GB522799A - Electric signal transmission systems - Google Patents

Abstract

522,799. Automatic volume control. STANDARD TELEPHONES & CABLES, Ltd. Dec. 16, 1938. No. 36709. Convention date, Feb. 18. [Class 40 (iv)] The time constant of an automatic volume control circuit decreases with increase of signal level and vice-versa in response to rapid changes in level. As described the resistance upon which the time constant depends comprises a bridge of non-linear resistances controlled by rectified signal current..The invention may be applied to the audio - frequency amplifier preceding a wireless transmitter to prevent over-modulation and the control effected may be signalled to the receiver by a pilot-frequency to effect an opposite control on the highfrequency stages. As shown, the input transformer'IT is shunted by a triode E the grid bias of which is controlled by rectification at R1 of output voltage, bias being applied over a rectifier bridge R2 ... R5 and a resistance V which may be adjustable. When the system is quiescent, the bias of E is negative and its signal shunting effect is negligible, but when the output exceeds a predetermined value determined by the bias on rectifier R1, the bias of E is reduced so that it exerts a shunting effect to maintain the output substantially at the predetermined value. Output signal energy is also applied over step-up transformer and rectifiers R6, R7 to control the resistance of bridge R2 ... R5 so that in the quiescent state the resistance is high and time constant large. e.g., 25 seconds, but when a predetermined maximum input is reached, the time constant is small, e.g., 30 milli-seconds. The rectifiers R6, R7 have a differential effect on the voltage over resistance V1 for biassing the bridge and serve to reduce the destructive effect of high-potential surges on the rectifiers and minimize variation' of the time-constant during normal conversation. The single shunt valve E may be replaced by a series of valves arranged as shunt impedances associated with series resistances connected in the line, the grid being connected in parallel. The transmission level of a pilot-frequericy generator B is also automatically controlled, the pilot being excluded from incoming speech by a filter T and from the control circuit by a filter Q.