Patent GB-248069-A (GB)
Title: Improvements in or relating to signalling systems using high frequency carrier waves
IPC: H04M7/16
Filed: 1924-11-26
Abstract: 248,069. Western Electric Co., Ltd., (Western Electric Co., Inc.). Nov. 26, 1924. Automatic and semi-automatic exchange systems.-In a system in which communication between any two stations of a number of stations is effected over carrier wave transmission lines extending from the stations, automatic means are provided for selectively interconnecting the transmission lines. Each transmission line is arranged for multiplex carrier working, sets of apparatus subsequently referred to as carrier channels, being used at each end of a multiplex line to receive and transmit the carrier waves of the particular frequencies used for a connection, and to reject other carriers which may be coexistant over the multiplex line in respect of other connections. The multiplex lines extending from the stations terminate at their other ends in a switching station where a separate selector switch, associated with each incoming channel, is adapted to be controlled bv a modulated carrier wave transmitted from a calling station, to select a group of outgoing channels belonging to a wanted multiplex line, and then to hunt for, and extend the incoming channel to, an idle channel of the selected group. Since the two channels thus linked may be allotted receiving and transmitting carriers of different frequencies, means are provided at the switching stations for converting the frequency of the carrier transmitted over one channel into the frequency of the carrier which the other channel is adapted to receive, and also, if necessary, for inverting the side band. The invention is described as applied to a telephone system in which trunk connections between manual exchanges are effected by operators over multiplex carrier lines, the connections being set up link by link over one or more intermediate automatic switching stations. General operation of system, Figs. 1 and 2. Three manual exchanges 5, 1, 3 are shown in Fig. 1 each connected to an intermediate switching station A or B, Fig. 2. over a carrier multiplex line to which access may be had over any one of three carrier channels C1 - - C3, each comprising a transmitting branch M and a receiving branch D and each allotted receiving and transmitting carriers of particular frequencies. A manual operator desiring to extend a calling subscriber to a distant exchange takes the multiplex trunk line extending to the switching station, say B, into use over an idle channel, by inserting the trunk plug of an idle cord into the jack of an idle channel. The transmitting carrier proper to the engaged channel is sent over the multiplex line and modulated at the calling exchange by a 150-cycle control frequency current. The resultant modulated carrier is demodulated at the switching station B and prepares for operation a rotary step-bv-step S1 - - S12 located thereat and associated with the incoming channel corresponding to the channel taken into use at the calling exchange. The calling operator then dials one or more digits appropriate to the wanted exchange, thereby withdrawing the control frequency from the transmitting carrier a corresponding number of times and causing the operation of the switch S1 - - S12, which selects and extends the carrier path to an idle outgoing channel of the multiplex line leading to the wanted exchange or to another switching station having access to the wanted exchange. In the latter case, a switch similar to the switch S1 - - S12 is operated at the second switching station by a second digit and the multiplex line to the wanted exchange is taken into use over an idle channel. Means are provided at the switching stations for transforming the modulated incoming carrier into a modulated carrier having the frequency appropriate to the seized outgoing channel. The modulated carrier enters the called exchange over a channel corresponding to the engaged outgoing channel from the switching station, is demodulated, and lights a supervisory lamp indicative of the incoming channel. An operator at the called exchange plugs into the corresponding jack, and a 150-cycle modulated carrier is transmitted from the called exchange and causes a supervisory lamp to be lighted at the calling exchange to indicate that the called exchange has answered. The calling and called operators are now able to converse by superimposing speech modulations on the carriers transmitted over the selected path. Having ascertained the designation of the wanted subscriber the called operator completes the connection over the cord, and the calling and called subscribers can then converse, the transmission between exchanges being effected by modulated carrier waves. Seizure of an idle carrier channel to the multiplex toll line 104, Fig. 4, by an operator at the calling exchange, Fig. 3. Assuming that a subscriber SS, Fig. 3, whose line terminates in the exchange shown in this