Patent GB-555118-A (GB)
Title: Improvements in or relating to sound records and sound reproducing systems
IPC: G11B7/00
Filed: 1941-09-11
Abstract: 555,118. Optically polarized sound records ; recording and reproducing apparatus. INTERNATIONAL POLAROID CORPORATION. Sept. 11, 1941, No. 11664. Convention date, Sept. 10, 1940. Drawings to Specification. [Class 40 (ii)] [Also in Group XX] A sound record comprises a film of liglittransmitting material having a surface layer the light-polarizing properties of which vary so as to form a sound-track. The polarizing layer may comprise a deposit of oriented lightpolarizing crystals, a set suspension of such crystals in a light-transmitting medium, or a 'transparent plastic having oriented long-chain molecules treated by staining to render it polarizing. The track may be variable-density type, in which the polarizing properties or density of the polarizing material vary as a function of the frequency and amplitude of the recorded sound, or it may be of variable width type in which the width of the polarizing area varies in a similar way. High fidelity sound reproduction is obtained owing to the grainless character of the image and the greater maximum density of the image in comparison with that on normal sound film. Reproducing or re-recording apparatus for use with the film comprises in addition to the usual components, an analyzer with its polarizing axis perpendicular to that of the film ; alternatively, a second film of uniform polarizing properties may be combined with the first, the polarizing axis of the two films being mutually perpendicular, this combined film being used with normal reproducing apparatus. The sound track may be produced by forming a gelatine resist on the film corresponding in thickness or hardness to the density of the track on the sound film negative, the film being then exposed to treatment which destroys or alters its polarizing characteristics in accordance with these variations in the resist. Or, a film of a transparent plastic having oriented long-chain molecules, such as polyvinyl alcohol, polyvinyl acetyl or regenerated cellulose may have the track printed on it by the application to it of a dye which renders it polarizing through a gelatine resist corresponding in hardness or thickness to the density of the sound-track negative ; alternatively the die may be applied from a gelatine matrix corresponding to the negative. In another method the film having oriented molecules is made light-sensitive by treatment with a solution comprising for example, ammonium or potassium bichromate, and is exposed through the negative. The bichromate in the unhardened portions is then washed out, and the film is treated in a bath containing an iodide and an acid, the bichromate and iodide reacting in the hardened areas so as to render them light polarizing. The surface molecules of the long-chain molecular material may be oriented in mutually perpendicular directions by a scraping or scrubbing action so that a sound track may be printed on one side and the other side be rendered uniformly polarizing to act as the analyzer. Or, a sound-track may be printed on each side, so forming a pair of stereophonic records for simultaneous reproduction. Similar superimposed records may be obtained by laminating together two' films each carrying one of the records. The reproducer comprises a beam splitting semi-transparent mirror and an analyzer in each beam with their polarizing axes so oriented that the light intensity in each beam is affected only by the corresponding one of the sound tracks. Other devices for splitting the beam into two components polarized in mutually perpendicular directions may be used, such as a Nicol prism modified to permit the emergence of both components. In another arrangement, the superposed tracks are longitudinally displaced in relation to one another and are reproduced as they pass separate gates. The stereophonic sound tracks may be used on film carrying stereoscopic pairs of images also formed by polarizing material.