GB605178A - Improvements in pulse-communication systems - Google Patents

Abstract

605,178. Pulse modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 17, 1945, No. 34177. Convention date, Jan. 8, 1944. [Class 40 (v)] In a pulse communication system, modulation is effected by varying the number of pulses transmitted within periodic intervals, without varying the spacing between successive pulses transmitted during said interval or any other characteristic of the pulses. In one embodiment a pulse generator 10, Fig. 4, feeds rectangular D.C. pulses of constant recurrence frequency, phase, amplitude and duration to one signal grid of a keying valve 11. Another signal grid is biassed by the anode current of a control valve 12 flowing in resistance R. When valve 12 is conducting, valve 11 is cut-off and no pulses pass to the transmitter 16 from its anode circuit. When valve 12 is cut-off, valve 11 can conduct to pass on pulses. The output from a saw-tooth generator 14 is combined with the audio modulating signal from amplifier 15 to control the conductivity of valve 12, the fraction of the negative-going saw-tooth cycle during which it is cut-off, being determined by the modulating voltage. In the example described, the pulse recurrence frequency of generator 10 is 36 Kc./s. and the frequency of the saw-tooth generator is 6 Kc./s. For ' the condition of no modulation, three pulses are transmitted for each cycle of the saw-tooth generator, six being transmitted for maximum positive and zero for maximum negative modulation. At the receiver, clipping circuits limit the pulses between two levels to remove noise on and between them and they are restored to constant width and phase by a gating circuit synchronized by the incoming pulses and having a pass period less than the received pulse duration. The resultant pulse train is fed to the signal reproducing device through a low-pass filter to remove pulsefrequency components.