This is an attempt to demystify audio compressors.
They are useful both in the music studio and when performing live.
It's all about reducing & smoothing the audio peaks and troughs.
I have a few different types, ranging from small modules to rack units.
They all however have similar controls.
There are also quite a few eurorack compressors.
My favourites are the WMD MSCL and the L-1micro compressor
You can actually build a compressor with a envelope follower, a an inverter and a VCA. A modular compressor is essentially a voltage controlled envelope follower tied to a VCA.
+ Modular Compressor -using eurorack modules
The MSCL is unusual in that it doesn't have the usual controls.It's inspired by the classic
DBX "over easy"style compressor . The threshold, gain & output are all controlled with one
knob and two switches.
This is an excellent stereo compressor for a small modular system. It packs in a lot for just 4HP
The Limit Switch limits the make-up gain allowing the MSCL to act like a peak limiter.
The Threshold Switch helps compress signals of different amplitudes. Both modular & line level.
Both switches have 3 settings:left (off), middle(half on),right (full on).
The side chain input helps create your ducking sounds.To turn on the compressor, I like to turn the LIMIT to the left &
THLD (Threshold) slide switch to the middle to start with.
The Limit is your final volume.
The Threshold is the volume the audio has to reach before compression begins.
I usually end up with both switches in the middle or maybe one of them
to the right (usually the threshold).
Often I'm mixing Line Level audio with Modular audio and these have
huge volume differences.
The lower toggle switch needs to be in the centre position.
I usually set the COMP knob to around 75% and GAIN to 50%
As you turn up the COMP knob you will see the LED change from red to green.
Red is your gain reduction. Green is gain addition.
(The GAIN knob is your Makeup... so use it at the end of the audio chain).
Play around with the envelope. Turning up the attack will let the
beginning of your kick drum through. These envelop knobs are a great
performance feature
The bottom 3 throw toggle switch has side chain, in, out.
IN = wet compressed signal
OUT = dry uncompressed signal.
Switching between these two is a great way to compare.
As WMD say in their video .. "don't be afraid to use your ears"
Just listen to how each sounds and make your adjustments accordingly.
Threshold
This sets the volume the audio has to reach before compression begins.
It acts like a gate.
Adjusting this control sets the level at which the gate will open and allow the signal
at the input to pass through to the output.
Ratio
This is the amount the audio is reduced (over the threshold).
It's as the name suggests a ratio.
When an input signal is above the THRESHOLD reference level, the setting of this control determines the number of decibels (dB) by which the input signal must change in level to produce a 1dB increase in the signal level at the output
Common ratios you will see are:
1:1
1.3:1
2:1
3:1
4:1
Infinity :1
---------------
1:1 is no compression
Infinity :1 , Here the compressor acts as a limiter as audio will never exceed the threshold.
3:1 here the audio above the threshold is compressed a third.
So if the sound peaks 15dB over the threshold, it will be attenuated to 5dB over the threshold.
I tend to set the ratio between 3:1 to 10:1
Above this, it tends to clip the audio.
Attack
The ATTACK control is an envelope. Just like a ADSR. It sets the amount of time it takes the module to begin compressing a signal once the threshold is breached. The ATTACK range is from FAST (for a tighter and more noticeable compression effect) to SLOW (for more delayed, gradual compression).
Release
The RELEASE control sets how fast the compression circuit returns the input to its original level.
Again this is very similar to the ADSR envelopes you are used to with modular synths. The RELEASE rate is from FAST (where the compression slope is steep) to SLOW (for very smooth, gradual compression).
There is no absolute right way to set the ATTACK and RELEASE controls.
MakeUp / Output Gain
The OUT-PUT GAIN control is also known as MAKE-UP.
It does not interact with the threshold of compression.
Use it to compensate or Makeup for any drops in the final volume which may have occurred during compression
Knee
While discussing the MSCL compressor above, I mentioned the DBX "over easy" style.
This is where Hard vs Soft knees curves come into the discussion.
"Over easy" seems to be dbx's way to describe a "soft knee".
A soft knee will yield a generally "softer" more subtle compression
effect.
It is, in effect, a variable threshold since there is no distinct point at which processing begins.
The compression ratio will start climbing gradually before you approach the
threshold as opposed to suddenly "kicking in" all at once at the threshold.
Some compressors have an adjustable knee. I haven't seen many of these in the eurorack format
Multi-Band Compression.
There aren't many of these in Eurorack.
Here, the sound from your modulars is split into multiple frequency bands using modules like
EQs. Three bands is the
most common number.
Each frequency band is then compressed
separately after which the audio streams are
mixed back together.
I particularly like the Endorphin.es golden master.
It's a 3 band equaliser with a compressor for each band.
Just 6HP
What it lacks in compressor controls it makes up for in size and ease of use.
Sounds great too.
2HP also make a compressor.
It might be fun to have a few of these chained to an EQ to make a multi-band compressor with lots of control
Peak vs. RMS
Sidechain
This effect was first developed in radio studios. The background audio could
be dropped automatically when the announcer spoke.
Thus, the volume level on one instrument is controlled by the volume
level of another instrument. A common example would be making the
compression level on a bass controlled by the output volume of the
kick drum. This is called Ducking.
I really like the endorphin.es Cockpit.
Though it's not marketed as a compressor, it's ducking is great for getting
that pumping sound.
It's really a mixer with a VCA.
Feedback (feed forward) compression.
Here, the input signal is split into two. One goes into the compressor itself.
The other goes into the sidechain. The sidechain is then fed by the output of the compressor.
This type of compressor was made famous by the Universal Audio 1176.
LIMITING
This is where you set the max volume your signal will reach.
It provides overload protection.
The Audio Damage ADM10 Kompressor is DSP based.
Just 6HP. Nice
Other eurorack compressor modules
+ Autodyne - SSF
+ Dynamo - Bastl
+ Mutant Hot Glue - Hexinverter
+ Mutable Instruments Streams
Links
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