PreSonus ACP22 - Manual (Page 25)

Appendix
A Word About Compression ....
Punch, apparent loudness, presence.., just three of many terms used to
describe the effects of compression/limiting.
Compression and limiting are forms of dynamic range (volume) control.
Audio signals have very wide peak to average signal level ratios (sometimes
referred to as dynamic range which is the difference between the loudest
level and the softest level). The peak signal can cause overload in the audio
recording or reproduction chain resulting in signal distortion. A
compressor/limiter is a type of amplifier in which gain is dependent on the
signal level passing through it. You can set the maximum level a
compressor/limiter allows to pass through, thereby causing automatic gain
reduction above some predetermined signal level or threshold.
Compression refers basically to the ability to reduce the output level of an
audio signal by a fixed ratio relative to the input. It is useful for lowering the
dynamic range of an instrument or vocal, making it easier to record without
distorting the recorder. It also assists in the mixing process by reducing the
amount of level changes needed for a particular instrument. Take, for
example, a vocalist who moves around in front of the microphone while
performing, thus making the output level vary up and down unnaturally. A
compressor can be applied to the signal to help correct this recording
problem by reducing the 'louder' passages enough to be compatible with the
overall performance.
How severely the compressor reduces the signal is determined by the
compression ratio and compression threshold. A ratio of 2:1 or less is
considered mild compression, reducing the output by two for signals greater
than the compression threshold. Ratios above 10:1 are considered hard
limiting. Limiting refers to the point at which the signal is restrained from
going any louder at the output.
The level of input signal at which the output is reduced is determined by the
compression threshold. As the compression threshold is lowered, more and
more of the input signal is compressed (assuming a nominal input signal
level). Care must be taken not to 'over compress' a signal. Too much