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Compression is a pretty big feature in music. But it's also on of the most difficult to use. Good compression can really make the music sound punchy and crisp, but bad compression (and let's say it's easier to f*ck up to get a real good result) can ruin things very easily.
A compressor will reduce the dynamics of the sound (the difference between loud and more silent parts). A compressor will reduce the level of the loudest parts (all will become more clear when I'll explain the terms, I hope), that way, you can turn the overall volume up, which in turn will make the overall mix sound louder.
Easy basics, less easy practically.
Let's see what everything means first :
- Gain : Good compressors usually have two gains. One for input (set the input gain so the unprocessed signal averages at 0 dB in the compressor) and one for output, called makeup gain. You see when you reduce the level of the loudest parts, the general level will be reduced too, with the makeup gain you'll "make up for that loss", hence the name. If you only have one gain control, it's most likely it's a makeup gain (input is controlled by the mixer).
- Threshold : this control defines at which level the compressor will start to work. If for example you set it at -10 dB, the device will compress every signal that's higher than -10 dB (so you see, right settings of the input gain are important too).
- Ratio : defines how hard the compression will be. Let's say for example you have a 1:2 ratio. This means that every signal that comes in above the treshold level will be come out halved. So for every 2 dB that comes in (above threshold), you'll get an increase of only 1 dB. If you have ratio's of 1:10 or more, we'll talk about limiting, because the output increase will be almost neglectable.
- attack : this defines how fast the compressor kicks in. Sometimes you'll want the compressor to start working pretty slowly (strings, etc...), sometimes you'll want it to start compressing very fast (drums/snares...).
In fact you can think of a compressor as a guy sitting behind a mixer. When loud parts are coming, he'll decrease the fader, and after a while he'll increase it again. Well, how fast he'll respond to decrease the signal, that's the attack. How fast he brings back the level to the nominal level, that's release.
- release : see above.
- predelay : don't really know what it has to do with compression in itself, predelay is used in reverbs mainly.
How to use your compression, that depends on what you wanna have as result. Do you want your mix to be louder overall, or to sound punchier? A different kind of compression is needed for both options...
- Punchier : always start with the Threshold on maximum. This bypasses the comp. A good starting ratio is 1:2 or 1:3. Attack and Release on medium. Now gradually decrease the Threshold until you notice the sound gets punchier (you'll see you'll have to increase the output to keep the level high enough). When it's good enough for you, try tweaking the attack and release settings. This depends on the music played, and your own preference. Just listen carefully until you get the result you want (it's possible that if you tweak one setting, you'll need to reajust another one to keep it good sounding). Just avoid that you hear the sound audibly pumping. Good compression is often the one you can't really hear.
- Louder : again start with Threshold on max and ratio on a pretty high setting (ranging to 1:5, up to 1:20 => hard limiting). Attack and release on fast. Now turn down the Treshold gradually.
This method will completely "stop" the loudest parts (as if they were smashing into a brickwall, nothing comes louder), and is much used to protect speakers or prevent the signal to exceed a certain level.
So you turn down the Threshold (and this one you do best on the loudest parts of your music) and you'll hear that the peaks will be cut down. At a certain point this will begin to sound unpleasant. When you get at that point, back of a little until it sounds good again. You'll have found a pretty good balance.
Now you can turn up the output gain to get your level back up to nominal. You'll hear now that the overall mix will sound louder than before.
You may reall hear the compressor "working", just tweak the release until that effect gets minimised (again I repeat, good compression is the one you don't notice).
This are pretty basic settings, and are the ones you should start of with (they have proven to have give good results). Start experimenting from there until you achieve the sound you want.
If you need extensive info about compression (also loads of other audio info : http://www.studiocovers.com/articles3.htm
Hope this helped
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