GB1190763A - Improvements in or relating to Automatic Level Control Arrangements - Google Patents

Abstract

1,190,763. Companders. ELECTRIC & MUSICAL INDUSTRIES Ltd. 21 July, 1967 [4 Aug., 1966], No. 34900/66. Heading H4R. A circuit for smoothing the control voltage in an automatic level control circuit comprises two circuits having different discharge time constants, the circuit of longer time constant having a lower output amplitude, and a circuit for deriving the control voltage from the circuit producing the minimum output voltage. Fig. 1 shows a circuit, fed with the control signal at terminal 1, to charge up capacitors 3 and 7 via the respective diodes 2 and 6. The potential on capacitor 3, which has a relatively long discharge time constant, is applied via an attenuating potentiometer comprising resistors 4 and 5 to transistor 9, while the potential on capacitor 7, which has a shorter discharge time constant, is applied to transistor 10. Transistors 9 and 10 provide a circuit for selecting the minimum voltage which is fed out at terminal 13. Transistor 14 is arranged to discharge capacitor 3 to the same level as capacitor 7 whenever transistor 10 conducts, i.e. the output signal is derived from the short time constant circuit. Fig. 4 shows waveforms existing at the output of the long time constant circuit C, Fig. 4 (b) and the output of the short time constant circuit A when the waveform, Fig. 4 (a), is applied to the input of the circuit. Fig. 4 shows the output of the minimum selector circuit, D indicating the output with transistor 14 omitted, and D<SP>1</SP> with transistor 14 present. The arrangement provides smoothing, by the long time constant circuit, of small amplitude beats while giving fast decay after major level changes.