Patent GB-339288-A (GB)
Title: Two-way terminal circuit for telephone cables
IPC: H04B3/20
Filed: 1929-09-03
Abstract: 339,288. Western Electric Co., Ltd., (Bell Telephone Laboratories, Inc.). Sept. 3,1929. Two-way repeating systems.-In a two-way repeater associated with a channel of great storage capacity, such as a submarine cable, and in which the transmitting and receiving circuits are never connected to the channel at the same instant, the connection of the transmitting circuit to the channel is effected by a relay operated by voice currents in-coming to the circuit, and the relay also initiates a sequence of disabling operations on the receiving circuit commencing at the end remote from the cable. The sequence of operations for rendering the receiving circuit operative commence at the cable end thereof. In the system described, voice currents incoming over a land line LL pass over line 22 through volume control circuit 23, delay network 19, amplifier 24, attenuation equalizer 25 to power amplifier 26 comprising two banks of valves in push-pull relationship, this (transmitting) circuit being normally disconnected at 10 from cable CL. Current incoming from the cable passes to a normally connected receiving circuit comprising filter and equalizer 32, screened-grid amplifier 33, amplifier 34, attenuator 35, equalizer 36, low pass filter 37 and further amplifiers 38, 39. Voice currents from LL rectified and amplified at 27 operate relay 51 which (1) operates relay 52 to connect the transmitting circuit to the output transformer 29 and (2) opens the circuits of relays 55, 54, 53 which release in turn so that the receiving circuit is short-circuited at 70, 60, 50 and is disconnected from the cable at 20, the connection of the transmitting circuit to the cable being completed at 71. When speech over land line LL eases, relay 51 falls back, relay 52 releases, and after a short time relays 53, 54 55 pull up in turn. Relay 53 is arranged to operate contacts 20 before opening contacts 50 so that for a brief time contacts 20, 50, 60, 70 are all closed and any energy in the cable is dissipated. The operating and release periods of the various relays are determined by the resistance and capacities shown in their circuits. The equalizer 25 may be of the type described in Specification 202,284. The cable is of the single core type with a long receiving sea-earth conductor 73 and a relatively short transmitting sea-earth conductor 73<1> and contact 71 prevents potentials in the transmitting circuit from affecting the receiving circuit. In a modification, Fig. 3, using a twin-core cable, contact 71 is omitted, but. it may be included to prevent loss of received energy in transformer 29. The disabling operations on the receiving circuit may alternatively be effected by the grid paralyzing method. Specification 338,973 also is referred to. To protect the cable from excessive voltages in the transmitting circuit, a grounded shield 61 is provided between the windings of transformer 29, the secondary circuit of which includes a fuse 31 and shunts 30 comprising gas discharge tubes arranged to break down at voltages somewhat below the safe voltage limit of the cable. The fuse 31 will carry maximum signaling current but is blown by discharge current from tubes 30. Specifications 339,269 and 339,270 also are referred to.