Patent GB-335046-A (GB)
Title: Improvements relating to intercommunications between telephone systems
IPC: H04Q1/446
Filed: 1929-09-06
Abstract: 335,046. Siemens Bros. & Co., Ltd., and Grinsted, W. H. Sept. 6, 1929. Semi-automatic and manual exchange systems. -In an inter-exchange system employing alternating currents of audio-frequency for impulsing and supervision, arrangements are provided whereby filter or rejector apparatus is inserted when such currents are traversing the junction to prevent their reaching the instruments, the apparatus being disconnected when conversation is to take place. A distinctive frequency is employed for each direction of transmission, and the frequencies are higher than those of the audible tone currents. As shown, the alternating currents are supplied from generators TG, TG<1> and are received by relays AR, AR' through tuned amplifier-rectifiers TR, TR<1>. The currents are normally flowing, and filters LPF, which pass the audible tone currents, are normally in circuit, being adapted to be cut out by relays X, X<1>. Call from the exchange A. On plugging into to jack OJ, sleeve relay CO connects the junction to the jack, and relay Y energizes and disconnects the generator TG to interrupt the flow of AC and release relay AR<1> at the exchange B, whereupon relay ARA<1> pulls up and completes the circuit to the automatic switches, from which a dialling signal is sent through the filters to the operator. Dialling controls relay Y to transmit corresponding A.C. impulses from the generator TG, relay Q being operated to prevent interference at the transmission bridge, and these are repeated by relay AR<1> at contact ar<1>2 to the automatic switches, relay D' being operated during each train to disconnect the preceding transmission bridge. If the called party is free, ringing takes place, and the operator hears ringing tone. Upon reply, ralay Y1 energizes and disconnects the generator TG1 to release relay AR at the exchange A, whereupon relay ARA pulls up, and relays X, X' operate to cut out the filters. The connection of battery to the a wire extinguishes the calling supervisory lamp. If the called party is busy, relay Y' energizes intermittently and causes flashing of supervisory lamp, while busy - tone is heard by the operator. Upon release by the called party, relay Y<1> falls back and A.C. is applied to the junction, energizing relay AR and disconnecting relays ARA, X to insert the filter and light the supervisory lamp. When the plug is removed, relays CO, Y fall back, and A.C. is applied at the generator TG to energize relay AR<1> and release relays ARA<1>, X'. Call from the exchange B. On plugging into jack OJ', sleeve relay CO<1> connects the junction to the jack, and relay Y' energizes and disconnects the generator TG<1> to release relay AR at the exchange A, whereupon relay ARA pulls up and lights the calling lamp CL. The operator plugs into the incoming jack IJ and throws her listening key. Relay Y energizes and disconnects the generator TG to release relay AR<1> and energize relay ARA<1>, and relays X, X<1> operate to cut out the filters and extinguish the supervisory lamps. The operator at exchange A restores her listening key after hearing the number required, but relay Y energizes again when the called party replies to cut out the filters and extinguish the supervisory lamp at exchange B, which lights again when the called party releases to give a disconnect signal. When the operator at exchange B removes her plug, relays CO', Y' fall back, with the result that relays ARA, X release at the exchange A and light the supervisory lamp to give a disconnect signal thereat.