Patent GB-709350-A (GB)
Title: Improvements in or relating to automatic telephone systems
IPC: H04Q11/04
Filed: 1950-12-21
Abstract: 709,350. Automatic exchange systems. STROMBERG-CARLSON CO. Dec. 21, 1950 [Dec. 24, 1949], No. 31111/50. Class 40 (4) [Also in Groups XL (b) and XL (c)] In a 100-line exchange, each line is allotted a separate channel of a pulse-multiplex system. The channel for a line A numbered 23 consists of a short pulse 84b, Fig. 37, recurring every 100 microseconds. The channel is derived from a common source of pulses repeated at 1 megacycle per second by gating under the control of a train of tens gating pulses 73a phased to represent the tens digit 2 and a train of units gating pulses 64b, phased to represent the units digit 3. A tens gating pulse lasts 10 microseconds and recurs every 100 microseconds. A units gating pulse lasts 1 microsecond and recurs every 10 microseconds. The system is shown in outline in Figs. 1 to 5. Channel pulses and gating pulses are generated in common equipment 17. When the line A (No. 23) is in use, a gating circuit 11 described as a multiplexer transmits and modulates in amplitude the channel corresponding to the line. A free link 12 comprising a finder 12a  and a connector 12b, is allotted by a circuit 24 which transmits enabling pulses cyclically to the finders 12a, 13a, 14a. The link remains operated in the intervals between the allotter pulses of successive cycles. Tens and units registers in the connector count the dialled impulses and open gates to transmit tens and units gating pulses which control the transmission of short channel pulses to the wanted line, B (No. 32). Modulations are transferred from one channel to the other by means of reconstructor circuits which in effect lengthen the pulses until they span the time positions of all the 100 channels of the system and can be used to modulate any one of them. The pulses are distributed to the wanted line by a distributer 15. The usual dial, ringing and busy tones are generated by means of oscillatory circuits such as multivibrators. Ringing is controlled by a 50 per cent. increase in the amplitude of the channel pulses transmitted by the connector. Positive pulses are used for transmission from the multiplexer to a link and negative pulses are used for transmission from the link to the distributer. Tubes used only to invert pulses and rectifiers used only to limit voltage changes are not all specifically mentioned in the abridgment. Figs. 7 to 24 when arranged as in Fig. 6 show the system in detail. Figs. 25, to 31. concerning the common equipment for supplying pulse trains and tone signals, are described but not shown. Common equipment, Fig. 4; a crystal controlled electron coupled master oscillator 18 controls two conventional pulse forming circuits, in circuit 19, which feed trains of pulses 95, 96 Fig. 37, over conductors 40, 41. The trains of pulses 95, 96 are adjusted by means of four-phase condensers so that the pulses 96 which are used as channel pulses fall approximately midway between the pulses 95 which are used to drive the units pulse ring 20. The pulse ring comprises a ring of Eccles Jordan flip-flops connected by inverters and amplifiers to conductors 30a ...30j 31a ... 31j. Ten pairs of tubes are arranged in a ring with the first tubes of nine pairs conducting and the second tubes of the same pairs cut off. The condition of the tenth pair is reversed. A pulse on 41 restores the reversed pair and reverses the next pair. As the ring is driven by pulses 95, each pair is reversed for the period between two pulses once in a cycle of ten pulses and trains of units gating pulses 62, 63 ...71 are produced on the conductors 30a ... 30j, 31a ...31j. The tens pulse ring 21 is identical with 20 but is driven by pulses from conductor 31j so that the tens pulses on conductors 31, 32 are ten times as long as the units pulses and the cycle is ten times as long as the units cycle. Channel pulses 96 on conductor 41 are amplified in commutator drive circuit 25 and are fed direct to the connector over conductor 35 for the called lines. They are continuously gated through a channel pulse commutator 22 for the calling lines. The channel pulse commutator comprises ten gating tubes the control grids of which are all fed with pulses 96 from the driving circuit 25. The suppressor grids are fed with positive units gating pulses over 31 and channel pulses are transmitted only when a channel pulse on the control grid of a tube concides with a gating pulse on the suppressor grid. A ringing carrier supply circuit 26 feeds a 100 kc/s. carrier to a ringing key circuit 27 and to a conductor 37 common to the line circuits. The circuit 26 comprises a flip-flop which is driven by units pulses on conductors 39 connected to 31a, 31#, producing a square wave at 100 kc/s from which the fundamental is derived by filtering. Continuous 20 c/s. ringing current is fed from a generator 60 of unspecified type to conductor 61 which is multiplied to the line circuits where it is