Patent GB-362416-A (GB)
Title: Improvements in and relating to sound and picture reproducing apparatus
IPC: G11B7/00
Filed: 1930-05-29
Abstract: 362,416. Optical sound-reproduction ; associated working with kinematographs. BRITISH THOMSON-HOUSTON CO., Ltd., Grown House, Aldwych,London. -(Assignees of Marvin, H. B. ; 121, Woodland Avenue, Schenectady, New York, U.S.A.) May 29, 1930, No. 16634. Convention date, May 29, 1929. Void [Published under Sect. 91 of the Acts]. [Class 40 (ii).] Relates to associated soundreproducing and kinematograph projection apparatus of the kind in which a combined sound and picture film is employed. According to the invention, the light-sensitive cell used for sound-reproduction is mounted on the projector, and the amplifier connected thereto is located at a distance from the projector, to isolate it from the vibrations of the latter, the connections between the light-sensitive device and the amplifier comprising a cable and suitable transformers for minimizing the capacity leakage effects in the adjacent conductors of the cable. In the apparatus shown, a photo-electric cell 4 together with a lamp 1 from which light is projected on to the cell through the sound record on the film 3 are mounted in a casing 12 on the projector (not shown). The cell 4 is connected through a current source 5 and resistance 6, shunted by a condenser 7, with the primary winding of a step-down transformer 14, the secondary winding of which is connected to a cable 13. The remote end of this cable is connected to the primary winding of a step-up transformer 15, the secondary of which is coupled to the input of an amplifier 8 which operates a loud-speaker. The circuit arrangements for the first stage of a suitable amplifier are shown, and a further example in which a screened-grid valve is used is also shown in the Specification. The transformers may each be constructed, as shown in Fig. 3, with an inner winding having a relatively few turns forming a relatively long coil 26 of small diameter. The outer winding has a larger number of turns, and forms a coil 27 of relatively short length and large diameter. The coils may be wound on an insulating spool 28, and supported between flanges 29 preferably of wood or fibre. A ratio of 12 to 1 between the numbers of turns in the respective windings is stated to be satisfactory, the small and large coils having approximately 1500 and 18000 turns respectively. The core 30 of the transformer may be made ot double L-shaped laminations, overlapped to form a closed member of substantially square cross section. The windings are arranged on one leg of the core, producing a design which gives rise to considerable inductive leakage. The leakage reactance of the transformer may be designed to resonate with the capacity existing in the cable circuit, to improve the transmission characteristic. The distance between the inner and outer layers of the " pancake coil 27 is large, reducing the distributed capacity in the windings; and the inner winding 26 is considerably spaced from the core 30, to reduce capacity effect between the coil and the core. Characteristic curves are given to show the improvement effected by the use of the transformers as regards prevention of high frequency attenuation due to capacity effect. An explanation is also given of the advantage arising from the use of a screenedgrid amplifier with the transmission system described.