GB477252A - Improvements in or relating to systems for reproducing film sound records - Google Patents

Abstract

477,252. Optical sound-reproducing. ELECTRICAL RESEARCH PRODUCTS, Inc. May 10, 1937, No. 13265. Convention date, June 4, 1936. [Class 40 (ii)] Relates to the reproduction of sound from film records 3 of constant-width variablealignment type, i.e. having a transparent or opaque track of constant width oscillating laterally along the centre of the film as shown in Fig. 2, by means of an apparatus comprising a slit 11, Fig. 1, and an optical device 6 for separating the beam of light passing through the film 3 from a source 1 into two parts corresponding to the two halves of the film, each part affecting separate light cells 7, 8, the output of which is combined in push-pull. According to the invention, in order to reduce ground-noise, the effective amount of the light passing through the film is reduced gradually from the centre to the edges of the film or vice versa according to whether the centre track is opaque or transparent. This may be obtained by making the slit 11 narrower towards its ends as shown in Figs. 5 and 6 ; in addition, the slit may be separated into two halves as shown in Figs. 3 and 4. Alternatively, an apertured screen 18, Fig. 8, may be used between the slit 19 and a lens system 15, 16 with a stop 17, the position of the screen 18 along the axis of the system determining the gradation of the light. In a modification, a single lens is used instead of the combined lenses 15, 16 and stop 17. The aperture in the screen 18 may have one of the shapes shown in Figs. 11, 12 and 13. The same result may also be obtained by the use of grey wedges or the like.