Dynamic Range Processing and Digital Effects

Compression is the automatic reduction of gain as the signal level increases beyond a preset level called the threshold. It is an automatic version of “gain riding”, where one might manually lower a fader as the signal grows larger to maintain a more constant output level. For signals above the threshold, the increase in output amplitude for a given increase in input amplitude is known as the compression ratio. Below threshold, the ratio is 1:1 while above threshold the ratio may be set anywhere up to ∞:1, which is considered limiting since in that case the output signal does not increase at all as the input signal increases. The time required for the gain to reduce when a signal crosses the threshold is referred to as the attack time and the time it takes to return to the original gain when the signal drops back below the threshold is the release time. Generally, the attack is fast and the release is slow to prevent audible artifacts of the gain change, however too short an attack time reduces the transient onset of a sound and noticeably alters the character of the sound: sometimes this may be desirable, but often it is not. The effect of compression is to reduce the overall dynamic range of a signal. This can be for musical effect, to make a sound stand out more in a complex mix, or for the purposes of noise reduction in noisy recorders or transmission systems.