Patent GB-543841-A (GB)
Title: Improvements relating to electrical communication systems
IPC: H03F1/54
Filed: 1940-07-12
Abstract: 543,841. Repeating systems. STANDARD TELEPHONES & CABLES, Ltd. July 12, 1940, No. 11628. Convention date, Sept. 28, 1939. [Class 40 (iv)] In a transmission system having regular and spare sections associated with a common circuit at each end, these sections are conjugate at either or both ends.. In some arrangements both circuits may be operative together. Conjugacy may be obtained in known manner by means of hybrid coils, e.g. as shown at the right hand end of Fig. 8, in which the points B, D are conjugate, At the other end it is arranged that points F and A are conjugate with the switch at C open. In another arrangement, Fig. 9 (not shown), a similar effect is produced with the aid of impedance bridges. In a further modification, Fig. 10, the points B, D are made conjugate by the provision of an impedance Z2 equal to the impedance ZA looking towards amplifier 8, the transformer having unity ratio. Switching may be controlled by a pilot current so that the spare section is cut in automatically. Preferably the switching is effected by a relay associated with an impedance network as shown in Fig. 4 since at high frequencies direct switching is unsatisfactory because an open relay contact has a comparatively low impedance. The Specification as open to inspection under Sect. 91, comprises also an arrangement in which the two transmission sections are permanently coupled to the common circuits, at both ends and signals are kept 120 degrees apart in phase so that the resultant current in the common circuit has the same value as the component in either of the single circuits. In another arrangement, Fig. 8 (Cancelled) (not shown), the single circuits are connected alternately to the common circuit by a differential relay controlled by pilot current. In another arrangement, Fig. 14 (Cancelled), the spare circuit is associated at the sending end through a high loss network 10 and a compensating amplifier A7. A hybrid coil is used at the receiving end and is so arranged that with the switch at D open, points B and G are' conjugate. At the sending end, the normal circuit is replaced by a network Z4 when the spare circuit is used. In other arrangements, disablement of valves is used instead of switching. In one of these, Fig. 15 (Cancelled) (not shown), the common circuits are associated at each end by hybrid coils and amplifiers in the normal and spare paths are controlled so that either can be disabled under pilot current control by disconnecting the anode battery or applying a paralysing grid bias. Both may transmit together, the messages being 120 degrees apart in phase, or the spare may be normally disabled. In another arrangement, Fig. 16 (Cancelled), the receiving-end hybrid coil is replaced by the arrangement shown in which one valve is normally disabled by the disconnection of its anode supply at S3. The. latter is preferably an arrangement including a gaseous discharge tube so that the spare circuit remains connected up indefinitely after switching has occurred although the reception of pilot current is resumed. In a similar arrangement, Fig. 17 (Cancelled) (not shown), the valves are disabled by a paralysing bias and in another, Fig. 18 (Cancelled) (not shown), the hybrid is omitted at the transmitting end. Other arrangements have one spare circuit common to several regular circuits. As shown in Fig. 19 (Cancelled) the upper valves in each of the pairs feed or are fed by the normal transmission circuits N1...N3 respectively and the lower valves are all coupled as shown to a common spare circuit S. The condition of ,the valves is controlled by switching apparatus SW1, SW2, the latter being responsive to pilot currents and the former to a signal from the latter. In another arrangement, Fig. 24 (Cancelled), if the pilot on No. 3 channel fails, the relays PC3 on the receiving side of the fault fall back and that nearest to the fault completes a circuit over SS3, SL3 at the station on the transmitting side of the fault, PS3, PL3 at its own station and back to battery on bus A at the first-mentioned station. SS3 and PS3 perform the necessary switching to the spare circuit and SL3, PL3 lock them in a circuit in series with CL which prevents the association of the spare circuit with another channel. Operation of reset key RES when the fault has been cleared restores normal conditions. Another circuit involves a switching arrangement, Fig. 22 (Cancelled), using " thermistors." While normal pilot current is being received, R is energized and a silver sulphide " thermistor " T1 connected in the normal transmission path and also in a bridge network including an A.C. source e is heated to its low resistance condition. When R3 falls back on the failure of the pilot, the element T1 assumes its high-resistance condition, blocking the normal path and increasing the current through the heater H2 of a " thermistor " T2 in the spare path. This subject-matter does not appear in the Specification as accepted.