Patent GB-583541-A (GB)
Title: Improvements in or relating to pulse modulation systems of electric communication
IPC: H04J3/00
Filed: 1944-06-22
Abstract: 583,541. Pulse modulation. STANDARD TELEPHONES & CABLES, Ltd., CHATTERJEA, P. K.. and SCULLY, C. T. June 22, 1944, No. 11920. [Class 40 (v)] In a transmitter for timemodulated pulses, the pulses are produced by applying an alternating voltage to a non- linear or saturable magnetic inductance, the timing of the pulses being varied by the signal voltages. Fig. 1 shows a single pulse system in which a base-frequency source P is connected through a series tuned circuit C1L1 to a coil L with a saturable magnetic core to produce alternate positive and negative pulses, as shown in full line in Fig. 7a at instants when the current wave passes through zero as described in Specifications 477,875, [Group XXXV], and 551,282. The instants of zero current are varied by a signal voltage from source Q applied through a low-pass filter LPF across coil L, thus to displace the positive and negative pulses in opposite directions as shown in broken lines, Fig. 7a. The negative pulses are eliminated by a rectifier RF and the positive pulses, Fig. 7b, are amplified and used to modulate a carrier wave from source O for subsequent transmission. Other ways of applying the modulating voltage to the saturable inductance L are described, Figs. 2-5 (not shown). Instead of eliminating one set of pulses, they may be reversed in polarity and combined with the other set to give a double pulse train in which -both pulses of each pair are time modulated in opposite senses, Figs. 15, 16 (not shown). These may be used to give duration modulated pulses by means of build-back circuits such as are described in Specification 583,511. Fig. 6 shows an arrangement for producing pairs of pulses, one pulse of each pair being fixed in time. The upper circuit produces across R3 pulses of the frequency of source P and time modulated in accordance with signal source Q as described for Fig. 1. The lower circuit is fed from source P through delay network D and produces in a similar manner across R<1>3 pulses at constant intervals. The two trains of pulses are combined in the common load resistance RC of valves V1, V2 to give pairs of pulses as in Fig. 7d, A being the time-modulated pulse and B the fixed pulse. The space between members of a pair is determined by the delay device D: Fig. 7e shows a solid pulse built up from pairs of pulses, the width being capable of modulation from 0 to 50 per cent of the transmission time for 100 per cent modulation. Fig. 8 shows a four-channel system in which a base frequency from source P is fed through a frequency multiplier M to feed quadruple frequency to a saturable inductance L and rectifier RF to produce a series of unmodulated positive pulses at four times the frequency of P. Source P also feeds inductance 1L and, through delay devices D2-D4, inductances 2L-4L to produce similarly trains of pulses modulated by sources Q1-Q4. All the pulse trains are mixed at MIX to give an output at O which comprises four spaced sets of pairs of pulses, the fixed pulse of each pair being derived from the quadrupled frequency output from M. If, instead of multiplexing, two channels are phase reversed and modulated by the same signal and the pulses combined with unmodulated pulses of doubled frequency, a train of double pulses are produced having a doubled frequency, and capable of time modulation over 100 per cent of the transmission time (Figs. 9-11, not shown). Fig. 12 shows an alternative arrangement for allowing 100 per cent modulation of the transmission time. A balanced bridge is formed of two saturable core inductances 110, 111 and impedances 112, 113. A carrier wave of pulse repetition frequency is fed from source 114 across one diagonal of the bridge, and a signal voltage from-source 117 is applied across the other diagonal. The signal currents flow in opposite directions in coils 110, 111, while the carrier currents are in the same direction. Thus, in the two trains of pulses produced, the pulses are modulated in opposite senses. However, the positive pulses from coil 110 and the negative pulses from 111 are modulated in the same sense. These are selected by valves 120, 121 biassed to cut off, and combined at 127 to give a time-modulated train of double frequency capable of modulation over 100 per cent of the transmission time. A saw-tooth or triangular wave form may be used as the base frequency wave.