Patent GB-166662-A (GB)
Title: Multiplex high frequency electrical signalling systems
IPC: H04J1/00
Filed: 1920-04-14
Abstract: 166,662. Western Electric Co., Ltd., (Western Electric Co. Inc.). April 14, 1920. Multiplex signalling systems and apparatus; thermionic relays.-A carrier-wave signalling system employing modulated high-frequency oscillations is superposed on two low-frequency signalling lines in such a way that carrier signals in one direction are transmitted on one line, and in the other direction on the other line. Several carrier frequencies are employed so as to convey a number of independent messages. At the existing repeater points on the low-frequency lines, one-way repeaters, connected in shunt to the lowfrequency repeaters, are provided for each line for amplifying the carrier currents. Filters are provided for directing the low-frequency and the carrier currents into their proper paths at the various stations. In a modified arrangement, carrier currents are transmitted in both directions along the same low-frequency line. Figs. 1, 2 and 3, when placed side by side in the order named, show one form of the invention. Two low-frequency lines, such as ordinary telephone lines ML1, ML2, extend from station A, Fig. 1, through repeating station B, Fig. 2, to station C, Fig. 3. The low-frequency signals are repeated by two-way thermionic repeaters AE1, AW1 protected from high-frequency currents by low-frequency filters LF2, LF2<SP>1>. The line ML1 is also used for conveying from station A to station C a plurality of signals in the form of modulated carrier oscillations, and the line ML2 is similarly used for conveying a plurality of signals from C to A. Dealing with the transmission from A to C, a series of low-frequency lines L1 - - L4 entering the station A are connected by balanced transformers 11 - - 14 to modulators M1 - - M4, which are supplied with carrier oscillations of different frequency, say 10000, 15000, 20000, and 25000. The modulated oscillations on the output side of the modulator M1 are filtered by a device TF1 which only permits passage of a narrow band of frequencies approximating to the particular carrier frequency 10000. They are then amplified in the device TA1, and if desired again filtered at T1F1, and then pass to the common line TL and through a high-frequency filter HF1 to the main line ML1. The filter HF1 prevents low-frequency currents in the line ML1 from reaching the line TL, and the filter LF1 prevents high-frequency currents in the line TL from reaching the terminal portion LT1 of the main line ML1. At the repeating-station B, Fig. 2, the carrier currents are diverted from the low-frequency repeaters AE1, AW1 by the filters LF2, HF2 and pass through a one-way repeater A1E to the line T1L<1>, and thence to the main line ML1<SP>1> on the other side of the low-frequency repeaters. The repeater A1E comprises two pairs of tubes 21, 22 and 23, 24, each pair being in tandem and in parallel with the other pair. The arrangement is such that variations in the input circuits do not affect the current flow in the common portions of the parallel circuits. At the station C, Fig. 3, the low-frequency signals pass through a filter LF1<SP>1> to the terminal portion LT1<SP>1> of the line ML1<SP>1>, and the carrier currents pass through a filter HF1<SP>1> to a common receiving line RL<1>. The carrier currents of a frequency approximating to 10000 are selected by the filter RF1<SP>1>, and are detected by the device D1<SP>1> which operates by the known homodyne method. The detected signals pass through an amplifier RA<1>, and a low-frequency filter R1F1<SP>1>, and are transmitted along a line RL1<SP>1> to a station D into which a number of low-frequency lines L1<SP>1> - - L4<SP>1> enter. If desired the stations C, D may be combined into a single station similar to A, Fig. 1. The apparatus for transmitting signals from station C to station A along the line ML2<SP>1>, ML2 is identical with that described above, the signals coming in on lines L1 - - L4 being transmitted through transformers 111 - - 141 to the carrier apparatus at station C, repeated through the apparatus A,W at station B, and received along line RL at station A, from which the various signals are selected and conveyed to the lines L1 - - L4. Networks N1 - - N4 at station A and N<1>1 - - N4<SP>1> at station D balance the lines with respect to transformers 11 - - 14, 11<1> - - 14<1>. Similar networks MN1, MN1<SP>1> balance the lines at the repeating-station with reference to the repeater transformers. Filters LF3, LF3<SP>1>, or corresponding networks, balance the filters LF2, LF2<SP>1>. A modified arrangement is indicated by placing Figs. 4, 5, and 3 side by side in the order named. In this arrangement, only a single low-frequency line ML1 is used for conveying the carrier currents in both directions between station A, Fig. 4, and repeater B, Fig. 5. The common transmitting line TL at station A is coupled by a transformer 10 to a line CB branched from the main line ML1. The common receiving line RL at station A is connected across the line CB. The line ML1 and filter HF1 are balanced with respect to the transformer 10 by a network MN and a filter or network H1F1. At the repeating-station B, Fig. 5, a branch line C1B is connected through a balanced transformer 33 to the input side of a one-way repeater A1E and to the output side of a one-way repeater A1W. The opposite sides of these repeaters are coupled respectively to the lines ML1<SP>1>, ML2<SP>1> extending to station C.