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Compressors are some of the most crucial and fundamental processors for audio. In the family of dynamics processors, they help alter and control a sound's dynamic range, or the difference between the loudest and softest level of a sound. They do this by reducing the loudest sound levels and then boosting overall level to compensate. For the best results on all your recordings, there should be a compressor somewhere in the line. In Part 1 we laid out some theory and history of compression -- Part 2 is a practical guide to the controls you'll see on most modern compressors and some tips and secrets on how to make them kick.
Input and Output
These are the easy ones: input adjusts the signal level coming into the compressor, before any of the magic takes place. Output controls the output level after all the processing has been done - this is the last adjustment before the signal squirts out the output jacks. This sounds simple enough, but there are a couple of tricks: As you change other settings on the compressor, you'll have to adjust the output control to compensate. Also, input level will affect how much compression you get -- this introduces the topic of threshold.....
Threshold is the level at which your widget starts to kick in. When the signal level is below the threshold, a compressor does nothing other than add a bunch of cable and ineffectual circuitry to your signal path. Once you exceed the threshold your compressor starts to do its thing, controlling output level (according to the ratio you've set).
Usually the threshold activity is accompanied by a satisfying LED light show, or a waving VU meter on the unit's front panel. If you've configured your device as a limiter by dialing in a very high ratio, the threshold is the maximum output (in a mathematical world this statement really holds true; in practice things don't work exactly this way, but close enough for jazz). Threshold knobs are labeled erratically from brand to brand, but most commonly, counterclockwise knob twiddling reduces the threshold, meaning that the compressor kicks in at lower sound levels (more lights flashing). Clockwise twiddling sets the threshold higher, allowing more sound to pass through unaffected. Think limbo dancing: the threshold is the height of the horizontal pole. Set it high enough and people just walk through. Set it absurdly low and enjoy the unnatural results.
But watch out. If you've set your compressor up just the way you want it and you start playing with the input knob, you'll be changing how much signal goes to the threshold control. These two knobs have to be considered in tandem: If you dial in more input signal, you need to raise the threshold or you'll be getting more compression than you want. Older compressors often used a fixed threshold and offered only the input knob to adjust the amount of compression. But nowadays, the fashion is for more knobs, of course.
Another control we didn't often find in the sixties is ratio, which adjusts the intensity of the compressor's reaction to incoming signal. (Getting back to our limbo analogy, it controls how hard the pole is-er, well, maybe we've exhausted this metaphor. Fuggeddaboudit.) Ratio can make the compressor react a lot or a little depending on how you twist the knob. 2:1 is a little, and, generally speaking, when you're recording basic tracks and looking for warmth, presence and enhanced detailing, you'll dial in ratio settings between 2:1 and 6:1. 6:1 on up to 20:1 is considered heavy compression, which can be good on vocals, bass and great for cool room effects for drums and percussive material, like those unbelievable drum sounds on Revolver. Beware, though: compression is very hard to undo in the mix. Unlike equalization, you won't be able to "uncompress" sounds that have gone to tape over-crushed.
A ratio setting of 20:1 or higher is called limiting, and is great for crazy effects like backward sounding guitar and cymbals. Or, if you set the threshold high, limiting is useful for automatic level control, to keep those peak lights from going off. This last use is especially important for recording to digital media - the sound of overloaded D/A converters is wretched!