PreSonus CL44 - Manual (Page 15)

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 compression destroys the acoustical dynamic response of a
performance. ('Over compression', however, is used by some engineers as an effect, and with killer results!)
Compressor/limiters are commonly used for many audio applications. A kick drum can get lost in a wall of electric guitars.
No matter how much level is increased, the kick drum stays 'lost in the mud'. Add a touch of compression and tighten up
that kick drum sound allowing it to 'punch' through without having to crank the level way up.
A vocal performance usually has a wide dynamic range. Transients (the very loudest portion of the signal) can be far outside
the average level of the vocal signal. It is extremely difficult to ride the level with a console fader. A compressor/limiter
automatically controls gain without altering the subtleties of the performance.
A solo guitar can seem to be masked by the rhythm guitars. Compression can make your 'lead' soar above the track without
shoving the fader through the roof.
Bass guitar can be difficult to record. A consistent level with good attack can be achieved with proper Compression. Your
bass doesn't have to be washed out in the low end of the mix. Let the compressor/limiter give your bass the punch it needs
to drive the bottom of the track.