Figure, desires to converse with a subscriber centering in another distant exchange. On removing his receiver, the line lamp LL is lighted and the operator at the position to which the calling line is allotted, inserts the local plug LP of an idle cord CC-2 into the answering jack AJ, operates her listening key LK, and ascertains the calling party's requirements. Having noted the condition of the three carrier channels, over which access is had to the multiplex line, from the condition of the busy lamps BL-1 - - BL-3, she then inserts the plug TP into the jack MJ-1 of an idle channel (assumed to be the first channel), thereby completing a circuit for relays 16, 15, Fig. 4, and a toll supervisory lamp TSL in the cord CC-2, Fig. 3. Slow-release relay 16 energizes relays 20 and 17. Relay 20 lights the valve filaments of the balanced modulator BM-1 and the balanced amplifier BA-1 in the transmitting branch of the seized channel, and connects the harmonic distributing circuit 105 to a tuned circuit TC-3, which picks out the 10000 cycle transmitting carrier appropriate to the channel and applies it across a potentiometer 23 for application to the. grids of balanced modulator BM-1. Relay 17 closes a circuit over conductor 53 for the busy lamps BL-1 of the seized channel at all the operators' positions. The relay 15 energizes a relay 19, which connects the 150 cycle frequency, derived from the distributing circuit 106 and used for the control of the switching apparatus at the intermediate switching station, to the grids of the balanced modulator BM-1 by way of a tuned circuit TC-2 and potentiometer 22. The harmonic carrier frequencies in the distributing circuit 105 are derived from a base frequency of 3333 cycles supplied over the multiplex line 104 from the harmonic generator HG-2 at the switching station, Fig. 6, through the separating circuits STC-3, distributing circuit 100, tuned circuit STC-2, hybrid coil HFH-2, Fig. 5, tuned circuit STC-1, Fig. 4, and an amplifier A-2, Fig. 3, to an harmonic generator HG-1, the harmonics from which are amplified by an amplifier A-3. The 150 cycle control frequency is generated by an oscillator 0-1, Fig. 3, and passed through an amplifier A-1 and tuned circuit TC-1 to a transformer 14, which is provided with a grounded sheet between its windings to prevent unbalance of the two sides of the distributing circuit 106. After passing through a high pass filter HPF-1, Fig. 4, and the balanced amplifier BA-1, the modulated upper side band of the 10000 cycle carrier proceeds through a band filter BF-1 into a common transmitting path, and thence by way of a hybrid coil HFH-1, associated with a balancing network N, into the multiplex line 104. Control of apparatus at the intermediate switching station in which the multiplex line 104 terminates, to select an idle carrier channel of the wanted exchange. The upper side band of the modulated 10000 cycle carrier passes over the multiplex line 104 and hybrid coil HFH-2, Fig. 5, into a common receiving branch, and thence by way of a 10000 cycle band filter BF-9 into the receiving branch of the first channel, where it is amplified by an amplifier A-5 and applied to a balanced detector BD-2, to which is also applied a demodulating 10000 cycle frequency derived from a common distributing circuit 109 by way of a tuned circuit TC-6. (The demodulating frequencies 10000, 13333, and 16666 which may be required, are passed to the common circuit 109, over a band filter BF-17, which separates them out from the set of harmonic carriers produced by the harmonic generator HG-2 fed by an oscillator 0-2 and amplifier A-8.) The 150 cycle control frequency is separated by the tuned circuit TC-5, Fig. 5, rectified and amplified in the rectifier R-2 and energizes relay 25, which closes the circuit of a slow release relay 26. Relay 26 grounds the wire 125 to mark the engaged channel busy, and energizes a relay 38, which lights the valve filaments of the balanced modulator BM-2 and amplifier A-6 in the receiving branch of the channel. The operator throws the dialling key CK. Fig. 3, preparatory to sending the impulses to control the switch S-1, Fig. 6, associated with the seized channel at the switching station: battery over a resistance 48 is thereby substituted for the suerpvisory lamp TSL at the make-before-break contacts of the key CK. The operator now dials the call number say "3," of the wanted exchange, on the dial OD, which causes the control relay 15, Fig. 4, to de-energize thrice, and by opening the circuit of relay 19, thrice disconnect the 150 cycle control frequency from the modulator BM-1. The dial OD is so regulated that each break at contacts 13 is for a short interval and each succeeding make is for a longer interval. On the first interruption in the application of the control frequency, the relay 25, Fig. 5, at the switching station also de-energizes, and closes a circuit for a relav 27, which completes a circuit over conductor 119, the switch brush 1 in its first position, and co