Patent GB-709399-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: BEALE CHARLES EDMUND
IPC: H04Q1/446
Filed: 1951-07-06
Abstract: 709,399. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. July 3, 1952 [July 6, 1951], No. 16050/51. Class 40 (4) ' A system employing voice-frequency impulse signals for setting up, control and supervision of connections over routes having " go " and " return " channels between exchanges is provided with a route terminating set having on the two-wire side thereof an impulseresponding device responsive to a train of d.c. loop impulses to control the transmission of corresponding v.f. impulses over the " go " channel; also on the two-wire side is a switching device which responds more rapidly to the first break in the d.c. loop than said impulseresponding device and causes line-splitting at the two-wire side of the terminating set sufficiently prior to the transmission of the first v.f. impulse to allow the v.f. receiver at the incoming end of the route to recover from any surges due to the d.c. loop impulsing. Figs. 1-3 show a terminating set in the form of a bothway relay set for use in the setting up, by subscriber or operator dialling, of calls between widely separated exchanges, currents of a single voice-frequency being employed in dialling, supervision and control. Outgoing call.-The calling subscriber's loop is applied over path LSM from the local selector multiple, bringing up relays AN, BN and SX, and the last-named relay connects a v.f. signalling current source VFS to the outgoing path OGP (and thence to the incoming path ICP of the remote relay set). After a delay of 40-60 milliseconds, relay SR operates to release SX and remove the v.f. seize signal, also bringing up relay IG which is followed by relays BB and J. Tube VA now fires to energize a high-speed relay HS which completes the audio path from the local selector to a 4-wire terminating and signalling unit TSU. Incoming call.-At the remote (incoming) relay set, the v.f. seize signal on the incoming path ICP acts upon a v.f. receiver over path VFR, bringing about the release of a normally energized relay X (not shown). Relay E is thereby energized to seize the incoming selector over path IS and bring up relays I and IG. When the seize signal is removed from ICP, relay X is re-operated, followed by relays A, AA, BG, EA and RC. Tube VA conducts so that relay HS is energized to advance the connection. Calling subscriber dials ; effect at outgoing relay set.-On the first break in the dialling loop, before relay AN completes its release, the decay in flux due to its line windings causes a build-up of voltage across its third winding so that a negative-going pulse is sent to the control electrode of tube VA which is thereby cut off. Relay HS is immediately released to effect line-splitting. Relay AN finally releases 12 milliseconds after HS, bringing up relay SX after a further period of 12 milliseconds. The first burst of v.f. dialling current is then applied to the outgoing path OGP, the remote v.f. receiver having had time to recover from the initial d.c. loop surge. Relay CN is energized on the release of AN and holds during each impulse train. Relays AN, SX follow the dial impulses and apply the corresponding v.f. impulses to the outgoing path OGP. When relay AN remains up at the end of the digit, CN releases and HS is reenergized. This process is repeated for each digit. Alternative arrangement for line-splitting, Fig. 4.-In this modification the tube VA is dispensed with and a further high-speed relay SP is introduced. On seizure, relays AN and SP are operated over the loop and the latter brings up relay HS. At the first break of the dialling loop, SP and HS release in advance of AN to bring about line-splitting approximately 20 milliseconds before the transmission of the first v.f. dialling impulse. Non-linear resistances NLR2, NLR3 are incorporated to reduce the amplitude of the surge. Effect of voice-frequency dialling impulses at incoming relay set.-Relay X releases on the arrival of each v.f. impulse over path ICP and relay HS is de-energized during each digital impulse train. Relays A and AA follow the impulses, and the first release of AA brings up relay C, which remains energized throughout the impulse train and releases relay T. The first re-operation of relay AA brings up relay CA. Relay A repeats the incoming v.f. impulses as d.c. impulses over path IS to the incoming selector, and the connection is completed in normal fashion. Called subscriber answers; effect at incoming relay set.-The reversal of potentials operates relays I and D. Relay DA is thereby energized and brings about the release of relay HS, splitting the line, and the operation of relay SX which initiates the v.f. answer signal. Relay IG, having been short-circuited on the operation of DA, releases after a delay of 70-100 milliseconds and short-circuits relay RC which releases after a further delay of 100-150 milliseconds. This releases relays DA and SX, terminating the v.f. answer signal which has lasted for 170-250 milliseconds, and brings up relay DD to re-energize relays HS, IG, and RC. Reception of v.f. answer signal at outgoing relay set.-Relay X is released for the duration of the signal and brings up relay VF to energize relay DA and release relay IG. Relay HS falls back to split the line, and relay AS comes up followed by JA which interacts with slow-releasing relay J to send a metering pulse. At the termination of the answer signal, X is re-energized, releasing VF and DA so that IG, HS, and AR are operated and the speech circuit is completed. Calling subscriber hangs up; effect at outgoing relay set.-Relay AN releases, bringing up CN and SX to start the v.f. clear forward signal while BN falls back slowly to initiate release of the preceding apparatus in the connection. Relays CN, BB, AS and AR are also released, so that differential relay AY is connected to interrupted earth lead IE. The first earth pulse operates AY, and relay AZ is energized at the end of the pulse. The second earth pulse releases AY, and relay RG is energized to light a supervisory lamp LP. At the end of the second earth pulse, AZ falls back followed by SR, IG, and SX, thus ending the clear forward signal which has lasted for 1.5-3.0 seconds, the intermittent earth pulses being 0.75 sec. long with 0.75 sec. separation. If the call had not been answered, however, relay JA would not have been energized, so that relay SR would not release following the release of AZ at the end of the second earth pulse. Relay AY would then operate on the arrival of the third earth pulse, followed by AZ at the end of the pulse, SR thereupon releasing. The fourth earth pulse would release AY and SX to end the clear forward signal which in this case would have lasted 3.75-5.25 secs. The end of the fourth earth pulse would finally release AZ. Reception of v.f. clear forward signal at incoming relay set. Relay X falls back, followed by A and AA, opening the loop to the incoming selector and bringing up relay C which releases HS to effect line-splitting. The loop is closed again after 70-100 milliseconds by the delayed release of IG, shortcircuited on the relapse of X. After a further delay of 100-150 milliseconds relay RC falls back, to be followed 400-600 milliseconds later by EA and 100-150 milliseconds later still by E. This releases relays D and I and also finally opens the loop to the incoming selector, initiating the release of the equipment ahead. When the clear forward signal is terminated, relay X is re-operated and BG releases slowly to bring up SX which causes the v.f. release guard signal to be sent back to release the outgoing relay set. After 100-150 milliseconds relays C and SX are released to end the release guard signal, and the two relay sets are ready to be taken into use for another call. A call may be set up by an operator, the relay sets operating as described above. An additional v.f. ring forward signal of 375 milliseconds on and 375 milliseconds off, repeated until acknowledged, may be employed, together with its associated acknowledgement signal which lasts 100-150 milliseconds. The ring forward signal may be used for trunk offering. Two relay sets of the type described may be incorporated in a tandem exchange, a pad AP in each relay set being automatically switched out during the setting up of the connection. That aspect of the system which relates to the control of the d.c. loop at the incoming relay set in response to v.f. impulses is claimed in Specification 709,406.