Patent GB-531424-A (GB)
Title: Improvements in volume control circuits in signal transmission systems
IPC: H04B3/06
Filed: 1939-07-21
Abstract: 531,424. Automatic volume control. STANDARD TELEPHONES & CABLES, Ltd. July 21, 1939, No. 21253. Convention date, Aug. 10, 1938. [Class 40 (v)] Apparatus for automatically controlling the volume of voice frequency signals to maintain the volume substantially constant is characterized in that a volume control device inserted in the transmission circuit has connected to its output two detector circuits each including a marginal device arranged to become effective at two different energy levels in said output circuit and arranged to thereby control said volume control device in such manner as to hold the ratio between the durations of the two energy levels in the output circuit substantially constant. A signal transmission line having input conductors 2, 3, and output conductors 4, 5 has a repeater in the form of two pentodes 7, 8. Two detector circuits 22, 23 are connected to the output conductors 4, 5 by means of a hybrid coil 24. Detector circuit 22 comprises a triode 25 the output of which operates a polarized relay 28 having a hangover provided by a condenser 29 and resistance 30. Armature 36 normally engages contact 38 and on operation moves to engage contact 37. Detector circuit 23 comprises an amplifier 40 and a detector 39, the output of which operates a polarized relay 44 having a hangover provided by condenser 45 and resistance 46. Armature 53 normally engages contact 54 and when operated engages contact 55. With a weak signal on the line relay 44 will be operated but not relay 28 as circuit 23 is made to respond to a lower energy level than circuit 22. Operation of relay 44 will charge a balancing condenser 56 with positive potential from battery 58 through a high resistance 60. When a predetermined potential is built up, the positive grid bias of amplifier 67 increases the plate current sufficiently to cause armature 64 of relay 61 to engage contact 65 and discharge a control condenser 18. This reduces the negative grid bias on valves 7, 8 to increase the gain on the transmission line. When, however, the energy level at the output 4, 5 increases sufficiently to operate circuit 22, relay 28 is operated to put a negative potential on balancing condenser 56 through a low resistance 59, and this reduces the anode current of valve 67 to move armature 64 of relay 61 to engage contact 66. This causes condenser 18 to be charged from battery 79 and so increases the negative bias on valves 7, 8 to reduce the gain. A relay 71 which is operated whenever relay 44 is operated, places a ground on armature 64 of relay 61. This ensures that the gain control is only operative when there are speech signals on the line. The speed of the gain changes is dependent on the dimensions of the condenser 18 and resistances 78, 80. In a modification, relays are put in series with relays 28, 61 to increase the speed of gain changes and to prevent over compensation. In another modification, a forwardly active syllabic frequency circuit connected to the input conductors is provided for raising the gain by controlling relay 44 when the voltage of the output signals is too low to operate circuit 23. This occurs when a weak talker follows a very strong talker.