GB345717A - Improvements in optical slits for use in sound recording and reproducing - Google Patents

Abstract

345,717. Recording and reproducing sound. GROSE, S. J., 6, Bream's Buildings, London. Jan. 15, 1930, No. 1386. [Class 40 (ii).] Optical apparatus.-An optical slit for use in apparatus for producing a sound record o f t h e varying density y and constant width type, or for use in reproducing from such a record, has a width which is not uniform along the length of the slit. This device is designed to improve the response in sound-recording or reproduction by eliminating certain points of zero response which, when an ordinary slit is used, occur at frequencies corresponding to " wave lengths of recorded sound " equal to the slit width or to sub-multiples thereof. A slit made according to the invention may be bounded by straight lines or curves, and it may decrease progressively from a maximum width at one end to a minimum width at the other ; or the widest or narrowest points may be intermediate the ends of the slit. The intensity of the light falling on the slit may be greater at the narrower parts of the slit than at the wider parts. In the construction shown, a transparent prism has two surfaces a, b relatively inclined at about 90‹ to each other, a tapering " flat " c being formed at the apex. Light passing through the base d of the prism on to the inclined surfaces is totally reflected thereby, whilst that falling upon the flat surface c is transmitted, the part c thus acting as a slit. According to the Provisional Specification, a plurality of sound records may be made simultaneously on separate tracks through slits of different widths, the records being preferably on a single film.