Patent GB-643378-A (GB)
Title: Improvements in and relating to electrical systems of communication
Inventor: GABOR DENNIS
IPC: H04J3/00
Filed: 1947-12-12
Abstract: 643,378. Secret transmission. BRITISH THOMSON-HOUSTON CO., Ltd. Dec. 12, 1947, No. 32823. [Class 40 (iv)] [Also in Group XL (c)] In a pulse code modulation system multiplexing is obtained by combining at the transmitter the instantaneous amplitudes of n independent messages into one resulting amplitude according to a code which is such that if the resulting amplitude is divided by a unit amplitude a number is obtained which represents simultaneously every one of the n independent message amplitudes. At the receiver the resulting amplitude is separated into its n independent components. Transmitter (Fig. 5).-The sampled incoming signals x, y, z are applied to amplitude compressors, in order that the levels to be discriminated may be made equidistant in intensity, and then to a comparator. As shown this consists of a cathode-ray tube with a plurality of pairs of deflecting plates to the left-hand plates of which the incoming signals are applied. The cathode-ray beam passes between the pairs of deflecting plates and through a number of aligned diaphragms and can reach the target electrode of the tube only if equal potentials are impressed on both plates of each pair of deflecting plates. The potentials impressed on the right-hand plates are generated by a number of scanners which jointly scan the complete range of possible level combinations of x, y, z within the limits defined by the code. The scanning cycle is preferably completed in a time numerically equal to one-half of the reciprocal of the available transmission band-width measured in cycles per second. At the instant in the cycle when the scanners reach the combination of values agreeing with that of the incoming signals, the beam strikes the target electrode whereupon a signal generator sends to line a signal whose amplitude is proportional to the time which has elapsed from the beginning of the scanning cycle. Receiver (Fig. 6).-The received signal amplitude is periodically sampled by a sampler of known type after amplification to standard attenuation by automatic volume control apparatus A.V.C. The sample is stored on a condenser which is discharged by a constant current to a datum level of charge. At the beginning of the discharge the sampler starts scanners x, y, . . . similar to those of the transmitter, and when the condenser charge reaches the datum level it signals to the samplers x, y,. . . These samplers thereupon sample the instantaneous values of x, y, . . . reached in the scanning cycle and these are fed via amplitude restorers (complementary to the compressors of the transmitter) to the various message channels. In a modification, Fig. 7 (not shown), the receiver comprises a cathoderay tube in which the beam either oscillates at high frequency in the vertical plane or is fanshaped in that plane, and is deflected horizontally by an amount proportional to the received signal amplitude. A mask is provided having sets of perforations so arranged that their vertical widths at a horizontal distance s are proportional to the respective component message amplitudes which correspond to a resulting amplitude s. Collecting electrodes are associated with the sets of perforations and receive charges proportional to the respective component amplitudes. Reference has been directed by the Comptroller to Specification 642,330.