Patent GB-494019-A (GB)
Title: Improvements in or relating to the recording and reproducing of sound
IPC: H03G7/00
Filed: 1937-04-22
Abstract: 494,019. Optical sound recording and reproducing. RADIO CORPORATION OF AMERICA. April 22, 1937, No. 11536. Convention date, May 28, 1936. [Class 40 (ii)] [Also in Group XXXIX] A plurality of records of the same sound is made at different ranges of level, preferred portions of these records are segregated, and the sound is reproduced from these segregated portions. For example, sound received by a microphone 10, Fig. 1, connected to an amplifier 11 with a gain control 12 and volume indicator 13 is supplied to three amplifiers 14A, 14B, 14C operating three recorders 15A, 15B, 15C working in synchronism or producing three parallel tracks on a single film. The amplification of 14B is 25 decibel greater than that of 14A, and that of 14C 25 decibel greater than that of 14B, so that the record of weak sounds is better on 15C, of medium sounds on 15B, and of loud sounds on 15A. These best sections of the recordings are determined by an examination of the records, and the other sections are blacked out. The records can then be passed through reproducers 21A, 21B, 21C, Fig. 2, working synchronously and coupled through volume controls 23A, 23B, 23C, amplifiers 24A, 24B, 24C, and volume control 25 to a loud-speaker 26. The amplifiers 24A, 24B, 24C have ratios corresponding inversely to those of the amplifiers 14A, 14B, 14C of the recording apparatus, so that the sound is reproduced in correct extensive range by the loud speaker, only one record being operative at a time and that the best record for the particular loudness of the sound. In a modification, instead of blacking out the unwanted parts of the records, the outputs of the amplifiers 24A, 24B, 24C are switched off or faded out. Various potentiometer devices are described for performing this fading operation. In the apparatus shown in Fig. 6, three identical photo-cells 50A, 50B, 50C controlled by the three records are connected to the system of resistances shown provided with sliders 43, 44 connected to the grid of a thermionic valve 35, this arrangement providing for fading out and adjustment of gain ratio simultaneously so that only a single amplifier is required. The fading out may also be performed optically, for example by means of a shutter 65, Fig. 7, at the optical centre between the lenses 61, 62. In this Figure, 60 is the light source, 63 a slit, 64 the film, and 67 the photo-cell. The fading out may be performed according to indications or code marks made on the records as a result of a visual inspection of the records. The indications may take the form of holes punched in the film record, these holes causing contacts to be made while the film is passing through the reproducing apparatus to give signals or to automatically operate the fading devices, time lag being allowed for by a corresponding advance position of the holes. The selection of the best recording, instead of being made by visual inspection, may be performed automatically for example by means of a bridging amplifier and a peak voltmeter on one or more channels. Specification 401,697 is referred to.