GB1096196A - Means for controlling the amplitude of electrical signals - Google Patents

Abstract

1,096,196. Compandors. BRITISH BROADCASTING CORPORATION. Nov. 12, 1965 [Nov. 13, 1964], No. 46362/64. Heading H4R. A compressor, expander, or limiter, comprises an attenuator including a photo-resistive element and a semi-conductor light source which is controlled in accordance with signal level in order to provide a ratio of output dB change to input dB change of ¢, 2, or 0. Fig. 1 shows an output controlled compressor, or limiter, depending on the circuit constants, the choice of which is mathematically discussed, using a gallium phosphide diode as a light source to control the resistance of a cadmium selenide resistor r. The level at which compression or limiting takes place is adjusted by varying a bias on the diode light source by adjustment of resistor 6. An input controlled limiter is described with respect to Fig. 10 (not shown), while an input controlled expander is provided, Fig. 4 (not shown), by including the photoconductive varistor (P.C.V.) in series with the signal path. Also described is a circuit, Fig. 7, providing expansion at low levels, for noise reduction during periods of no signal, compression at intermediate levels, and limiting at high levels. The low level expander comprises a P.C.V. V2 which provides a " step " change in attenuation at a predetermined low level. Compression is provided by P.C.V. V which introduces compression above signal levels set by attenuator 10, an attenuator 12 being ganged with 10 to provide compensation for gain changes introduced by the adjustment of 10 when the compression level is changed. Limiting is provided by the P.C.V. V1 at a level set by the attenuator 13.