Patent GB-673354-A (GB)
Title: Improvements in or relating to electric signal communication systems
Inventor: EARP CHARLES WILLIAM
IPC: H04J3/00
Filed: 1950-12-01
Abstract: 673,354. Multiplex pulse signalling; valve circuits. STANDARD TELEPHONES & CABLES, Ltd. Dec. 1, 1950, No. 29464/50. Classes 40 (v) and 40 (vi). In a communication system, an electrical wave is periodically sampled and each sample represented by a plurality of indices, from which the sample can be reconstituted, these indices each representing on a continuous scale the same function of the sample, at least one of them ambiguously. In the embodiments described, two indices are used which may comprise two position modulated pulses occurring in succession in one channel period of a time division multiplex system and one or both of which may be ambiguous. Alternatively two phase modulated carrier pulses of different frequencies may be transmitted simultaneously, both indices being ambiguous. In a further embodiment, the frequency of the pulses of a pulsed frequency modulated carrier constitutes one unambiguous index, and the phase change from one pulse to the next, the second ambiguous index. The first embodiment may be modified so that the indices are distinguished not by their position in the channel period but by positive and negative changes in the transmitted carrier frequency. In an embodiment in which successive ambiguous position-modulated pulses occur in one multiplex channel period and are controlled by a single primary pulse, modulated in position by the channel input wave, the channel recurrence frequency is 10 kc/s., the twelve channel periods, graph A, Fig. 2, being eight microseconds long and a four microsecond period being allotted to the two microsecond synchronizing pulse 23. The two modulated pulses 33, 34 for channel seven, for example, occur respectively in the gating periods corresponding to pulses 24, 25. A 10 kc/s. master oscillator 1, Fig. 1, controls the synchronizing pulse generator 3 feeding the outgoing conductor 4. The primary position modulated 0.1 microsecond pulse 36, graph B, Fig. 2k having time limits 27, 28 is produced by phase modulating the master oscillation in circuit 8, by the input wave applied at terminals 9, 10, and squaring, differentiating and limiting the output wave in circuit 11. The unmodulated position of the primary pulse is shown at 26. The primary pulse is caused to generate two short trains or " combs " of positive pulses, graphs C, D, with recurrence frequencies of 550 kc/s. and 500 kc/s. respectively, by feeding it to two blocked valves 14, 15 which conduct to shockexcite resonant circuits 16, 17 respectively, the outputs of which are converted into the combs of pulses in generators 18, 19. The gating circuits 20, 22 are fed with gating pulses 24, 25, graph A, respectively, derived from generators 12, 13 controlled by the master oscillator through conductor 2 and phasing circuits 6, 7 respectively, and select one pulse each from the two pulse combs. These pulses (33, 34, when the primary pulse is the unmodulated pulse 26) are fed to the output conductor 4 through the inverter 21. When the primary pulse is modulated, for example to position 36, the pulse combs are shifted the same amount, graphs H, J, and the pulses 31, 37 are gated by pulses 24, 25 for transmission, graphs H, J, G. The other channel circuits are connected across conductors 24 and comprise duplicates of the channel seven circuits 5 ... 21, the only difference being in the settings of the phase shifters 5, 6, 7. The functions of circuits 14, 16, 18 and 20 are performed by valves 85, 93, 96, Fig. 6, and their associated components. Pulses from circuit 11 unblock valve 85 to shock-excite the narrow band-pass filter 87 ... 91, which produces a train of waves the amplitude of which expands and then contracts and which contains about fifteen cycles of appreciable amplitude. These waves are squared in valve 93 and differentiated in circuit 94, 95, to produce a train of narrow pulses the positive ones of which form the pulse comb and are effective at the control grid of the normally blocked valve 96, when a gating pulse is applied to the suppressor grid from terminal 98. The resulting negative pulse at the anode is taken from terminal 100 and is also fed through condenser 102 and rectifier 103 to charge a time-constant circuit 104, 106 in the grid circuit. This cuts off the valve for a sufficient time to prevent its responding to a second pulse of the same pulse comb. A similar circuit in which the band-pass filter is tuned to a different frequency forms the elements 15, 17, 19 and 22 of Fig. 1. At the receiver, the incoming pulses from terminal 38, Fig. 3, are applied over conductor 39 to all the channel circuits, each of which comprises two gating circuits 45, 46, which select and separate the two channel pulses. The gate pulses are derived from generators 43, 44 respectively, which are controlled through delay networks 41, 42 from the synchronizing pulse at the output of the selector 