XO Wave is now discontinued
as we prepare to bring you the next generation
Digital Audio Workstation called
This site remains available for anyone who
purchased XO Wave in the past.
However, please keep in mind that as discontinued software:
- This site may not contain up-to-date information.
- Technical support is discontinued, though we will do our best to continue to provide
email support, especially to anyone who purchased recently.
XO Wave: Compressor
Note: The term "level" is used extensively in this document, and in
XO Wave. In other contexts, it might also be called "volume" or
The Compressor in XO Wave is a powerful tool for mixing, mastering,
and recording; and can even be useful when editing. If you have never used a
compressor, you may want to start with the How to Use Compression
tutorial. If you are experienced with compression already, you
may be able to just jump right in and use the XO Wave Compressor
without any difficulty. Either way, this page explains XO Wave's
Compressor, a powerful and flexible tool for controlling
dynamics in your recording projects.
The diagram at the right shows the signal path for XO Wave's Compressor.
The main signal first goes through a gain reduction step, then a make-up gain step,
and finally a Limiter. There is also an additional signal path
called the Side Chain, which is an alternate signal that controls
the behavior of the compressor, but is not heard directly in
the Compressor's output.
The amount of gain reduction at any time depends on the
ratio and threshold
settings, and on the strength of the signal in the Side Chain
(which by default is the same as the input signal). To make up for the
gain lost during gain reduction, make-up gain is available to boost
the level of the entire signal. Because the gain reduction step
tends to reduce the loudest parts more than the softest parts and the
make-up gain increases all sounds equally, the net effect of these two steps
is usually to make the whole track
sound "louder" or "hotter", where the quieter
segments are closer to peak volume.
The first two stages (gain reduction and make-up gain) both modify the
signal in a smooth way, without jarring changes in gain or amplitude.
As the signal gets stronger, the gain reduction steps in
little-by-little to reduce the signal. For example, with the
ratio set to 1:2, a 2 dB increase in signal strength
results in a 1 dB increase in the output.
The smoothness of the gain-reduction step over time is controlled by the
attack, and release settings.
The third stage, the Limiter, modifies the signal according to
a strict rule: never let the signal go above its threshold.
When the signal goes above the threshold, it engages the Limiter
which reduces it by the amount needed to keep it in range.
This is useful for digital recordings, which are
often constrained to specific amplitudes, but it can result in a
very harsh sound because the sudden changes in gain can distort the audio
significantly. Because of this, the limiter in XO Wave uses
lookahead and release controls
to smooth the transition from normal to limiting behavior.
In the case of signals that are above the threshold for long periods
of time, however, audible distortion may still occur.
Common Interface Features
There are 3 parts of the Compressor's graphical interface panel.
The top provides common
meters and the Meter Hold control. The middle
provides a tabbed area which can switch between
Limiter, and Side Chain
controls. The bottom provides additional common controls. The
top area includes the following:
- Compression State indicator: The three
simulated LEDs in a traffic-light configuration show the
state of the Compressor. Green
means that the signal is below the threshold and
therefore no compression is occurring. Yellow means that
the signal is below the main threshold but above the
"knee" of the Compressor, meaning that partial
compression is taking place. Red means that the signal
is loud enough that the Compressor is fully
- Limit indicator: The limit indicator
activates when the Limiter has
detected audio signal which exceeds its
threshold, and the reduced the gain to keep
the signal in range. Note that because of the Limiter's
advanced "lookahead" feature, an active limiter does
not necessarily mean that hard clipping has occurred.
- Gain Reduction Meter: This
meter (a long strip of squares) indicates how much the
signal's gain has been
reduced by the Compressor.
- Meter Hold: Here you can adjust the
"ballistics" of the gain reduction meter, meaning how fast
the meter changes value. Like the main XO Wave meter,
the gain reduction meter is a peak meter, meaning it
measures the peak value recently experienced. The
Meter Hold setting controls how long (in
milliseconds) the meter "remembers" a peak before it
"forgets", so a new peak can register.
