Compressors

Perhaps the most commonly encountered dynamics
processor, a compressor reduces (or "compresses") the
dynamic range of an audio signal. A compressor functions by
reducing the level of all signals above a user-defined point (the
threshold), by a specified amount. A ratio
defines the amount of reduction that occurs above the
threshold. A ratio of 2:1, for example, will allow an audio
signal to exceed the threshold by only half as much as what it
would have without compression. Assuming a threshold setting
of 0 dB, a +10 dB signal is output at +5 dB. Similarly, a
4:1 setting will reduce the output by one-quarter of the original
signal level. This reduction limits variation between the lowest
and highest signal levels, resulting in a smaller dynamic range.
A common myth concerning compressors is that they make
quiet signals louder. While this may be the perceived effect,
reducing the dynamic range of a signal allows the user to
boost the overall level of the signal, yet keeps loud signals from
getting "too loud" and causing distortion further down the
audio chain - or simply annoying listeners. The compressor
itself does not boost lower signal levels, but simply allows them
to be perceived closer in level to louder signals.
Other compressor settings include attack, release, and
decays. A compressor’s attack time relates to how quickly the compression takes effect once the signal exceeds thethreshold. Shorter attack times offer greater transient control.
Longer attack times generally sound more natural, and are
often employed in musical applications. Too long an attack
time can cause the compressor to miss signals that otherwise
should be compressed. Release refers to the time it takes for
the compressor to return the signal level to its original value
after the level drops below the threshold. Too short a release
time can result in "pumping" and "breathing" with signals that
have rapid level changes. Too long a release time can render
quieter passages inaudible since gain reduction is still being
applied to the audio signal.
A compressor’s knee is the point where the signal
crosses the threshold. Standard compression schemes
reduce the signal by the same amount once the signal has
passed the threshold. This is known as hard knee
compression. Some compressors allow the user to select
soft knee compression instead, where the onset of
compression near the threshold occurs more gradually than
the more aggressive hard knee compression. The compression ratio near the threshold is actually
less than specified. Audibly, soft knee compression creates
a more gradual transition from uncompressed to
compressed signals, making the compression less noticeable.