Compressors -- sometimes called compressor/limiters -- do essentially two things, and it's all about thresholds:
First, they boost low signals so that the signal does not fall below a certain 'floor' threshold.
As a string is plucked it vibrates intensely. Over the span of several seconds, the vibrations gradually decrease and the string eventually stops moving.
But as long as the string is still vibrating, no matter how subtly, the compressor boosts the level of the signal gradually, corresponding to the decay of the vibrating string. This creates the perception of increased sustain as the signal does not decay as quickly.
Secondly, compressors create a 'ceiling' so that the volume and dynamic do not go above a threshold that has been set -- they limit loud signals. If the note is hit harder than the setting on the pedal allows, the compressor kicks in. Which can create a desirable effect as we shall describe below.
Depending on how we set the floor and ceiling thresholds, the signal can come out very even, characterized by increased sustain and and a characteristic compressed 'squash' if the note is played hard. This squash can be used to good effect for funk rhythms, for smooth-jazz type solos and for country pedal-steel-type bends.
But since compressors raise the floor threshold of a signal, they will also raise noise and hiss levels of booster, overdrive and distortion pedals if placed after these. They should therefore always be placed first in the chain, immediately after the guitar.
Let's look at what we have to work with on a typical compression pedal. On the classic Boss CS-series Compressor/Limiter pedals there are controls for Level, Attack and Sustain.
Level controls the overall amount of volume coming out of the pedal. What most players don't realise is that Level also determines the 'ceiling threshold' described earlier. Setting it lower, let's say at 10 o'clock, also lowers the ceiling, which means that we do not have to play as hard for the compressor to kick in, resulting in more squashing of the signal. Raising the Level beyond 12 o'clock causes the compressor to kick in less, allowing for a more neutral sound.
The Attack knob controls attack time. Attack time determines how soon (in miliseconds) the compressor will kick in when the input exceeds the threshold set by the Level. Setting the Attack low allows more of the initial attack of the note through, which sounds more natural. Set higher, the signal becomes very even, with the attack smoothed out.
Sustain controls the 'floor threshold' described earlier. Set lower, the compressor does less to amplify any signal that falls below a certain level, resulting in faster note decay. Set higher and the compressor becomes more sensitive to any signal that falls below a certain level, raising their apparent loudness, and increasing sustain.Use these concepts as a basic guideline, then break out the ol' compressor and have at it..
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