Patent GB-558758-A (GB)
Title: Improvements in or relating to gain control circuits for carrier systems
IPC: H04B3/20
Filed: 1942-07-17
Abstract: 558,758. Repeater systems. STANDARD TELEPHONES & CABLES, Ltd., THORNTON A. M., and BALY, W. F. July 17, 1942, No. 9961. [Class 40 (iv)] [Also in Group XL] In a multi-channel carrier system in which pilot currents are employed for automatic gain control, abnormal changes in pilot level at a repeater cause the effective interruption of the transmission circuit. Conversely, a spare line which is normally blocked can be brought into service by the transmission of a normal pilot current over it. In the system shown, currents from line B are directed by a filter DFB to an amplifier RAB, the gain of which is under the control of two pilot currents, selected respectively by circuits SPB, GPB, for controlling the slope and the general level of the amplifier characteristic. Abnormal changes of pilot level are recognised by a circuit ARB which co-operates with a delay circuit ADC so that if the abnormal condition persists, transmission is cut-off by circuit TCB. Since the relays recognising the abnormal pilot level have small operating margins, it is arranged that they are periodically released by a pulsing circuit PC throughout an alarm condition. Similar operations are controlled by the pilots in the other direction. Circuit details.-Rectified pilot currents are taken in series through the a windings of relays HS, LS, the opposing b windings of which are fed by a local source. The b winding of HS however is normally short-circuited and the arrangement is such that both relays are normally inoperative. An abnormal rise in pilot level energizes HS and an abnormal fall allows LS to be operated by its b winding. These relays energize corresponding relays HHA, LLA (common to both pilot currents) which operate a relay AL. This completes a circuit for charging a condenser C2 in circuit ADC. and after, say, 5 seconds, the tube V1 therein breaks down, energizing relay D which energizes relay M. The latter energizes PAS to give an alarm and relay T in either TCA or TCB according to the direction from which the abnormal pilot was received. The relay T inserts a balanced bridge, Fig. 2,. in the transmission circuit. If the resistance R1 is provided as shown, the relay T is not operated when the alarm condition results from too strong a pilot. It is held, however, over contacts of HHA when an abnormally strong pilot follows immediately on the absence of pilot as when a fault is cleared or when a spare circuit is first switched on. When the automatic gain adjustment has become effective, HHA will fall and the transmission circuit will be made effective. Relays HHA and LLA also complete a charging circuit for C2 in the pulsing circuit PC and after, say, two seconds, the auxiliary gap in the tube therein ionizes and the main discharge thereupon energizes D which discharges C2 and energizes E. The last relay breaks the discharge circuit and energizes FA, locking HHA and LLA and removing the shunt from the b winding of HS. This so reduces the b current in LS and increases that in HS that whichever relay is operated falls back. After an interval, FA falls and if the abnormality is still present, HS or LS pulls up again and the cycle is repeated.