Patent GB-584729-A (GB)
Title: Improvements in or relating to electrical signalling systems
IPC: H04J3/04
Filed: 1942-12-04
Abstract: 584,729. Pulse modulation; multiplex signalling. BUTEMENT, W. A. S., and OXFORD, A. J. H. Dec. 4, 1942, No. 17308. [Class 40 (v)] Multiplex pulse signalling systems, of the type in which the channels are distinguished by time displacement of a sequence of pulses, employ at the transmitter a saw-tooth voltage source feeding in parallel a valve chain for each channel, each chain being arranged to develop a pulse when the saw-tooth voltage rises above a fixed bias voltage which is different for the respective channels; the receiver comprising a similar synchronized saw-tooth voltage source arranged to produce similar sets of pulses which are applied to operate electronic switches or to open gate valves for the,respective channels at the moments when pulses of that channel are arriving. The pulses are modulated in duration by the respective signals, and if desired only the rise or fall, or-both, of each pulse may be transmitted ; and synchronization of the sawtooth sources is effected by a longer pulse. As shown in Fig. 2, the saw-tooth generator 1 is coupled in parallel to the grids of valves VIA B1B, V1C the cathodes of which are positively biased to varying amounts by tappings on a potentiometer 4. During the fly-back, the grid of valve VIA cuts off the anode current and the rise in anode voltage is transferred through condenser 3 to the grid of the valve V2A the anode of which is connected to a mixing valve V4. A synchronizing pulse of a length depending on the time constant of the condenser 3 and resistance 2 is thus transmitted to the short-wave sender 9. Subsequently the valves V1B, V1C ... are opened successively by the applied saw-tooth voltage, and as each one opens, the grid of the second valve V2B of the chain' is driven negative for a short period and a condenser 7 is charged. The resultant positive voltage on the grid of valve V3B opens that valve for a period dependent on the voltage applied to its cathode by the signal input 8. The resulting pulses modulated by the signals are fed to the mixer valve V4. and thence to the radio transmitter 9. At the receiving station, Fig. 4, the carrier wave is detected at 10 and the pulses are applied to a polarity-reversing valve 11 feeding negative pulses to the grid of the valve 12. While this valve is blocked a, condenser 13 charges up, but only the long synchronizing signal allows this charge to rise sufficiently to open the valve 14. When this occurs, a pulse passes to a saw-tooth source 15 and synchronizes it with the similar source at the transmitter. The sawtooth voltage is applied to pulse generators G1 ... G8, the output of which is applied to the suppressor grids' of pentode valves PI ... P8 one for each channel, so as to open these valves in turn. The signal pulses, from the receiver 10 are applied in parallel to the inner grids of these valves, so that each valve is operative to transfer to the pulse demodulators O1 ... 08, only those pulses which belong to the relevant channel. In a modified system the long synchronizing signal is transmitted during the flyback period of the. saw-tooth oscillator which is controlled by a sine-wave generator. The crest of this wave trips the condenser-discharge valve of the saw-tooth oscillator, and also serves to generate the long synchronizing pulse. The received pulses may be passed through a limiting valve which passes on only a thin horizontal slice of the received pulses to the subsequent stages (Figs. 6 and 8, not shown). The channels may be scrambled to ensure secrecy by synchronous switches arranged to alter the order of succession of the channels.