The bottom of the Compressor panel offers three buttons.
- Bypass Compressor disables the
Compressor (including makeup gain).
This is useful for comparing the compressed and
uncompressed signals, or if you just want to use the
- Bypass Limiter disables the
Many people only use a limiter towards the end of their
- Side Chain Listen allows you to listen to
the side chain, rather than the main output of your
Compressor/Limiter. This is most useful while you are
setting up the Side Chain EQ.
When you open the Compressor, it initially shows the most commonly used
controls. The center of the window has a tabbed panel. By
selecting tabs other than the initial Basic
tab, you can expose the more advanced controls, described
in detail below.
The Basic tab contains a threshold control, which
works just like most compressors' threshold control; and a gain
control, which allows you to set the gain applied after
compression. This is sometimes called "make-up gain" since it is
typically used to compensate for the reduction in signal caused
by the Compressor.
Just as importantly, the Basic tab contains a
Preset pop-up menu,
which lets you select from a list of Compressor presets for
controls such as Ratio,
Attack, Release, and Side Chain
settings. Although we cannot provide presets for every
situation, they offer a good starting point, and can be a
real time saver. It is important to note that when you apply a
preset, all the parameters will be reset -- including any
automation you may have. Fortunately, like most things in XO
Wave, changing presets is undoable, so if you
apply a preset by accident or change your mind, you can
Usually, the first thing you want to do when using the Compressor is
select the most appropriate preset from the list (Note: we at
XO Audio are still tweaking the presets, so please let us know
if you have any suggestions!) After that, lower the threshold
until you get the amount of gain reduction you want. Finally,
add some gain to make up for the signal you lost during
compression (The gain reduction meter can give you an idea of
how much the gain is being reduced at any given time.)
When adjusting gain, you should realize that this occurs after
the compression stage, and before the Limiter stage. This means
that if you add too much gain, the Limiter will step in and reduce
the gain further. When the Limiter is reducing the signal level,
the red "limit" indicator at the top of the Compressor becomes
active. The Limiter is a "hard" or "brickwall" limiter, meaning
that it will do whatever it has to do to keep the signal level
in range. For tiny sections of audio, this is usually not a
problem, but if you hear audible distortion or see the limit
indicator stay on for more than just a flash, you should probably
either increase the amount of compression, or decrease
the gain setting.
Most of the time, that's all you'll need to do. For finer control of
the Compressor, Limiter, or Side Chain, a variety of other
controls are available by selecting the appropriate tabs, documented
in detail below. In addition, the later sections of this
document describe use of the Compressor for special
purposes, such as de-essing.
The functions of the Basic controls are:
- Preset: The Preset pop-up menu allows
you to select a preset for the settings. This is
especially useful when first setting up the Compressor.
Note that if you select a preset, existing Compressor
settings (including automation) will be cleared. If
you accidentally select a preset or you change your
mind, you can Undo the change.
- Threshold: This controls the Compressor's
trigger threshold. Signal which is below the threshold
is not compressed. Lowering the threshold results in
compressing more of the signal, while raising it exempts
more of the signal from compression. For more info, see
our tutorial: How to Use
- Gain: The gain control allows you to add
gain (volume) to make up for the gain removed by the Compressor.
The Compressor reduces the signal level only when the
signal is above the threshold, but the gain control adds
the same amount of gain to the entire signal (otherwise
it would just negate the Compressor).
In addition to the common and basic controls above, the Compressor
offers additional options grouped under 3 tabs:
Compressor, Limiter, and
The Compressor tab allows you to fine-tune
compression settings. The Threshold &
Gain settings are shared between tabs, so a
change here will also be reflected in the Basic
tab, and vice-versa.
- Attack: You can control both the attack
time and the algorithm used when the signal goes above
the threshold. These control how long it takes for the
compression to take effect after the signal goes over
- Release: You can control both the release
time and the algorithm used when the signal falls below
the threshold. This controls how long it takes for the
Compressor to stop compressing after the signal drops
below the threshold.
