The Benefits of M/S Processing

Read Time: 5 min

Audio engineers like to think in terms of warmer, punchier, and brighter. They can also think in terms of Mid and Side. By working in Mid and Side instead of left and right, a whole new world of options becomes available.

What is M/S?

For those unfamiliar with Mid/Side (M/S) processing, the idea is to convert your left and right stereo channels into a Mid and Side. What you are left with is one channel containing any information that could be considered the "center" of your stereo field, and the outside edges in another channel.

But how exactly does it work?

How M/S Works

The basic idea of M/S encoding from a mathematical standpoint is very simple.

• Mid = Right + Left
• Side = Right - Left

And to go back to decode back to stereo...

• Left = Mid + Side
• Right = Mid - Side

Unfortunately setting up math inside a DAW is not so simple, but it is far from impossible! Thankfully today we have convenient M/S encoders to turn our stereo files into M/S. This was not always the case.

For those of you looking to do it yourself, here are the steps...

M/S Encoding Without a Plugin

Here's how to encode a stereo file as M/S:

• First load your stereo file onto a stereo track.
• Next create two submix tracks. One submix will be your Mid and the other Side. Make sure these tracks have dual pan knobs (one for each channel).
• Send your stereo file to both submix tracks.
• In both the Mid and Side submixes, pan both pan knobs to center.
• On the Side submix, add an effect that can phase invert one side of the submix. Most utility stereo gain, pan, and phase effects should have this feature.
• Phase invert one side of the Side submix.
• Finally, lower both the Mid and Side submixes by 6dB each. The encoding process will consistently add 6dB of gain if done properly.

If you followed the above steps correctly you should now have successfully encoded your stereo file into M/S. Try boosting the Side submix or taking away the Mid to test. If something sounds wrong double check your phase plugin. Remember we only want to phase invert one channel of the Side submix.

M/S Decoding Without a Plugin

To get ourselves back to stereo we need to do just a few more steps:

• Create two more submix tracks. These will be our Left and Right. Unlike encoding, having dual pans here is not necessary.
• Send both your Mid and Side submixes to both the Left and Right submix tracks.
• Pan the Left submix hard left.
• Pan the Right submix hard right.
• Place your phase inversion plugin on the Right submix and invert the left channel.

What to do with M/S Processing

With your stereo file successfully converted to M/S, now the fun can begin. But what exactly can we do with M/S anyway?

EQ the Stereo Field

One of the biggest benefits of M/S processing is how it lets us EQ different parts of the stereo field. If you are handed a stereo file and asked to make the vocals brighter, your first reaction would (should) probably be, "I can't do that—the voice is mixed in with the other instruments."

However, most vocals are dead center in the stereo field. By placing an EQ on the Mid portion of the M/S we can EQ the voice more directly while leaving the rest relatively unchanged. Obviously other dead center instruments will be affected as well so make sure to do this on a case by case basis.

Another fun option is to brighten up a dull reverb that tends to get lost in the mix. Simply place the EQ on the Side submix instead. Simple!

Multiband Control

One of the best forms of dynamic and frequency control we have is a multiband compressor. The ability to control specific frequency ranges is a huge boon to our workflow. However, sometimes we need even more control.

Traditionally, a multiband compressor will work on a regular stereo field. So if your goal is to tame an unruly kick drum, every time the compressor hits the kick, it will be taming the entire low end across the stereo field.

Most kicks are panned hard center. By placing the multiband on the Mid submix, you will just be taming the low frequency content of the middle. For electronic tracks with large amounts of stereo bass, this can be a life saver!

Saturation

If you want to give a sound a larger than life quality, saturation is always a good bet. When dialed in correctly it gives a larger perceived stereo field, and vocals that pop out more. However, if something in the mix is already over the top, it is just going to be overkill. Usually that something are the vocals.

What if we wanted the reverb to match it? Simply placing a saturation plugin on the file would make the vocals and guitar jump out even more. However, if we place the saturation on the Side submix, we can get that larger than life feel on just the reverb. Awesome!

Conclusion

As you can see M/S processing, while slightly tricky to set up without a dedicated plugin, is immensely powerful. Traditional mixing does not allow us to work with a center, which ironically is where most of the important tracks are!

Be careful with using plugins that abuse phase when doing M/S processing, as they can throw off the encoding and decoding. Thanks for reading!