Patent GB-640099-A (GB)
Title: Improvements in or relating to receiving equipment for pulse modulation communication systems
Inventor: RANSOM DAVID HIRAM
IPC: H04J3/04
Filed: 1947-10-24
Abstract: 640,099. Multiplex pulse signalling. STANDARD TELEPHONES & CABLES, Ltd. Oct. 24, 1947, No. 28479. [Class 40 (v)] A sinusoidal wave having a frequency equal to the recurrence frequency of a pulse train and generated from a pulse in said train, is used to generate a plurality of phase-displaced waves for application to a plurality of non-linear coils arranged to generate new pulses related to the original pulse by delays characteristic of the phase-spacing for use in demodulating and separating the other pulses in the train. In each magnetic channel pulse generator and modulator of a multiplex transmitter, Fig. 6, a transformer T1 is designed so that a 10 kc/s. sinusoidal voltage applied to the primary winding P1 overloads and saturates the core to produce voltage pulses in the secondary winding S every half-cycle, alternate pulses being of opposite sign. The series inductance L2 together with the average value of the inductance of P1 and the series condenser C2 are tuned to the input frequency to produce heavy current surges and a band-pass filter effect to prevent the pulses being passed back to the source and audio frequencies which would give rise to cross-talk. Alternatively, a shunt anti-resonant circuit or a buffer amplifier may serve the same purpose. The lower amplitude audio modulating source is applied to the primary winding P2 through a low-pass filter L1, C1 and modulates the timing of the generated pulses. The rectifier W1 removes the negative-going pulses from the output circuit. The pulse trains from the various channel modulators are interlaced in time by supplying the 10 kc/s. waves from the different phases of a phase transformer. The transformer T1 of the second channel has no modulating winding, this channel being used for the transmission of fixed marker pulses which may be distinguished from channel pulses by their reverse polarity, greater amplitude or greater duration. Thus the channel pulses may be shortened by differentiation. The outputs from the generators are fed to a common line. Multiple phase generator.-The basic 10 kc/s. wave is generated by a crystal-controlled oscillator V1, Fig. 9, and passed through valves V2a, V2b, with their associated anti-resonant circuits L1, C1, L2, C2, to remove harmonics. The output of V2b is split into two phases, displaced by 90 degrees, in the circuit R1, R2, C3, C4, which are separately fed, by cathode followers V3a, V3b, to push-pull power amplifiers V4 ... V7 and V8 ... V11. The latter feed the star-connected primaries of transformers T3, T4, and the ring-connected primaries of transformers T5, T6. The secondaries of both transformer nets are Scottconnected, giving 12 phases #1 ... #12 in all, which are spaced apart by 15 degrees. By using the anti phases of the generated waves a total of 24 channels may be used. Multiplex receiver.-For separating the channel trains from the received interlaced pulse train, the synchronizing pulses are separated by applying the received pulses to valve V1a, Fig. 15, which has a 1.5 microsecond delay line DL in its anode circuit, so that the 3 microsecond synchronizing pulse a, Fig. 14, has added to it a delayed pulse b giving a resultant c which is fed to valve V2a. The latter is biassed beyond cut-off and selects the overlap portion of the composite pulse. The 3/4 microsecond channel pulses do not produce an overlap in the output of V1a and thus are rejected by V2a. The tuned circuit L3, C3, in the anode circuit of V2a produces a substantially sinusoidal output at synchronizing pulse recurrence frequency, which is further smoothed in amplifiers V1b, V2b with their tuned grid circuits L1, C1 and L2, C2. The 24 phases necessary to gate the receiving channels are produced as at the transmitter, Fig. 9, and applied to magnetic pulse generators similar to those at the transmitter, Fig. 16 (not shown). Demodulation of the time-modulated pulses is carried out in flip-flop circuits which produce, in known manner, duration-modulated pulses, the leading edge of which is initiated by a narrow gating pulse and which are terminated by the received signal pulse, Fig. 17 (not shown). Alternatively a wide gating pulse applied to the suppressor grid of valve (a) of the pentode pair, Fig. 19, enables it to conduct upon receipt of a narrow received signal pulse on its control grid, the valve returning to its non- conducting condition on the termination of the gating pulse. The duration-modulated pulses thus produced at the anode of valve b are passed through a low-pass filter to recover the modulating signal. A somewhat modified circuit of this type is also described, Fig. 25 (not shown). Group demodulation.-In an alternative receiving arrangement, Fig. 21, a single demodulator 45 deals with the signals of channels 1, 4, 7 &c., demodulator 46 serves channels 2, 5, 8 &c. and demodulator 47 the remaining channels 3, 6, 9 &c. The multiphase source 38 of 10 kc/s. waves synchronized from the synchronizing pulse separator 35, as in Fig. 15, feeds gating pulse generators 39, 40, 41. Generator 39, for example, provides, through circuit 42, pulses which cause the demodulation of channels 1, 4, 7 &c. in circuit 45 and individual channel gating pulses which select the individual demodulated outputs in output gating circuits, two of which 48, 49, are shown. Specifications 635,509 and 640,100 are referred to.