GB657028A - Improvements in or relating to automatic gain control arrangements for carrier current communication systems - Google Patents

Abstract

657,028. Telephone transmission systems. STANDARD TELEPHONES & CABLES, Ltd. July 16, 1948, No. 19125. [Class 40 (iv)] In a carrier current communication system in which the repeater gain is controlled by a pilot current transmitted over the communication channel, comprising one or more groups of repeater stations, each comprising a master station and a number of slaves, the master station provides continuous gain control over a predetermined range beyond which the level is maintained by a step on a stepped gain control at all the stations in the group in response to a signal initiated in the master station. The continuous automatic control at the master station is effected by the action of the rectified output from a 2000 cycle oscillator, which is under the control of the pilot signal, on a thermistor 20, which is also heated from a steady D.C. source 22. When the gain adjustment at the master station reaches a predetermined limit in either direction, further change of pilot level operates relay B to prepare a local gain controlling selector switch S1 or S2 for stepping via D and F or E and G and completes a circuit to the first slave, which circuit includes a balanced Wheatstone bridge comprising resistors 42-47, one of the relays N or P and the line 18 and a relay J across the null points. The operation of N or P prepares a gain controlling selector T1 or T2 at the first slave for stepping and completes a circuit to the next slave in the manner above and so on to the last slave, in which one of the relays corresponding to N and P causes the appropriate selector to step directly. The stepping of the selector unbalances the Wheatstone bridge in which its own N or P relay is a component and so operates the relay at the preceding station corresponding to the J relay above, which steps the selector at that station and similarly the other selectors at the other stations back to the master station. Should further gain adjustment still be necessary, the whole process is repeated, but if the level should be such as to cause excessive gain change, i.e. beyond a predetermined limit, then the relay C operates and the gain of the system is held at the level determined by the D.C. source at terminals 51, 52. An alarm is given to the master station in the event of a failure of the control line 18 or if a Wheatstone bridge is permanently unbalanced due to a selector failing to step, by a relay K at the master station or by similarly positioned relays at the slave stations. The slave stations are described as preceding the master station in the direction of transmission and, although a single selector switch is shown, a modification, Fig. 5 (not shown), incorporates separate selectors for raising or lowering the gain. A system in which the gain is adjusted at the slave stations by individual signals sent over separate wires in a predetermined sequence is also described.