40, which is also fed from terminal 38. Each separated channel pulse is made to give rise to a comb of pulses, graphs E, F, or K, L, Fig. 2, of 550 kc/s. and 500 kc/s. recurrence frequency respectively, by circuits 47, 49, 51, 53 and 48, 50, 52, 54 respectively. These pulse combs are applied to a circuit 55 which produces an output pulse on the coincidence of two input pulses. The position of this output pulse corresponds with that of the primary position-modulated pulse derived from the input wave at the transmitter. The resulting pulses are demodulated in circuit 56 which is preferably of the type described in Specification 581,005. In another embodiment, Figs. 7, 8, 9 (not shown), the first pulse of a transmitted pair is an unambiguous position-modulated primary pulse and the second is ambiguous, being produced by gating a pulse from a pulse comb initiated by a pulse modulated in position by the input wave but having ten times the range of modulation of the primary pulse. A coincidence circuit at the receiver restores this widely modulated pulse unambiguously by comparing a 500 kc/s. pulse comb produced by the second ambiguous pulse with a 50 kc/s. train of gating pulses produced by the first unambiguous pulse. In a third embodiment, Figs. 10, 11, 12, 13, 14 (not shown), similar to the first, the two indices consist of trains of phase modulated waves of different frequencies initiated by index pulses and which may be transmitted direct or used to modulate a common carrier of higher frequency. The two gating periods are simultaneous instead of occurring in succession. The coincidence circuit at the receiver receives pulse combs derived from the two incoming wave trains which correspond in frequency to the two pulse combs which are gated to produce the indices at the transmitter. The channels may be multiplexed by using different index wave train frequencies for each channel or by a time division arrangement. In another embodiment, pulses modulated in amplitude by the input wave are converted into a step-wave which frequency modulates a carrier wave, the latter being pulsed to provide the transmitted signals. The indices used to reproduce the sample wave amplitudes at the receiver, are the carrier frequency of the pulse, which is unambiguous, and the phase change from one pulse to the next, which is ambiguous. The 10 kc/s. master oscillator 204, Fig. 15, controls an eight-microsecond gate pulse generator 206 through a phase adjuster 205. The positive gate pulses 229, 232, Fig. 17, are inverted and differentiated in circuits 209,210 and the resulting short positive pulses are used to discharge a storage circuit 215. The latter is charged one microsecond later by an amplitude modulated pulse produced by modulator 212, the modulating wave being fed to terminals 213, 214 and the input pulse, from circuit 210, through the delay network 211. The output of the storage circuit is a series of long rectangular pulses, graph B, Fig. 17, the short gaps therebetween being removed by a low-pass filter 217 to produce a stepped wave which modulates the frequency of an oscillator 208. The output of the oscillator is gated on by pulses 229, 232, &c., graph A, and passes to the line 219 as pulses of a carrier frequency which is proportional to the wave sample amplitudes. The system may be multiplexed by duplicating the circuits 205 ... 208 for each channel in a time division series. At the receiver, the first and unambiguous index is the frequency of each wave pulse, which is measured by comparing the phase of two samples of the pulse spaced apart by four microseconds. The other and ambiguous index is the phase change measured by comparing the phases of two samples, spaced apart by one hundred microseconds and taken from the ends of successive wave pulses. The I.F. circuits 183, Fig. 18, operating at 4 Mc/s. for example, feed a synchronizing pulse selector 184 which controls two gate pulse generators 43, 44 through adjustable delay networks 41, 42 respectively. Generator 43 produces eightmicrosecond pulses which control gate circuit 45 to pass the channel pulses applied thereto from the I.F. circuits 183. Generator 44 produces one-microsecond pulses which control gate circuit 46 to select a short sample of the wave pulse selected by the gate circuit 45 as near as possible to the trailing edge. The synchronizing pulses are also fed to the harmonic generator 189, the filter 190 selecting a 100 kc/s. wave therefrom, the frequency of which is multiplied in circuits 191, 192 by twenty-four and twenty-five respectively to produce heterodyning waves of 2.4 and 2.5 Mc/s. respectively. The 4 Mc/s. output pulses from circuit 45 are mixed with the 2.4 Mc/s. wave in circuit 187 and the 1.6 Mc/s. sideband selected by filter 193. This 1.6 Mc/s. wave is then mixed with the 4 Mc/s. output from circuit 45 in circuit 195 and the 2.4 Mc/s. sideband selected by filter 197. The resulting 2.4 Mc/s. wave is mixed with the 2.5 Mc/s. wave from circuit 192 to produce a 100 kc/s, pulse which is selected by