Patent GB-1008512-A (GB)
IPC: H04N9/69
Abstract: 1,008,512. Colour television. ELECTRICAL & MUSICAL INDUSTRIES Ltd. Dec. 1, 1961 [Dec. 8, 1960; March 2, 1961; Oct. 12, 1961], Nos. 42268/60, 7568/61 and 36594/61. Heading H4F. The invention relates to a colour television receiver for use with a transmission in which in place of the conventional gamma corrected luminance signal E Y 1 made up of individually gamma corrected colour components [i.e. E Y 1 = .59E G <SP>1/γ</SP> + .30E R  + .11E B <SP>1/γ</SP>] there is em-   ployed a signal E Y <SP>1/γ</SP> in which the component colours are added before gamma correction [i.e. E Y <SP>1/γ</SP>= (.59E G + .30ER +.11E B )<SP>1/γ</SP>]. The transmitted signal comprises: (a) a first colour   difference signal E R <SP>1/γ </SP>- E Y <SP>1/γ</SP>, (b) a second colour   difference signal E B <SP>1/γ</SP> - E Y <SP>1/γ</SP> and (c) the luminance signal E Y <SP>1/γ</SP>. If conventional matrixing is employed to derive the red, blue and green component signals, the red and blue signal are obtained correctly, but the green signal is obtained incorrectly in the form E G <SP>1/γ</SP> + 1À7 (E Y <SP>1/γ</SP> - E Y <SP>1</SP>), where the last term denotes the error. It is shown that the error is zero for reference white and increases with colour saturation. The green signal is too large and should be decreased as the colour saturation increases. The relationship between the error and saturation is a complex one, but the invention is based on the principle that a correction sufficiently accurate for practical purposes may be obtained by decreasing the green signal as saturation increases. Where the colour difference signals are transmitted as quadrature-phase modulations of a sub-carrier, as in the N.T.S.C. type of transmission, a measure of the saturation may be obtained by detecting the instantaneous amplitude of the sub-carrier. This is employed in a first embodiment, Fig. 3, where the sub-carrier amplitude is detected in circuit 21-28 and the resulting signal applied in opposition to the   green colour-difference signal E Y <SP>1/γ</SP> - E Y <SP>1/γ </SP>obtained on lead 1 at the output of the receiver processing circuits 31. The same method is employed in Fig. 5 which shows a detecting and matrixing circuit operating on the XZ axes of the subcarrier wave (see " Colour Television " by Carnt and Townsend, page 246). The subcarrier wave amplitude is detected in circuit 121, 123, 128 and applied via stage 131 between the grid and cathode of the valve 102 in the circuit which develops the green colourdifference signal. Stage 114 is a pulsed D.C. restorer. In a second similar circuit, Fig. 6 (not shown), the detected sub-carrier amplitude is applied directly to the grid of valve 102 and by common cathode feedback to the cathodes of valves 101 and 103. The effect is not only to reduce the green colour-difference signal but also to increase the red and blue colour-difference signals as saturation increases. The combined effect is said to provide improved correction. Fig. 7 illustrates a method of multiplicative correction in a system where the green signal is obtained (in accordance with normal matrixing methods) by combining the luminance signal E Y <SP>1/γ</SP> obtained on lead 203 with - 0À3E Y <SP>1/γ </SP>and - 0À11 B E<SP>1/γ</SP> obtained via amplifier or attenuators 211, 212 and multiplying the result by 1À7 in stage 213. In accordance with the invention, the luminance signal is first passed through an amplifier 206 whose gain is normally unity and decreases as saturation increases. The control for the amplifier gain is obtained by detecting the sub-carrier amplitude in detector 208. A more accurate reproduction of saturation is obtained (i.e. the saturation per unit of luminance) by dividing the detector output by the luminance value in a divider stage 209. Fig. 8 illustrates a further form of multiplicative correction applied to a system in which the sub-carrier is demodulated in stages 225, 226 and 227 at the appropriate phase angles to obtain the colour difference signals. In accordance with invention the gain of demodulator 227 for the green signal is decreased with increase of saturation by a control signal obtained by detecting the subcarrier amplitude in detector 229 and dividing the resulting signal by the luminance signal E Y <SP>1/γ</SP> in divider 230. Where the colour signals are not transmitted as quadrature modulations of a sub-carrier, for example in a colour monitor system where the signals E Y <SP>1/γ</SP>, E R <SP>1/γ</SP> and E B <SP>1/γ</SP> are available directly, or in the SECAM system where the sub-carrier is modulated in amplitude only, detection of a received sub-carrier wave cannot be employed to derive saturation information. In such circumstances it is proposed that the saturation information may be obtained from the root mean square value of the sum of the square of red and blue colour difference signals. Conveniently, this may be obtained by modulating red and blue colour-difference signals in quadrature on a generated sub-carrier wave and then detecting the resultant amplitude.