Patent GB-729261-A (GB)
Title: Improvements in or relating to voice-frequency signalling systems
IPC: H04Q1/446
Filed: 1953-05-01
Abstract: 729,261. Semi-automatic exchange systems. WESTERN ELECTRIC CO., Inc. May 1, 1953 [May 1, 1952], No. 12119/53. Class 40 (4). In a V.F. signal receiver which discriminates between a signal frequency and signal imitation in speech, the incoming currents are preamplified and applied to signal and guard circuits arranged to be anti-resonant and series-resonant respectively at the signal frequency, the outputs of the two circuits being separately rectified and combined to control two signal relays through individual thermionic valves. In the following description, V.F. signal receivers form part of a system in which calls are set up between two identical toll offices, Figs. 1, 2 and Figs. 4, 5. Two.wire toll lines, Fig. 3A, may be provided and different signal frequencies used for the two directions of signalling. Alternatively, the provision of four-wire toll lines, Fig. 3B, permits the use of the same signal frequency in both directions. General arrangement. In the normal condition of the line a low-level signal frequency is transmitted in both directions. In the office shown in Figs. 1 and 2, signal frequency from a source OSC1 in a signal receiving and transmitting circuit SC1 is connected to outgoing conductors 301, 302 over resistors 231, 232. At the distant office, Figs. 4 and 5, the signal frequency incoming over conductors 335, 336 is applied over a transformer T41, volume-limiting amplifier 450 and transformer T43 to a network 454 comprising series-resonant and antiresonant circuits 456, 455 connected in series and tuned to the signal frequency. Circuit 455 and a rectifier 453 constitutes a signal-detecting channel while circuit 456 and a diode formed by one section of a double triode valve 470 constitutes a guard channel. The rectified signal and guard voltages are combined through resistors 459, 461 and are applied to signal relays 485, 490 over amplifiers comprising a valve 480 and the other section of the double triode 470 respectively. The response of signal relay 485 is immediate, while the response of 490 is delayed by a delay circuit comprising condenser 467 and resistors 468, 469 in the grid circuit of the double triode. Due to the continuous application of a low-level signal, relays 485, 490 are both operated so that earth is disconnected from an outgoing signal conductor 570 and relay 496 disabled. A filter network 410 comprising anti-resonant and series-resonant circuits 411, 412 tuned to the signal frequency prevents forward transmission of the signal through a speech amplifier 418. A continuous return signal from a source OSC2 is transmitted to SC1 over resistors 431, 432 so that a similar normal condition prevails at the other office. Seizure of junction. The toll operator answers with a plug AP1, Fig. 1, a call incoming from a local office or another toll office and takes a toll line into use by inserting calling plug CP1 into jack J1. Relays 22, 104 and 140 pull up, followed by 110 which is operated by 104 in a circuit which is not illustrated and disconnects a signalling conductor 170 from a twoway trunk circuit TWT1. Relay 110 also pulls up 1004, and by connecting earth to conductor 168, marks TWT1 busy to toll route selectors. Operation of 140 pulls up 220 which disconnects source OSC1 from the outgoing circuit to constitute a seizure signal and operates 204, 230 in turn, the latter relay disabling 204 which falls back slowly. While 204 is operated the outgoing toll circuit is disconnected and terminated by resistors 205, 206. At the distant office, cessation of the signal frequency causes 485 to fall back, followed by the delayed release of 490 which operates 496 and earths conductor 570 so that relay 603 in trunk circuit TWT2 responds. Relay 496 grounds a permanent signal alarm circuit 499, connects a varistor 493 across resistor 468 to aid in dissipating the charge on condenser 467, connects the guard channel into circuit by opening a short circuit across the series-resonant circuit 456 and short-circuits a resistor 457 to improve the selectivity. In the trunk circuit TWT2, relay 603 grounds conductor 568 to guard the outgoing trunk circuit from seizure and pulls up 605 in series with the line relay 657 of toll selector TS2. This selector and any additional selector is held by ground on conductor 653 applied by relay 607. Dialling. When the toll operator dials the first digit, relay 140 repeats the impulses to 220 which has its operate and release times equalized by a potentiometer 221. When both relays fall back at the commencement of the first impulse, slow-release relay 230 is disabled and slowrelease