If you've ever played with an overdriven or distorted sound, you will have noticed that it feels like less effort to make a consistent sound, as opposed to when playing with a clean sound. This is because of compression. Compression happens whenever we overdrive or distort our sound, but there are ways of achieving it even without the distortion. So, let's have a look at what compression is, how to achieve it, and when it might be useful.
In brief, compression (in the context of a sound wave; we're not referring here to reducing digital file sizes, which is also called compression) is the reduction of the dynamic range. The dynamic range is the difference between the loudest and quietest parts of a signal. So when we compress a sound, there's less of a difference between the loud and quiet parts.
A clean guitar sound is very dynamic i.e. it's possible to articulate a note very quietly or very loudly, and the volume is high when the note is plucked, and quickly decreases as the note sustains.
We can use a compressor to even out these volume differences. This will give each note more sustain i.e. the volume of the note will decrease less over time, and it will make the initial volume spike as the note is plucked (known as the transient) less prominent. All this adds up to a smoother sound. From the player's perspective, playing with compression feels easier, requires less effort in the plucking hand to even things out. It's hard to describe, but you'll feel it when you try it.
Some compressor pedals have very few controls, and some give more options to sculpt the sound. For example, the MXR Dyna Comp has only Output and Sensitivity controls. The Output is fairly self explanatory, it simply controls the overall amplitude/volume of the signal that the pedal outputs. The Sensitivity controls the threshold at which the compression starts happening. Any part of the signal that is quieter than the threshold passes through unchanged. When the signal gets louder than the threshold, it is brought down towards the threshold level.
Some pedals give more control over how the compression works. For example, some have an “attack time” (sometimes just “attack”) knob. This controls how quickly the compressor responds to the signal going above the threshold. So, if you want the smoothest possible sound, set the attack low, if you want some of the transient to make it through, set the attack to be slower.
Sometimes you'll see a “blend” knob, which allows you to mix the compressed signal with the uncompressed (or “dry”) signal. This is sometimes referred to as parallel compression.
Another control that is available on some models is the “ratio” knob. This control how strongly the compressor attenuates the signal once it goes above the threshold. A low ratio will have a smaller effect than a large ratio. An extremely high, or even infinite ratio, will no allow any of the signal to poke above the threshold.
All this was a bit of a mystery to me when I first encountered compression, so if you're in the same boat, hopefully this will help you decide if you need some compression in your signal chain, and how to adjust it if you do.
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