An automatic gain control is not a compressor and not a limiter. With a relatively large time constant the input level is adjusted to the nominal level that is set during programming the unit. Incoming low signals will be (slowly) increased, incoming high levels will be (slowly) decreased to match the nominal level.
An automatic gain control unit is very valuable in voice alarm systems to keep the announcement level within the appropriate range.
|input and output level||control circuit|
|background noise level and system gain|