GB363117A - Improvements in and relating to sound recording apparatus - Google Patents

Abstract

363,117. Optical sound - recording apparatus. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Hewlett, C. W.; 1770, Wendell Avenue, Schenectady, New York, U.S.A.) Dec. 4, 1930, No. 36617. Convention date, Dec. 4, 1929. Addition to 361,907. [Class 40 (ii).] Optical sound-recording apparatus is designed to produce a sound record which substantially eliminates background noise, especially during the silent parts of a sound reproduction, by a modification of the method described in the parent Specification, in which a sensitive film is exposed to a light beam which is varied in accordance with the wave form of the sound to be recorded and the mean value of which beam is also varied in accordance with the volume of the sound. According to the present invention, a thermionic amplifier has its plate circuit connected with a mirror galvanometer and means are provided for varying the grid potential of this amplifier in accordance with the wave form of the sound to be recorded and for supplying a biasing potential from a rectified current source to the grid of this amplifier, which potential varies in accordance with the volume of the sound. The input signal to be recorded is applied to the circuit 18 and amplified by two thermionic amplifiers 22, 25. A third thermionic device 30 acts as a rectifier and is fed from the amplifier 25. The rectified current from the device 30 is applied to bias the grid of the thermionic amplifier 34. To the grid of the amplifier 34 is applied a portion of the amplified signal from the amplifier 25 of such magnitude that the peak values of these signals are equal to or a little less than the bias produced by the operation of the valve 30. The conductor loop of an oscillograph galvanometer 11 is arranged in the plate circuit of the amplifier 34. When no sound is being recorded there is no circuit through the galvanometer and it is arranged that the entire width of the soundrecord track on the moving film 1 is illuminated by light reflected by the mirror 10 from the source 7. When sound comes on the negative bias on the device 34 is increased, so that the mean plate current is reduced by an amount equal to or slightly greater than the peak value of the current impulses. This alters the mean position of the recording light beam to shift the edge of the light beam just enough or a little more than enough to accommodate the full swing of the vibrations of the beam. In this way a negative film record is produced as shown by Fig. 2.