Although manually maintaining audio levels is generally the best
approach, there are some automatic devices that can help, and
even do some things that you can't do manually. We'll start with
a simple audio control circuit, one that is built into most
consumer audio equipment. If the average audio level is low, an
AGC (automatic gain control) circuit will raise it; if the
average level is too high, the circuit will bring it down. Even
though AGC circuits can free you from having to worry about
manually controlling audio levels, they can't intelligently
respond to different audio needs. When no other sound is
present, as, for example, during a pause in dialogue, an AGC
circuit will attempt to bring an audio level up to a standard
setting. This can momentarily make unwanted background sounds
louder. If subsequent audio processing circuits (in editing
equipment, for example) have AGC circuits, the problem can get
progressively worse as each piece of equipment further increases
background noise. AGC circuits can