DBX Compression 101 User Guide
Compression 101
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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 compr essor,
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 a ware that signal pr ocessing is being used.Only if y ou 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.[RA TIO] tells him how f ar 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. Shor t
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
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again after a loud signal has stopped;when it’s turned counter-clockwise,he pushes the fader
back up instantly,and when it’s full clockwise,he’ll take longer to push his fader back up to
unity gain.
The [OUTPUT] control is simply a gain control located after our “automatic engineer in the
box.”The [INPUT/OUTPUT] switch allows you to see the levels before the engineer does his
job,or after.
The most important controls are the [THRESHOLD] and [RATIO] knobs.They both interact
to get the effect you want, and that requires some experimenting. For example, if your
average input signal is 0 dB,a ratio of 2:1 with a threshold of -12 dB will give you 6 dB of
gain reduction,as will a ratio of infinity with a threshold of -6 dB.But the latter setting will
sound more “squeezed”than the former.
Avoid common compressor mistakes
Extreme settings will lead to extreme results. If you set an infinite ratio and turn the
threshold down to -40 dB,the compressor will do what it’s being told to do:tur n the level
way down. If you then try to compensate by cranking the [OUTPUT] control to its
maximum,you’ll amplify the noise of your mixer,EQ,Mic preamp,and the compressor itself.
The noise will fade itself in whenever the input signal stops, resulting in the classic
“pumping”and “breathing”problems.Noise is present in every system,and improper use of
any compressor will amplify it to an obnoxious level.
If the ratio is set to 1:1,it doesn’t matter where the [THRESHOLD] control is:the compressor
is being told not to change the gain at all,even if it’s above the threshold level.None of the
REDUCTION LEDs will light, and you may as well have the compressor in BYPASS mode.
Similarly, if the ratio is infinite and the threshold is high,or the input trim of the mixer or
microphone preamp is too low,you will get no compression (and,if you raise the [OUTPUT]
level control,you’ll be amplifying the noise f loor).For low noise operation,make sure your
mixer ,compressor,and amplifier settings are set properly.As a g eneral rule,you want as muc h
gain as possible in the front of the system (at the microphone preamp),so that a good linelevel signal is travelling through the whole signal path.if you hav e a weak signal to start with,
and then amplify it at the end of the signal path (by turning the main outputs of the mixer
all the way up,for example) it will be excessively noisy.
When using a compressor on a live P.A.system,improper settings can cause feedback.Make
sure that a channel is well below the feedback point when there is no gain reduction active.
If you hear feedback every time the music stops, you must lower the overall level of the
system.
About stereo compression
For great technical information on stereo processing, see the dbx white paper called True
RMS Power Summing®,written by Roger Johnson,Director of dbx Engineering.
Otherwise read on...
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Compression 101