[Good.Times.Studio.Lincoln]

I understand the basic gist of what a compressor does... It makes soft parts louder and somewhat limits louder parts so that the sound is more balanced.  I also use Logic 9's compressors frequently.  With all that being said, I'd love to learn the technical side of what's ACTUALLY happening inside of the compressor. Can someone please explain to me HOW they work and what the different knobs and sliders actually do on a technical level?

[Ramshackles]

Thats actually a very broad question, with the most fancy compressors capable of many operations.
At its most basic though, a compressor might have 2 controls - input and output volume. For it to act as a compressor, you would basically set the input volume to higher than the output. Any part of the waveform going in that exceeds the waveform going out (and depending on the difference between input and output this will vary) will be squashed back down to meet the output level. Basically, it reduces the dynamic range of a track.

A more modern one would consist of a threshold control - at what level (in dB usually) does the compression kick in? And then a ratio - after the waveform has exceeded this level, by how much should it be squashed down? There is also usually something called 'make-up' gain. This is used because reducing the maximum peaks in a track reduces the apparent overall volume, so you will want to push it back up. So the actual 'compression' doesn't make the quieter parts louder, it just squashes the peaks. Its the make-up gain that then brings the WHOLE thing back up - but because those peaks have already been reduced, it seems like just the quieter parts are getting loud.

Other controls a compressor might have could be attack (how fast does the compression kick in?) and release (after the compression has kicked in, how long is it on for?).

Even more complicated are multiband compressors, which allow to apply compression over specific frequency bands.

[Good.Times.Studio.Lincoln]

Freakin' sweet!  Thanks so much RamShackles!  That makes a TON of sense!!!  :-D 


I'm using a totally digital Compressor (all on screen) so I'm used to seeing the Threshold control, Makeup gain, attack, release, etc.

So as I understand: the Threshold control works as a "gate" as it won't kick in until a particular dB level is reached.
 
As far as having total control over the "louds" and the max peaks in the song you can use the Make-up Gain to bring those down which in the end makes it seems the quiets have gotten louder when in reality the louds have gotten softer.


Did I write/understand that correctly? 

[Ramshackles]

yeah, the compression only kicks in when the threshold is exceeded, so in that sense it is kind of a gate.
The make up gain doesnt bring the max peaks down, thats the job of the compression (the amount of compression being controlled by the ratio). The make-up gain is simply a volume knob, I'll try and illustrate what goes on:

Imagine you have a track that is 15db for 5 secs and 25db for 5 secs. Your threshold is at 20db and you have an reasonably large ratio giving some pretty heavy compression. This means that for the first 5 secs, the track is the same, but for the next 5 secs it is compressed. This reduces the overall apparent volume of the track it's still 15db for 5 secs, but because there is quite a lot of compression on the next 5 secs, it is something closer to 20db. The make-up gain is there to push the volume back up, but it affects the whole track. So pushing it back up so that the last 5 secs is back at 25db will also push up the first 5 secs by the same amount (to 20db).

Now, before you say that 'hey, why dont you just automate the first 5 secs so it is louder', remember that the compression has shaved off the peaks of the loud parts and in doing so has changed the sonic character of the track.

In that respect, compression can be seen as having 2 functions - it can reduce the dynamic range of the track (like you said - make the quieters seem louder by reducing the loud parts and increasing the volume on the whole thing), but it can also be used subtly to change the sonic character of a track. It takes some playing round to get used to that, and really I am still a novice when it comes to that. For the most obvious effect, putting compression on a kick drum can make it 'pound' more or give it more 'punch' or give a snare more 'snap'. It can tighten up guitar tracks and make bass warmer or more pounding again.
It requires a lot of experimenting with primiarily the ratio and attack and release times, and to a lesser extent the threshold and make-up

[Good.Times.Studio.Lincoln]

Shweet!  Thanks so much for you're input!  :-D