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We have established that a compressor reacts by turning down the signal whenever it gets louder than its threshold setting. But just as it takes you a few seconds to turn down the volume of your TV when those loud commercials come on, and a few more seconds to turn the volume back up when the program returns, a compressor takes time to react to a signal also. A compressor's attack and release controls determine the reaction time of any gain reduction once the threshold is breached. When a signal is determined to be too loud and shoots over the threshold, the attack time is how long it takes for the compressor to grab hold of the signal and turn down the volume.
Likewise, when that same signal falls back below the threshold and stops being too loud, the release time is how long it takes for the compressor to let go of the signal and return it to its uncompressed level. Well, it might take you five to ten seconds to grab that remote and adjust the volume on your TV, compressors generally work much faster, sometimes even reacting instantly to the incoming signal. A compressor's attack and release are usually measured in milliseconds, or even microseconds. Some compressors offer look-ahead processing, where the signal is buffered and previewed by the compressor's algorithm to preempt a threshold breach before it can happen.
Attack and release settings are based on the signal you are trying to process, but at a minimum, the attack should be set fast enough to grab hold of a signal before it completely decays, and the release should be quick enough for the compressor to recover before the next note or beat. For example, if the compressor is set for too long of an attack time, the signal might sneak through completely before the processor has time to grab it, kind of like trying to catch a ball after it's already passed through your arms. Likewise, too long of a release time may have the compressor holding on for too long, continuing any gain reduction into the next note or phrase, even if that note is below the threshold.
Definitely watch out for this with percussive signals like drums that tend to have very short decay times, sometimes well under 100 milliseconds from attack to complete decay. Now that we understand what attack and release are, in the next movie we will cover strategies for applying attack and release settings within a compressor.
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