Patent GB-751919-A (GB)
Title: Improvements relating to colour television
IPC: H04N11/22
Filed: 1954-08-12
Abstract: 751,919. Television. COLUMBIA BROADCASTING SYSTEM, Inc. Aug. 12, 1954 [Aug. 19, 1953], No. 23493/54. Class 40 (3). Sequentially developed colour television signals are transformed into signals of the simultaneous type (e.g. the type employed in the NTSC system) by supplying the sequential signals to picture reproducing tubes individual to each colour component and re-scanning the reproduced pictures by means of respective picture signal generating tubes. The picture reproducing tubes may be of the long-persistent type and the picture signal generating tubes of the image orthicon type or alternatively the former tubes may have faster decay phosphors and long storage tubes of the vidicon type employed in place of the latter. In one embodiment (Fig. 1) R (red), B (blue) and G (green) component signals developed sequentially by camera tube 11 at a line scanning rate of 47,250 per sec. and a field scanning rate of 180 per sec. are " gated " to respective picture-reproducing tubes 32R, 32B and 32Y (operating at the same line and field rates), the R and B signals being supplied unchanged and the G signal combined with selected portions of the R and B signals to form the luminance signal Y of the NTSC system and fed to tube 32Y via a crispener circuit 36. The (double interlaced) component images thus produced are then re-scanned by means of camera tubes 38R, 38B and 38Y, operating at conventional line and field scanning rates (e.g. the 525 line 60 field U.S. standard) and with the line scanning direction at right angles to that employed in camera tube 11. In this manner the R, B and Y signals are simultaneously derived and fed to a matrixing circuit 44 in which the R and B signals are converted to the Q and I signals of the NTSC system, the Y signal being preferably fed through a " crispener " circuit 60 to improve its definition since it is this signal which carries image detail. The Q and I signals are then employed to modulate quadrature phases of a sub-carrier wave which is combined with the Y signal and modulated onto a main carrier for transmission in conventional manner. In a modification (Fig. 3, not shown) of the arrangement described in connection with Fig. 1 the video signal on lead 26 (Fig. 1) is supplied directly to amplifiers 31R and 31B and via a colour mixer circuit which supplies the proportion of the R, B and G signals which form the Y signal to amplifier 31Y tubes 32R and 32B being blanked " off " during the occurrence of the other respective colours. This arrangement is stated to compensate for changes in the D.C. level of the outputs of separator 19 (Fig. 1) which result from voltage and temperature variations. In another modification (Fig. 8, not shown) the R and B signal outputs of separator 19 (Fig. 1) are fed directly to amplifiers 31R and 31B respectively and the R, B, G signals on lead 26 (Fig. 1) are supplied directly to amplifier 31Y a rotating apertured shutter with apertures proportioned to allow the relative proportions of the image light of the R, B and G images which are required for the production of the Y signal being interposed between picture tube 32Y and the associated camera tube 38Y. This arrangement is stated to avoid the difficulty occasioned by the difference between the relative proportions of the R, B and G signals supplied via potentiometers 33R, 33B and 33G (Fig. 1) to tube 32Y and the proportions of these signals in the output of camera tube 38Y this being caused by the effect of the gamma of tube 32Y which usually is greater than unity. In a further modification (Fig. 10, not shown) the video signal on lead 26 (Fig. 1) is supplied (via an amplifier) to a single picture-reproducing tube the field and line scanning rates of which are similar to that of camera tube 11 and the reproduced image is directed by means of an optical beam-splitter on to respective R, B and Y signal camera tubes via individual rotating apertured shutters the apertures in the shutters in the R and B image paths serving to allow only light from these component images to reach the associated tube and that in the Y image path having different light transmissive properties for each of the R, B and G component images so as to cause the associated camera tube to produce the correct Y signal. Specifications 627,592, 647,714, 647,715 and 648,304 are referred to.