Patent GB-632892-A (GB)
Title: Improvements in or relating to two-way electric pulse communication systems
IPC: H04B14/02
Filed: 1947-11-19
Abstract: 632,892. Multiplex pulse signalling; pulse delaying circuits. STANDARD TELEPHONES & CABLES, Ltd., and SCULLY, C. T. Nov. 19, 1947, No. 30695. [Classes 40 (v) and 40 (vi)] In a two-way multi-channel pulse timemodulation communication system using interlaced pulse trains, at any station a transmitted and received pulse occur in a channel period, part at least of the channel period being common to the periods of maximum excursion of the transmitted and received pulse. In one arrangement, the two pulses in their unmodulated positions divide the channel period into three nearly equal sections and each, when modulated, employs nearly two thirds of the channel period. In another arrangement, the pulses are caused to occur near the beginning and end respectively of the channel period when no signals are passing in the corresponding direction, but move to the centre of the period when transmission begins, the possible period of overlap then covering nearly the whole of the channel period. The type of modulation may be that described in Specification 535,384. A four-channel system is described, in which a master sawtooth generator 23A, Fig. 2, controls the timing of the pulses produced by the synchronizing pulse generator 24A and channel pulse generators 19A ... 22A. The pulse outputs from these sources are mixed, 25A, amplified, 26A, and passed to the communication medium 10. A.receiver blocking circuit 27A is connected to the output of amplifiers 26A and is fed over conductor 28A with received pulses which are fed to the synchronizing pulse separator 29A and the channel demodulators. Each of the latter comprises a gating circuit 32A, controlled by the master generator 23A, a build-back circuit, 33A, for producing durationmodulated pulses, a low-pass filter 34A and amplifier 35A. The received synchronizing pulse is passed through a phasing device 30A and switch 31A to control the master generator 23A if the station is not a master station. The synchronizing pulse generator 24A produces a triple synchronizing pulse as described in Specification 596,658, or if the second arrangement of signal pulse allocation is used, a quadruple synchronizing pulse is necessary. A channel pulse generator for the second arrangement previously referred to comprises a valve 126, Fig. 7, to the grid of which the modulating signal is applied. One secondary of the output transformer 130 is connected to one pair of diagonal points of resistance-bridge 41 ... 44, the other diagonal points being fed with the sawtooth wave from the master generator through terminal 38. The combined voltage across resistance 43 is fed to the pulse generating valve 51, which is normally biassed to cut-off by the potential divider 52, 53. When the grid voltage exceeds the cut-off bias, the valve conducts and, owing to the feed-back through transformer 55, generates a half sine-wave, the remainder of which is damped out by diode 56 and subsequent anode current saturation, and which is fed out from terminals 58, 59, connected to a third winding on transformer 55. The pulse position is set by adjustment of the cathode biassing current at resistance 52, when no signals are being fed to valve 126. The mean pulse position is automatically brought to the centre of the channel period when signals are applied, by adding the output of signal rectifier 131, 132, to the grid-bias of valve 51. This feature is omitted when the first-mentioned arrangement of signal pulse allocation is used, Fig. 3 (not shown). The blocking circuit 27A, Fig. 2, comprises a valve 63, Fig. 4, to the grid of which the strong transmitted pulses and weak received pulses are applied through terminal 60. The valve is biassed beyond cut-off-so that the received pulses are unable to unblock it but so that the transmitter pulses are passed from the anode to the control grid of valve 65 to block that valve. Terminal 60 is also connected through delay line 66 to the suppressor grid of valve 65 so that the received pulses unblock it to pass output pulses to terminal 69. Transmitter pulses reaching the suppressor grid find the valve blocked at the control grid through valve 63 and produce no output, the blocking pulse being maintained for a suitable period by the grid time-constant circuit 71, 72. The receiver gating and buildback circuit, Fig. 6, is supplied with the sawtooth wave from the master generator at terminal 117 and fed to the control grid of valve 102. The time-modulated signal pulses to be demodulated are applied to the control grid of valve 103, the suppressor grid being supplied with a gating pulse produced by valve 102 and of duration determined by the short-circuited delay line 106 in the anode circuit of valve 102. The grid bias for valve 103 is produced across resistance 111 by the normally conducting triode 112 which is cut off when valve 103 produces a pulse at its anode and restores after a time determined by condenser 113 and resistance 114. The resultant duration-modulated pulses across resistance 111 are taken from terminal 124. In a circuit for producing a quadruple pulse synchronizing signal, the sawtooth wave from the master generator is applied to valve 140, Fig. 8, and a short pulse in the secondary of anode transformer 142 due to the sawtooth flyback is applied to four parallel circuits including resistances 146 ... 149 respectively. Three of the circuits also include delay lines 150, 151, 152, which introduce delays of t, 2t and 3t, respectively, t being the pulse spacing required. The four output pulses are mixed in pairs in valves 153A, 153B, whose outputs are combined at terminal 158.