Patent GB-746345-A (GB)
Title: Image-reproducing system for color-television receiver
IPC: H04N9/20
Filed: 1954-06-18
Abstract: 746,345. Television. HAZELTINE CORPORATION. June 18, 1954 [July 16, 1953], No. 17962/54. Class 40 (3). A compatible colour-receiver is provided with means for controlling the reproduction of each (colour) component image at brightness levels near the extremes of the brightness range so that a monochrome image free from colour distortion is produced when monochrome signals are applied to the receiver. The invention is described in connection with a tri-gun colourpicture tube of the "shadowmark" type but is applicable to other systems in which the composite image is produced by the superim-position of a plurality of component images. In the described embodiment (Fig. 1), received signals of the NTSC type are supplied to a tri-gun colour-picture tube 31 of the "shadow-mark" type, the brightness signal Y being fed directly to the cathode of the " red gun and via voltage dividing circuits 32, 33 (to produce the desired relative magnitudes for correct colour reproduction) to the cathodes of the " green " and " blue " guns respectively and the colour difference signals R-Y, G-Y and B-Y fed to the control electrodes of their respective guns by means of individual circuits each of which contains a D.C. restoration diode and the latter two of which (i.e. the G-Y and B-Y channels) are provided with bias controlling moans for controlling the reproduction in accordance with the invention. Similar bias controlling means may also be provided in the R-Y channel but because of the small contribution which the R-Y signal makes to the combined image such means may be dispensed with in this channel. Referring to the G-Y channel (the bias control circuit for the B-Y channel being identical) the bias controlling means comprises a source of variable potential 34G (designated 234G in Fig. 2 which Figure shows elements similar to those in the relevant part of Fig. 1 and designated by similar numbers with the addition of 200 and which may comprise variable- Á valves, the output of which are fed to the different respective guns of the picture tube so that the colour signals are combined in the variable-Á valves instead of in the guns of the picture tube as in Fig. 1) connected as shown via a variable resistance 36G, fixed resistance 37G and part of the "green" D.C. reinserter diode load resistance to the first grid of the " green " gun in tube 31. The junction of resistances 36G and 37G (in effect, the first grid of the "green" gun) is connected via a variable resistance 40G and a fixed resistance 39G (shown as 240G in Fig. 2) to the second grid of this gun which is also connected to a positive voltage source via a resistance 38G, the value of which is arranged (by variation of 36G) to be Á times resistance 36G (Á being, the amplification factor for tube currents near beam cutoff). In operation the black level is first set by variation of 34G (also variation of 34B and 34R) when a low intensity (monochrome) video signal is applied to the tube and then with an applied video signal equivalent to a high light-intensity (i.e. max. white) the resistances 40G (and 40B) are adjusted so that the composite image produced by the combination of the three component (colour) images has the correct brightness and is free from colour distortion.