Patent GB-441293-A (GB)
Title: Improvements in or relating to systems for recording modulated electric currents, such as microphone currents in sound recording
IPC: G11B7/00
Filed: 1934-08-18
Abstract: 441,293. Optical sound - recording apparatus. WESTERN ELECTRIC CO., Ltd., Bush House, Aldwych, London.-(Electrical Research Products, Inc. ; 195, Broadway, New York, U.S.A.) Aug. 18, 1934, No. 23866. [Class 40 (ii)] A method of recording modulated electric currents, such as microphone currents in sound recording, consists in making two records, one being a variable density record of only the positive half waves of the variations in amplitude of the modulated currents and the other being a variable density record of only the negative half waves of these amplitude variations. The exposure of the recording medium in the absence of modulated currents is preferably arranged to lie at the lower end of the region of correct exposure of the medium as determined by its Hurter and Driffield curve. Figs. 1 and 3 show a lightvalve arrangement for carrying out the method according to the invention in which currents from a microphone 1 are fed to ribbons 6, 8 arranged in planes displaced from each other and secured at their centres to a conductive bridge 18. The lead 5 is connected to one end of the ribbon 6 whilst lead 7 is connected to the other end of ribbon 8 so that the right -hand portion of ribbon 6 and the left-hand portion of ribbon 8, Fig. 3, do not carry any current and will not be displaced during the operation of the device. The ribbons are arranged as shown in Fig. 1 between the poles of a magnet 11, the poles being apertured at 12, 13 so that light from a source 9 can pass through these apertures and the lenses 10, 14 to form two images 15, 16 on a moving film 17. As shown in Fig. 3, the lower edge of ribbon 6 is preferably aligned with the upper edge of aperture 13 whilst the upper edge of ribbon 8 is aligned with the lower edge of aperture 12. In operation, one half wave of the modulated currents will cause the displaceable portions of the ribbons to move upwards so as to increase the exposure of record 15 whilst exposure of record 16 will decrease slightly and then remain unexposed whilst the other half wave causes the ribbons to be displaced downwardly to produce a reverse effect. Other light-valve arrangements are described, one in which two ribbons connected in parallel are employed and another in which a single ribbon is used, whilst, according to a further arrangement, the recording is effected by means of two light sources of varying brilliancy. Fig. 6 shows a construction of light valve employing three parallel ribbons 101, 102, 103 connected as shown to the leads 106, 107, a pair of masks 104, 105 being provided to limit the area of the beam transmitted past the ribbons. The ribbons 101, 103 are preferably tuned to have a natural vibration frequency slightly higher than the highest frequency to be recorded whilst the ribbon 102 has an appreciably higher natural vibration frequency. The ribbons 101, 103 may then be respectively connected in parallel relationship with impedances 108, 109 so as to equalize the response of the ribbons at low frequencies. The complete light valve may, if desired, have the general form of the light valve described in Specification 382,663 suitably modified, Figs. 8 and 9 (not shown).