Patent GB-405078-A (GB)
Title: Improvements in or relating to apparatus for photographically recording sound
IPC: G11B7/00
Filed: 1932-10-19
Abstract: 405,078. Optical sound &c. recording apparatus; television and telegraph receivers. WESTERN ELECTRIC CO., Ltd., Bush House, Aldwych, London.-(Electrical Research Products Inc. ; 195, Broadway, New York, U.S.A.) Oct. 19, 1932, No. 29300. [Class 40 (ii).] Relates to methods of and apparatus for use in recording sound or telegraph signals, picture transmission, television and the like, wherein a beam of radiant energy, which is adapted to fall upon a relatively moving surface, is modulated by means of a pair of shutters or ribbons which move parallel to the direction of motion of the surface. According to the invention, a delay or phase difference is introduced between corresponding movements of the two shutters or ribbons. The delay may be constant, or it may be a function of frequency of modulation. Where a substantially constant delay is introduced, it is preferably equal to the time taken by an element of the moving surface to pass across the unmodulated beam. Fig. 1 shows one application of the invention to photographic sound recording apparatus, employing a light valve of the type having two ribbons 11, 12 movable in the field of a magnet 4, for modulating a beam of light from a lamp 1 transmitted to a moving film 6. Amplified microphone currents are supplied to the primary winding of a transformer 16, the secondary of which is centretapped and connected in circuit with the ribbons 11, 12 in the manner shown. One half of the winding is connected to the ribbon 11 through a non-inductive resistance 18, while a delay network consisting of inductances 22, 23 and a condenser 24 is included in the circuit comprising the other half of the secondary winding and the ribbon 12. With this arrangement, the movements of the ribbons effected by the induced currents are out of phase, and with suitably chosen components, the delay time may be made such that the latent image of each movement of the ribbon 12 reinforces that of the corresponding movement of the ribbon 11. Several modifications are described, employing differing arrangements of transformers and of delay networks. For example, in Fig. 3, each of the ribbons 11, 12 is associated with a separate transformer 66 or 72, a delay network formed by inductances 70, 71 and condensers 73 .. 76 being included in the circuit through which currents are supplied by the amplifier 15 to the primary winding of the transformer 72. Two arrangements are also described, of which Fig. 5 shows an example, in which the invention is applied to a recording system of the kind having means for supplying a static biassing current to the light valve, this current serving to vary the mean time of exposure of the film in accordance with the variations in volume of the sound. The output of the amplifier 15 is branched, one portion feeding a rectifier network 36 .. 39, which is connected in known manner to a condenser 40, resistances 41, 59, and biassing battery 51. Another part of the currents from the amplifier 15, suitably amplified at 60, is applied to the primary winding of a transformer 53. The secondary winding is tapped and connected as shown to the ribbons 11, 12, a delay network 63, 64, 65, 55, 56 being included in the circuit containing the ribbon 12. A condenser 62 of large capacity is connected to the junction of the ribbons, to the delay network, and, through a rectifier 58 and an adjustable resistance 59, to the battery 51. The static biassing current from the battery thus passes in opposite directions through the halves of the secondary winding of the transformer, and thence through the ribbons 11, 12. In the application of the invention to television, the modulated beam of light may scan over a stationary screen by means of a revolving perforated disc or drum, or by a system of revolving or tilting mirrors.