Further Stereo Adventures with Ableton's Utility
Although EQ Eight has great built-in Mid/Side functionality, we can isolate sum and difference signals for processing with any plugin. Start by opening Ableton's 'Utility' on any track you wish to process. Now right-click on Utility's header and select 'Group'. This will put the Utility device inside an Audio Effect Rack. Click the Show/Hide Chain button to show the racks chains, and then duplicate the existing chain by right clicking on its name and selecting Duplicate (or CTRL/CMD+D). Name one chain 'Mid' and the other chain 'Sides'. Now select the Mid chain and set Utility's width to 0%, then set the same parameter on the 'Sides' chain to 200%.
What you have now are two chains working in parallel, one providing the 'sum' and one providing the 'difference'. Note that this is not M/S as such, as both are still stereo chains - as before, what you have are the Mid and Side channels desperately decoded to L/R stereo, which means that the Side chain still contains the same signal in the left and right channels, but in opposite polarity.
Before we go any further, save your new stereo processing rack to your Ableton Library by clicking the save icon in the audio effect rack header. Now you'll be able to quickly apply the rack to any audio track.
At this point, no real processing has been added, so if you bypass the whole audio effect rack, you won't hear a difference. But if we add a stereo plugin such as Ableton's Reverb on the Sides chain alone, we can produce an extremely wide effect, adding space and width without the Mids being muddied up with reverb.
Try experimenting with just about any effect on the Sides signal, such as delays, phasers, even glitchy processors such as Beat Repeat. The effects of processing the Side signal can be very subtle, so it's often best to work on buses or groups of tracks. Try grouping your drums and applying EQ only to the Side signal, or group all your synth tracks together and apply some delay to the Sides signal to add depth without clouding the main content.
All in all, Mid/Side processing can change the width and depth of your mixes in subtle and effective ways, many of which would otherwise be impossible. Once mastered, these techniques could start appearing in all your mixes. Go forth and widen!