Patent GB-362915-A (GB)
Title: Improvements in or relating to methods of and apparatus for recording sound and in sound records
IPC: G11B7/00
Filed: 1930-09-04
Abstract: 362,915. Photographic sound recording; valve amplifying circuits. WESTERN ELECTRIC CO., Ltd., Bush House, Aldwych, London.-(Bell Telephone Laboratories, Inc. ; 463, West Street, New York, U.S.A.) Sept. 4, 1930, No. 16361/31. Divided application. [Class 40 (ii).] Consists in a particular method of recording sound on a photographic film or the like, whereby " ground noise " occurring during reproduction of the sound is reduced. According to the invention, the film is moved at a uniform speed through light of constant intensity, and the time of exposure of the elements of the recording material is varied directly with the volume of sound recorded, whilst the light is modulated virtually completely for all volumes of sound recorded. The apparatus shown comprises a light valve of the kind having filaments 15 carrying the varying currents disposed in a magnetic field, light from a constant source being transmitted to a film (not shown) through the space between the filaments, which lie transversely to the direction of travel of the film. The light passes through holes 18 in the field magnet 19. The average distance between the filaments is varied in accordance with the amplitude or " envelope " of the sound waves, and in addition, instantaneous variations corresponding with the sound frequencies are produced. For this purpose, a circuit is used which comprises two branches from an amplifier 13, to the input side of which is coupled a microphone 10. One branch of the output from the amplifier includes a transformer 14, by which sound modulated currents are applied through leads 35 to the filaments 15. The other branch is connected to a tapped transformer 24, a suitable portion of the secondary winding of which is connected to the grid and filament of a detector valve 25. Resistances 28, 29 are arranged in the plate circuit of this valve, and the pulsating voltage across the resistance 29, which voltage is smoothed by a choke coil 42 and condenser 43 and thus follows the envelope of the sound-modulated currents, is applied to the grid-filament circuit of an amplifying valve 30. An adjustable bias is applied to the grid of the valve 30 by a battery 32. The filaments 15 are connected in the output circuit of the valve, so that the mean width ofthe light-transmitting slot formed thereby varies in width in accordance with the volume of the sound. By adjusting the grid bias of the valve 30 and a resistance 39 in the plate circuit, and with suitable adjustment of the input, it can be arranged that when no sound modulated currents are applied the slot in the light valve is practically closed, and that the amount by which the slot enlarges with increase of volume may be made equal to the amplitude of vibration. Thus virtually complete modulation is effected. In a modification, a steady current from a battery is applied through a variable resistance to the filaments 15 of the light valve. This current, which can be so adjusted that the normal spacing between the filaments is very small, opposes the output current from the valve 30 which flows through the filaments 15, and which is partially shunted by an adjustable high resistance. Both the systems described produce a record consisting of striations having a constant length and density in a direction perpendicular to the direction of travel of the film, the mean density of the record over several striations varying in accordance with the volume of the sound. Specification 361,907 is referred to.