Patent GB-783143-A (GB)
Title: Improvements in and relating to colour television apparatus
IPC: H04N9/68
Filed: 1955-09-28
Abstract: 783,143. Television. PHILCO CORPORATION. Sept. 28, 1955 [Sept. 28, 1954], No. 27638/55. Class 40 (3). In a colour television receiver utilizing a luminance component and a colour modulated subcarrier chrominance component (e.g. the N.T.S.C. system) the gain of the chrominance channel is increased relative to the gain of the luminance channel when the received chrominance signal increases (from a predetermined initial value) by a factor which is proportional to the relative amplitudes of the two (received) components. By this means desaturation of the reproduced colours due to the (finite size) phosphor elements in the picture tube being excited in response to portions of the composite signal which are not truly representative of colour intelligence is reduced. Fig. 1 shows one embodiment in which the chrominance signal is selected by means of a resonant circuit 15 and supplied, together with the D.C. component (which is restored by means of condenser, diode and resistor arrangement 12, 13, 36 respectively to a datum level determined by potentiometer 14) to the control grid 16 of one section of a double triode valve 18 and, without the D.C. component (due to the presence of condenser 19) to the control grid 17 of the other section of valve 18. Due to the action of the D.C. restorer a negative bias (of value determined by the setting of potentiometer 14) is developed across resistor 36 which keeps that section of valve 18 controlled by grid 16 cut-off until the ratio of chrominance component to luminance component (the latter being related to the bias developed across resistor 36) equals or exceeds a predetermined value. In this manner the gain of the chrominance channel is increased when the ratio exceeds and is reduced when the ratio is less than the predetermined value which, preferably, is approximately unity, the precise value being determined by visual appraisal of the reproduced image. The chrominance signals developed across resistor 22 are then supplied to the control electrode 24 of the picture tube 25 together with the luminance signal from filter 11 are additional improvement in the saturation characteristics of the image being achieved by reducing the amplitude of the luminance signal by an amount proportional to the (modified) chrominance signal. As shown, this reduction is effected by rectifying the (modified) chrominance signal in rectifier 23 and combining subtractively the components in the luminance signal frequency range (selected by filter 26) of the rectifier output with the luminance signal via potentiometer 27. In a second embodiment (Fig. 2) the gain of the chrominance channel amplifier 43 is controlled by means of a signal derived from a circuit 41 and dependant on the difference between the luminance signal and the rectified chrominance. In a third embodiment (Fig. 3, not shown) the chrominance channel has a fixed gain and the variation of relative gain of the chrominance and luminance channels is effected by employing the output of the circuit 41 (Fig. 2) to control a variable attenuation circuit in the luminance channel. Specifications 749,118 and 753,447 are referred to.