Patent GB-185795-A (GB)
Title: High frequency multiplex signalling system
IPC: H04J1/10
Filed: 1921-05-10
Abstract: 185,795. Western Electric Co., Ltd., (Western Electric Co., Inc.). May 10, 1921. Thermionic relays and repeaters. - Relates to line systems carrying modulated high-frequency signals. A single wave-modifying device is arranged to act either as an amplifier, modulator, or detector, or to serve all these functions, in a two-way circuit, both the transmitted and received high-frequency waves passing in common through the wave-modifying device. The range of frequencies passed through to one line-section is different from that passing to the oppositely-directed line-section, one range being chosen to lie, say, above 20,000 cycles whilst the other range lies below that frequency, and filtering-circuits of the kind described in Specification 142,115 are arranged to control the incoming and outgoing currents so that they pass through the wave-modifying device from one line section to the other, but are prevented from returning to the same line-section. In the two-way circuit shown in Fig. 3, the wave-modifying device consists of a balanced modulator-detector circuit DM comprising two discharge-tubes 10, 11 connected in balanced relation, in the manner described in Specification 130,432, [Class 40 (v), Wireless signalling &c.], and coupled to a high frequency generator 9. This circuit bridges two parallel branches 5, 6, which contain low frequency filters f, f1 and high-frequency filters F. The branches connect a low-frequency line-section LL, which may be an ordinary telephone line, to the high-frequency section LH. Speech-frequency currents from the line LL pass by the filter f and transformer 13 to the grids of the valves 10, 11, and thereby modulate the high frequency carrier-wave supplied by the source 9. Owing to the opposed relation of these valves, the carrier-wave is not transmitted to either of the branches 5 or 6, but the modulation components, comprising side bands or frequencies both lower and higher than the carrier wave, pass through the transformer 14 into the branch 6, where they are prevented from passing back to the line LL by the filter f1 but are passed by the filter F into the high-frequency line LH. On the other hand, high-frequency modulation components from a distant station on the line LH pass through the filter F and input coupling 13 to the circuit DM, where they are homodyned in the manner described in Specification 102,500, [Class 40 (v), Wireless signalling &c.], by the generator 9 and appear in the output coil 14 as low-frequency signals which pass through the filter f1 into the line LL. The line LH may simultaneously carry ordinary telephone currents which are diverted into a telephone circuit 17 through a filter f2, which blocks high-frequency currents. Balancing networks and balanced repeating-coils may be applied either to the highfrequency line alone or to both lines. As shown in Fig. 6, a single balanced repeating-coil arrangement H serves for both high-frequency and lowfiequency transmission. A network N1 and duplicate filter f1<SP>1> serve to balance the repeating coil H against the low-frequency line LL and filter f1, whilst a network N, and filter F1 similarly balance the coil H against the high frequency line LH and filter F1. Another lowfrequency line such as LL1 feeding a second twoway circuit DM1 is balanced by a network N3, and filter f2<SP>1>, whilst the previous network N2 together with the filter F2<SP>1> suffices to balance the high-frequencies carried by the branch 43. Speech waves from the line LL pass through the bridged winding 44 of the repeating coil H to the modulator, and the speech-modulated components of the carrier wave from the generator 9 pass into the coil 46 and through the winding 47 - - 50 into the series windings of the repeating-coil and thence through the filter F1 and branch 42 into the line LH. Back leakage into the coil 44 is prevented by the balanced bridge action of the coil H together with the networks. N1, N2. Conversely, signals from the line LH are bomodyned in the circuit DM, and the corresponding low frequencies appearing in the coil 46 are impressed through the windings 47 - - 50 on the coil H and so to the line LL. Fig. 7 shows two terminal stations for the line LH. both of which are connected to low frequency lines such as LL and LL2. These stations contain duplicate amplifier-generator sets G, one of which is arranged to generate continuous oscillations whilst the other is set just below oscillation point and acts to amplify selectively the received carrier wave. The tubes 10, 11 coupled to the generator G form a modulator-detector circuit, and are arranged in balanced relation in the manner described in Specification 151,928, [Class 40 (v), Wireless signalling &c.], so as to pass only high-frequency components, when acting as a modulator, or low-frequency components when acting as a detector. The arrangement is such that only the lower band of modulated components is transmitted over the line LH in one direction, whereas the corresponding higher band is transmitted d between the stations in the opposite directions. To ensure this, the filter F- passes only the lower band of modulated components from the out-put coil 56. The filter F+ transmits the carrier wave from the generator G, but stops the lower band from passing through to the local circuit. Also a filter F prevents the carrier wave from reacting upon the local circuit. Consequently, only the carrier and lower band frequencies are transmitted to the right hand station where they are blocked by filters F, F+ and are passei through only by the filter F- to the input coi. 55. Correspondingly, the lower band of outgoing signals from the right-hand station is blocked locally, whilst only the carrier wave and higher band components are transmitted to the line LH and can pass only through the filter F+ ;o the input coil 55 of the left-hand station.