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Compression 101
Introduction
This paper offers a basic explanation of compression for beginners. If you've just purchased
a compressor and aren't sure how to use it, or are considering the purchase of a compressor,
this will help you understand what compressors are used for.
What a compressor does
Most types of signal processors, such as reverbs, equalizers, and delays, are designed to make
an obvious change in the sound. But a compressor's action is much more subtle; when used
properly, most listeners won't be aware that signal processing is being used.Only if you hear
the original dynamic range of a signal and compare it to the compressed version will the
effect be noticeable. yet, compressors are essential in modern audio work.Almost every lead
vocal on a pop record is compressed during tracking or mixdown. Often the entire stereo
mix maybe compressed or limited during the mastering process. Finally, when you hear the
song on your favorite radio station, it passes through yet another compressor before it's
transmitted.
A compressor/limiter is essentially an automatic volume control. Imagine an engineer with
his hand on a fader and his eyes on an input level meter.As long as the meter stays below a
certain point (the threshold), he leaves the fader all the way up and the gain is unchanged.
But the instant the sound gets louder, the engineer pulls down the fader by a certain
amount.After the sound gets soft again, the engineer will push the fader back up.That's what
the compressor is doing, except much faster and more accurately than humanly possible.
Paradoxically, by cutting the peak levels, a compressor allows you to raise the average level
of a sound using the Output control to make it sound louder. By using the threshold and
ratio controls, you can set a stable sound that will hold its position in the mix whether the
singer is whispering or screaming.
What the controls do
Let's go back to the "engineer with his hand on a fader and eyes on the meter" analogy.The
front panel controls simply tell the "engineer" what rules he should follow. [THRESHOLD]
tells him how high the input meter can rise before he has to start pulling down the fader:
if it's turned full clockwise, he won't pull down his fader until the highest red LED comes
on; if it's turned counter-clockwise, he'll have his hand on the fader even before the lowest
green LED lights. [RATIO] tells him how far he should "pull the fader down" when the signal
is above the threshold level: should he pull it down just a little bit (compression) or pull the
fader as far down as necessary to make sure the output level is never higher than the
threshold (limiting)? The [OverEasy®] switch affects how he reacts as signal approaches
and travels through the threshold: does he reduce it exactly by the ratio only after it crosses
the threshold, or does he gradually ease into the full ratio as it passes through? The LEDs of
the gain reduction meter tell you how much the "engineer" is pulling down the "fader" at
any time. If these LEDs aren't on, his hands are in his pockets.
The [ATTACK] and [RELEASE] controls involve the speed of the engineer's response. Short
attack times order the engineer to get his hands on the fader 1/10,000th of a second after
he sees a too-loud signal; long attack times tell him to let transients less than 1/5th of a
second pass. [RELEASE] tells the engineer how quickly he should push the fader back up
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Compression 101 - November 1998
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