Aside from panning left and right, there’s another way we can separate stereo signals for processing that’s frequently overlooked or not understood by budding amateurs - the middle and sides. Middle is essentially the mono part of a stereo signal - the frequencies that come out of both left and right. Sides are the bits varying stereo information - the sounds that populate left and right fields only. Separating a track into middle/sides can be both useful for mixing and as a creative tool with a little imagination.
A common use for mid/side processing is to EQ a stereo bass line - knocking out the low-end frequencies in the sides to maintain a tight mono feel in the low-end, but retaining some harmonically rich high-frequency ‘fizz’ in the sides.
It’s very simple to hear this mixing technique in effect if we grab a professionally mixed commercial track and jump into your DAW of choice (which for me is Ableton Live). Load up your chosen track and drop an EQ8 on the master. Change the EQ8’s mode to M/S (middle/side) then under edit click to change between M and S. In M mode turn on the first filter, change it to lowpass. Slowly turn that filter’s frequency up to maximum and listen to the mono sound disappear. You’ll notice that due to the convention of mixing bass frequencies in mono, the low-end of kickdrums and bass instruments disappear very quickly. However quite often there’ll be higher harmonic frequencies of those parts left in the sides - especially in modern electronic music. Using this technique on tracks with mixing and production you’re fond of is also a great educational insight into the producer’s mixing techniques.
As well as basic M/S EQing for cleaning up your mix, we can also split our signal into two chains in Ableton Live for a mid/side setup that allows for effects and other processing. The quickest way to do this is by dropping in a Utility module, grouping it with itself to create a chain in the audio effect rack, and duplicating that chain so you now have two chains with just Utility on it. Now, set one chain’s Utility to width: 0% and the other to width: 200% - creating a middle chain and a sides chain. It’s best to name each chain here too, before it gets confusing. The resulting summed signal should be exactly what your signal started as, but now we have the ability to apply effects to the sides and middle separately. For example, delays and reverbs work well on the side chain as they’re not muddying up your middle low-end, whereas compression on your mid chain can help to tighten things up further and give your track a sense of focus. It should be noted however, that if you put stereo effects on your mid chain, they’ll no longer be mono. As with basically everything in Ableton Live, remember to experiment with automation, particularly as stereo effects are a very easy way to mess with people’s heads.