Compression
Compression reduces the dynamic range of a signal. In audio production, compression is often used to limit the peaks on transients, preventing them from clipping when increasing the volume of the master copy. For guitar playing, the main usage is similar, bringing the harsh, percussive spike in volume from the attack of a clean guitar tone so it is more in line with the sustain volume. For clean or mildly distorted guitar, it can be used to get longer sustain or thickness in the tone. It can be used to augment high-gain distortion, so that less distortion can be used without losing thickness - the tone gets clearer without losing saturation.
Common Parameters
- Threshold - the volume level that the signal needs to reach before the compressor actually does anything.
- Ratio - Determines how much dynamic range is squashed for signals that exceed the threshold. At high settings, the compressor acts as a limiter, preventing the signal from exceeding some volume limit.
- Soft-Knee - this is a switch that alters ratio based on the signal's proximity to the threshold to make the compressor more transparent.
- Attack - this delays the compressor from kicking on until the signal level is above the threshold for this amount of time. Useful to allow the transients of a guitar signal through untouched while compressing sustained notes.
- Volume/Output - simple output volume, but necessary to adjust in line with compressor settings. Can also be used as a solo boost when only using the compressor for solo's.
Signal Chain Placement
Compressors can be placed pretty much anywhere in the signal chain, having a slight difference in tone depending on placement.
A compressor at the front of the chain is going to send the distortion stage a more consistent volume level, meaning the amount of distortion saturation is going to be more constant. This may be preferable to thicken up a rhythm tone, but it may hurt a mildly distorted lead or crunch tone, where you want accents and attack to distort more.
Placing it towards the end of the chain is simply going to level out the final volume a bit. This is useful to add sustain to the signal without altering the tone. You can use it to get palm mutes to sound more prominent when the distortion isn't fully saturated. It can also be useful to help the tone fit a mix better. Engineers typically use a compressor on the master bus to prevent strong peaks from clipping while boosting overall volume.