GB1149891A - Improvements in or relating to the electrical control of audio frequency gain - Google Patents

Abstract

1,149,891. Volume compressors. ELECTRIC & MUSICAL INDUSTRIES Ltd. 5 May, 1966 [6 May, 1965], No. 19087/65. Heading H4R. In a volume compressor the control voltage is derived from the compressor input by a circuit, Fig. 2, which comprises a pair of emitter followers 18 and 19 fed with antiphase signals from terminals 14 and 15 via D.C. restoring circuits including diodes 20 and 21 so that a full wave rectified, negative going signal, appears across the emitter resistance 26, and is fed to the complementary emitter follower 27. The signal appearing across emitter resistance 30 is applied to a pulse elongator 8 comprising a capacitor 32 which charges negatively quickly and decays slowly. The pulse from the elongator is applied via a differentiating circuit 10 to one input of a maximum selector 13 comprising transistors 37, 44 and 52. A second input to the maximum selector is fed from the elongator circuit 8 via an integrator and differentiator 11, and a third input to the maximum selector is fed from a second elongator circuit 9 and an integrator 12. Fig. 3b shows the composition of the output signal when a rectangular pulse is applied to the elongator circuit inputs, 57 being the waveform passed to transistor 37 by elongator 8 and differentiator 10, 58 is the waveform from circuit 11, 59 is the waveform from circuit 12, while the output from the maximum selector is as shown in the full line. The arrangement prevents repetitive variations in an output signal envelope, after an initial transient, causing higher harmonics to be generated in the compressor output due to fluctuations in the control signal.