A compressor is a device that reduces the dynamic range (i.e. differences in volume) of an audio signal (see compression). In a compressor, the ratio of change above a threshold in input level to the change in output level is measured in dB. For example, for a 3:1 ratio the output level changes by 1 dB for every 3 dB of change in input level above a certain threshold.
Basically, a compressor reduces the difference between the loudest and quietest levels of an audio signal. In other words, it makes the quiet bits louder and the loud bits quieter in order to make the sound more even in volume, giving the impression of tight, punchy power or smooth, slick motion. This reduction in the dynamic range of the signal is a powerful tool in avoiding distortion levels and adding sustain – as well as raising the volume during moments of quieter playing.
Many instruments, including the guitar, have a very wide dynamic range that needs to be controlled in the mix of a recording, and sometimes live, too. For a tight, funky clean guitar, a compressor can give an even, constant feel. In heavy metal, it can create long sustains for solos or giant-sounding power chords, and it can give riffs a wall-of-sound quality.
Compressors are often judged by their ability to control the dynamic range without creating noticeable audible side-effects. Heavy compression can cause a signal to "pump and breathe" with the onset of release of the compression. Some compressors can also dull the signal and lose the top end slightly. But the benefit of compression, when applied properly, is that it can add punch and excitement to instruments, as well as fatten up sounds and create a professional-sounding recording.
As well as guitars, compressors – like reverbs – are used on all instruments in the mixing process. Heavy-metal LPs are among the main beneficiaries of the compression process, from the pedals on instruments in the initial recording to mixing and then mastering.
Compressor and compressor/sustainer pedals are often avoided by guitar players because it's not always obvious what they're doing – unlike the immediate apparent effect of a fuzz or pitchshifter. Using a compressor wisely can enhance clean country leads and choppy funk rhythms, or create expansive clean chords. Studio rack compressors have greater and more complex range of controls, while the pedal variety generally has only the bare essentials. The use of pedal compression in a live performance enables increase in the overall level of sound without damaging the audience's eardrums when a loud note or chord is hit.
Among the controls that may be found on a compressor pedal are: