Compression is crucial to sound recording and live sound mixing, but many musicians don’t understand exactly what it does. Compression is simple in theory but can be complex and subtle in practice.
A compressor reduces the dynamic range of sounds that pass through it. That sounds complicated, but it isn’t. “Dynamic range” is just the distance between the loudest and the quietest sounds in a recording or performance. Compression reduces this range by reducing the gain on sounds louder than some threshold, and/or increasing the gain on sounds quieter than some threshold.
In compressors designed for studio use, the user has detailed control over the low and high thresholds. In stompbox compressor sustainers, there is usually a single knob that controls both low and high thresholds.
Picture it this way: turn the knob all the way left and there is a low floor and high ceiling to your sound, turning it towards the right gradually raises the floor (volume below which no sound can go) and lowers the ceiling (volume above which no sound can go).
There are four main settings on most compressors: threshold, ratio, attack, and release.
Threshold: the volume (high or low) at which the compressor will kick in to boost or reduce the gain.
Ratio: how much the compressor will reduce or increase the output when it kicks in.
Attack: How quickly the compressor acts when a sound goes above or below the set threshold. Set it fast and you only hear the compressed sound, set it slower and you hear the uncompressed attack before the compressor kicks in.
Release: how quickly the compressor stops acting once the sound goes back above or below the threshold at which it will kick on.
The “sustainer” part of “compressor/sustainer” is the low-threshold boost. “Sustain” is how long note played on an instrument rings before decaying to the point that it can no longer be heard. A compressor increases sustain by boosting gain when the note starts to decay.
So, in short, a compressor does three kinds of thing:
- Acts as a sustainer, boosting the signal as sounds decay so that notes ring longer.
- Forces a sound to stay within a range of volumes. This can be used to ensure that, for example, a soloist cuts through and doesn’t get swallowed up by the band, or that no instrument in a rhythm ensemble jumps out front in an awkward way.
- Creating an “effect” on your sound, by some combination of the way compressing the dynamic range produces a signature sound, and the way timing of the attack and release produces a recognizable effect. For example, the chicken-picking’ twangy country sound, the classic eighties ‘sheen’, and high-pressure high-volume distortion sounds found in contemporary metal are all produced in part by using compression.
How to shop for a compressor pedal:
The most important thing to do before shopping for a compressor or compressor/sustainer is to determine what you want it to do. Do you want it to be a tool that helps you control your sound while staying out of the way of your tone, or do you want it to be an effect that changes your sound in a particular way? Or do you want both?
There are two significant characteristics of different pedals: 1) the quality of the compression, 2) the amount of control that the user has over settings. Both of these considerations are relevant to the “tool” vs. “effect” question.
Some say that there is “good” compression and “bad” compression. This is usually based on whether or not the compression sounds artificial and forced (“bad”) or natural (“good”). “Natural” here means one of two things, either the compressor does not significantly change the tone of the instrument and amp combo, or it produces a warm and full sound. Now, of course, you may be looking for an artificial, even ‘chintzy’ sound, so you may want a “bad” compressor.
Or, you may be looking for a unit that works seamlessly with your sound, and doesn’t change it. It depends on what you want the pedal to do. More natural sounding compressors tend to be more expensive, topping out with the Keeley units north of $250.
All of the compressors have a control that adjusts some combination of threshold and ratio. Sometimes the release speed is folded into one or the other of these controls as well. The exact kind of control and the degree of variation you get when twisting the knob from left to right differs with each box too widely to detail here.
The big difference in terms of control between different stompbox compressors is whether the user can control the attack time. Several, like the DigiTech, and BOSS models, have an attack control. Some, like the Electro Harmonix Nano Soul Preacher, has a selector switch between three preset attack speeds. Others, like other Electro Harmonix models, Red Witch, Hartman, and Akai do not allow you to control this but come with a preset (usually natural-sounding) attack speed. Finally, some makers, like MXR, Rocktron, and Keeley, sell two models for different prices, one with an attack control and one without.
If the pedal doesn’t call itself a “sustainer” there’s a good chance that it will not allow you to produce an “endless” low-threshold boost. It will increase sustain, of course, but not in that “forever and ever” way that a sustainer does. (So, soloists who want notes that go on forever: get a sustainer.)
Kinds of compression technology
Finally, there are several different kinds of technology used to produce compression. Most use one or more FET transistors to produce compression, but two other kinds of technology are sometimes used.
Vacuum Tubes: The Electro Harmonix Black Finger uses vacuum tubes to produce compression, which gives a warmer sound, and has its evangelists, just like tube amps. EH also produces a FET version of this compressor, the White Finger Multi FET Compressor.
The “Metal Can” Amplifier: Two currently available models of the compressor, the BBE Bench Press and the MXR Custom Comp Effect, use a novel circuit technology, the CA3080 OTA (Metal Can) amplifier. This is a circuit that regulates output by creating a negative feedback loop with the input, thus avoiding the need to have a processor intervene in the signal chain. Its proponents claim that it produces clean and transparent compression that does not remove the essential character of the source tone. The CA 3080 OTA was first used in a compressor from ROSS that is long out-of-production but still inspires clones (and lawsuits from its inventor).