Patent GB-323014-A (GB)
Title: Improvements in volume control for sound recording and reproducing
IPC: H04N3/36
Filed: 1928-09-17
Abstract: 323,014. Beatty, W. E., (Bell Telephone Laboratories, Inc.). Sept. 17, 1928. Optical apparatus and records.-Relates to systems for recording and reproducing sound, particularly applicable to apparatus employing photographic film records, wherein the amplitudes in the sound-trace are intentionally limited when necessary (i.e. when sounds exceeding a certain minimum in volume are to be recorded), on account of the limitations inherent in the material and apparatus used, compensation being effected during reproduction by restoring the amplitudes of the reproduced sounds to their proper proportions. According to the invention, the means which serve to control the compression of amplitudes on the record also produce markings on the record .to indicate where such control has been applied, and its extent; these markings may also serve automatically to effect the necessary compensating control of the reproducing apparatus. The invention is particularly described as applied to the production and use of a photographic record, which may be made on a kinematograph film. In the recording system, Fig. 1, a microphone 2 which picks up the sound to be recorded is connected to an amplifier 3, the output terminals of which are connected to a potentiometer consisting of resistances 7 - - 13 in series. A recording-device 17, past which the film 72 moves, is adapted to be connected across the whole or any available fraction of the potentiometer, so that by diminution of the currents supplied to the recorder the amplitude of the record trace may, when necessary, be limited at any time in a predetermined degree. The required connections of the: recorder to the different tappings on the potentiometer are effected by a system of interconnected relays R, under the control of a set of keys K, these corresponding in number and arrangement to the tappings on the potentiometer. A set of three lamps L is located adjacent to the film, and these are flashed singly or in a predetermined combination each time a key is operated to change the connection of the recorder 17 to the potentiometer. The lamps produce markings such as 111 - - 113 alongside the sound trace 110 on the film, Fig. 3, and the manner in which these markings-are arranged indicates the character of the " volume control " employed at any part of the record. The reproducing apparatus, in which the sound record is passed between a source of light and a photo-electric cell connected to an amplifier for operating a loud-speaker, embodies a system of relays and a potentiometer arranged in a manner analogous to that in the recording apparatus. The lamps L are replaced by a set of adjustablymounted contact fingers which so determine the operation of the relays that the " volume level " is automatically controlled, and a loss introduced at any point in recording is compensated for by a proportionate gain in volume during the reproduction. The requisite movements of the contact fingers are effected by perforations in the film, made at the places where the indicating markings occur. The invention may be applied to gramophone disc records, by forming in a second groove parallel to the sound-groove a special indication of the changes in volume control.