relay 204 pulls up. Both relays remain held for the duration of the impulse train and with each release of 220 the signal frequency from source OSC1 is directly connected to the outgoing conductors 301, 302. At the distant office 485 and 490 respond to each V.F. impulse. Relay 485 connects the filter 410 in the through transmission circuit and causes a condenser 491 to charge over a resistor 486. The response of 490 is delayed and so ensures that the impulse is of sufficient duration, while its release is delayed by temporary positive grid potential derived from condenser 491. Relay 490 repeats the impulses to relays 605, 657 by disconnecting the operating circuit over conductor 570. Relay 496 is also disabled by 490 but is slow to release and remains held during impulsing. If a short-duration V.F. impulse is received so that the signal relay 485 falls back prematurely, the release time of 490 is increased by temporary positive grid potential applied to the grid of valve 470 from the anode circuit of valve 480 through condenser 491. This circuit is also utilized to hasten the release of 490 in the event of an overlong impulse, the anode potential of 480 being considerably decreased while a V.F. signal is received so that negative potential is applied to the grid of 470. Answering of call at tandem toll office. Relay 660 pulls up if the call is routed through selector TS2 to an incoming trunk circuit IT2. Line relay 705 then operates in series with 605 in the two-way trunk circuit TWT2 and lights answering lamp L7. When an answering plug is inserted in jack J7, relay 734 pulls up in a circuit which is not illustrated and operates 611- Battery is then substituted for ground on conductor 564 and relay 420 operates, disconnecting OSC2 from conductors 321, 322 so that the return signal frequency is terminated. Relay 430 is also operated by momentary operation of 404. The band-elimination filter 410 is removed from the speech path by 420 and 485. Relay 420 also disconnects a resistance shunt 422 across the guard circuit to increase the guard sensitivity, and connects a resistor 423 in series with resistor 492 and condenser 491 to increase the release time of 490, the latter feature ensuring that a recall signal to the answering operator is repeated over lead 570. Backward signals. If TS2 or a succeeding selector encounters all trunks busy or the called party is engaged, busy signals received from conductor 564 at 60 or 120 impulses per minute are repeated by 420 as V.F. impulses over the toll line and cause corresponding operation and release of signal relays 285, 290. The impulse length is greater than that of dial impulses so that 296 falls back between successive operations of 290. When released, relay 296 short-circuits the guard network and maintains equality between the operate and release times of 290 by open-circuiting condenser 291 and connecting resistor 294 between the grid and cathode of triode 270. Relay 290 repeats the busy signal over conductor 170 to relay 105 so that supervisory lamp CS is busy-flashed. Momentary premature operation of 290 by positive transients is prevented by a varistor 263. If a stop-dialling signal is received over conductor 564 during the setting up of a call, relay 420 disconnects the return signal frequency from the toll line. A start-dialling signal to the calling operator is effected by release of 420 whereby the return signal is reinstated. Release. If the answering operator removes the plug, relays 611, 734 fall back, followed by 420 which connects the signal frequency to the toll line. At the calling office the V.F. signal pulls up relays 285, 290, and when 296 falls back, lamp CS is energized over relay 105. If the calling operator clears down, relays 104, 140 falls back and a forward V.F. signal is transmitted by release of 220. In the distant office 485 and 490 operate and the circuit is restored to normal. Permanent signal alarm. The alarm apparatus 499 is energized in the condition of 490 and 420 released and 496 operated so that an indication is given if a large number of signalling channels are simultaneously disabled. Two-wire toll line, Fig. 3B. Different frequencies are used for signalling in the two directions, hybrid coils HC31, HC32 providing the incoming and outgoing transmission paths to the 4-wire exchange circuits. Blocking filters 356, 376 are tuned to the signal frequency of the adjacent toll office to prevent signal return. Each filter comprises anti-resonant networks 357, 358 and a series-resonant network 359 and are disconnected during speech by the operation of relays 368, 388. Specifications 20860/11, [Class 38 (v)], 204,028, [Class 35], 289,543, [Class 38 (v)], 561,364, 640,032-and 640,108 are referred to.