Patent GB-640108-A (GB)
Title: Improvements in or relating to signalling systems
IPC: H04Q1/446
Filed: 1948-02-20
Abstract: 640,108. Automatic exchange systems. WESTERN ELECTRIC CO., Inc. Feb. 20, 1948, No. 5098. Convention date, June 15, 1945. [Class 40 (iv)] [Also in Group XL (c)] In a voice-frequency signalling system, V.F. current normally flowing over a trunk line is terminated as a seizure signal, and selective impulses and supervisory signals are sent by means of current of the same frequency but of greater amplitude, the normal amplitude being restored after the disconnect signal has lasted for a brief period. As shown, a two-way 4-wire trunk line TL1, 2 is connected at its ends through hybrid coils HC1, 2 to outgoing trunk circuits OT1, 2 and incoming selectors TS1, 2. Voice-frequency current normally flows over both branches of the line from generators 230, 330 to operate relays 374, 290 controlled by signal receivers SR2, 1. When an operator plugs into the trunk circuit OT1, sleeve relay 104 in a manner not shown brings up relay 110, which energizes 1004 and connects wire 170 to relay 105 in place of 1003. Relay 140 operates over the ring and energizes relay 200 over wire 164 to disconnect signal current from the branch TL1 and bring up 220 which opens a shunt 235 across the generator 230. The consequent release of relay 374 at the called office connects up 503 over wire 470, marking the trunk busy, and line relay 557 of the selector TS2 energizes in series with 505 which brings up holding relay 507. At the calling office, relay 140 responds to dialled impulses and controls 200 to send impulses of signal current of increased amplitude which are repeated by 374 to line relay 557 to set the selector TS2. Switching relay 130 comes up at the end of dialling. Assuming that the connection is extended to toll position circuit IT2 at the called office, plugging in by the answering operator results in the operation of 511, which over wire 464 energizes relay 300 to disconnect signal current from the branch TL2 and bring up 320 which opens a shunt 335 across the generator 330. The release of 290 at the calling office energizes 105 over wire 170, dimming the supervisory lamp CS. When the answering operator disconnects, the release of 511 releases 300 which restores the signal current and disconnects 320 to replace the shunt 335 across the generator after a brief period. The operation of 290 at the calling office lights the lamp CS. In the event of a busy condition, 511 operates intermittently and causes flashing of the lamp CS. Similarly, when the calling operator disconnects, the release of 140 disconnects 200, 220 to restore the signal current and replace the shunt 235 across the generator. The operation of 374 at the called office disconnects 505, 605, 507 and 503 to bring about release. Signal receivers. Although different arrangements are shown for the signal receivers connected to the branches TL1, 2, either arrangement may be employed for both. The receiver SR1 comprises an amplifier 260 acting as a limiter and a detector 270 controlling the signal responsive relay 290 and feeding parallel-tuned and series-tuned circuits 273, 274 and 277, 278. The detector is biassed to cut-off, and the voltage across 273, 274 is rectified at 272 and fed back to give positive reaction while that across 277, 278 is rectified at 282 and fed back to give negative reaction as a guard against non-signalling frequencies. When energized, relay 290 connects up a slow-operating relay 285 which reverses the rectifier 282 to prevent release of 290 byvoice-currents on calls to an intercept operator. In the receiver SR2, the parallel-tuned and series-tuned circuits 381, 382 and 383, 384 are fed by the limiting amplifier 355 and control the grid bias of the detector 370 in opposite senses through a voltage-dividing network including rectifiers 363, 368, change in bias being delayed by a condenser 371. A slow-operating relay 375 connected up by the signal responsive relay 374 cuts out the seriestuned circuit. Specifications 561,364 and 640,032 are referred to.