Patent GB-374996-A (GB)
Title: Improvements in and relating to apparatus for making photographic sound records
IPC: G11B7/00
Filed: 1931-04-08
Abstract: 374,996. Photographic sound records. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Greentree, C. D. ; 1112, Van Curler Avenue, Schenectady, New York, U.S.A.) April 8, 1931, No. 10378. Convention date, April 12, 1930. [Class 40 (ii).] A photographic copy of a sound record is made in such a manner that the film upon which the copy is made receives during the printing operation an additional exposure, to increase the density of the normally clear portions of the copy and thus reduce the cause of " ground noise." The additional exposure is, according to the invention, controlled by means responsive to a current which is derived from the record, and which varies in accordance with the average wave amplitude of the sound record trace. A printing apparatus for copying a record of the varying-width type is shown. A negative record film 1 and a sensitized film 4 are fed together in contact by the sprocket wheels and guide rollers shown past a printing aperture 12, through which light passes from a lamp 11. Between the film roll 2 and a sprocket 14 the film 1 passes a slit 20 at which light from a source 21 is projected through the film on to a photo-electric cell 24. The cell is connected to an amplifier 25, rectifier 26, and filter 27, so as to produce a direct current which varies approximately as the amplitude of the record passing the slit 20. This direct current is caused, by connection between the filter 27 and a suitable control device, to produce corresponding variations in a light beam which is transmitted from a source 31 through an aperture 30 on to the sensitized film 4. By this means, the film 4, before moving past the printing aperture 12, receives a preliminary exposure in accordance with the output from the filter 27, such exposure being a maximum when the amplitude of the waves recorded on the film 1 is a minimum, and vice versa. The device for controlling the supplementary exposure consists, in the construction shown, of a shutter 33, Fig. 3, movable transversely over the aperture 30. The shutter is mounted on an arm 34 attached to a spring-controlled spindle 36, on which a coil 35 is pivoted between the poles 37 of a magnet. The coil receives the varying direct-current output from the filter 27. To compensate for the time lag in the response of the shutter 33, due to the filter circuit, the distance along the film between the printing aperture 12 and the aperture 30 is appropriately less than that between the aperture 12 and the slit 20. Various modifications in the method of controlling the auxiliary exposure are described. For example, in applying the invention to a variable density record an arrangement is preferred in which the shadow cast by the shutter is not sharply defined. Instead of the shutter apparatus, a light valve of another type or an oscillograph may be used. The operator may cut out the action of the auxiliary exposure device when desired, by means of a switch 28, or the switch may be controlled automatically, e.g. by notches in the edge of the film. Specification 288,225, [Class 40 (ii), Phonographs &c.], is referred to.