Editing Automatic gain control (section)

=== Audio/video ===

An [[audio tape]] generates a certain amount of [[noise]]. If the level of the [[Signal (electronics)|signal]] on the tape is low, the noise is more prominent, i.e., the [[signal-to-noise ratio]] is lower than it could be. To produce the least noisy recording, the recording level should be set as high as possible without being so high as to [[clipping (audio)|clip]] or [[distortion|distort]] the signal. In professional [[high-fidelity]] recording the level is set manually using a [[peak-reading]] meter.  When high fidelity is not a requirement, a suitable recording level can be set by an AGC circuit which reduces the gain as the average signal level increases. This allows a usable recording to be made even for speech some distance from the [[microphone]] of an audio recorder. Similar considerations apply with [[VCRs]].

A potential disadvantage of AGC is that when recording something like music with quiet and loud passages such as classical music, the AGC will tend to make the quiet passages louder and the loud passages quieter, compressing the [[dynamic range]]; the result can be a reduced musical quality if the signal is not re-expanded when playing, as in a [[companding]] system.

Some [[reel-to-reel]] [[tape recorders]] and [[cassette deck]]s have AGC circuits. Those used for high-fidelity generally don't.

Most VCR circuits use the amplitude of the [[vertical blanking interval|vertical blanking pulse]] to operate the AGC. Video copy control schemes such as [[Macrovision]] exploit this, inserting spikes in the pulse which will be ignored by most [[television]] sets, but cause a VCR's AGC to overcorrect and corrupt the recording.