controlled by channel pulses of large amplitude and by interrupted or keyed 100 kc/s. carrier from circuit 27. The ring-back tone generator 29c comprises a multivibrator producing a signal output at 200 c/s. which is mixed with the output of a saw-tooth oscillator operating at a frequency of 20 c/s., Fig. 25A (not shown). The outputs of both the ringing current generator and the ring-back tone generator are interrupted or keyed in the circuit 27 in which a multivibrator is employed to cut off the transmission during four of every five seconds. A busy tone generator, Fig. 31 (not shown), in circuit 29 comprises multivibrators operating at 1 c/s. and at 180 c/s. the outputs being mixed. The dial tone generator, Fig. 30 (not shown), in circuit 29 also employs two multivibrators operating at 180 c/s. and 15 c/s. to produce a wobbly tone. The link allotter 24 comprises a pulse ring with three pairs of tubes, which is driven by a blocking oscillator operating at 1 kc/s. to feed 1 millisecond pulses (i.e. 1;000 microseconds or 10 pulse frames) cyclically to the finders 12, 13, 14. Line circuit 10, Figs. 11A, 12; when the receiver is lifted at a calling station A, a rectifier 1125 is biased over the loop to transmit the 100 kc/s. supervisory carrier current always on conductor 37 to a limiting amplifier 1131 which transmits a substantially constant control potential over conductor 53 to the gating tube 1323 corresponding to A in the multiplexer. The multiplexer then transmits pulses forward in the channel allotted to A. Dial tone is received through the distributer, conductor 44a, amplifier 1204 and transformers 1200, 1111. A balancing network 10a prevents reversion of the tone over conductor 53 to the multiplexer. Dialled impulses are transmitted to the multiplexer as blocking potential on conductor 53. If the line is busy, busy tone is received over the same circuit as dial tone. If the line is idle, ring-back' tone is returned. When the line is called, large amplitude negative pulses received over 44a reduce the volt drop in anode resistance 1202 and bias rectifier 1216 sufficiently to transmit the keyed 100 kc/s. ringing control current on conductor 38 to the control grid of ringing control tube 1208 which fires during positive half-cycles and. transmits the 20 c/s. ringing signals on 61 during the "on" periods of the control current. The ringing current fires a neon tube 1107 during alternate half-cycles and the ringer 1106 is operated. When the call is answered, a ring trip circuit operates in the connector, the amplitude of the pulses returns to normal and ringing stops. Voicefrequency modulations on the incoming pulses are detected by 1204 and any high frequency components remaining are removed by a low pass filter 1114 with a ceiling about 3.5 kc/s. Outgoing voice-frequency signals derived from the secondary of transformer 1111 are applied to the multiplexer tube 1323 over conductor 53 and a network comprising a resistance 1144 and rectifiers 1142. Line circuits 10, 16 are identical.  Multiplexer 11, Figs. 13, 14.-This circuit is fed continuously with channel pulses and tens pulses from the common equipment over conductors 34, 32 and under the control of a line, modulates and gates pulses in the time position allotted to the line. The circuit includes 100 units gating tubes 1323, one for each line. All the tubes in a vertical column correspond to lines with the same units digit and all the tubes in a horizontal column correspond to lines with the same tens digit. In one pulse cycle, ten channel pulses 84a, 84b &c. are fed to each of the tubes 1323 in the column corresponding to the digit 3. As tube 1323 has a control potential on the suppressor grid following the initiation of a call at A (No. 23), it passes all ten pulses each cycle. Only the second pulse of each cycle is gated by the tens gating tube 1420 which is controlled by tens pulses on conductor 32b and the single pulse transmitted in each cycle is in the time position 23. Tube 1421 rectifies the inversion in the first gating tube 1323. A master inverter 1470 rectifies the inversion in the tens gates. Distributer 15, Figs. 21, 22; tens and units gating pulses are fed to the 10 tens gating tubes 2130 and to the 100 units gating tubes 2232 which are arranged substantially as in the multiplexer but in reverse. Pulses in the time positions 3X are accepted only by tens gating tube 2130 and of these pulses only those in the position 32 of station Bare accepted by tube 2232. Finder 12a, Figs. 2A, 15, 16; when an allotter pulse is applied over 36a to the input circuit 200, multiplexer pulses are transmitted to circuits 203, 204, 201. The +B switching circuit 201 then provides anode potential on 12c for circuits 203, 204 and for numerous circuits in the connector and is slow to release so that it remains operated during the intervals between allotter pulses. Finder gating pulses 64b are transmitted over conductor 12d to identify the calling station in the connector and busy pulses are applied to busy conductor 5