Patent GB-456734-A (GB)
Title: Improvements in or relating to sound film recording and reproducing systems
IPC: G03B31/00
Filed: 1935-05-11
Abstract: 456,734. Optical sound recording and reproducing apparatus. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A., MACINERNY, A. E. F., Flat 284, Grove End Gardens, St. John's Wood, London, and VAST, E. C., 73, Beverley Drive, Edgware, Middlesex. May 11, 1935, Nos. 13892/35 and 9019/36. [Class 40 (ii)] [See also Group XXXIX] Relates to sound-reproducing systems for films having thereon a sound record and an auxiliary record, the latter being used during reproduction to produce expansion of the volume range of the sound record to compensate for compression introduced during recording. According to the invention, a single light source and a single photo-electric cell are arranged so that light from the source passes through both records to the photo-electric cell, the output of which thereby corresponds to the combined variations of both records. Fig. 7 is a circuit diagram showing how the output of the cell 28, to which light passes through both the sound record and the auxiliary record on the film 11, is used to apply to the control grid 32 of a variable-Á valve 33 signals corresponding to the variations of the sound record and also biassing variations derived from the auxiliary record, the variations in bias of the control grid effecting compensation for the volume compression introduced in the recording. The output of the photo-electric cell 28, amplified by a direct current amplifier 29, is applied to a low-pass filter 30 and a high-pass filter 31. These serve to apply separately to the grid 32 of the variable-Á pentode 33 potential variations produced by the simultaneous light-modulating effects of the two records, with the result that the output of the valve 33 at the terminals 34 can be used to give a faithful reproduction of the recorded sound. A condenser 40 and resistance 41 in parallel with the cell 28 enable adjustment to be made for variations in the print density of the records. The volume of the sound output may be controlled by means of a resistance 42 in the circuit of the filter 30. Various alternative forms and arrangements of the sound-track and of the auxiliary record are described. Fig. 1 shows an example, in which the auxiliary record 10 lies in the centre of the sound track 12, the total width being that of a normal sound track. The auxiliary record and the sound track in this case have no relative longitudinal displacement, i.e. corresponding parts of both are adjacent to each other. For scanning a film of this kind, the slit is in the form of an inverted T, Fig. 5, or a cross, forming narrow rectangular beams, that passing through the sound track having its greatest dimension transverse to the film, while that passing through the auxiliary record has its greatest dimension longitudinally. The light slit may be made adjustable, so that the effectiveness of the volume control exerted by the auxiliary record can be altered if desired by varying the length of the auxiliary record that is scanned. In Figs. 3 and 4 is shown an arrangement using a record of the variable density type having the auxiliary record 16 alongside and displaced relatively to the sound track 15. The film passes vertically downwards at a, Fig. 4, through the light beam which falls on the photo-electric cell 22, and is looped back upon itself so as to pass horizontally through the beam at a point b nearer the exciter lamp 18. The arrangement is such that the sound record 15 is scanned at a and the auxiliary record at b, an image of a slit 19 being focused in the two positions successively, by objectives 20, 21. To compensate for loss of light, the slit 19 may be abnormally wide, the size of the image at a being suitably reduced by the objective 21. Figs. 8 and 9 show means for making records of the kind shown in Fig. 1. The output from a microphone 50 passes through an amplifier 51 combined with a high-pass filter, to two amplifiers 52, 53 in parallel. 52 rectifies and smoothes the current passed through it, so that its output operates a light valve 64 which records the volume variations. The amplifier 53 comprises two valves in parallel, to the grids of which are applied the potentials derived from the amplifier 52. Thus, the output of 53 is applied to a light valve 60 to record the sound with the desired volume compression. The manner in which the light valves 60, 64 are arranged to produce the centrally-disposed auxiliary record is shown in Fig. 9. Light modulated by the valve 64 is reflected by a mirror 62 disposed in the path of the light beam between the valve 60 and the film 11.