PreSonus ACP22 - Manual (Page 9)

Controls & Connections
Ratio
Ratio sets the compression slope. This can be defined as the output level
versus the input level. For example, if you have the ratio set to 2:1, any
signal levels above the threshold setting will be compressed at a
compression ratio of 2:1. This means that for every ldB of level increase into
the compressor, the output will only increase 1/2dB, thus producing a
compression gain reduction of 0.5dB. As you increase the ratio, the
compressor gradually becomes a limiter. A limiter is defined as a processor
that limits the level of a signal to the setting of the threshold. For example, if
you have the threshold knob set at 0dB, and the ratio turned fully clockwise,
the compressor becomes a limiter at 0dB. This means that the signal will be
limited to an output of 0dB regardless of the level of the input signal.
Attack
Attack sets the speed at which the compressor 'acts' on the input signal.
A slow attack time (fully clockwise) allows the beginning envelope of a
signal (commonly referred to as the initial transient) to pass through the
compressor uncompressed, whereas a fast attack time (fully counter-
clockwise) subjects .the signal to the ratio and threshold settings of the
compressor instantaneously.
The Attack control is only active when the Auto
button is not pushed in. When the auto button is pushed in, the
compressor automatically determines the appropriate attack time
for compression.
Release
Release sets the length of time the compressor takes to return the gain
reduction back to zero (no gain reduction). Very short release times can
produce a very choppy or 'jittery' sound, especially in low frequency
instruments such as a bass guitar. Very long release times can result in an
over compressed sound, sometimes referred to as 'squashing' the sound. All
ranges of release can be useful at different times. You should experiment to
become familiar with the different sound possibilities. (Refer to the
applications section of this manual for some ideas.)