The Technical Stuff
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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, is especially
egregious 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.
Input Signal and Threshold
The first and perhaps most significant factor in compression is the level of the input signal. Large
(loud) input signals result in more gain reduction, while smaller (softer) input signals result in less
gain reduction.
Threshold
is another important factor. It is a term used to describe the level at which a
compressor starts to work. Below the threshold point, the volume of a signal is unchanged; above it,
the volume is reduced. For example, if a compressor’s threshold is 0 dB, incoming signals at or above
0 dB will have their gain reduced, while those below 0 dB will be unaffected.
In the 2-LA-2, the Peak Reduction
knob in each channel controls both the threshold and the amount of
input signal being routed to the compressor circuit. As it is turned up (clockwise), the overall degree of
compression increases; as it is turned down (counterclockwise), the overall degree of compression
decreases. At the 0 (fully counterclockwise) setting, no signal enters the compression circuit, hence no
gain reduction.
Ratio
Another important term is compression
ratio
, which describes the amount of increase required in the
incoming signal in order to cause a 1 dB increase in output. A ratio of 1:1 therefore means that for
every 1 dB of increase in input level, there is a corresponding 1 dB increase in output level; in other
words, there is no compression being applied. A ratio of 2:1, however, means that any time there is an
increase of 2 decibels in the loudness of the input signal, there will only be a 1 dB increase in output
signal. A ratio of 4:1 means that even when there is a full 4 decibels of increase in loudness, there will
still only be a 1 decibel increase in output signal. (Bear in mind that decibel is a logarithmic form of
measurement, so a 2 dB signal is not twice as loud as a 1 dB signal; in fact, it requires approximately
10 dB of increased gain for a signal to sound twice as loud.)
