Patent GB-454904-A (GB)
Title: Improvements in or relating to the recording of sound
IPC: G11B7/00
Filed: 1936-01-20
Abstract: 454,904. Optical sound-recording apparatus and processes. TOBIS TONBILDSYNDIKAT AKT.-GES., 43, Mauerstrasse, Berlin. Jan. 20, 1936, No. 1863. Convention date, Jan. 19, 1935. [Class 40 (ii)] Relates to a method for the photographic recording or re-recording of sound on a film by a variable density trace of constant mean density, the width of the trace being altered in accordance with the intensity of the sounds being recorded, and consists in changing the sensitivity of apparatus controlling the recording light beam in accordance with changes in the sound intensity so as to vary the modulation of the sound trace to compensate for the alterations effected to the width of the trace, whereby the sounds are reproduced at the correct volume level. The light control device may take the form of a light valve (string oscillograph, Kerr cell) or of a Braun oscillograph tube. As shown in Fig. 1, light from a source 1 is applied through the lens system 2, 3, 4 to the film 5, a slit member 7 and the string 8 of a string oscillograph, whose magnetic field is formed by the current traversed coils 9, 10, being located between the lenses 2 and 3. The width of the sound trace is altered by means of a shutter 12 of the shape shown in Fig. 2a which is carried by a string 11, the magnetic field of this system being produced by the current traversed coil 13. Coils 9 and 13 are fed with constant current from a battery 14 through control resistances 15, 16. The amplified microphone currents are applied to the string 8 through the secondary 17 of the output transformer whose primary 18 is connected to the output of the recording amplifier. One end of a second secondary winding 19 of this transformer is connected to the grid of a rectifying valve 20 whilst its other end is connected through a grid-bias battery 21 to the cathode of valve 20. The anode of the valve is connected through coil 10 and string 11 to the anode battery 22. The grid-bias battery is so proportioned that in the normal condition, i.e. when no alternating currents are transmitted to grid of valve 20, no anode current flows in the anode circuit. In this condition the string oscillograph is energized solely by the coil 9 and, as coils 9, 10 are arranged in opposite relation, has in this condition the most intense magnetic field, that is the greatest sensitivity. The string 11 with shutter 12 thus has such a position that the sound track is completely masked off. When, however, alternating voltages are applied to the grid of valve 20 through the recording amplifier, the string 11 and coil 10 are traversed by a direct current of alternating intensity corresponding to the particular recording strength. This results in a weakening of the magnetic field of the string oscillograph and a reduction of its sensitivity whilst on the other hand there will be a change of position of shutter 11 in such a way that the sound track will be more or less exposed in accordance with the particular mean recording strength. If necessary, the string 11 and coil 10 may be fed by two separate rectifiers which have characteristics differing from each other or the magnetic field of the shutter 12 (and also that of the oscillograph) may be influenced in suitable manner by the variable rectified current. In order to avoid disturbing noises due to an over-rapid alteration of the sound trace width, the frequency of these alterations is kept below the range of audibility by connecting a condenser 23 or a combination of condensers and resistances in parallel with the valve 20. By using a shutter 12 of the form shown in Fig. 2a, a sound track of the form shown in Fig. 2b is produced. Alternatively, by using a shutter with a V-shaped notch, Fig. 3a (not shown), a sound track which is masked and uncovered symmetrically from both sides, Fig. 3b (not shown), is obtained.