That being said, a general "follower" plugin which could output control data based on different characteristics of the signal being fed into it -- now, that might indeed be something. That way you could assign a control data stream to any arbitrary parameter in any other plugin, based on the qualities (amplitude, frequency content, etc) of another signal (than the one being effected by the target plugin).
I have an idea for a new plugin to be called Enveloper (or something like that), which sounds like what you may have been talking about. Its controls would include basic ones like threshold, ratio, attack, release, etc., but it would behave similarly to a Live Rack in that you can drop plugins onto it and map that plugins controls to it. For instance:
You drop Live's Reverb device on the Enveloper and map the Reverb's Dry/Wet control to Enveloper. Now the Dry/Wet knob will respond to the track it's on based on the envelope you dial in with Enveloper's Threshold, Ratio, Attack, and Release controls. Basically Enveloper makes any plugin program dependent. And of course, like a Rack, you could map multiple parameters to it as well (like Dry/Wet, Room Size, and LP filters, or whatever you desire). Icing on the cake: SIDE-CHAIN INPUTS.
Imagine this bad boy on a compressor. Not only could you side-chain awesome compressors that don't have side-chains built in, but you could dynamically change threshold, ratio, and release controls! I've run into the situation where I love the borderline extreme compression settings I've dialed in, but can't use them because of a few stray transients that crap out. Would be awesome to be able to dynamically increase the threshold or decrease the ratio for such stick-out/breakthrough transients. This would really allow some deeper, smoother compression...
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