GB545401A - Amplitude range control in a signal transmission system - Google Patents

Abstract

545,401. Automatic volume control. STANDARD TELEPHONES & CABLES, Ltd. April 25, 1941, No. 5353. Convention date, May 2, 1940. [Class 40 (v)] To improve the operation of a volume range expander or compressor so as to minimize the effects of noise or fading, the time constant of the volume control circuit of an amplifier is automatically varied in accordance with the rate of variation of the input of the amplifier. In Fig. 1, a telephone circuit T.C. feeds through an input transformer T1 a variable amplifier VA having an output T2. A delay circuit D2 prevents the application of a signal to the amplifier VA until after the forward-acting control circuit has adjusted the gain. The common part of the grid-cathode circuits of the variable amplifier VA include a condenser C3 the voltage across which determines the gain of the amplifier. Across C3 are connected in series a gas discharge tube CT, circuit C4, R6 and a thermionic impedance V3. Gain control potentials are fed to C3 by a circuit comprising transformer T5, full wave rectifier F3 and discharge tube CT. The time constant of the control circuit is adjusted by varying the impedance of V3 by means of voltages from a comparison circuit comprising transformers T3, T4 fed from either side of a delay network D1 and feeding full-wave rectifiers F1, F2 so that the difference in voltage across condensers C1, C2 is proportional to the rate of variation of a signal in TC. This differential voltage is fed through a reversing switch S to the grid of V3 so that the impedance-of V3 and hence the time constant of the gain control circuit is varied according to the rate of variation on the signal input to the amplifier VA. When switch S is set to the left-hand position, the arrangement is such that a sharp signal such as a key click operates to increase the time constant of the control circuit and the gain is not increased, whereas a slowly varying signal reduces the time constant and allows volume expansion. If switch S is set to the right conditions are reversed and are suitable for volume compression. A modification is described (Fig. 2, not shown) for a single side-band radio receiver in which the variable amplifier is in the single side band channel and the control circuit is operated by the carrier. In this case the time constant of the control circuit is a function of the rate of change of the carrier envelope, and a marginal relay is provided to prevent any change in gain after the carrier has faded below a predetermined value.