What is use of Automatic gain control circuit?

Answer 1

Automatic Volume Control (AVC) automatically adjusts the volume of a TV/ radio according to the surrounding noise or environment with the intelligibility of speech or audio signal from the audio device.

Watching television or listening to the radio may be interfered by various sources noise.

Aircraft noise is extremely loud and researchers succeeded to reduce a few notches on noise meter. Railway noise also causes disturbances to local residents. In addition noise may come from traffics or viewers of television may start talking suddenly. Viewers of television may sit in a noise insulated room. Noise of that room may increase due to sudden opening of door. Automatic volume control will help user in understanding programs of television in all mentioned noisy situations. When user-defined volume high level is smaller than volume level required avoiding noise, user may feel further obstruction. While one or more viewers of television/ listeners of radio are talking or making noise volume will raise. Listeners will be warned about their noise through raise of volume automatically.

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Response #2:
(Answer #1:)

The above treatment is a very nice write-up on the subject of Automatic Volume
Control. The question, however, inquires regarding Automatic Gain Control (AGC).

AGC is implemented in a feedback loop inside any device where amplification takes
place, usually a radio receiver, video, or audio amplifier

, where portions of the circuit
work best only within some narrow range of signal level. When sub-circuits like that
are present, the signal level input to them is kept constant by preceding circuits
that sense the signal level, and adjust their gain appropriately so that their output
level ... and the input level to the following stages ... is held steady.

Often, the extent to which these control circuits are reducing their gain is brought
out to a single measuring point, and the voltage at that point is the source of all
the information available to indicate the strength of the signal arriving at the antenna.