Patent GB-501587-A (GB)
Title: Improvements in or relating to electro-optical systems for recording sounds and like oscillatory signals
IPC: G11B7/00
Filed: 1937-08-25
Abstract: 501,587. Optical sound - recording. WESTERN ELECTRIC CO., Ltd. (Electrical Research Products, Inc.) Aug. 25, .1937, No. 23286. [Class 40 (ii)] Relates to systems for recording sound or like oscillations on a film by means of an oscillograph, light-valve or the like, of the kind in which ground noise is reduced by shifting the mean position of the oscillatory member or members in dependence on the signal volume, in such a way that the degree of transparency of the positive record is reduced as the amplitude of the signals is decreased. According to the invention, an arrangement is provided whereby no alteration in the mean position of oscillation of the recording member or members occurs during the recording of a small range of amplitudes which are the least of those in the whole range of signal amplitudes, alteration in the mean position being effected only by larger signals which are outside the said small amplitude range. With this arrangement, a greater degree of modulation of the recording light beam over a wide range of amplitudes is effected. Fig. 1 (Prov.) is a diagram showing the opera- 'tion, in accordance with the invention, of a ribbon light-valve. A-C represents the spacing of the ribbons when no signals are being recorded. Small signals are recorded without any change in this sotting of the ribbons, but beyond the point D to prevent overloading the mean position is altered by the signals, as indicated by the line D-E. A recording system embodying the invention is shown diagrammatically in Fig. 2 (Prov.). A portion of the signal currents is applied to the lightvalve 3 through an amplifier 1 and a transformer 2. Another portion is amplified at 4 and applied through a transformer 5 to a double diode rectifier 6. The output from the rectifier, smoothed by a filter 7, is employed in a modulator 9 to control the modulation of a constant frequency oscillating current generated at 8. The modulated current is amplified at 10, rectified at 11 and smoothed at 12. The output from the filter 12 biasses the light-valve 3 in dependence on the envelope of the amplitude variations of the signals, being applied to the light-valve in opposition to a steady biassing current from a battery 13. The output from 12 is a maximum when no signals are being recorded, and causes minimum separation of the valve ribbons. A battery 14 is so connected to the cathode of the rectifier 6 that the cathode is initially positive with respect to the anodes. Thus, it is necessary for the input to the rectifier to reach a certain amplitude before rectification occurs, and so the modulation 9 is not affected by signals having the lowest amplitudes. For signals above an amplitude limit determined by the bias on the rectifier 6, the modulation of the oscillating current supplied by 8 is varied in accordance with the envelope of the amplitude variations of the signals, whereby the mean spacing of the light-valve ribbons then increases with increasing signal amplitude. A recording system embodying the features shown in Fig. 2 (Prov.) is described with reference to a drawing accompanying the Complete Specification. In this drawing (not shown) are indicated the circuit arrangements for the system in which the control of the high-frequency modulator is effected by the filtered output of a double-diode triode and a pentode which act together to amplify and rectify a portion of the signal currents. The grid bias of the double-diode triode and the cathode potential are controlled by a double-slider potentiometer. The filter circuit may be so designed as to cause the bias on the light-valve ribbons to be altered in dependence on the peak value of the signals rather than on the average amplitude.