Compressors
A compressor is a very useful tool and probably the most commonly used out of all processors. As instruments never produce a continual sound with the same volume, velocity or tone, a compressor can be used to make it more constant. It can be seen as a person constantly the volume dial of the output of an instrument so that the level always stays the same.
Compression is expressed in terms of a ratio - the compression ratio. This ratio describes how much the signal appearing at the output of the compressor changes in relation to a given change in the level of the original signal applied to the input. If no compression is applied and the input signal doubles in level, then the output signal will also double in level, precisely following the change in the input signal. This is known as a ratio of 1:1. A compression level of 2:1 would mean that the level of the output signal will change only half as much as the input signal. Expressed as decibels, a compression ratio of 20:1 would mean that a -20dB change in the level of the input signal would result in only a -1dB change in the level of the output signal.
Limiting is basically an extreme compression which is set to affect only signals above a certain level. This is useful for limiting peak in the signal which exceed the handling capacity of certain equipment. The compression ratio for a limiter is to to infinity which means that no change in the output level will occur no matter how much the input level changes.
Basic Parameters
Threshold - This sets the level above which compression takes place
Ratio - Controls the amount of gain reduction that takes place one the threshold has been crossed/exceeded.
Attack - This sets the amount of time the compressor takes to respond to an input which exceeds the threshold setting.
Release - One a signal peak undergoing compression falls back below the threshold setting, the release control sets the time taken for the signal to recover to its original level.
Gain - This is used to 'make up' for any signal loss incurred during compression.
Peak limiter - This sets the threshold of the 'peak limiter'