Patent GB-778022-A (GB)
Title: Improvements in communication systems employing gas discharge tubes
IPC: H04Q3/52
Filed: 1955-08-11
Abstract: 778,022. Automatic exchange systems. WESTERN ELECTRIC CO., Inc. Aug. 11, 1955 [Aug. 18, 1954], No. 23180/55. Class 40 (4). The talking path from a telephone subset contains a gas discharge tube, a gas discharge counter having alternate rest and transfer cathodes being provided to a rest cathode of which the gas discharge tube is connected so as to be fired when dialling pulses from the subset are applied to the transfer cathodes to step the counter discharge to that rest cathode. In the embodiment of Fig. 3 talking paths can be set up between any two of the subsets 10-13 by striking one gas discharge tube in each of the groups 151, 161, 171, 181; and 191, 201, 211, 221. If subset 13 calls subset 12 discharge tube 181 is struck by raising the receiver to close contact 39 in the subset and connect tube 181 across the potential sources 31 and 32. As only one tube in the group containing tube 181 can sustain high current discharge at any one time, non-conducting tubes are locked out. When tube 181 strikes, a pulse produced across resistor 42 is passed by capacitor 50 to strike a discharge tetrode 51 providing anode potential for a counter 27 (of the kind described in Specification 708,424) in which conduction is set up to a normal cathode 34. Voltage in the leg resistance 76 of cathode 34 fires a discharge tube 74 which acts as a relaxation oscillator with an anode circuit 75 tuned to 1200 cycles passed as dialling tone to a wire 24 common to all subsets. To gain access to subset 12 three pulses are put on to the wire 24 by a dial mechanism which operates a switch 43 to increase resistance in the conducting path of tube 181 thereby altering the potential across resistor 41. These pulses are passed over capacitor 25, and a low-pass filter 77 which blocks dial tone, to a wire common to the transfer cathodes of counter 27 and transfer the counter discharge from normal rest cathode 34 to the third rest cathode 29. When the discharge settles a potential is applied to the starter anode of tube 211 which draws low sustaining current. When the receiver of subset 12 is raised switch 39 is closed and tube 211 conducts across its main gap to connect subset 12 in series with subset 13 and capacitors 48 and 60 to complete a low-impedance talking path. As soon as dialling pulses steps discharge from the normal cathode 34 the dial tone discharge tube 74 is extinguished. As shown in Fig. 4, the tube 211 acts as a ringing tone relaxation oscillator the tuned anode of which includes the receiver 71. When the receiver at subset 12 is raised to answer a call the switch 39 closes to shunt the anode circuit and stop ringing tone. To extinguish the talking path the receiver at subset 13 is replaced to extinguish tube 181 the consequent reduction in current through resistance 42 applying a positive pulse to the firing anode of a discharge tube 67 which shunts tetrode 51 and counter 27 cutting out both while acting as a self-extinguishing relaxation oscillator. The same positive pulse is applied to a similarly arranged discharge tube 63 to extinguish tube 211 or any other conducting tube of its group. A varistor 62 placed across a common transfer cathode resistor 44 shunts the positive pulse put on wire 24 whenever a receiver is lifted to initiate a call. Fig. 2 (not shown) gives a simplification of the arrangement of Fig. 3, the talking path discharge tubes being replaced by diodes without starter anodes so that the cathode resistances 55 of counter 27 are connected directly to the anodes of tubes 191, 201, 211 and 221. In this event each of the resistances 57 is connected in series with a varistor and an inductance, the varistors preventing conduction through the diodes 191-221 in series with the counter 27 and the inductances reducing a tendency to cross-talk. Figs. 5a and 5b show an arrangement whereby two talking paths may be established at the same time by the use of two counter tubes 271 and 272, each of which is associated with one of the groups of discharge tubes 801-871 and 802-872. If the subset 10 initiates a call to subset 11, say, either of the tubes 801, 802 will strike, the common load inductance preventing sufficient conduction for both. Suppose 802 fires, then relay 982 operates and strikes tetrode 512 to initiate conduction in counter 272. Conduction through 802 locks out 812, 822, and 832, and establishes a dialling path from subset 10 via reversing transformer 96 to the transfer cathodes of counter 272 whereby tube 851 can be selectively fired by dialling to complete the talking circuit to subset 11 over condenser 99. A call between subsets 12 and 13 may be set up at the same time by seizing counting tube 271 and its associated talking path discharge tubes. To prevent breaking in on an established talking path resistors 98 at each subset produce a voltage when in a talking path to operate two of the relays 100 which switch negative potential on to the rest cathodes of each subset. Alternatively an arrangement of gas discharge tubes and diode gates may be used to prevent breaking in. Specification 686,427 and U.S.A. Specifications 2,515,361, 2,518,319, 2,578,370, 2,635,810 and 2,646,523 also are referred to.