- Threshold: This threshold control is the
same control as is seen in the Basic
tab. It controls the volume level above which the
- Knee: This control allows the Compressor to
start working before the signal reaches the threshold.
This allows slightly "softer" or "gentler" compression,
and can make the effect considerably more
- Ratio: This control works the same as the
ratio control found on most compressors. When the signal
goes above the threshold, the gain of the (virtual) circuit is
reduced by this amount.
For example, with the ratio set at 2:1, an increase in
input of 2 dB above the threshold will result in an
increase in the output of only 1 dB. Higher ratios yield
more compression, but only signals that reach
the threshold (knee) are affected.
The Limiter prevents the signal from ever going above a certain
level. The simplest limiter design would simply "clip" the
signal if it went above a certain level. This can create a very
harsh sound, so XO Wave's Compressor has a "lookahead" feature
which, by delaying the signal slightly, is much less
noticeable. When driven to an extreme, however, no limiter can
avoid clipping. Therefore it is important to use the Limiter
- Lookahead: This control allows the Limiter
to start reducing the signal before the peak occurs.
This can make it
considerably less obtrusive; it corresponds to the
Attack & Release
settings of the Compressor proper.
- Release: This control allows the Limiter to
continue reducing the signal even after the signal is
below the threshold, again to avoid jarring volume changes.
- Threshold: Controls the threshold at which the
Limiter will cap the signal level.
The Compressor's Side Chain allows you to modify the
control signal for the Compressor (which it compares to the
specified threshold) without modifying the 'real' audio signal
which the Compressor adjusts, based on those comparisons. This
is useful for precise control of the Compressor's behavior, such
making the compressor more or less sensitive to
The Side Chain Listen option plays back the modified
side chain signal instead of "real" input signal; toggle it on
to confirm that your Side Chain EQ is having the desired effect;
toggle it on and off to compare the side chain to the unmodified
The two most common uses of side chain are:
- Preventing bass from "punching out" the rest of a mix.
Compared with our ears, compressors tend to be overly
sensitive to bass frequencies. When compressing a stereo
mix, the bass can cause the compressor to over-react,
resulting in an overly compressed sound, often with
other instruments sounding like they've been "punched
out". The solution to this is to reduce the bass going
through the Side Chain.
- "De-essing" a vocal track. Often, recorded vocals have very
strong "sibilant" sounds (consonants such as "s" and
"th") which can be unpleasant, especially when other
effects such as reverb are added. Generally, it is best
to use proper mike selection and mike technique to avoid
sibilants from the start, but there are many
situations where sibilance was not avoided during
tracking, or where the effects added after tracking
enhanced the sibilance more than desired, so a good
de-esser is always good to have. Sibilant sounds occupy
a frequency range from around 3kHz to 10kHz, and
sometimes even higher. Your first instinct might be to
use EQ to get rid of it, but that tends to dull the
sound of the whole track. Instead, you can use a
compressor with a Side Chain. The
Side Chain should be set to
emphasize the undesired frequencies, so the
compressor will take care of them. In XO Wave, we just
go to the Side Chain tab and use the EQ
settings to turn up the frequencies that are most
objectionable. The Side Chain Listen
feature is helpful for finding those pesky frequencies.
Turning up the frequencies we want to get rid of may
seem counter-intuitive at first, but the compressor does
a much better job at smoothly reducing sibilance than
Many people also wish to use the Compressor's Side Chain for "ducking". In XO Wave,
Envelope Generator effect provides more convenient control over ducking than the Compressor.
The controls available in the Side Chain are the
same as those available in the Basic Butterworth Filter. Please
see our EQ Tutorial for more detail
on this filter.
The XO Wave Compressor is a powerful and versatile tool that can act
as a compressor for individual instruments or a whole stereo
mix, or provide special functions such as a de-essing. By carefully
selecting a preset, you can usually do most of what you need
with very little effort. If the presets don't suit you, XO
Wave offers a wealth of controls for fine-tuning the Compressor
to meet your needs.
-- Bjorn Roche