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How to Use M/S Processing in Mastering

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Read Time: 7 mins

Mastering is always a hot topic. Most of us love to use the latest technique to make our masters bigger, badder and usually louder. Unfortunately, not all mastering techniques get as much airtime as others, and M/S processing is one of these lesser known subjects.

It's likely that a lot of budding producers who do their own mastering won't be aware of M/S processing and its benefits. If you're not taking advantage of what this technique has to offer you could be missing out, so let's take a look at how it works and its practical applications.

Step 1 - What is M/S Processing and How Does it Work?

If you are experienced in the areas of recording and microphones, you may already be aware of M/S (mid/side) technique. Using the M/S method, a single mono source is recorded using two microphones. This will then produce a stereo signal — nothing fantastic there, you may think, but when you consider that stereo width can be adjusted, retaining perfect mono compatibility, you may think again.

This type of recording basically involves mixing one mic dead center (essentially going to both left and right sides) and the second sent to two channels. These two channels are then panned hard left and right, with one side phase correct and the other phase reversed. The stereo width can be adjusted after the performance has been recorded, but the fact that it uses a complex set-up and requires specific kinds of microphones may cause some to find alternative methods.

M/S microphone technique can get a bit complicated, and as we are concentrating on using M/S in mixing and mastering here I don't really have the space to explain it fully. If you are interested in this side of it you can have a look at articles such as this. Here you can learn about the specific microphone types required and how to set them up.

M/S microphone technique diagram

Step 2 - Using M/S Processing with Stereo Recordings

All this talk of microphone technique is all very well but how does this relate to treating an entire mix? Well, once our three M/S mic channels are mixed, they are bounced down or decoded to stereo, and in turn the reverse can be achieved relatively easily using the right encoder and decoder software.

With our stereo audio encoded into an M/S matrix we can not only manipulate the stereo image of the signal but actually treat the stereo information separately from the mono. For anyone who has not used this technique in their mastering chain before, you may just find it revolutionary.

Imagine a situation where your track has been mixed down and during the mastering stage and you want to be able to raise the level of stereo string, or EQ a mono kick drum. Using M/S plug-ins this is all possible. Some plug-ins even allow you to de-ess either mono or stereo vocals that are buried in the mix, which is pretty impressive. All this magic happens under the hood as the M/S decoding is all internal.

Of course these processors are also at home when used on individual mix elements. For example you could manipulate the mono and stereo percussion instruments within a drum loop to allow the loop to fit perfectly with your mix. The applications of this technology are pretty diverse.

Step 3 - Which Plug-ins Support M/S Technology?

There is one small downside in all of this and that is the fact that there aren't too many plug-in manufacturers developing in this area. Luckily the ones that are do it very well and the results they produce are often really impressive.

The German company Brainworx appears to be the current leading authority on plug-in based M/S processors. They have an impressive array of products ranging from the free 'bx_solo' to the excellent, full blown 'bx_digital'. The bx_solo allows beginners in this field to get a taste of what the process is capable of without spending a penny, well worth a download. You can also download 14 day demos of all their plug-ins at their website.

The bx_digital is a bit of a monster and incorporates a full M/S matrix, an EQ for both mid and side elements, de-essers, a 'mono maker' and even high and low end enhancers. Bar dynamics control there is not much you can't do with your mix here.

The well equipped Brainworx bx_digital.

A plug-in that has only recently introduced M/S support is iZotopes Ozone mastering suite. Pretty much every page of iZotope now has an M/S option and allows you to manipulate your mono and stereo signals separately, adding width, EQ and dynamic changes to each section of your audio.

For more of this all-in-one M/S magic be sure to check out the Flux 'Alchemy'. This plug-in is a fully fledged mastering processor and has a great little M/S section that incorporates widening and a bitter/sweet enhancement section.

The Flux Alchemy plug-in

As most of you will undoubtedly be aware, none of these plug-ins are free — in fact none of them are even that cheap, but if you are serious about delving in this area of processing, it's worth investing in one of these. My personal choice would be the Brainworx range as this is what I use for all my mastering but if you prefer an all in one option and are on a tight budget you might want to opt for the iZotope Ozone.

iZotope Ozone has full M/S processing capability.

If all you need is an encoder/decoder then Voxengo's free 'MSED' plug-in should be right up your street. It's available for download here.

The free Voxengo MSED plug-in.

Step 4 - Basic M/S Processing in Mastering

The main thing you will find useful during a mastering session is the M/S processors ability to raise and lower the level of stereo or mono information in your mix. I often find that by boosting the stereo information by just a few db, the mix is given a lift and a wider, more open sound is imparted to the track.

You may want to make more precise adjustments than just a general stereo width enhancement. Imagine your mix has been recorded with a mono lead vocal but it was mixed too high, giving the overall track a very narrow feel. By reducing the level of the mono material with the M/S processor you should be able to go some way to rectifying this. If you find this effects other levels too much you may need to go back to the mix or seek out other options.

Brainworx include other useful tools for basic processing of your tracks. The mono maker for example will take any audio below a certain frequency and make it strictly mono. I find this often gives a track a more focused and coherent low end mix. It also allows you to process this new area in the mono section and gives you the piece of mind that you won't color other instruments when doing so.

Step 5 - Using M/S Based Equalization

Another advantage of having control over the mid/side portions of your mix is the ability to equalize them independently.

This can allow you the freedom to enhance the upper mids of stereo strings, or even reverbs, whilst leaving the mono, core instruments untouched. Or you could enhance the low end of your kick and bass and roll off some top end from stereo percussion or backing vocals. These can be indispensable tools during a challenging mastering session.

Another great trick here is to use the high pass filters to remove the low end from the stereo signal. When this is used in conjunction with the mono maker you can be 100% sure that the very low frequencies in your mix are focused and in mono.

The M/S EQ in iZotope Ozone 4

Step 6 - M/S Dynamics Processing

Although M/S compression and limiting has been around for as long as compression itself, it is a relatively new area in virtual realm. Brainworx are doing some very exciting stuff in this area and it's worth checking out.

Their full blown M/S dynamics processor the 'bx_dynEQ' allows the user to home in on specific elements in the mix and compress not only certain frequencies, but also the mono or stereo sides. This is the perfect tool for boosting a kick drum or bass part in your mix. An absolute godsend to dance and hip hop producers.

Brainworx dyn_eq

There is also a fun cut down version of this plug-in, featuring the same technology but with a far simplified interface. This plug-in is aptly named Boom! and a demo can be downloaded from the Brainworx site at the link earlier in this piece.

Brainworx Boom!

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