Patent GB-332567-A (GB)
Title: Improvements in or relating to the modulation of a beam of light, for use for example in the recording of sound or in the reproduction of recorded sound
IPC: G11B7/00
Filed: 1929-01-23
Abstract: 332,567. Robertson, J. R., and Vielle, J. Jan. 23, 1929. Vacuum tubes.-Relates to apparatus for modu. lating or increasing the modulation of a beam of light, for use, for example, in recording and reproducing sound. A beam of light is passed through a glow discharge device from an external source, and electrical variations are impressed upon the discharge device so as to produce corresponding variations in the glow. The electrical variations may be caused, ior example, by the passage of the beam through a photographic sound record, or by means responsive to sound which is to be recorded. Fig. 1 shows a soundreproducing system, in which light from a lamp 1 passes through a discharge tube 2 on to a lightsensitive cell 3, a film sound record being moved across the beam of light at either of the positions shown at 4, 4<1>. The vacuum tube may, as shown, have a central tubular portion 5 sealed from the surrounding annular portion 6. The cell 3 is connected in series with the primary winding of an output transformer 7, a battery 8, end electrodes 9, 10 in the vacuum tube. A polarizing discharge may be maintained by a battery 13 between electrodes 11, 12 in the central tube 5, but these electrodes may be omitted. The modulation of the light beam by the sound-record produces electrical oscillations in the circuit of the cell 3, and these are impressed on the electrodes 9, 10. An oscillatory glow is thus produced in the tube, and according to the nature of the gas in the tube the glow may obscure to a greater or less extent the beam passing through it from the source 1, or may itself directly affect the photo-electric cell 3. Thus, in either case, the original modulations of the light beam are amplified or augmented by the effect on the glow produced by the electrical oscillations which are fed back from the cell 3 to the discharge, tube. An additional regenerative effect may be obtained in either or both of two ways. One of these consists in providing a coating of photo-electric material 16 on one of the walls of the vacuum tube 2, another wall having a metallic coating 17. These coatings constitute electrodes which are connected in series with a battery 18 and a coil 19 coupled to a coil 20, the latter being in series, as shown, with the primary winding of the transformer 7. The other method of increasing the regenerative effect consists in providing a second vacuum tube 21, out of the path of the beam from the lamp 1, and so arranging it that light from it falls on the cell 3. The electrodes of the tube 21 are connected in series with a battery 22 and with a coil 23 coupled to the coils 19, 20. Fig. 2 shows a sound-recording system in which acoustically modulated light from a source 1 is passed through a glow discharge device 2 on to a sensitized film 24. Electrical oscillations, derived either from a recording microphone or from a photo-electric cell so arranged as to be affected by a part of the modulated light beam, are impressed upon a coil 25 which is coupled to a coil 26 in circuit with electrodes 9, 10 in the tube. 2, the effect produced being similar to that produced with the apparatus shown in Fig. 1. A second tube 21 having electrodes coupled to the coils 25, 26 by means of a coil 23 may be arranged as shown so that the effect of the fluctuations of the light emitted thereby is superposed upon that produced by the device 2. Photoelectric coatings 16, 17, are provided in the tube 21 in a similar manner to those in the tube 2 shown in Fig. 1, the regenerative effect from these coatings being fed back through a coil 19 coupled to the coils 25, 26. Compound vacuum tubes with more than two separate chambers and an appropriate number of electrodes may be used, any of the chambers being used to conduct the "carrier" or polarizing discharge.