GB587941A - Improvements relating to multi-channel electrical pulse communication systems - Google Patents

Abstract

587,941. Multiplex signalling pulse modulation. ' STANDARD TELEPHONES & CABLES, Ltd., and LEVY, M. M. May 26, 1944, No. 10308. [Class 40 (v)] In multiplex signalling systems of the kind in which the several channels are rendered operative in turn by rectangular pulses, these pulses are combined with saw-tooth or like pulses and are applied together with the signal wave to a pulse generator in each channel to produce timemodulated pulses for that channel. At the transmitter, Fig. 1, a generator 2 feeds rectingular pulses 3 to a delay line 1 which supplies gating-pulses in turn to the several channels 11..... 15, these channels feeding the common output line 5 through diodes 4. In the channel 15, the pulse 21 is applied to the grid of a valve 20, the output of which is taken across a resistance 23 and a condenser-shunted resistance 22, 24 in the cathode lead. The output pulse 0 shows a rising voltage and is applied, together with a signal voltage from a microphone 25, to trip a multivibrator 16. The multivibrator valves have a common cathode resistance 35, across which is developed a rectangular pulse 34, the leading edge of which occurs at a time dependent on the signal voltage from the microphone 25. Differentiation is effected by the network 36, 37 and the resulting short positive pulse is transmitted through a diode 40 to the line 5. Should the multivibrator not be tripped in the normal way, a sharp positive pulse is applied to a grid of the valve 17 from the anode of the valve 20 at the conclusion of the pulse 21. In a modified modulating circuit, Fig. 9, the gating-pulse is applied to a pentode valve 50, the output circuit of which includes the resistance-capacity network 52, 53, 54 connected in the cathode lead of the first valve 57 of the three-valve multivibrator 57, 58, 60. Limitation of the input pulse is obtained by resistance 69 and diode 70 ; and the network 52 .... 54 may be replaced by more complex networks. The back-coupling is effected by connecting the cathode of the valve 60 to the input grid of the pentode 57 through the signal-producer 59. Distortion is introduced into the signal owing to the fact that the rise in voltage across condenser 22, Fig. 1, or 53, Fig. 9, is not linear, but is of exponential form. This is neutralized, however, by the use in the circuit of the receiver of elements having the same time constant as the corresponding elements at the transmitter, so that the duration-modulated pulses built up in the receiver from the received time-phased pulses have a falling amplitude following the same exponential law. Fig. 11 shows one form of demodulator, in which channel pulses are applied in negative sense to block one valve 74 of a three-valve multivibrator. The positive anode pulses are applied through Cathodefollower 73 to the grid of the valve 72. This valve is normally blocked by positive voltage on its cathode, and can only be unblocked when a gating-pulse, Fig. 12b, is applied in negative sense to its cathode. The gating- pulse is derived through valve 71 and network 55, 51, 75 which gives the pulse an exponential slope 92. A channel pulse arriving on the grid at the time 93 opens-valve 72 to reverse the multivibrator until the end 91 of the gating- pulse. The signal is taken off through a lowpass filter LP from the screen circuit. In a modified receiver, Fig. 13, positive gating-pulses 86 are applied to the grid of a gas-filled tube 83, causing the condenser 85 to discharge exponentially through a resistance 87. Signal pulses 88 are applied to the grid of a gas-filled tube 84 connected across the resistance 87. The first pulse 88 occurring after the gating-pulse 86, opens the tube 84 and shorts the resistance 87. The voltage across resistance 87 thus consists of a pulse, Fig. 14, rising rapidly at 80, falling exponentially to a point 81 determined by the timing of the signal pulse, and then abruptly ceasing. Specifications 551,282 and 587,939 are referred to.