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Compression 101
Terms
Compressors are often difficult to grasp for beginning musicians and engineers. Where an
effect like pitch shifting or delay is easy to hear, compression often performs subtle changes
on a signal and can be more difficult to learn.This introduction is intended as an expanded
glossary, which shows what the controls on the compressor's front panel are doing to the
input signal.
Threshold
The THRESHOLD knob sets the level where compression will begin. As long as the input
signal level is below the Threshold level, the compressor will do nothing to the signal. Once
the input signal crosses the Threshold, the compressor will begin compressing at a ratio set
by the ratio control.
Ratio
The RATIO knob controls the amount of compression, which will happen once the input
signal crosses the Threshold level, described above. Ratio controls how much the input
signal will be reduced as a ratio of the input signal level. For example, if the compression
ratio is set for 6:1, the input signal will have to cross the threshold by 6 dB for the output
level to increase by 1 dB. the maximum setting is typically labeled: 1 (infinity to 1),a nd is
also called Limiting.This means that the output signal won't increase at all, no matter how
far above the threshold the input signal goes.
Attack
The ATTACK knob controls the amount of time before compression starts after the
threshold is exceeded by the input signal.The range of this control can go anywhere from
very slow (about 1dB/Sec) to very fast (about 400dB/Sec). Long attacks are useful for
percussive sounds, where shorter attacks are good for melodic parts like vocals and strings.
The Attack control is also useful for keeping the transients on percussive drum or bass
sounds. Experiment with different short attack times on snare drums to get more or less of
the "stick" attach.
Release
The RELEASE knob controls the amount of time the compressor takes to stop compressing
after the signal crosses below the threshold. the range of this control can be from approxi-
mately 4000dB/Sec to 10dB/Sec. Short release times are good for percussive, punchy
sounds, where longer release times can make compression less obvious on vocals.Adjusting
the release time may be necessary when using extreme compression and "pumping" or
"breathing" is audible, or if lower level signals after peaks are getting lost. See also the
section on pumping and breathing explained below.
Output
The OUTPUT knob controls the level of the compressor's output. The Output control is
useful for making up gain, which was reduced by the compression circuit, or matching the
input level of a mixer or recorder. If the Gain Reduction meter shows that the input signal
is being attenuated by -6dB, then the Output control generally should be set around +6dB.
This control is disabled if the [BYPASS] button is pressed.
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Compression 101 - November 1998
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