Patent GB-305245-A (GB)
Title: Method of and means for recording sound on and reproducing sound from phonofilms
IPC: G11B7/00
Filed: 1927-08-27
Abstract: 305,245. Jackson, L. Mellersh-, (Sie. mens & Halske Akt.-Ges.). Aug. 27, 1927. Optical apparatus; electric circuit applications. -Relates to the recording of and reproduction of sound, by photographic and photo-electric processes, of the kind wherein the record is distorted whilst it is being made, in such a manner that vibrations of large amplitude are recorded on a scale less than that of their true value, relatively to those of small amplitude, while in reproducing, this distortion is corrected by emphasizing the effect of the vibrations of large amplitude. According to the invention, this effect is produced independently of the frequencies, over the whole range of sound vibrations to be recorded and reproduced. In this way the record can be compressed on to a smaller width of film than would otherwise be necessary. Fig. 1 shows diagrammatically an apparatus for producing a record of the kind shown in Fig. 2, comprising a shutter 3, 4 with a wedge-like opening between the parts thereof through which light passes from a source 7 to the film 6. One or both parts of the shutter is or are oscillated by electromagnetic means 5 controlled by an amplifier 2 with which a microphone 1 is connected. The distortion in recording may be produced either by special modification of the electrical devices or by special formation of the shutter. As an example of the electrical method, a valve circuit as shown in Fig. 4 may be employed in the amplifier. A rectifier valve 14 is used in association with the amplifying valve 2, the output circuit of the valve 2 being connected, through a transformer 13, to the shutter-controlling device. The output potential variations are applied to the input circuit of the valve 14, the plate circuit of which contains an impedance 15, preferably a resistance and condenser in parallel. This impedance is in the input circuit of the valve 2. With this arrangement, the grid potential of the valve 2 is controlled in accordance with the amplitude of the vibrations to be transmitted, in such manner that the vibrations of greater amplitude are transmitted to a less extent than the smaller ones. The necessary electric distortion can also be effected by using one or more amplifying valves having a characteristic curve with a very short straight portion, the normal grid potential being chosen at a point on the centre of the straight, part of the characteristic. The described methods of electrical amplitude distortion can also be used where the sound record is in the form of blackenings of the film of varying intensity but of constant width, instead of in the form of a curve. In accordance with an optical method of producing the desired effect in recording, a specially constructed shutter, shaped as shown in Fig. 7, is employed, the effective edges of the shutter being curved so that the rate of increase in the extent of the aperture is less for the greater amplitudes than for the smaller ones. In the sound-reproducing apparatus, which is of the kind in which light from a steady source passes through the record and falls on a photo-electric cell, connected to an amplifier which works a loud-speaker, the necessary correction of the distortion introduced in recording may be effected electrically by using a special amplifier. For example, two amplifying valves arranged " in push-pull connection," as shown in Fig. 8, may be used. In this system, the anode and grid batteries 19, 20 are each connected to the centre of a winding of a transformer 22 or 21. With this arrangement, the vibrations corresponding to the greater amplitudes on the distorted record are amplified to a greater degree than the small ones. Fig. 10 shows another arrangement, for use in reproduction from the record, in which a valve 31, having a characteristic such as is shown in Fig. 5, is employed in conjunction with an amplifying valve 32, which may be the first valve of a cascade amplifier. The grid of the valve 32 is connected to a point on a resistance 33 arranged between the anode and the grid of the valve 31. The effect produced by this arrangement is similar to that of the system shown in Fig. 8. Various optical methods of correction during reproduction for the distortion of the record are described. A slit shaped as shown at 23 in Fig. 11, past which the record is moved, may be used. Such a slit may be made by photographic reduction of an enlarged model, the photographic image itself or a galvanically produced deposit thereon being utilized as the slit. The combination of a bent bar with a rectangularly slotted member could also be used for the special slit. Instead of using, in the reproducing apparatus, a slit having a varying width, an optical device such as a plate of graded opacity, may be interposed in the path of the rays. Optical and electrical correcting devices may be used simultaneously.