Patent GB-1025924-A (GB)
Title: Improvements relating to colour television apparatus
Inventor: PERCIVAL WILLIAM SPENCER
IPC: H04N9/69
Filed: 1961-07-06
Abstract: 1,025,924. Colour television. ELECTRIC & MUSICAL INDUSTRIES Ltd. June 22, 1962 [July 6, 1961], No. 24498/61. Heading H4F. [Also in Division G4] Colour television apparatus comprises means for setting up three signal components which together represent a coloured picture; plural networks each having input terminals to which at least the common frequencies of respective ones of the signal components are applied and each network being arranged to produce a respective output signal to represent a different approximation to a non-linear function of the signal components applied thereto; a series of one-way devices each connected from the output of a respective network to a common point so as to set up at such point the one of said output signals which lies at an extreme of the range of values of said output signals at the time; the signal set up at said common point being the best available approximation to said non-linear function among said output signals, and means utilizing the signal at the common point with at least two of the three signal components which together represent the coloured picture. In a colour television system, signals are derived from the received waveform to represent functions of the luminance Y and colour difference components rj, bj, where (1) Y 1/γ  = (1R + mG + nB) 1/γ with an upper frequency limit of 4 Mc/s.   (2) rJ = R 1/γ - Y 1/γ with an upper frequency limit of 1À5 Mc/s. (3) bj = B 1/γ - Y 1/γ with an upper frequency limit of 0À5 Mc/s. and R, G, B are red, green, blue colour components; l, m, n are predetermined coefficients preferably, 0À3, 0À59, 0À11; so that for unit luminance, (4) y<SP>2</SP> = 0À3x<SP>2</SP> + 0À59 w<SP>2</SP> + 0À11 z<SP>2</SP> where x = R 1/γ, w = G 1/γ, z = B 1/γ, y = Y 1/γ and j#2 so that y<SP>2</SP> = 1 and (5) 1 = X<SP>2</SP> + W<SP>2</SP> + Z<SP>2</SP> where X<SP>2</SP> = 0À3x<SP>2</SP>, W<SP>2</SP> = 0À59 w<SP>2</SP>, Z<SP>2</SP> = 0À11z<SP>2</SP> representing a sphere the surface of whose positive octant (Fig. 1, not shown) is a surface of constant luminance corresponding to the colour triangle, on which any point x<SP>1</SP>, w<SP>1</SP>, z<SP>1</SP> represents a colour whose luminance is given by It is shown by solid geometry (Figs. 1, 2, not shown) that any linear function of x, y, z represented by colour component signals in a TV receiver is represented by a plane and any set of linear approximations thereto is represented by a set of noncoincident planes. The equation of the plane tangential to the spherical surface at the point representing full white is (7) 0À55Y + 0À33Z + 0À77W = 1 or (8) W r  = 1À7y - 0À51x - 0À19z or (9) Gr¢ = 1À7Y¢ - 0À51R¢ - 0À19B¢ where W r  = Gr¢ and G r  is a linear approximation to E r  when R, B, Y are correct values. While a linear approximation to w r  given by equation (8) is correct for received colour signals representing white, it is incorrect for other colours, and a linear circuit is provided for making a series of linear approximations to W r  such that in each case R, G r , B define a point on a plane horizontal to a constant luminous function defined by Y 1/γ for the received signal, and selecting the signal representing the minimum output. A series of values for W r  are obtained by selecting different coefficients of y, x, z; those shown in equation (γ) above being those appropriate to the plane tangential to Y at the white point. Signals representing Y r , -X r , and -Z r  are obtained from the video waveform of a colour TV receiver and are applied over four distinct sets of summation resistors 11, 12, 13,14; 21, 22, 23, 24; 31, 32, 33, 34; 41, 42, 43, 44 (Fig. 3) respectively producing four distinct linear approximations to which are combined at an output over four diodes unilaterally conducting D 1 , D 2 , D 3 , D 4 . The resistors have predetermined values such that the signal at each diode represents a value of W r  related to the values of -X r  and Z r  in accordance with the equation of a tangent to the constant luminance function sphere (Figs. 1, 2, not shown) at a point containing or adjacent to that defined by the luminance value Y r , and the minimum of the four values is selected by the diodes for the output. One resistor set is chosen to define the tangential plane at white and the other define tangential planes at three other points. Fig. 4 shows a colour TV receiver wherein a transmitted signal at 50 is demodulated at 51 to give a luminance component Y 1/γ and colour components rj = - (R 1/γ - Y 1/γ), bj = - (B 1/γ - Y 1/γ) of which rj, bj are applied directly to   red and blue grids 52, 53 of three gun-picture tube 54 while the luminance signal is applied as Y 1/γ to the red cathode 56 and the blue and   green cathodes 57, 58 over potentiometers 59, 60. A signal for the green grid is derived from a function generator of the kind shown in Fig. 3 receiving input signals representing Y 1/γ;   from the outputs of 51 and the signal on the green grid is given by The function generator is applicable to a receiver yielding components of the form so as to produce a signal representing Y<SP>1</SP> for the three cathodes of the picture tube; the green gun signal G 1/γ - Y<SP>1</SP> being produced   directly from the receiver components.