GB681023A - Improvements in or relating to automatic level control devices for electric carrier current communication systems - Google Patents

Abstract

681,023. Automatic volume control. STANDARD TELEPHONES & CABLES, Ltd. Feb. 27, 1950, No. 4969/50. Class 40 (iv). [Also in Group XL (c)] In a pilot-controlled gain regulation system for a carrier communication circuit, of the type in which the rectified pilot controls the output amplitude of an oscillator used to effect the gain regulation, there is provided rectified reaction for the oscillator to limit the oscillation amplitude and a bias circuit for the pilot rectifier so that it produces a rectified voltage only when the pilot amplitude exceeds a given value. In the circuit shown, the oscillator output is applied to the heater of a thermistor 9 controlling an attenuator or an equalizer in the communication circuit. The pilot is applied to terminals 1, 2, amplified by a high impedance valve 3 and passed to a rectifier circuit 4, 5 which is biased by means of a current derived from a high resistance 28. For low pilot levels the rectifier 5 is effectively short-circuited by rectifier 4, but for higher levels the current drawn by valve 3 exceeds the current flowing through resistor 28 and rectifier 4 is blocked. The rectified output is derived from a low resistance 6 and is applied to the grid of an oscillator valve 7 in series with the frequency determining circuit 15, 16 to control the oscillation output. In order to ensure that when the pilot level is low the oscillation amplitude does not rise sufficiently to burn out the thermistor, part of the output is applied to a rectifier circuit 20, 21. This circuit is biased by a potentiometer 24 so that as the pilot decreases below a certain level and the oscillator output correspondingly increases, an increasing negative voltage appears across resistor 22. At the same time the voltage across resistor 6 is decreasing and when the former voltage exceeds the latter, rectifier 23 conducts and applies a bias to the grid to limit the oscillation amplitude. The anode load of the amplifier valve 3 may be replaced by a tuned circuit or a transmission line resonant at the fundamental and a harmonic of the pilot frequency.