Patent GB-594539-A (GB)
Title: Secret radio-communication system
IPC: H04K1/02
Filed: 1944-09-26
Abstract: 594,539. Pulse modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Sept. 26, 1944, No. 18418. Convention date, Dec. 7, 1942. [Class 40 (v)] Secrecy is obtained in a radio communication system by producing at the transmitter pulses of variable width and frequency, the pulse width varying in accordance with the intelligence to be transmitted and the frequency varying in accordance .with a predetermined (secret) code. Impulses are transmitted which correspond only to either the leading or trailing edges of the variable width pulses and at the receiver a local reference impulse generator, the frequency of which varies in accordance with the above-mentioned predetermined code, is provided for regeneration of the pulses so that they have a duration determined by the phase difference between the incoming transmitted impulses and the locally generated reference impulses. The intelligence to be transmitted is, as shown, fed by a microphone 1 through an audio amplifier 2 to a mixer 4 also fed with a triangular wave generated at 5 and frequency modulated at 6 in accordance with a predetermined code determined by the coding device 7. The mixer output is converted by a square wave amplifier 8 to constant amplitude pulses as at B, Fig. 1, the pulse width being modulated according to the intelligence wave A to be transmitted and the frequency of the pulses from centre to centre being varied as determined by the units 5, 6, 7. A differentiator 9 converts each variable width pulse into discrete leading and trailing edge impulses, and a half-wave rectifier 10 suppresses either the leading or trailing edge impulses as at C, Fig. 1, the others being fed to a keyer stage 11 .where the impulses key sub-carrier current from a generator 14 which is then fed over line 12 to and radiated from radio transmitter 13 over antenna 15. At the receiver, Fig.3, the impulses collected at 20 are converted by the radio receiver 21 into D.C. impulses corresponding to the transmitted impulses at C, Fig. 1, and fed to a known trigger circuit 23 comprising a pair of thermionic tubes having two conditions of stability, the change from one condition to the other being effected by applying suitable potentials to the grids of one or the other of the two tubes. The local oscillator 24 for supplying references impulses D, Fig. 1, may be a sine wave tone generator and is frequency modulated at 31 under the control of a coding device 30. The sine wave oscillator output is fed through a limiter 25 which flattens the wave tops, a differentiator 26 which converts the flat-topped waves into discrete positive and negative impulses, and a half-wave rectifier 27 which suppresses the negative (or positive) impulses. The local oscillator 24 and the triangular generator 5 in the transmitter both have the same frequency, and the coding devices 7, 30 being similar, the impulses fed to the trigger circuit 23 are as indicated at D, Fig. 1, cophasal with the instantaneous centres of the variable width pulses B. These impulses D are fed to the grid of one of the tubes in the trigger circuit, the grid of the other tube being fed with the received impulses C over line 22. The output of the trigger circuit 23, which compares the phase of the received impulses with the local reference impulses, is indicated at E, Fig. 1, in which the variable width pulses correspond to the leading halves of those at B. These pulses are fed through a buffer amplifier 28 and low-pass filter 29 from which the original intelligence modulation as shown at F, Fig. 1, is obtained.