Patent GB-684413-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: BEALE CHARLES EDMUND
IPC: H04M15/10
Filed: 1950-01-17
Abstract: 684,413. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Jan. 9, 1951 [Jan. 17, 1950], No. 1155/50. Class 40 (iv). In a director system in which a call routed outside the originating exchange area is assessed on a time basis at a tandem exchange by a toll relay set which transmits periodic alternating current metering signals back to the originating exchange, the signals are inaudibly transmitted over the two speech conductors in balanced relationship and are detected over a rectifier bridge connected to a centre-tapped shunt relay, forward signal transmission being prevented by a balanced repeating coil. First code selector. Line relay L, Fig. 1, responds to a calling condition and causes a single motion switch CH, Fig. 3, to hunt for a free register-controller RCH. Operation of relay K at a free outlet stops the switch and pulls up JA to transmit dialling tone from lead FR. The incoming impulses, which comprise two exchange code digits, followed by the numerical designation of the wanted line, are repeated by L to the register-controller. After translation of the code digits into a maximum of six routing digits, the impulses are retransmitted over contacts k3, k4 to the first code selector, Fig. 2. This selector comprises a 200-point Strowger switch having rectangular release as described in Specifications 391,151 and 450,019 and its levels are individually allotted to local calls, manual board positions and outgoing toll junctions. The first routing digit steps the switch over relay A to the level required and hunting for a free outlet commences under control of a pair of test relays HA, HB. When a free outlet is found, relays BA and HA or HB extend the impulsing circuit over the selector. Completion of impulse transmission releases relay K, so that a homing circuit is completed for the hunting switch CH and the code selector connected to the calling line. If all the outlets required are in use, busy tone from lead BT. is extended over overflow contacts S1, while an overflow meter OM is operated over the unequipped eleventh outlet. If the called subscriber fails to dial, a timing cycle in the register controller releases relays BB and A so that NU tone is extended to the calling line and the code selector is raised one step. Relay K then falls back to release the register-controller followed by relay I which lights a fault lamp LP. The selector is then maintained in the alarm condition until the calling line is cleared. Metering local calls. A unit fee is charged for a local call. When routing is completed and the called subscriber replies, battery reversal on the speech wires operates relay D so that MH pulls up. A positive battery pulse terminated by the delayed release of JA is then applied over resistor YA to the calling subscriber's meter on the P wire. A rectifier MRA maintains the busy earth. Metering calls to manual boards. The two code digits only are dialled, the call being free or chargeable according to the service required. In the selector levels allotted to these calls, normal post contacts NPA release JA so that a unit answering fee is not recorded when the operator replies. Metering, if required, is effected by transmission of alternating current pulses from the manual position to relay MM, which also controls the positive battery connection to the P wire. If the calling party clears, relays MH, B and BA remain operated so that the metering circuit is still effective. Calls through tandem exchanges. These calls are extended over an outgoing toll junction to a tandem switching centre provided with two motion tandem selectors giving access, according to the second routing digit, to a separate group of toll lines on each level, each toll line including a toll relay set. Normal post springs NPA suppress the unit fee charge at the code selector. Toll relay sets, Fig. 6. Line relay AA pulls up when the set is taken into use, followed by AB which busies the P wire and starts a common meter-pulse generator set over lead ST. Relay H extends the speech conductors to the toll line TL and disables relay JF for the duration of the call, extinguishing an indicator lamp LJF. Relay AA then repeats the impulses to the toll line, and a dialling relay AC which pulls up for the duration of each impulse train short-circuits the windings of a repeating coil TRA. When the called party answers, battery reversal on the speech conductors pulls up relay AD, followed by DD which reverses the potential of the incoming line. Metering assessment at tandem exchanges, Three different classes of metering are provided. according to straps on the terminal group T1-T5. Periodic earth pulses are applied to lead MP by the meter-pulse generating set and control relay PC, the time interval between pulses being adjustable according to the time of day. Periodic metering throughout duration of call. When the called subscriber answers, relay DD connects PC to lead MP. Relay PC then responds to the earth pulses and applies alternating current (preferably 50 c.p.s.) over transformer TRB to both speech conductors. Called subscriber answering fee. Terminals T1 and T2 are strapped. Relay MD remains operated momentarily with DD when the called subscriber answers so that PC pulls up and a single metering signal is transmitted. Metering starting when the toll relay set is taken into use. This class of metering enables a charge to be made for prolonged or repeated calls to absent subscribers. Terminals T4 and T5 are linked and complete an operating circuit for relay PC from lead MP immediately the line relay pulls up AB. At the code selector, the reverted A.C. signals are not effective until relay K falls back at the completion of digit transmission. Preventing suppression of metering by dialling during conversation. Relay DD remains held by relay AC over its second winding ,so that relay PC remains connected to lead MP, the A.C. signals being effective over only one speech conductor. Alternating current metering signals. Forward transmission is prevented by the repeating coil TRA. The balanced transmission of the signals to prevent audibility during conversation is provided by symmetrical loading of the speech conductors at both ends and by applying the A.C. signals in parallel over both conductors. At the first code selector metering is effected by relay MM which is energized from a centre tapping on relay D over a bridge rectifier MRB. A repeating coil may be included at the code selector to block the A.C. signals from the calling line circuit. Release. If the calling party clears, relay L falls back and releases the operated relays in the code selector. Relays HA and HB together complete a rectangular release homing circuit for the selector. At the toll relay set relay AA falls back and restores the set to normal. If the called party clears first, relays AD and DD fall back to isolate the metering circuit. The code selector clears only when the calling subscriber hangs up.