Patent GB-381898-A (GB)
Title: Improvements in or relating to television and like apparatus
IPC: H04N3/08
Filed: 1932-05-30
Abstract: 381,898. Television. BAIRD, J. L. and BAIRD TELEVISION, Ltd., 133, Long Acre, London. May 30, 1932, No. 15292. [Class 40 (iii).] In a television &c. system in which the scanning means comprises a mirror drum with mirrors canted to different degrees so as to cause a spot of light systematically to pass over the object or screen or to cause an image of the object to pass over a light-sensitive cell with a limiting aperture, the scanning movements occurring in two senses at right-angles, means are provided for causing the spot or image to partake of an additional movement for the purpose of effecting a displacement of the lines of traversal of the object or screen in one revolution of the drum relative to the lines of the previous revolution. Overlapping of the strips traversed in successive revolutions of the drum may occur, but in a complete cycle all parts of the screen or object are equally exposed. In the arrangement shown in Fig. 1, the mirror drum 101 with series of progressively canted mirrors 102, 103, 104 is adapted in each revolution to scan the screen 114 in a series of parallel but non-contiguous strips. Light from a modulated arc 109 before reaching the drum passes through an aperture 111 in an opaque mask 112 by a limited area which is defined by an archimedean spiral slot 106 in a rotary disc 105, the portion of the slot in use determining the extent of the secondary displacement above referred to. The drum 101 may, c.g. make three revolutions for each revolution of the disc 105, and the successive strips traversed on the screen in the first revolution of the drum mav be distanced from each other the width of three strips ; during the second revolution of the drum a series of strips contiguous with the first series is traversed ; during the third revolution the scanning of the screen is completed by a third series of strips contiguous with the first and second series. In place of the modulated arc a constant arc co-operating with a Kerr cell may be employed, and the single spiral 106 may be replaced by three complete spirals. Fig. 2 (not shown), co-operating with separate Kerr's cells or equivalents. In a further modification, Fig. 3, three similar segmental spiral slots 306, 307, 308 of selected width and commencing at suitable different distances from the edge of the rotary disc 305 are employed to scan the screen &c. in a series of overlapping strips. Other constructions are described in which the secondary displacement of scanning lines or strips is effected by means of one or more mirrors coacting with the mirror drum, e.g. in Fig. 4 (not shown) a mirror disc with a progressively varying cant reflects light to the mirror drum ; in Fig. 5, the inclination of the tiltable mirror 124 is varied by means of a rotary cam 126 engaging a lug 127 on the mirror 124 ; while in still further arrangements a second mirror drum with differently canted mirrors is intermittently revolved to co-operate with the main mirror drum, or a disc. with differently canted mirrors fitted on one face is continuously revolved. In the case of the spirally slotted disc arrangements the edges of the slots may be serrated with the object of allowing the edges of contiguous traversal strips to overlap without producing lines of increased illumination. Specifications 253,957, 289,307, 321,389, [all in Class 40 (iii), Telegraphs, Electric], 322,841, [Class 80 (ii), Gearing, Variable-speed &c.], and 326,251, [Class 40 (iii), Telegraphs, Electric], are referred to.