Quote Originally Posted by rounser
sidechaining is when the compressor works indipendantly to the sound its compressing?
Not exactly, from what I understand...it's more a way of using the compressor's "smarts" to trigger some other effect as well. Compressors are "smart" because they listen to the incoming audio and choose when they should do their thing, based on the threshold setting. A sidechain lets you send that "effect ON" command that the compressor sends to itself to another effect as well.

So you can be compressing one track quite happily, but rigging up a sidechain from that compressor to another effect lets you say "oh, yeah, and whenever that compressor gets triggered, I also want you to trigger a ducker effect on the bass on this other track." So your compressor is also controlling other effects, and the sidechain makes sure that the other effect gets triggered when the compressor does.

In theory, you could hook a compressor sidechain up to reverb (or whatever), such that something gets reverb whenever compression kicks in, but there wouldn't be much point to that...a ducker, gate, or another compressor seem to be the most useful things to sidechain to (I've heard of sidechaining a compressor up to a gate on the high hats, for instance).
i've never heard of a sidechain that works as you describe.

in hardware compressors, it works like this:

signal comes in, and is split into two channels. one goes to the sidechain circuit, and one goes to the compression circuit. in the sidechain circuit there is a level detector of some kind. the detector is controlled by the attack, release and theshold nobs. the output of the detector in the sidechain circuit runs to an amplifier in the compression circuit, and the amplitude of the audio coming through the compression circuit is made to go up and down depending on what the detector is doing in the sidechain.

with an external sidechain, there is an insert point in the sidechain circuit, after the signal is split, but before the level detector. on a stereo cable, audio will be sent out on one channel, and returned on the other. the audio being sent will be exactly what is coming into the compressor, with no compression applied. an eq is typically put in this insert loop, which allows you to shape the signal before getting to the detector. you could do things like cut out the highs so that a stereo bus compressor will pump with the bass, even though the hats are ringing loud.

but the more common use is to feed a totally different signal on a mono cable into the return leg of the insert. this in effect replaces the original signal with a completely different one in the sidechain circuit, before the detector, so that the compressor is triggered by the second sound.

maybe some software compressors work differently, but this is how the hardware compressors i've used have all worked. it would be cool if they put out a gate signal like you describe though. i could definitely see that coming in handy.