Patent GB-455386-A (GB)
Title: Improvements in and relating to methods of recording sound
IPC: G11B7/00
Filed: 1935-04-25
Abstract: 455,386. Optical sound - recording apparatus. MIDGLEY, A. M., and MIDGLEY, F., 9, Priory Hill Avenue, Wembley, Middlesex. April 25, 1935, Nos. 12409 and 25590. [Class 40 (ii)] Apparatus for recording variable width sound records comprises a light source c, the normal intensity of which is such that the resulting negative record produced will make a positive of the necessary density to mask ground noise when no sound, or sound of low intensity, is being recorded, and a gas-filled grid-controlled relay e which controls the circuit supplying current to the light source and operates to increase the current supply when sound is being recorded or when the intensity of the sound increases. As shown in Fig. 1, sound is recorded on a film b by means of a mirror oscillograph a reflecting light from a lamp c which is fed with alternating current by means of a transformer d connected in the anode circuit of the relay e. Current is fed to the transformer d from a transformer f, g, which is provided with an additional secondary f<1> for supplying current to the heater h of the relay e. The undulating potential from an amplifier i, which is connected in the circuit of the oscillograph a, is applied to a rectifier l connected to a potentiometer m, the movable contact of which is connected to the grid of the relay e. The potentiometer m is connected with the movable contact of a second potentiometer n, which is connected across a bias battery o and to the cathode of the relay e. The voltage of the grid of the relay is adjusted by means of the potentiometer n so that the normal intensity of the light beam is such that the resulting positive record will be of the necessary density to mask ground noise when no sound or sound of low intensity is being recorded. When sound is being recorded, or the intensity of the sound increases, the increased positive charge produced on the grid of the relay e serves to increase the intensity of the light beam. The recording lamp may be supplied with direct current instead of alternating current. In the modification shown in Fig. 2, the anode of the relay e is connected to the primary of the transformer d, with the interposition of an adjustable resistance r for limiting the maximum intensity of the light source, and an adjustable resistance s for limiting the minimum intensity of the light source when the relay is not working is connected between the primary of the transformer d and the cathode of the relay e. A potentiometer u is connected through a step-up trans. former t with the oscillograph circuit, the movable contact of the potentiometer being connected to the grid of the relay e, and one end of the potentiometer resistance being connected to the movable contact of a second potentiometer v connected across a bias battery o and to the cathode of the relay e. The negative bias voltage of the grid of the relay is adjusted so that the relay will not operate when there is no sound, or only sound of low intensity being recorded, and the minimum intensity of the light source is then adjusted as above described. When sound is being recorded, or the intensity of the sound increases, the relay operates and supplies additional current to the lamp.