Patent GB-550409-A (GB)
Title: Improvements in or relating to electric signalling systems
IPC: H04Q1/453
Filed: 1941-12-17
Abstract: 550,409. Selective signalling. WESTERN ELECTRIC CO., Ltd. (Western Electric Co., Inc.). Dec. 17, 1941, No. 16279. [A Specification was laid open to inspection under Sect. 91 of the Acts, March 8, 1942.] [Class 40 (i)] [Also in Group XXXIX] In a multi-frequency signalling system, the signalling frequencies used are equally spaced in cycles and the receiving filters have equal band-widths and the same phase shift characteristics in their pass bands with the result that current! due to a particular signal reach a maximum in the corresponding filter when they have fallen to a minimum in adjoining filters and it is arranged that the receiving devices are disabled until this condition is reached. They are then rendered sensitive for a period which is determined by the receiving apparatus and not by the duration of the signal. The invention is described with reference tc selection in automatic exchange systems. Three arrangements are described. Circuit arrangements.-In response to a signal comprising a number of frequencies received from substation A, Fig. 3, via limiter 311, amplifier 308 and rectifier E, relays 314, 315 operate to prepare the circuit of the receiving tubes and disconnect the energizing circuit of the lower winding of 316 which, however, remains in the position shown during the charging time of a condenser 306. It then changes over to complete a current for charging a condenser 318 through the lower winding 317 which thereupon changes over temporarily to complete the operating circuits of the receiving tubes. Those corresponding to the received signal break down and operate their relays. In spite of the restoration of 317, the discharge continues in the operated tubes until the end of the signal releases 315 but the operation of other tubes by transients at the end of the signal is prevented. At the end of the signal, relays 314, 315 fall, 306 discharges and 316 moves to the position shown, discharging 318. Discharge in the tubes Ga ... Ge is interrupted and the current is now ready for the next digit. In a modification, Fig. 4, the cathodes of the detector tubes f1 ... f5 are normally disconnected and the relays 417 ... 420 are in the position shown. On the arrival of a signal, 417 changes over, disconnecting the upper winding of 418 which, after a delay caused by the discharge of 421 through this winding, disconnects the upper winding of 419, discharges a condenser 429, and makes an energizing current for 420 which completes the cathode circuits of the detector tubes. After a delay period during which a condenser 424 is charged via the upper winding of 419 the latter opening the discharge circuit of condenser 429 and energizing circuit of 420 (upper). Release of 420 is delayed for the charging time of 429. In a further modification, Fig. 5, the arrival of a normal signal breaks down a gas-filled tube 550 whereupon relay 517 changes over to cut out a loss pad 518, 519 in the input circuit and relay 529 completes the anode circuit of another gas-filled tube 551 via a relay 528 but this tube is unable to discharge while signals are being received owing to the biassing effect of rectified signals. Relay 529 also disconnects the upper winding of 537 and, after a delay due to the discharge of a condenser 536, 537 changes over and energizes 538 which completes the cathode circuit of the receiving tubes. When signals cease, 551 discharges through 528 which releases 529, 537; the relapse of 538 is delayed by the charging of 539. In the case of a strong signal, 554 breaks down and 517 is held in the position shown to prevent the removal of the loss pad. In the case of a very strong signal, 546 also breaks down and 548 changes over to cut a resistance 552 into the cathode circuits of the receiving tubes.