The freeware plugin Voxengo MSED is a great way to add audio processing on just to the the sides of a stereo signal, or compress the middle, while leaving the sides untouched.
In the tradition of nearly all freeware, Voxengo MSED is missing a bit of documentation and getting it setup in any meaningful way is a bit tricky. So I’ve gone ahead and made a Track Template for use in the excellent DAW…. Reaper.
So go ahead and download the Reaper Track Template MidSide Stereo with Voxengo MSED. (to use, right-click in Reapers track pane on the left and select ‘Open template’)
If you are thinking “Mid-side! Whazz Dat?”, then Emusician has a great article on a variety of mid-side techniques.
UPDATE – Since writing this post I’ve realised that Reaper itself can easily be setup to do mid-side encoding and decoding. In fact I found a Track Template for Mid-side encoding and decoding, simply by searching on the Reaper forums.