GB478137A - Improvements in and relating to apparatus for automatic compression and expansion of volume range in electro-acoustic systems - Google Patents

Abstract

478,137. Automatic volume control. PAWLEY, E. L. E. July 10, 1936, No. 19234. [Class 40 (iv)] A system in which the volume range is compressed during transmission makes use of impedances which vary in non-linear fashion in accordance with the direct current flowing through them, an impedance of this kind being in shunt across the transmission path in the compressor and in series in the path in the expander, and the control currents being derived from the signal currents through a rectifier which is such that uniform compression and expansion is obtained. In the compressor, Fig. 1, one of the secondary windings of the output transformer 13 feeds through an amplifier 16 giving uniform amplification a bridge rectifier 17-20 which supplies direct current to a reservoir condenser 21. The latter is connected through an adjustable resistance 22 to a pair of shunt rectifiers 7, 8 and to a tapping on a balancing resistance 5 interposed between the secondary windings of the input transformer 1. If the bridge rectifier is such that the direct current is proportional to the square of the voltage applied thereto it is found that the output voltage of the compressor is proportional to the input voltage in decibels, the compression factor being one half, provided the resistance pads 6, 9, 10. 11 and the transformer ratios are suitably chosen. In general the compression factor is dependent on the gain of the amplifier 16 and on the non-linear law of the rectifier. In the expander, Fig. 2, the control is obtained from the input to the transformer 23 and the rectifiers 36, 37 controlled by the reservoir condenser 32 are in series in the transmission path and are in balanced circuits joined by tappings on balancing resistances 26, 30 associated with the input and output transformers. Additional resistance pads may be employed if desired. Since the effect of the compressor is to raise the relative level of the weaker signals, background noise tends to increase. The modification of Fig. 3 avoids this effect by the addition of a pair of rectifiers 38, 39 in series in the transmission path to attenuate the signals below a certain level. If one side of the path is earthed, the variable impedance may be constituted by a single shunt or series rectifier.