Patent GB-362993-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: 362,993. 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. 16362/31. Divided application. [Class 40 (ii).] Consists in a particular method of producing a photographic sound record of such quality that undesired noise occurring during reproduction of the sound is minimized. According to the invention, the mean amount of light acting upon the photographic film is varied in accordance with the volume of the sound being recorded, and this mean amount is subjected to a substantially constant degree of modulation by the sound. In the apparatus shown a microphone 10 is connected to an amplifier 11, the output of which is connected through a transformer 12 to a multivalve amplifier 13, 15. The output of the valve 15 is applied through a transformer 16 to the filaments 17 of a recording light valve, the filaments being so ar- . ranged in the field of a magnet 21 as to form a slot through which light passes from a lamp 18 to a moving film 23. The plates of the valves 13, 15 are connected to a battery 26 through resistances 29, 27 respectively, the grid bias of the valves being determined by the potential drop in a resistance 30 included in the plate circuits. A portion of the sound-modulated current is applied through a transformer 32 to a detector 33, the output current of which varies with the envelope of the amplitude variations of the audio-currents. This pulsating current is applied to vary the grid bias of the amplifying valves 13, 15, and thus the gain of the amplifier is varied inversely with the envelope of the changes in amplitude of the sound-modulated currents. By this means, the output of the amplifier, and consequently the amplitude of vibration of the light-valve filaments 17, is maintained substantially constant over a wide range of sound amplitudes, and by suitable design of the amplifier substantially complete modulation of the light is effected for all volumes of sound. A further portion of the sound-modulated current is applied through a transformer 37 to a detector 38. The output current from this valve, varying with the amplitude of the audio-currents, is applied through a coil 41 to a circuit controlling the intensity of light from the recording lamp 18. By this means, the light intensity changes in accordance with the volume variations of the sound being recorded, whilst the sound-frequencies are registered by the vibrations of the filaments 17. In the arrangement shown, the lamp 18 is supplied with current from the armature 42 of a generator driven by a motor (not shown). The generator is excited by a field winding 43, the current through which, from a battery 44, is modulated by the current flowing in the coil 41, and may be so adjusted that in the absence of sound-modulation, the current supplied to the lamp is insufficient to cause the emission of light. Instead of using the lighting arrangement shown, the rectified sound-modulated currents from the valve 38, amplified if necessary, may be applied directly to a lamp filament heated by a battery. In a modification, only one detector valve is employed, and this performs the functions of both the valves 33, 38. In this modified arrangement, Fig. 2 (not shown), the coil 41 is included in the lead 36. Specification 361,907 is referred to.