The Technical Stuff
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19
Compressor / Limiter Basics
The function of a compressor is to automatically reduce the level of peaks in an audio signal so that
the overall dynamic range—that is, the difference between the loudest sections and the softest
ones—is reduced, or compressed, thus making it easier to hear every nuance of the music.
Compression is sometimes referred to as
peak reduction
or
gain reduction
, because a compressor (or
“limiter,” when acting more severely) “rides gain” on a signal much like a recording engineer does by
hand when he manually raises and lowers the faders of a mixing console. Its circuitry automatically
adjusts level in response to changes in the input signal: in other words, it keeps the volume up during
softer sections and brings it down when the signal gets louder. The amount of gain reduction is
typically given in dB and is defined as the amount by which the signal level is reduced by the
compressor.
Compression or limiting enables even the quietest sections to be made significantly louder while the
overall peak level of the material is increased only minimally. The dynamic range of human hearing
(that is, the difference between the very softest passages we can discern and the very loudest ones we
can tolerate) is considered to be approximately 120 dB. Early recording media such as analog tape and
vinyl offered much less dynamic range, so compression was a virtual necessity, raising the overall
level of the material (making it “hotter”) without peak levels causing distortion. While many of today's
digital recording media approach or even exceed 120 dB of available dynamic range, quiet passages
of recorded music can still be lost in the ambient noise floor of the listening area, which, in an average
home, is 35 to 45 dB.
Despite the increased dynamic range, compression is especially important when recording digitally, for
two reasons: One, it helps ensure that the signal is encoded at the highest possible level, where more
bits are being used so that better signal definition is achieved. Secondly, it helps prevent a
particularly harsh type of distortion known as
clipping
—something that, ironically, only occurs in
digital recording, due to the inherent limitations of digital technology.
During recording, compression is customarily used to minimize the volume fluctuations that occur
when a singer or instrumentalist performs with too great a dynamic range for the accompanying
music. It can also help to tame acoustic imbalances within an instrument itself—for example, when
certain notes of a bass guitar resonate more loudly than others, or when a trumpet plays louder in
some registers than in others. Properly applied compression will make a performance sound more
consistent throughout. It can tighten up mixes by melding dense backing tracks into a cohesive whole,
can make vocals more intelligible, and can add punch and snap to percussion instruments like kick
drum and snare drum, making them more “present” without necessarily being louder. It can also
impart tonal coloration, making a signal warmer and fatter. Compression can even serve as a musical
tool, enhancing the sustain of held guitar notes or keyboard pads, or providing a snappier attack to
horn stabs or string pizzicato.
