What is mid-side processing? Learn the basics of how to EQ and compress your mid and sides separately, and how small adjustments can improve your overall mix.
Also available for download: M/S Processing for Pros – 10 Advanced Tips.
As even novice audio recordists know, a stereo file consists of two channels: left and right. Signal that is routed simultaneously and at equal level to both channels appears as a ‘phantom’ center image, while signal that is panned unequally to the L/R channels appears to the left or right of center.
What you may not know is that modern digital algorithms are capable of further separating stereo content into a monophonic mid channel (sometimes called a “center” channel) and a stereo sides channel. This means that you can apply equalization, compression or other effects to the sounds in the middle independently of the sounds panned to the sides – a technique called mid-side (MS) processing.
Bet now you’re interested. Here are five basic concepts about MS processing that will get you up to speed:
(Important: What we’re discussing here is not the mic technique of a similar name, where an array of two microphones at right angles – one of which is inverted in phase – is used to record a sound source in stereo while preserving mono compatibility.)
1. It’s not just a mastering tool… Try it in your mix
MS processing has become routine for many engineers during the mastering stage, but it can really come in handy during mixing too. In mastering, it’s usually used to reach into a flawed mix and make necessary changes in balance and width, but you can achieve the same results by applying a plugin that converts a L/R signal to mid-side (such as Waves Center) to the master buss on your mix. Just be sure not to do any MS processing until after you have all the elements balanced across the L/R stereo field. Trying to do so during mixing will make your head spin!
Of course, you can also apply MS processing to individual stereo sources such as sub-busses, stems, synths or loops. Doing so gives you the ability to alter or enhance without affecting the entire mix.
Whether you use it in mastering or mixing, MS processing can change the width and depth of a track in a variety of subtle, effective ways:
2. Start with simple level adjustments
Before you begin getting fancy with MS processing – and there are plenty of ways to do so – start with the simplest usage of all: Play with the relative levels of the mid and sides channels. This simple operation can bring focus and stability to the center, and excitement to the sides.
So if you have a track that sounds narrow overall, insert Waves Center, turn down the center channel a little, then turn up the sides a little. Instant extra width! If you have a track that is wide and needs some overall tightening and focus of the stereo image, try dipping the sides and up the center channel to taste.
Note that when the mid and sides channels are at equal levels, you’ll hear the mix in its original form – exactly as it was before being converted to an MS signal.
In this video example, you can hear how boosting the sides channel and attenuating the center instantly adds width to an already-stereo track:
3. Use equalization on the mid and sides channels
Being able to independently equalize the mid and sides portions of your mix can yield some incredibly cool and useful results. For example:
The following example shows how to give a stereo track extra width and have it stand out from within a dense mix:
4. Use MS for selective compression
The PuigChild Compressor, modeled on the famed Fairchild 670 compressor, offers separate compression for the mid and sides channels, referred to on the original unit as vertical and lateral.
Get things pumping on your effects return by applying a compressor to the vertical, or mid channel only with a fast attack time to squash the transients – a technique that greatly reduces the risk of artifacts on the reverb and room ambience, both typically panned to the outer edges of a mix. If you compress both mid and sides channels on a reverb buss, compressing the mid channel more than the sides will have the effect of making the reverb wider.
Or, on your mix buss, try compressing the sides channel to bring out the softer, textural elements. This will serve to accentuate the ambiance without overly squashing the backbeat – sort of like compressing the room mics on a drumkit. When compressing the lateral, or sides channel, you’ll want to just ‘kiss’ the gain reduction by a dB or two and use fairly slow attack/release times so that the transients (i.e., cymbals, high-hat) get through; otherwise, things will start sounding very unnatural. (Although, on second thought, that might be exactly the effect you’re after!)
5. MS processing + Dynamic EQ = Killer combination
Dynamic equalizers such as the Waves F6 Floating-Band Dynamic EQ are powerful tools that provide precision equalization along with band-specific compression/expansion and sidechain triggers, allowing you to craft EQ curves that move with the music. What’s more, each band of the F6 can process a signal in left/right stereo or in MS mode, enabling you to EQ the center of a mix – the singer, for example – without affecting the color or shape of the overall stereo image.
Suppose some side-panned rhythm guitars are not prominent enough in the mix. An obvious solution would be to try to bring them out a little by adding some midrange EQ. However, if you apply this across the left-right mix, you’ll likely end up thickening in the lead vocal’s range – probably not a good thing. Much better to work with the MS signal instead and add some subtle EQ and compression to selective frequencies in the sides channel only, along with possibly a slight complementary cut to the mid channel.
Or let’s say that some instruments and the lead vocal are competing for the same space in a mix – a pretty common problem. With a dynamic equalizer like the F6, you can carve out a spot for the lead vocal without affecting the stereo width of your mix while at the same time maximizing the blend, or ‘glue,’ between the instruments and the lead vocal.
Here’s the approach taken by mixer Brad Divens (Kanye West, Enrique Iglesias):
Alternatively, you can use the snare or kick as the sidechain source to create momentary frequency “duckings” that allow other instruments (such as bass guitar) to poke through. If you’re working in the EDM genre, you can tame aggressive white-noisey synth sounds in sides channel by ducking their high-frequency content whenever handclaps, cymbal crashes, or other transient sounds panned to the center are heard – a great way to transparently open up the top end.
We hope this helped in understanding the wide and wonderful world of mid-side processing. For a more advanced guide, download M/S Processing for Pros: 10 Advanced Tips.