M-S or Mid-Side processing melted my brain for a while!
I have to admit that the whole concept of M-S processing kind of melted my brain at first. It is almost embarrassing how long I was a full time pro before I started to wrap my head around it. It is pretty whacky… but a godsend when you need it.
The basic concept of M-S or mid-side processing is that it is possible to treat the center information (the stuff in the middle of the mix) differently from the sides (the stuff out on the left and right). I do not think I will ever be smart enough to know what goes on under the hood of an M-S processor, but M-S is becoming an increasingly important part of my work, especially now that mastering has become such a big part of my business.
Our standard stereo EQ or compressor can usually adjust the left and right sides independently, but all of those changes have a big impact on the center information. M-S can adjust the sides separate from the center, but you cannot adjust left and right independently within the same processor. M-S can be used to make things artificially wide (or less wide), but I am rarely interested in that. I mostly care about M-S because it lets me do EQ and compression like a ninja! You might ask why one would need M-S processing to do EQ and compression, and the answer is that most of the time we do not actually need it. The overwhelming majority of processing we usually need in a mix or master can be done with the same traditional hardware and plugin tools that we have been using for years. Most of the time standard left-right stereo compression and EQ is all we need to sculpt a track just the way we want. Where M-S becomes so valuable is when we are dealing with stereo sources that have problems (or creative goals) that are specific to elements that are in the middle of the of the stereo image OR out on the side. If the problem is across the whole stereo field we are best off using our traditional tools. But when needed, M-S kicks ass (and the good news is that if you find a plugin you love that does M-S, it will almost certainly do standard left-right processing as well). Let me give you some examples of how I have been using M-S lately.
A few weeks ago I was mastering a great album for an artist from Northern California. The mixes were great, but there were some problems with sibilance on the lead vocal (that harsh, spiky ‘ess’ sound that can jump out in vocal recordings). The best way to deal with that is on the individual track when mixing, but this mix was done and there was still too much sibilance when it came to me to master. So I applied a de-esser across the whole track. This helped with the vocal problem, but a de-esser in this situation is going to compress the whole mix every time there is a big ‘ess” in the vocal, and when it fixed the lead vocal, the guitars that were panned out left and right lost some presence. M-S to the rescue!! I was able to use an M-S de-esser and only processed the center channel. That way the de-esser did not affect the guitars and cymbals that were panned out to the sides. This allowed me to be far more aggressive with the de-easing than I could have normally been with a standard stereo de-esser. Sure, the snare drum was affected a little bit, but since the guitars still stayed big and exciting the artifacts of my de-essing were not that noticeable.
Another project I was mixing recently was a hard rock album where the drummer really hit the snare hard. The stereo overheads sounded great but there was too much snare in them and I wanted to get most of my drum sound from the overheads. Compression is the usual remedy for that and often works great. It worked well for this, but I found that when the drummer was playing the ride cymbal it was starting to pump a little too much with the amount of gain reduction I was doing to tame the snare. So I switched over to M-S compression and just compressed the center channel. This allowed me to compress the snare drum with less impact on the ride cymbal. (the ride cymbal was not hard right so it was still affected a bit, but much less dramatically).
Lastly, one trick I have been doing in mastering lately is using M-S EQ to do a little boost in the high end of some rock and pop records. This allows me to bring the high end presence of the guitars and cymbals a little bit forward without running the risk of making the vocal too harsh or sibilant. Pushing up the high end of the sides can make the mix sound a little bit wider without really changing the balance of the overall mix and without some of the negative effects of those phasey wider-izer type processors.
If you want to start to wrap your head around M-S processing there is a cool tool that can help you understand it a little better: bx_solo from Brainworx. It allows you listen to different aspects of a stereo signal (center, sides, left right, etc.) and you can even play around by adjusting mid and side balance to do some interesting widening tricks without major downsides. The best part is that it is free. Download it here.
So if you are looking to expand your palette of options, M-S or mid-side is a great thing to explore. If, in the end, M-S still melts your brain, do not worry about it. Lots of amazing records are made every day without it.