This has been copied over and revised from this post:
"Help with sidechain compression?"
viewtopic.php?f=16&t=2470
**EXPLANATION OF COMPRESSOR SETTINGS:**
--Slope--
This controls the amount the compressor reduces the volume when activated (the threshold is the point where the compressor is activated).
In SunVox it is measured by a percentage, but in other DAWs it is called "Ratio". They have the same effect, it's just that the measurement is different (Example: 50% vs 2:1 ratio)
For example:
I have a kick and a synth going to a compressor. I set the sidechain input to the kick module, so when the kick's volume goes over the threshold, the sidechain compressor activates and decreases the volume of the synth.
Now let's get into some numbers.
Suppose the slope is set to 50%. If my kick goes 2 decibels over the threshold, the compressor will reduce the volume of the synth by 1 decibel (2*.5=1). If my kick goes 4 decibels over the threshold, the compressor will reduce the volume of the synth by 2 decibels (4*.5=2). If I want less volume reduction, I reduce the slope. So now let's set that slope to 25%. If my kick goes 4 decibels over the threshold, the compressor will reduce the volume of the synth by 1 decibel (4*.25=1).
--Attack--
This value controls how long the compressor takes to reduce the audio when activated. Note that (when activated) the compressor will IMMEDIATELY start to reduce the volume. It's the attack that tells it how long to take to get to the full reduction.
Suppose I have a kick that lasts 32 milliseconds and exceeds the threshold by 4 decibels. My slope is set to 50%, and the attack time is 10 milliseconds. The volume of the synth connected to the compressor will be reduced by 2 decibels (4*.5=2) over 10 milliseconds. The biggest change in volume will be at the end of the attack.
Here is a more visual example of the effect of the compressor on the synth:
--- = Volume of synth
Threshold = 2 db
Assuming the synth's volume starts at 8 decibels:
A-------- <==(kicker exceeds the 2 db threshold by 4 db, sidechain compressor is activated and begins reducing the volume of the synth)
B------ <==(the synth's audio is reduced by 2 db 10 milliseconds after being activated at A. The volume of the synth is now 6 db)
Note that the length of the kick doesn't matter (when changing the volume), its volume just needs to exceed the threshold to activate the compressor (you could have a kick that is 1 millisecond long and it'll still activate it). To sustain the compression, your kick will have to stay over the threshold for a longer period of time.
--Release--
This controls how long the compressor takes to return the audio to its original volume after the attack (it's essentially the same as attack, but backwards).
Using the example above (from the attack portion of this post), let's set the release to 45 milliseconds. After the sidechain input audio stops being over the threshold, the release returns the synth to its original volume over 45 milliseconds.
VISUAL EXAMPLE:
--- = Volume of synth
Threshold = 2 db
Assuming the synth starts at a volume of 8 decibels:
A-------- <==(kicker exceeds the 2 db threshold by 4 db, sidechain compressor is activated and begins reducing the volume of the synth)
B------ <==(the synth's audio is reduced by 2 db 10 milliseconds after being activated at A. The volume of the synth is now 6 db)
C-------<==(the kick's volume has decreased and is no longer over the compressor's threshold, which triggers the release)
D-------- <==(45 milliseconds AFTER C, the audio is back to its original volume)
What your attack/release/slope values are depends on what you want. If you want a hard, pumping sidechain compressor, try an attack of 10 and a release of 45, then adjust the slope until it really pushes the synth down (just before it hits the bottom of the waveform box). This will reduce the volume over 10 milliseconds, then return it to its original state 45 milliseconds after the sidechain input audio stops being over the threshold.
If you want the kick to be louder, but don't want it to affect the compressor, you can link it to a distortion module (which is linked to out) and increase the volume (of the distortion module). The sidechain just uses the input you give it. The sidechain compressor functions the same as a regular compressor, the only difference is that the sidechain asks for an input instead of affecting
everything linked to it.
**TROUBLESHOOTING:**
The first thing you should check is if there is any audio being played. If there is no audio...well...there's no audio.
Assuming you're using a kick as the input, your linkage should look like this:
kick=>compressor (set kick module to input)
kick=>out
synths=>compressor
compressor=>out
Note that you aren't forced to use a kick as the input. You can use any audio for it. Below I've made an example using a generator as the input instead.
If you can't hear the kick or the synths, check to make sure everything is correctly linked. If you're still having a problem after reconnecting everything, it could also be that your kick is too quiet, your slope is too low, or your threshold is too high. If you're using the kick module with its default volume, you probably need to either increase its volume or increase the slope of the compressor (either one will affect the change in volume from the sidechain. You can do both too). You can link the kick to a distortion module, link the distortion module to the compressor, unlink the kick (the module itself) from the compressor, then increase the distortion module's volume and set it as the input instead.
So now your linkage should look like this:
kick=>distortion=>compressor
kick=>out (or you can link the same distortion module to out if you need a volume increase on the kick's out volume [kick=>distortion=>out])
synths=>compressor
compressor=>out
Depending on what your attack/release times are, you might have to adjust those as well (if the attack is too long it can be hard to hear/see a change, since it controls how long it takes to reach full compression).
If you're
still having problems after that, ask for help. Many of us on the forums have been using SunVox for years and would be more than happy to help you out. If we can't answer your question, ask NightRadio (it might be a bug).
**QUICK SUMMARY:**
-Threshold is the volume where the compressor is activated. The length of the input audio doesn't matter, it just has to exceed the threshold.
-The amount of volume above the threshold is what determines the effect of the slope (this is why hard limiters are handy, they
limit the volume instead of reducing it temporarily)
-The attack controls how long it takes to get to full volume reduction (after activation)
-The release controls how long it takes to get back to original volume (after the sidechain input audio stops being over the threshold)
-Reconnect everything to ensure the modules are linked correctly
-Check the sidechain input's volume, the sidechain's slope, or its threshold
-Check attack/release times
-If all else fails, ask Nightradio (and if he's not available, myself and the older members can help you)
