Patent GB-647714-A (GB)
Title: Improvements in colour television
IPC: H04N9/22
Filed: 1946-10-11
Abstract: 647,714. Television. COLUMBIA BROADCASTING SYSTEM, Inc. Oct. 11, 1946, No. 30449. Convention date, Sept. 7, 1940. [Class 40 (iii)] [Also in Group XX] In a colour television system including means for scanning an entire image area from one limiting boundary to the other by parallel lines extending fully across said area and advancing progressively at a uniform speed, a rotatable colour filter disc has a number of sets of a plurality of non- radial segments corresponding to the colours employed and equally spaced around the disc, the segments extending over central angles substantially greater than their peripheral angular extent and the leading edge portions of each segment are convex and so proportioned with respect to the full length of the scanning-lines that they obscure progressively and completely each successive scanning line at approximately the same, preferably uniform, speed at which the lines are advanced in scanning. As shown in the transmitting arrangement, Fig. 1, the colour filter disc 14, rotated by a motor 20, is located adjacent a coloured slide 12 which is focussed by lens system 16 on the screen 17 of an image dissector 18. A heat filter 13 is included between the projection light source 10 and the disc 14 and an infra-red filter 19 is located between the object (slide) 12 to be transmitted and the image dissector 18. The object to be transmitted may be a coloured film. Fig. 3 shows a colour filter disc 22 having six colour segments and located near the image field produced at the end of a cathode-ray tube 21. The derivation of the contours of the edges of the segments of a disc having two sets, each comprising red, green and blue, is indicated in connection with Figs. 4, 5. The peripheral extent of each segment is 60 degrees and a radius 34 making an angle of 6 degrees with the horizontal diameter 33 is drawn to allow for the frame fly-back time. The arc of 54 degrees between the radii 34, 35 is divided into eight equal parts, and each lateral edge of the image area is subdivided into eight equal parts 0<SP>1</SP>1<SP>1</SP>, 1<SP>1</SP>2<SP>1</SP>, 2<SP>1</SP>3<SP>1</SP> ... 7<SP>1</SP>8<SP>1</SP> and 0<SP>11</SP>1<SP>11</SP>, 1<SP>11</SP>2<SP>11</SP> ... 7<SP>11</SP>8<SP>11</SP>. A circular piece of tracing paper equal in size to the disc is marked with a radius line A which is rotated to be superposed on the radius 34. The points 0<SP>1</SP>L, 0<SP>11</SP>L coinciding with the points 0<SP>1</SP>, 0<SP>11</SP> are then marked. The tracing paper is then rotated so that the line A moves to the position 1 and the points 1<SP>1</SP>L, 1<SP>11</SP>L coincident with the points 1<SP>1</SP>, 1<SP>11</SP> are marked. The radius A is then moved to the position 2 and points 2<SP>1</SP>L, 2<SP>11</SP>L coincident with points 21, 2<SP>11</SP> are marked. This operation is repeated until the position 8 and points 8<SP>1</SP>L, 8<SP>11</SP>L coincident with points 8<SP>1</SP>, 8<SP>11</SP> are obtained. A smooth curve drawn through the points 0<SP>1</SP>L, 1<SP>1</SP>L ... 8<SP>1</SP>L, 0<SP>11</SP>L, 1<SP>11</SP>L ... 8<SP>11</SP>L is then drawn delineating the leading edge of the segment. A further leading edge spaced by sixty degrees from the first leading edge is similarly derived, and so on until the six segments have been formed. As indicated in Fig. 4, the central angle subtended by the leading edge is 62¢ degrees and the segment extends over a central angle of 123 degrees. In a modification, Fig. 6, the segments are reduced in extent and are such as to cover a limited portion of the scanned area. In a further modification, Fig. 7 (not shown), the segment is reduced in extent and is suitable for cathode-ray tubes with appreciable afterglow, the segment being such that a line when covered by a segment of a particular colour is still covered by that segment during the period of persistence of the afterglow. As indicated in Fig. 6, the colour segment may have its leading edge formed by straight lines 39, 39<SP>11</SP> so that its convex form is still maintained-the trailing edge being indicated by the line 39<SP>1</SP>. In a further construction, Fig. 8, the scanned area 31 is above the horizontal plane through the axis 32 of the filter disc, and in modification of this arrangement, Fig.. 9 (not shown), the segments are reduced in extent so that the coloured segment covers a limited area on either side of the line on the screen being scanned at any instant. In the cases in which the coloured segments do not extend over the whole surface of the disc, the intervening areas are preferably opaque. Specification 647,715 is referred to.