GB1046777A - Improvements relating to colour television systems - Google Patents

Abstract

1,046,777. Colour television. ELECTRIC & MUSICAL INDUSTRIES Ltd. May 9, 1963 [May 16, 1962; June 15, 1962; Nov. 9, 1962], Nos. 18771/62, 23013/62 and 42514/62. Heading H4F. The invention relates to a colour television system comprising means for developing a gamma-corrected luminance signal and two colour-difference signals and is concerned with the modification of the relative amplitudes of the luminance and colour signals so as to reduce errors in the colour reproduction at a receiver which uses linear circuit means to derive component colour signals, when the luminance signal is a true luminance signal, i.e. a signal of the form Y = (0À30E R + 0À59E G + 0À11E B )<SP>1</SP>/γ as distinct from the N.T.S.C. type of signal Y<SP>1</SP>= 0À30 E R <SP>1</SP>/γ+0À59 EG<SP>1</SP>/γ + 0À11 E B <SP>1</SP>/γ. If the colour difference signals are of the form R-Y and B-Y, the red and blue component signals which are derived at the receiver are correct, but the green signal is incorrect and has a value G + 1À7 (Y - Y<SP>1</SP>). If the colour difference signals are of the form R-Y<SP>1</SP> and B-Y<SP>1</SP>, all the derived component colour signals are incorrect and have the values R+(Y-Y<SP>1</SP>), G+(Y-Y<SP>1</SP>) and B+(Y-Y<SP>1</SP>). The error (Y-Y<SP>1</SP>) is zero for white and grey elements and increases as the product of luminance and saturation increases. In carrying out the invention the modification of the relative amplitude of luminance and chrominance signals may be effected by modifying either signal alone or both, although only arrangements for modifying the signals individually are described. In Fig. 1 the colour difference signals are of the form R-Y and B-Y. Red and blue signals E R and E B derived from cameras 1 and 3 are combined in a matrix 4 with a luminance signal E Y from a camera 2 to obtain the third colour signal E G , and the four signals E R , E G , EB and Ey are gamma corrected in circuits 5-8 respectively. An N.T.S.C.-type luminance signal Y<SP>1</SP> is derived by a matrix 9, and the luminance difference " error " signal (Y-Y<SP>1</SP>) is derived by difference circuit 10. Matrix circuits 12 and 14 develop the colour difference signals R-Y and B-Y. In accordance with the invention each of these latter matrix circuits also receive the " error " signal (Y-Y<SP>1</SP>) so as to modify the amplitude of the colour difference signals. The coefficients a 2 and a 3 are determined by circuit 11 and 13 and in practice may each be made equal to 0À5. The final colour difference signals, which are restricted to relatively narrow frequency bands compared with the luminance signal, are modulated in quadrature on a subcarrier wave from an oscillator 15 in the conventional fashion by modulator 16- 17 and then combined with the luminance signal Y in circuit 18. Fig. 2 (not shown) illustrates a modification in which a fourth camera tube is provided for generating the EG signal and matrix 4, as a consequence, is dispensed with. Fig. 3 (not shown) illustrates a similar four-camera system arranged to derive colour difference signals of the form R-Y<SP>1</SP> and B-Y<SP>1</SP>. The amplitude correction is again applied to the colour difference signals, but the coefficients a 2 and a 3 in this case are negative and equal to 0À5. Fig. 4 illustrates an arrangement for developing colour difference signal of the form R-Y<SP>1</SP> and B-Y<SP>1</SP> but here the amplitude correction is applied to the luminance signal Y, the colour difference signals remaining unmodified. The arrangements utilize the amplitude of the quadrature modulated subcarrier wave as a measure of the correction to be applied. The wave becomes zero on white and grey and increases in amplitude with both saturation and luminance. The arrangement uses a fourth camera 20 to obtain signal EG but is otherwise generally similar to Fig. 1 as regards developing the colour difference signals. The outputs of modulators 16 and 17 are combined in circuit 23 to obtain the complete subcarrier wave and its amplitude is measured in an amplitude modulation detector 24 to obtain an " error " signal a, A. This is then subtracted from the luminance signal Y in subtractor 25 before it is combined with the sub-carrier wave in circuit 26. The coefficient a 1 may be made equal to 0À2. Fig. 5 shows an arrangement utilizing four camera tubes where the tube 31 for the luminance signal E Y may have a different spectral response from tubes 32-34 which provide the colour signals E R , E G and E B . Tube 31 may, for example, be an image-orthicon and tubes 32-34 may be vidicons. The signal from tube 31 is a wide-band signal (denoted by suffix W) whereas the signals from tubes 32-34 are relatively narrow band (denoted by suffix N). A luminance signal Y<SP>1</SP> N is developed by matrix 38 and colour difference signals of the form R N - Y<SP>1</SP> N and B N - Y<SP>1</SP> N are developed by matrix circuits 39 and 40. A further matrix circuit 45 followed by gamma corrector 46 develops a luminance signal of the form YN, the coefficients l, m and n in the expression (l E R + m E G + n E B ) being chosen, however, to be the same as those in the luminance signal developed by camera tube 31. An " error " signal Y N -Y<SP>1</SP> N is then developed by a difference circuit 47 and subtracted from the output of camera tube 31 in circuit 44. Reference is made to modification of the coefficients involved in matrix circuits 38 and 45 for the purpose of correcting colour errors due to failure of separation filters associated with tubes 32-34 to match the theoretically correct spectral curves. Reference is made to the application of the invention to line sequential and P.A.L. (phase alternation line) colour systems.