Apple Final Cut Pro 5 - Manual (Page 942)

Chapter 9 Using Audio Filters 155
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The Compressor/Limiter filter has five controls:
 Threshold: This parameter defines how loud the signal must be before the
compressor is applied. This is the most important setting you need to adjust.
 Ratio: This slider determines how much compression is applied. Don’t overdo the
compression; a little goes a long way. Too much compression can reduce the
dynamic range to a flat, unvarying signal.
 Attack Time: This setting determines how quickly the filter reacts to changes in
volume (the default is usually acceptable, but you may want to experiment).
 Release Time: This defines how slowly the filter lets go of the change in volume that
it made (again, the default should work well, but feel free to experiment).
 Preserve Volume: Compensates for the attenuation of the clip caused by compression
by raising the level of the entire clip by a uniform amount.
Expansion
An expander increases the dynamic range of an audio signal by attenuating (reducing the
gain of ) the signal when it drops below a certain level (the threshold). This has the effect
of making relatively quiet portions of the audio signal even more quiet proportionally, so
the difference between the loud and quiet parts of the audio are increased.
An expander makes quieter portions of audio even quieter by decreasing the volume if
it drops below a specified level. The lower a level is relative to the specified volume
threshold, the more it is decreased, depending on the Ratio setting. An expander with a
very high ratio value is called a noise gate, and is used to make the level of all sound
below the specified volume threshold as close to silence as possible.