Patent GB-440114-A (GB)
Title: Improvements in or relating to optical sound recording, reproducing and like systems
IPC: G11B7/00
Filed: 1934-06-20
Abstract: 440,114. Optical sound recording and reproducing apparatus. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A., and SMALL, C. J. P., White Cottage, Burton Hole Lane, Mill Hill, London. June 20, 1934, No. 18294. [Class 40 (ii)] [See also Group XX] In an optical sound recording, reproducing, re-recording, or printing system comprising a light source and means for projecting light therefrom on to a moving record strip, the light after emission from the source is so modulated in accordance with the current supplied to the source that variations in the intensity of the emitted light due to fluctuations in the current supply are substantially neutralized. Figs. 2 and 4 show a construction of optical unit embodying the invention which may be used in a system for reproducing sound from a film record in conjunction with the projection of pictures, a single lamp being used both as projector lamp and exciter lamp. Light from the lamp (not shown), is received by a lens 7, Fig. 2, and directed by an adjustably mounted mirror 8 and lenses 9, 10 towards a slit 11, the light passing through the slit being focused by lens systems 12, 13, 14 on the sound track carried by film 5. The slit 11 comprises two jaws 17, 18 mounted by screws 20 on a soft steel member 19 so as to project across an elongated opening 21 therein, the slitbeingcurvedtocompensate for curvature in the focal plane of the light projecting means. The member 19 is mounted on a support 22 which also carries a mask 24 for limiting the effective portion of the slit, the mask being adjustable by means of a screw from the exterior of the apparatus. An electromagnet 25, 26 located below the member 19 is connected by leads 27 to the same source of current as the lamp and co-operates with armatures 29 formed as extensions of the member 19 which is provided with recesses 30 at the ends of the part carrying the jaw 18. The slit unit comprising the jaws 17, 18, member 19, support 22, mask 24 and electromagnet 25, 26 is removably mounted inside the lens tube assembly. The winding 25 preferably comprises a large number of turns of fine insulated copper wire and the desired phase relationship between the current in the winding and the applied voltage is obtained by an adjustable external resistance, as large a mass of conductive material as possible being provided in the path of the magnetic flux to allow for the production of eddy currents which tend to limit the reactance of the circuit. Thus, the brass end flanges of the magnetizing coil are preferably made as thick as possible. According to a modification, the slit is formed by a pair of slit-defining members caused to vibrate in a magnetic field under the control of the lamp current whilst, according to a further modification, a fine wire is connected to a shutter in the light path, which is pulled in the opposite direction by a spring, a current derived from the lamp current being passed along the wire thus heating it and causing it to expand and contract in accordance with the current fluctuations and thereby causing the shutter to oscillate with a period of oscillation half that of the current, thus modulating the light in the manner required to smooth out the fluctuations.