Well, hopefully Part 1 was simple enough at explaining how compressors basically work.  Now comes the techy-stuff.  The Basic Control covered here are the ones you’re most likely to encounter on any compressor.  Some have fewer and some have more controls than these, so I’ll just cover the ones that are fundamental to understand how to start controlling your dynamics.

1) Threshold – The threshold is the most basic of compressor controls.  This the “listening volume” that you setup for your compressor to look for.  If audio get’s louder than this volume level, then the compressor knows to start working.

2) Ratio, Gain Reduction, etc. – The Ratio usually looks like 2 : 1 (spoken “two to one”) or 4 : 1 up to 100 : 1 or infinity : 1.  These ratios refer to how much to turn your audio down when it passes the threshold.  Basically if a snare hit is 4 dB above the threshold and you set the ratio at 2:1, then the compressor will turn it down by 2dB (or divide the amount that passes the threshold by 2).  If the Ratio was 4:1, then the compressor will divide that 4 dB by 4 leaving you with 1dB above the threshold.

An important thing to note here is that the compressor doesn’t say “this is the threshold and audio will NEVER pass it.”  Rather it turns things down by dividing them if the audio passes the threshold.  In both examples above, the snare hit is still louder than the threshold, just by less. What this means practically speaking, is that if you can still clip a track with a compressor on it because the threshold isn’t like a “safety line” destroying all audio that passes it (that’s a limiter/compressor with infinity:1 ratio).

3) Attack – The Attack acts similarly to the attack controls on a synth’s ADSR envelope – it’s a measure of time that determines how quickly a unit will start to act.  On a compressor that means the amount of time it takes a compressor to “hear” audio go past the threshold and turn down the volume to the full amount of attenuation.

The trick to understanding this is that compressors aren’t perfect units who instantly grab audio and turn it down.  They’re more like a hand on the volume knob.  You can’t skip straight from 50 dB down to 0dB by turning the knob.  You have to pass through 49-1 dB first on the way to 0dB.  Compressors have the same limitation, although some of them can work exceptionally fast – there’s still a certain amount of time necessary for turning down the volume.

4) Release - The Release control is a bit like the opposite of the Attack.  It is the amount of time it takes the compressor to turn the volume back to “normal” after the audio has gone back down below the threshold.

The Attack and Release controls are pretty important controls in that if you make them too quick you might hear some clicking and clipping, but if you make them too slow you can hear funny “sucking” or “pumping” sounds.  I know that doesn’t adequately explain how to use the controls but it really comes down to listening to the track that they’re on.  If you’re hearing weird things in you A/B the compressor on and off, then perhaps a tweak of the Attack and Release will make it sound more natural.

5) Makeup Gain - This is an “output volume” control desgined to bring the whole track up in volume after it’s loudest parts have been compressed.  The basic idea being that if the loudest peaks are now quieter, you can bring everything up a bit louder without those peaks clipping the track.  I’d say use this sparingly, especially if the compressor is one of many plugins on your track.  There’a bunch of math/programming behind why, but it basically puts your audio through unnecessary processing because your fader will do the same thing.  If using your fader, you can’t get the track where it should be, then I’d go to the makeup gain and use that to help the fader.

6) Auto-Gain – Crap.

Sorry, that’s just my opinion.  But Autogain is basically an “automatic makeup gain.”  As you compressor your loudest parts, the auto-gain turns up the whole track a corresponding amount.  The basic idea is to turn up the track after it’s been compressed, but using the amount of reduction applied in the compression phase as automation points for the makeup gain.  If that is kind of confusing, I’d suggest you just let it go and not worry about it.  Instead try turning it on and seeing if you like it.  Then turn it off but DON”T compare that to the auto gain.  It’s quieter and therefore your brain will tell you it’s worse.  instead, turn up the makeup again and compare THAT to the auto-gain setting.

Then just use what sounds better to you.

7) Knee - The knee control of your compressor basically blurs the threshold a bit, allowing the compressor to apply some compression BEFORE the audio hits the threshold.  It doesn’t work EXACTLY like this, but as an example it will help explain what it’s doing.

If you set your threshold at -10 dB and the ratio at 4:1, then when the audio is at -6 dB (4 dB above the threshold) the compressor will output at -9dB (just 1dB above the threshold).

If the audio coming is -12dB (2dB below the threshold) the compressor will do nothing because the audio is below the threshold and the knee is set to a “hard knee” setting which means it will act as usual.

Now turn your knee control to a soft knee setting on the imaginary compressor.  Now if the audio coming is is -12dB (2dB below the threshold) the compressor will apply PART of the compression to the track.  The compressor would see the audio that is 2dB below the threshold as approaching the threshold, so it would turn the audio down by say 1dB and output at -13dB.  That’s just an example and not PRECISELY what it would do.  But basically as the knee gets “softer” the range over which the compressor will apply “some” compression expands.

I like to use the knee when I’m using a compressor to add some character and pop to a track – setting it so that the compressor “rides” the track a bit, always applying a tiny bit of compression.

Well, those are the basic controls of a compressor, although there’s lots more to be done with them.  Also lots of compressors combine the controls into weirdly named things like Yamaha’s lower-end compressors on mixers (which sound surprisingly good).  Those compressors have just one control called ‘compression” which simultaneously affects threshold, attack release and ratio, etc.

Stay tuned for Part 3 of Intro to Compressors which will feature more interesting uses and techniques, and come of the weirder controls.

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