What is compression, and how do I use a compressor?
Please read these very useful articles, they explain everything with nice graphics:
• From GuitarNoise.com
• From Harmony Central
• From Electronic Musician
• From Mix Magazine
• From Jim Carr
• From Rane
• A .pdf from dbx, "Compression 101"
• A .pdf from TC Electronic, "Compressor Applications"
• A .pdf from Rane, "Dynamics Processors"
A compressor is just an automatic volume control. At a basic level, it reacts to volume spikes from your music, and turns down the volume a bit as your input levels go up. That's really all there is to it. The reason compression is so widely misunderstood and confusing is because the various circuits that actually do the volume control almost always have some other impact on the tone which you may like or dislike, and also because it can honestly be quite difficult to set up any unit so it compresses your signal enough but not too much.
People (including me) talk about compressors fattening their tone, increasing sustain, making their sound "punchy", or other improvements; while those effects are possible, they are really just side effects that may be achieved, depending on your settings and the particular unit being used. But simply putting a comp in your signal path will not necessarily give you any of those desired effects--you have to make some educated choices.
You need to decide first which effects (such as sustain, peak limiting, or more consistent levels) you want to achieve; and what you are willing to pay--or put up with--to get those results. One key thing to know is that compression is always a compromise of some kind. Every compressor, from the cheapest to the most expensive, is a compromise. Selecting a compressor is largely about choosing the compromises that work best for you. For example, if you want a compressor that can give you both a natural transparent sound and a funky/dirty fat sound, be prepared to carry a rack unit that costs a lot of money and has a dozen knobs. Or if you want something very simple to use, be prepared to accept the fact that the sound and response of that simpler unit may not ideally match your instrument or your playing style. If you want lots of sustain, understand that sustain usually comes with a lot of noise. For every benefit there is a cost. But it's like anything else in life, the benefits are totally worth the trouble if you choose wisely.
There are a few core elements to compression; once you understand them, you'll have an easier time setting up any compressor with your rig. In fact, to use a compressor well and get good results at all, you must first familiarize yourself with these concepts:
• Input level is critical because it determines the range of signal that the compressor "hears" and reacts to. Some comps don't have an input level control, so you'd have to use the output volume controls on your instrument, preamp, or another pedal to regulate the level going into the compressor. Finding and setting the correct input level is probably the number one problem that frustrates compressor pedal users; the wrong levels will make your new pedal seem like a noisy waste of time.
• Threshold is how loud your signal has to get before the compression kicks in. This is essential because you may not want your entire signal to be compressed. Some units have no threshold knob- for them, the threshold is "fixed" and can only be adjusted by changing the input level of your signal.
• Ratio is how much your signal gets compressed once it passes the threshold. Ratios are figured in decibels (dB); a dB is a unit of how much a signal increases or decreases relative to where it started. With a ratio of 4:1 for example, the idea is that for every 4 dB your signal goes over the threshold, the output level will only go up by 1 dB. Generally, ratios of 2:1 to 4:1 are considered light or moderate compression, and ratios of 10:1 or higher are considered heavy compression or limiting. A hard "brick wall" limiter has a ratio of infinity:1, meaning that once your signal crosses the threshold, the output will not increase more than 1 dB no matter how high the input signal spikes.
• Attack controls how quickly the compression reacts to your signal, and Release controls how long it takes to "let up" and stop compressing after it's triggered. These controls are interactive, and the right settings for them will vary depending on the music and your playing style. You'll have to experiment, but a decent rule of thumb is to start with the attack and release knobs in their middle position, and adjust from there.
• Output Gain controls how much the volume of your signal is increased coming out of the comp, and this is necessary because compression lowers the overall average levels of your signal. So almost all compressors have a booster at the end, which provides "makeup gain" to bring your signal back up to the level you want. This is what accounts for both the increase in audible sustain and harmonics, and also the increase in the noise floor.
For compressors with only two knobs, "Compress" (or "Sustain") and "Level", the Level knob controls the output gain, while the Compress knob may control the ratio, the threshold, or the gain of your signal being boosted into a fixed threshold. It varies with different circuit designs, so you'll need to find out which function it is for any specific pedal you're using.
Please see this article for further discussion of how to set up each of these parameters, as well as an explanation of "hard knee" versus "soft knee". And check out this article if you are still wondering whether you even need a compressor at all.