour image-reproducing apparatus 16. This apparatus comprises a single-gun, shadowmask, cathode-ray tube and is supplied at 17a and 17b with the normal horizontal and vertical scanning waveforms. Additionally the tube is subjected to a circular scanning motion at the sub-carrier frequency of 3.5 Mc/s. and pulsed at a frequency of 10.5 Mc/s. so as sequentially to impinge on the green, red and blue phosphor dots aligned with each mask aperture. The received signals comprise a luminance component Y=.67G+.30R+.03B, where G, R and B are the individual colour signals, together with a 3.5 Mc/s. sub-carrier modulated at the 180‹ and 270‹ phase points respectively with signals r 1  and b 1 , A first channel receives the whole signal and comprises a synchronous modulator 52 and filter 335. The luminance component passes through this channel without modification whilst the sub-carrier component is effectively remoulded as described in Specification 772,792 to the symmetrical form 67G/ ‹ ‹+.67R/120‹+ 67B/240‹ suitable for direct application to the cathode-ray tube. This modification is brought about by combining the original 3.5 Mc/s. signal with another of one-third the amplitude and appropriate phase but having the modulation components in opposite sequence. The other signal is generated by beating the original with a 7 Mc/s. signal derived from a synchronized 3.5 Mc/s. generator 322 and applied through harmonic amplifier 45 and phase delay circuit 47. The resultant sub-carrier at the output of the modulator is of incorrect amplitude and is boosted relative the luminance component to the correct amplitude by filter network 335. In accordance with the invention, a correction signal is derived which cancels the brightness produced by the sub-carrier signal. With the particular received signals considered the correction signal is -.34G+.03R+.30B and is derived by detecting the sub-carrier at a phase of -103‹ with a gain of .92. This is effected in a synchronous detector 31 receiving the subcarrier through band-pass filter 30 and applying an output through filter 32 to an adder 28 where it combines with the luminance signal and remoulded sub-carrier before application to the cathode-ray tube. The 3.5 Mc/s. input for detector 31 is derived from the synchronized source 322 through delay network 40. In a second similar embodiment, Fig. 1 (not shown), a correction signal is derived as in Fig. 3, but the sub-carrier signal is applied directly to the cathode-ray tube without remoulding. In a third similar embodiment, Fig. 4 (not shown), the 0 - 2 Mc/s. components of the luminance signal are excluded from the channel including synchronous modulator 52 and are allowed to pass through the correction signal channel by the omission of filter 30. A fourth embodiment, Fig. 5 (not shown), operates in a similar manner, but the synchronous detector 31 is replaced by a synchronous modulator which received an input of 7 Mc/s. The amplitude and phase of the 7 Mc/s. signal are selected to produce the desired correction signal which appears at the modulator output as a modulation of the sum signal of 10.5 Mc/s. The signal is then combined with the luminance and remoulded sub-carrier in the adder circuit and provides correction effectively occurring at the 0, 120‹ and 240‹ phase points of the subcarrier. In a sixth embodiment, Fig. 2, the received signal is sampled asymmetrically and recombined with appropriate phase delays to provide a symmetrical signal suitable for direct application to the cathode-ray tube. The signals are derived by sampling at the phase points of 180‹ and 270‹ in stages 36b and 36c respectively, and a signal g=2(G - Y) is derived by sampling in stage 36b at 14‹. It is necessary to offset the gain factors and 2 introduced at the transmitter to produce the constant luminance characteristic and this is effected by the characteristics of input filters 235a, 235b and 235c. The luminance component passes without modification through each channel and combines with the sampled sub-carrier signals whereby the outputs of sampler 36a, 36b and 36c are respectively G, R and B. The green signal is delayed by 46‹ in network 37a and the blue signal by 30‹ in network 37c. The final signals then occur at the symmetrical phase points of 60‹, 180‹ and 300‹ and may be applied directly to the cathoderay tube 16.