Patent GB-1008979-A (GB)
Title: Improvements in or relating to circuit arrangements for colour television systems
IPC: H04N9/18
Filed: 1963-11-08
Abstract: 1,008,979. Colour television; automatic gain control. PHILIPS ELECTRONIC AND ASSOCIATED INDUSTRIES Ltd. Nov. 8, 1963 [Nov. 13, 1962; May 2, 1963], No. 44177/63. Headings H3Q and H4F. In a colour television system three colour difference signals R-Y, B-Y and G-Y are applied to the control-grids 3, 6 and 9 of the three electron guns of a picture tube 1, and a luminance signal Y is applied to the cathodes 2, 5 and 8 of the guns via a D.C. connected line 13 and negative feedback resistors 14, 15 which attenuate the luminance signal supplied to the cathodes 5 and 8 relative to that supplied to cathode 2. The luminance signal supplied to cathodes 5 and 8 is attenuated because the slopes of the three guns and the efficiences of the three phosphors are not equal. Arrangements for attenuating the luminance signals applied to two of the three guns are known, but this arrangement reduces colour substance which occurs when the supply voltage +V 6  varies. The luminance signal Y is supplied to the control grid of amplifier valve 11 which may be the video output valve, the anode of which is connected through an anode load resistor 12 to the supply voltage + V 6 . Simultaneous brightness control for the three guns comprises a single potentiometer circuit 16, 17, 18. To avoid colour unbalance resistors 14, 15 are adjusted so that a black and white picture is produced due to the luminance signal Y only when the colour difference signals are reduced to zero. When the current through valve 11 is such that a black image is reproduced, i.e. three guns cut off, and the supply voltage V 6  increases, the mode voltage of valve 11 increases to an extent determined by whether the anode current of the valve is stabilized. For this purpose the cathode of valve 11 is connected to earth through a cathode resistor 19 and through line 20 to the cathode of a further amplifier valve 21 by means of which the control voltage for automatic gain control is obtained, line flyback pulses being applied along line 22 and the resultant negative control voltage being supplied through line 26 to the control grids of at least some of the intermediate and/or high frequency amplifier valves. The flyback pulses 22 may be delayed so that control is effected on the black level porches, i.e. the black level is stabilized. Thus the voltage drop across resistor 12 for this black level remains constant and the voltage variations at the anode of valve 11 are equal to the variations of the supply voltage V 6 . If the black level is not entirely stabilized by the automatic contrast control, when the supply voltage varies the anode current through the valve 11 varies. The variation of the direct voltages at the controlgrids 3, 6 and 9 is much smaller than that of the anode voltage and thus when supply voltage V 6  decreases the three guns are not cut off and thus a voltage drop is caused across resistors 14 and 15 in proportion to their value and in a sense such as to reduce the current so that although the luminance signal is zero a greyish image is reproduced on the screen thus giving the same effect as if the direct voltages were brightness control voltages or luminance signals, i.e. instead of colour unbalance the variations in the supply voltage cause variations in the background brightness. For feedback at high frequencies the capacitors 30 and 31 may be omitted. To adjust the system the exact cut-off voltages for the three guns are adjusted by means of the resistors 32, 33, 34 and 35 connected between supply voltage V 6  and the larger booster voltage Vc and then the colour balance is adjusted by means of resistors 14 and 15. Besides attenuating the signal, negative feedback causes any non-linearity in the circuit to be partially removed and this is undesirable as the incoming television is " gamma corrected " and also the three guns must be supplied with signals containing the same degree of distortion. Thus in order to reduce the linearizing action of the signal supplied to cathodes 5 and 8 non-linear elements, e.g. non- linear resistors 36 and 37, are connected in series with the variable resistors 14 and 15 respectively. Non-linear resistors 36 and 37 may be voltage-dependent resistors or resistors with negative temperature coefficients and may be connected in parallel with the ohmic resistors 14 and 15. Further, the ohmic resistors 14 and 15 may be omitted (see Specification).