Patent GB-610774-A (GB)
Title: Improvements in or relating to multiplex electric communication systems
IPC: H04J3/16
Filed: 1946-04-16
Abstract: 610,774. Multiplex pulse signalling. STANDARD TELEPHONES & CABLES, Ltd., CHATTERJEA, P. K., and REEVES, A. H. April 16, 1946, No. 11671. [Class 40 (v)] In a multi-channel time modulated pulse communication system, in which the trains of channel pulses are interleaved to form a combined train, a train of unmodulated pulses is also interleaved therewith, and the pulses are so timed that the period separating each modulated pulse from the preceding pulse depends only on the instantaneous signal voltage corresponding to the said modulated pulse. At the transmitter, a master pulse generator 8, Fig. 2, produces a train of synchronizing pulses supplied over conductor 15 to a channel mixing valve 13 feeding the communication channel from output terminal 23. The channel pulse generators 10A, 10B ... 10N are connected in tandem, each being triggered over conductors 14A, 14B, &c. by the output pulse from the preceding generator except 10A which is triggered from the generator 8. Each channel pulse generator comprises a triggered flip-flop multi-vibrator, Fig. 3 (not shown), the inversion time of which depends on the channel modulating signal applied to the terminals 11A, 11B ... 11N, the short pulse arising at the instant of inversion serving as the channel pulse and the triggering pulse for the next channel pulse generator. Rectifiers such as 16 are provided in all the individual pulse output leads to isolate the various output circuits. At the receiver, Fig. 4, the pulse separator 52 derives the synchronizing pulses, which are transmitted with a greater amplitude than that of the channel pulses, and they are used to synchronize a pulse generator 54. The output of the latter is fed to the first 56A of a series of channel selectors 56A ... 56N to switch a relaxation oscillator, Fig. 5 (not shown), from its first to its second stable condition. The receipt of the first channel pulse returns the circuit to its first stable position so that a rectangular pulse, of duration equal to the interval between the two operating pulses, is generated and passed to the Pulse demodulator 59A ... 59N. A differential pulse derived from the trailing edge of this rectangular pulse is fed to the next channel selector to initiate its operation which is terminated by the next channel pulse and so on. The circuit of the demodulators 59A ... 59N is designed to eliminate cross-talk between the channels arising from phase modulation of the duration modulated pulses with which it is fed. It comprises two condensers 87A, 87B, Fig. 6, charged by odd- and even-numbered pulses respectively from the line 60 through circuits 96A ... 99A and 96B ... 99B respectively, each of which comprises a rectifier. The change due to one pulse raises the potential of the condenser to a value just insufficient to break down the gas-filled diodes 100A, 100B respectively and persists until the next ineffective pulse raises the potential above the break-down voltage to discharge the condenser. The persisting potentials thus produced on the condensers are proportional to the pulse durations only and are mixed in the double triode 88 and fed through transformer 92 to the signal reproducer. Rectifiers 102A, 102B prevent negative potentials from being applied to the condensers and biassed rectifier 115 clips the voltages in the output circuit due to the extra momentary changes the condensers receive to break down the diodes 100A, 100B. To initiate the correct action of the circuit, the ganged switches 110, 111 are closed to charge condenser 87A and discharge condenser 87B.