Patent GB-694757-A (GB)
Title: Secrecy communication system
IPC: H04K1/00
Filed: 1951-04-20
Abstract: 694,757. Pulse code modulation systems; pulse circuits. GRETENER, E. April 20, 1951 [April 22, 1950], No. 9295/51. Classes 40 (v) and 40 (vi). In a pulse code communication system for speech signals the rule relating the signal amplitude and the pulse code combinations is varied continuously and substantially at random, for secrecy, the decoding at the receiver being subject to the same varying rule. The methods described depend on the use of a normal binary coder, successive code groups from which are combined with successive enciphering binary pulse groups which vary substantially at random, the combination being effected by the multiplicative law + X +=+, + X - = - and - x - =+, where + and - represent the two possible pulse states, e.g. marking and spacing. In one embodiment, at the transmitter, a master oscillator 41, Fig. 2a, controls a circuit 40 which produces groups J of five pulses which are fed to a known coder 39. The resulting code groups B are determined by speech signals from source 43, 44 and, together with a high frequency wave from source 33, 34, are fed to the grid G2 of the enciphering valve 31. The enciphering pulse groups A are generated in circuit 38 under the control of a sawtooth generator 42, controlled by the oscillator 41. They are applied to valve 30 and thence to the grid C1 of valve 31. When a pulse is present on both grids no high frequency output is produced in the anode circuit output transformer 35 as the valve is operating wholly within the anodecurrent saturation region. When a pulse is present on neither grid the valve is cut-off during the whole H.F. cycle, so that there is no output. When, however, a pulse is present on one grid and not on the other, a H.F. pulse is produced in the output circuit 35, 36, feeding the transmitter. In another enciphering circuit, the balanced valves 46, 47, Fig. 2c, have applied to their control grids G1, G2, respectively, the code pulses and enciphering pulses. The common anode circuit contains a highfrequency source 49 which develops a voltage between the two anodes across the separate anode loads R1, R2 only when a pulse is present at one of the grids only. This H.F. pulse is rectified by elements 51, 56 and applied to the output terminals 52, 53. To produce the enciphering pulse groups in the circuit 38, Fig. 2a, a number of film strips 64, Fig. 3b, divided into opaque and transparent strips are passed continuously between the screen of a cathode-ray tube 66 and a photo-cell 68 feeding an amplifier 70. The number of strips is equal to the number of elements in the code group and each linear scan of the beam in tube 66 produces a group of pulses in the cell 68. The deciphering process at the receiver is identical with the enciphering process, and the equipments are synchronized by known methods. Further enciphering of the speech signals, by known methods, may precede the coding arrangements of the invention. The coding and enciphering may be performed in a single cathoderay coding tube. For example, in a 5-unit binary coder the beam may traverse two identical coding screens, each with 32 pulse group positions, the deflection being controlled by the speech signal plus a substantially random enciphering signal to traverse the beam to any one of 64 amplitudes. At the receiver the subtraction of the enciphering signal from the decoded signal reproduces the speech.