Automatic Volume Control

0I The proposed AVC gives weaker com- ponents of the input signal extra amplifi- cation while ensuring that this dynamic compression is not disconcerting. It therefore eliminates those annoying dif- ferences in loudness between speech and music on radio and television. The principle of the circuit is fairly simple. Field-effect transistor T1 is used as a variable resistance. The value of this resistance, rDS(on), can vary from infinity to about 150R. It is in parallel with R3 and, in conjunction with R4, determines the gain of Ai. Without the effect of the FET, the gain of Al is about 20 dB. Opamp A2 is connected as a straightfor- ward amplifier, whose gain may be varied by Pl. The negative part of the output signal of A2 is connected to the gate of Ti via a rectifier formed by Di, Cl, R7, and R8. Resistor R8 ensures that the switching of T1 happens gradually. This means that it takes a short time before T1 operates; in other words, momentary differences in input level do not affe...