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When A/Bing different models of compressor (usually DAW plugins since that's what I generally have access to), I can tell that different compressors with similar settings have a somewhat different sound, but I have a difficult time qualifying this beyond vague terms like "creamier" or "harsher".

Are there more precise terms than this? What properties might I be looking for when evaluating compressors?

For example, Ableton Live includes a Compressor device which models three kinds of compressors. They have the same settings, but each of the three models clearly sounds different. What might be changing?

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As some of you know, this field can get a bit subjective. I'll try to stay as focused as possible. Without going into many details, you should be looking into 2 things.

  • the reaction of the compressor
  • the colour that that reaction might add.

By this i mean (and we'll have to look at analog gear for a second here):
There are many types of compressors; namely, the mechanism that allows them to react to the change in level above the threshold. Let's try and make this simple:

  • Opto compressors (optical compressors) use a light emitting diode (LED) and a photocell. These are, by design, slower and less "abrupt".
  • FET. Using transistors, these guys can have a high impedance input design. You have your famous 1176 on this category. IMO, these guys can be extremely brutal.
  • VCA use a voltage controlled amplifier. For this reason, they are usually really clinical, precise.

Ok, now.. all of these can have different behaviours. You can probably begin to see a pattern here if you happened to play with some of these before. If not, Logic has an emulation of these and other designs on their integrated compressor. I believe that opto compressors can be easily "disguised". Maybe the "creamier" you used? some coloration but nothing much. They can get a bit out of sorts when pushed harder. They won't be very exact either. VCA are usually really clinical like i said (maybe my choice for vocals? Think SSL G series bus compressor or dynamics). When pushed hard they are... interesting (then again, I listen to a lot of heavy compressed stuff :P)

FET bring, from my experience, loads of colouration to the signal. The design itself brings some limitations (the 1176 was 600ohms input - which will bring some coloration right at the input stage for impedance mis-matching) and they are a bit brutal. Strangely enough, they seem to do a really good job when pushed harder but it can vary on the models a lot.

On a digital domain: Well - a lot of manufacturers are going to great lengths to emulate some of the precious compressors that became legendary (fairchilds, old neves, UREI's, old DBXs etc).

Hmm i guess my answer is getting a bit convoluted better see some feedback or if other questions arise and we'll take it from there.

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Thanks for the info! When you say "disguised" do you mean that it's somewhat difficult for people to hear that compression is going on, even though the dynamic range is reducing? –  Warrior Bob Jan 23 '11 at 3:26
    
you could probably get away with a bit more on optical based compressors than you can with VCA or FET. It is definitely "lazier" and not in the attack/release setting way - it's own reaction is slower - but it won't help you in more extreme settings (imo). –  jlebre Jan 23 '11 at 11:18
    
since it's reaction is slower, it can help you avoiding pumping and breathing or that awful "crushing" artifact. –  jlebre Jan 23 '11 at 11:19

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