Most camcorders have an automatic setting that theoretically delivers perfect audio levels with no fuss and no muss. Often, it works fine. But this feature, called "automatic gain control" has a dark side. That's because automatic gain control will always try to ensure "pointing straight up" audio levels. That's fine while the subject is speaking. But what if he takes a long pause? In that quiet moment, the automatic gain control will hunt for whatever sound it can find--say the humming air conditioner--and bring up that sound to high levels. This is bad because you don't want to increase the volume of the air conditioner during the pause. You want it quiet during the pause.

As a result, professionals don't use the automatic gain control.