Patent GB-673360-A (GB)
Title: Improvements in or relating to electric code modulation systems
Inventor: EARP CHARLES WILLIAM
IPC: H04B14/04
Filed: 1950-12-20
Abstract: 673,360. Multiplex pulse signalling. STANDARD TELEPHONES & CABLES, Ltd. Dec. 20, 1950, No. 31025/50. Class 40 (v). A signal wave is sampled periodically and a characteristic of each sample is first represented by a plurality of " indices " each representing the characteristic on a continuous scale, and at least one ambiguously, the index values then being converted into " digit " signals representing said characteristic on a discontinuous scale, the digit signals being transmitted over a communication medium. As described, the signal samples are represented by time modulated primary pulses which are each used to provide two successively occurring time modulated index pulses, the deviation of which is less than that of the primary pulse, as described in Specification 673,354 and which each select for transmission one of a continuous train of digit pulses of higher recurrence frequency. At the receiver, each digit pulse gives rise to a " comb " of pulses, of different repetition frequencies, which are applied to a coincidence circuit, the output pulses of which correspond to successive primary signal wave samples and are converted into an approximate but unambiguous replica of the signal wave. In one embodiment, twelve multiplexed speech channels, having a sampling frequency of 10 kc/s., have interlaced eight microsecond channel periods leaving a four microsecond period for a two microsecond synchronizing pulse. The 10 kc/s. master oscillator 1, Fig. 1, feeds a syncrhonizing pulse generator 3 supplying pulses 28, graph A, Fig. 2, to the outgoing line. A digit pulse generator 8 operating at a recurrence frequency of 5.5 Mc/s. is controlled by generator 1 through a circuit generating the 100 kc/s. tenth harmonic and frequency multipliers 6, 7 with factors of eleven and five respectively. The digit pulses are supplied to the gate circuit 9 whose output is fed to the line 4 through a pulse shaper 10. The remainder of the circuits of Fig. 1 comprise the equipment individual to one of the twelve channels, for example the seventh, that already described being common to all the channels. The two index pulses are selected in gating circuits 26, 27, the gating pulses 29, 30, graph A, being produced by generators 18, 19, respectively, controlled by generator 1 through adjustable phase shifters 13, 14 respectively. Pulse 29 is of 1.8 microseconds and pulse 30 of two microseconds duration. The signal wave is applied to the terminals 16 of a phase modulator 15, the carrier from generator 1 being fed thereto through an adjustable phase shifter 12. Circuit 17 converts the phase modulated wave into a correspondingly modulated pulse train by squaring and differentiating. These primary pulses, such as 31, graph B, shock excite the resonant circuits 22, 23, through normally blocked valves 20, 21, respectively, to produce short trains of waves having frequencies of 550 and 500 kc/s. respectively. These waves are converted by circuits 24, 25 into " combs " of pulses, graphs C and D, which are applied to the gating circuits 26, 27, respectively. The pulse 31 is in the unmodulated position and the gating circuits 26, 27, select the pulses 38, 39, respectively from the two corresponding pulse combs to form the index pulses. When the primary pulse is modulated to position 41, graph B, the pulse combs shift to the positions of graphs F, G, respectively, and pulses 36, 42 are selected to form the index pulses, graph E. The index pulses from all the channels are fed to the gating circuit 9 over conductor 11. Shown on a larger scale in graph H, Fig. 3, are the gating pulses 29, 30, which select the index pulses 38, 39, which are applied to the gating circuit 9, with the digit pulses, graph J, so that digit pulses 43, 44 are selected for transmission corresponding to the primary pulse 31. For the primary pulse 41, index pulses 36, 42, graph K, select digit pulses 45, 46 for transmission. At the receiver the incoming pulse train on conductor 47, Fig. 4, is applied to the synchronizing pulse selector 48 which feeds gate pulse generators 61, 62 through adjustable delay networks 59, 60 respectively. A train of digit pulses, with a recurrence frequency of 5.5 Mc/s. is derived from the synchronizing pulses by circuits 50... 55 similar to those producing the digit pulses at the transmitter. The incoming pulse train is passed through shaper 58 to the gating circuit 56 where each pulse selects one of the locally generated digit pulses for application to conductor 57 which feeds all the channel circuits only one of which is shown. The gating pulses 75, 76, graph L, Fig. 5, produced by generators 61, 62 respectively select the pair of channel digit pulses 71, 78 in gating circuits 63, 64, respectively connected to the conductor 57. From the channel digit pulses are derived the pulse combs, graphs M, N, of recurrence frequencies 550 and 500 kc/s. respectively, in circuits 65, 67, 69, 71 and 66, 68, 70, 72. These pulse combs are applied to the coincidence circuit 73 which produces an output pulse 83, graph S, due to the coincidence of pulses 81, 82 of the two combs, thereby reproducing an approximation of the unmodulated primary pulse 31, graph B, Fig. 2. The modulated primary pulse 41 is represented by the output pulse 90 from circuit 73, due to the coincidence of pulses 88, 89, from the pulse combs, graphs Q, R, derived from the pair of channel digit pulses 84, 85, graph P. The reproduced primary pulses are fed to a demodulator 74 of the type described in Specification 581,005 and to recover the channel modulating signal. The invention may be adapted to systems having more than two digit pulses per signal sample and to certain other embodiments of the invention described in Specification 673,364.