Patent GB-586585-A (GB)
Title: Electric-pulse communication system
IPC: H04B14/02
Filed: 1944-12-11
Abstract: 586,585. Leakage signalling ; pulse modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 11, 1944, No. 24841. Convention date, June 24, 1942. [Class 40 (v)] Signals, such as telephony, modulate a characteristic of a train of D.C. pulses applied to a pair of earthed electrodes and are received at a distance by a receiver connected to a further pair of earthed electrodes. In one embodiment a source T, Fig. 2, of D.C. power supplies anode potential for valves 1, 2 through a constant current choke X and, when valves 1, 2 are suddenly made non-conducting, it generates a pulse which is fed through condenser C and matching transformer TR to the spaced earth electrodes E, E<1>. The valves 1, 2 are fed in push-pull from a sinewave voltage source 3 of half the required pulse recurrence frequency and are biassed by a common D.C. source 5, the modulating voltages from source 4 being also applied in push-pull in series with the sine-wave source. With no modulation, there are short periods when neither valve is conducting, and ' these cessations of current flow cause equally spaced voltage pulses to develop at the point A, which are fed to the earth electrodes: Modulation causes each pair of pulses to be displaced nearer or farther apart in time. At the receiver, Fig. 4, the pulse voltages picked up by earth electrodes E<11>, E<111>, after amplification are passed in parallel to two blocking oscillators 13, 14, having a common anode resistance R to cause them to operate in opposite phase and adjusted to have a period substantially corresponding to half the received pulse recurrence frequency. The oscillators are locked in by the received pulses and their average anode currents are thus differentially modulated with the required signal corresponding to the output of modulator 4, Fig. 2, which appears in the secondary of the output transformer feeding the telephones 10. In a second embodiment, the transmitter consists of a blocking oscillator comprising the first triode of valve 21, Fig. 5, producing pulses the recurrence frequency of which is modulated by signals from transformer T1. The second triode of 21 and valve 22 amplify the pulses and feed them through transformer T3 to the earth electrodes E, E<1>. At the receiver a push-pull amplifier 23, Fig. 6, feeds a circuit 34, broadly tuned to the pulse recurrence frequency which forms the input circuit of an amplifier-limiter 24, followed by a frequency demodulator 25, 26, audio push-pull amplifier 27 and telephones 28. The sensitivity of amplifier 23 is reduced during the intervals between pulses by a voltage fed back through condenser C6, from the anode of valve 24, to the grids of 23 in parallel. The overall gain of - the circuit may be fixed and the output signal amplitude adjusted by a variable resistance across the primary of the input transformer. For pulse length modulation the transmitter of Fig. 2 may be used the modulation being applied to valves 1, 2 in phase instead of pushpull and the receiver, Fig. 6, being modified to operate without limiting and with an amplitude modulation detector instead of that shown. Preferably, however, the modified receiver has a threshold and limiter type amplifier inserted before valve 24. For signalling in a submarine the earth electrodes are replaced by the submarine hull and an electrode at the end of a long insulated conductor run out from the submarine.