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Compression is one of the most useful tools that a
recording engineer can use in recording and mixing. After you are
done compressing, the level fluctuations of the audio will become less
than they were before applying the compression. This will make the
instrument more audible and present in the mix. After compression,
the lower-level passages or notes won't get lost. In this
article I'm giving you an explanation of the common parameters found on
compressors and how to set them to the correct value. |
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Threshold
Parameter |
| What It
Means: The level at which
a compressor will begin to change gain. Compressors work by reducing
signal strength anytime the incoming signal rises above the
threshold level. The threshold is usually adjustable by a
control marked "threshold." Reducing the threshold level means
that more peaks of the signal will trigger the compressor to turn
down the gain. Increasing the threshold level means that less peaks
will cause the compressor to reduce gain. |
| How It Is
Set: You start with the threshold set high (so that it
doesn't cause any gain reduction). As you are watching the
gain reduction indicator, reduce the threshold until the desired
amount of gain reduction is present on the louder passages or
sounds. For general compression, 6 dB of gain reduction
is a usual goal. For louder types of music or where the audio
has larger fluctuations, sometimes 10 dB of gain reduction is used.
Special techniques may require differing amounts of gain reduction. |
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Gain Reduction
Parameter |
| What It
Means: The working of a
compressor, reducing gain during high-level passages exceeding the
threshold level. There is usually a gain-reduction meter
that shows the compressor lowering signal strength on the loud
passages. This meter (usually a strip of lights) could read
"-6dB" on on loud passages indicating that the level was turned down
6 dB on the loud passages. The meter would indicate "0dB"
(showing no gain reduction) on lower-level passages. |
| How It Is
Set: How to set this parameter is set is covered
above, under "Threshold." Too much gain reduction causes the
louder passages to sound restricted and "squashed." |
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Compression Ratio
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| What It
Means: The
compression ratio is how many dB the input signal has to rise above
the threshold for the compressor to allow one more dB out of the
compressor. A ratio of 2:1 means that when the input signal
rises to 2 dB above the threshold level, the compressor only allows
the output to rise 1 dB [if the input rises 4 dB above threshold,
the output only rises 2 dB]. When the compression ratio is
10:1 or higher, the compressor becomes a "limiter" and effectively
prevents the output signal from increasing above the threshold
level. |
| How It Is
Set: The compression ratio is set
according to your application: |
| General
Compression for more presence: 2:1 to 3:1 |
| Level Control for audio
with sudden peaks: 4:1 to 5:1 |
| Protection from severe
overload or damage: 10:1 or more. |
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The illustration on the right shows a swell of level in a signal
and how this swell would be changed by a compressor. |
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Attack Time |
| What It
Means: In a compressor,
the attack time is how fast the compressor reduces gain when the
input signal rises above the threshold. |
| How It Is
Set: There are two approaches to setting attack time,
as follows. |
| 1. Attack
times very close to 0 ms and less than 5 ms tends to bring down the
percussive transients of the audio. This type of setting is
used to reduce transients or to provide maximum protection for
overload and damage to equipment (such as tweeters) that could be
caused by those transients. Digital audio compressors
sometimes have "look ahead" circuits to get a true 0 ms attack time,
something that is impossible with analog compressors. |
| 2. Attack
times over 5 ms (usually 10 - 30 ms) are often used to let the
transients and the attack of instruments through. This type of
setting is often used for general compression. |
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Release Time |
| What It
Means: In a compressor,
the release time is how long the compressor takes to restore gain
after a high-level passage (that exceeded the threshold) is over.
Generally the fastest release time that sounds good is best. |
| How It Is
Set: One sets the release time by listening. For
general compression of mixed music a release time between 50 and
100 ms usually works well. |
| Too long of a
release time creates a breathing effect where you hear gain being
restored during softer passages. If there is noise on the
track being requested, you can hear the noise change also. The
"squashed" sound of higher-level passages (sometimes called
"pumping") is more noticeable with longer release times.
Optimum release time would be fast enough to restore gain as the
passage being controlled dies down. |
| Two quick of a
release time causes a reoccurring, unnatural surge in the
instruments at the end of their sound. It occurs because the
release time of the compressor is quicker than the dieing down of
the passage being controlled . Release times under 25 ms may
distort bass instruments because the compressor changes gain in the
middle of a cycle. |
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Makeup Gain (Output
Level) |
| What It
Means: -The makeup gain is a level control at the output of the compressor.
The compressor reduces signal strength of the louder passages. After this
compression, the peak level of the signal is less. This makeup gain control allows
the compressed signal to be turned up so that its peaks are as high as they were before
compression. This action of "making up" gain increases the lower level
passages of the signal. |
| How It Is
Set: Looking at a meter during higher-level passages,
adjust the make-up gain so that louder passages peak at the same
place with the compressor in or out of the circuit. It is best
to use a "VU" meter for this or the "VU" or "RMS" meter mode on
digital consoles and mixers (when available). |
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Knee - |
| What It
Means: The knee of a
compressor is how abruptly or gradually the compression ratio is
applied to the signal when the signal is near the set threshold
level. A Hard
Knee means that the full compression ratio is applied to any signal above the
threshold level. A Soft Knee means that a mild compression ratio is applied
to signals approaching the threshold level and that the ratio is increased as the signal
rises above the threshold level; the full compression ratio is applied to signals well
above the threshold level. The soft knee makes the gain reduction less obvious - sort of
like turning down the volume of a stereo slowly rather than abruptly. Soft knee is
used in applications where you are using a compressor to even out volume changes in an
instrument. DBX, a well-known brand of compressors, trademarked their soft knee
function with the name Overeasy. |
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The graphs to the
right are the traditional way of showing the input/output
characteristics of hard knee and soft knee compressors, and
the graphs resemble "knees" (hence the name). The
above graph shows how the output signal would be different in
these two types of compressors. |
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| How To Set
It: Use a soft knee to make the compression less
noticeable. Use a hard knee for maximum protection. Some
compressors have a variable knee from very soft to no knee.
Use a softer knee with lower compression ratios and less knee with
higher compression ratios. |
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RMS (Average)
Detecting - |
| What It
Means: There are two ways
you could define the "level" of a signal. One way would be to
use the absolute peak level of the waveform; the second way would be
to use the "average" level of the waveform. In percussive
instruments, such as drums, there is a huge difference between the
peak level and the average level. In other instruments, such
as a flute or violin, the two levels are much closer. The term
RMS is an acronym for the mathematical process of determining a
signal's effective average level [the "M" stands for "mean" -
another word for average]. Many compressors allow the user to
chose between average and peak detecting.
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| How It Is
Set: Electronic circuits overload according to the
"peak" level but your ear responds to the "average" level. If
you are using a compressor, with a high ratio, to help prevent
overload (say of a digital recording) you would want the compressor
to respond to the peak level. If you are using a
compressor to make an instrument more even in volume, you would want
the compressor to respond to the "average" level. |
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Auto Release - Auto Release is variable release
time feature in compressors and helps make the signal's volume even to the ear. With
auto release activated, the compressor's release time is longer for peaks going well above
the threshold and shorter for peaks going slightly above the threshold.
Auto release is often used for general compression. |
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