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Sides Splitting

Pro Tools Tips & Techniques By Mike Thornton
Published January 2014

Learn to set up M/S matrixing in Pro Tools and you open up a world of creative processing possibilities.

In this month's workshop we are going to take a look at using Mid/Side processing in Pro Tools. As in most DAWs, stereo signals in Pro Tools are usually represented as separate left and right channels. If you play back a simple stereo file, the left channel will be output from the left speaker and the right from the right speaker. However, as Alan Blumlein showed many years ago, there is an alternative way to represent a stereo signal on two channels, where one channel carries everything that is identical in the left and right channels — the sum of the left and right signals — and the other carries everything that differs between the left and right channels. This way of representing stereo is known as Mid/Sides, or sum and difference in some circles. What Blumlein demonstrated is that a conventional stereo signal can be 'matrixed' into M/S format and back without losing any information.

For most recording and mixing scenarios, conventional L/R stereo is the most useful format. After all, it's how your mixes will eventually make their way to the loudspeakers! However, there are quite a few circumstances where it's useful to work with stereo signals in the Mid/Sides format. Any signal recorded using an M/S mic technique will need to be decoded before it can be played back, but the ability to turn an L/R signal into M/S temporarily opens up valuable processing opportunities at the mixing and mastering stages — see the 'M/S Applications' box.

Stuck In The Middle With You

Quite a few plug-ins now support M/S processing internally, meaning they can take an L/R input, matrix it to M/S, apply separate processing to the M and S components, and then convert the output back to L/R. A few Pro Tools-compatible examples include equalisers such as Blue Cat Audio's Liny EQ, FabFilter's Pro-Q, Flux's EPure, Brainworx's bx_digital and DMG Audio's Equilibrium, along with dynamics plug-ins such as Blue Cat Audio's Dynamics or FabFilter's Pro-C and multi-function plug-ins like iZotope's Ozone and IK's T-Racks.Screen 1: a typical M/S processing chain set up using instances of HOFA's free 4u plug-in to handle encoding and decoding. There are also some dedicated plug-ins for rebalancing the width of stereo material using M/S processing. Brainworx's bx_solo and Blue Cat Audio's Gain Suite in Dual mode are both free; the former is more about monitoring what is happening with the Mid and Sides channels, but does include a useful stereo width control, while the Blue Cat plug-in allows you to adjust the gain of the Mid and Sides components separately. Moving on to a plug-in you need to pay for, Waves' Center has controls to adjust the centre (Mid) and Side levels separately, as well as EQ controls that skew different frequency areas towards the Mid or Sides channels while keeping the overall EQ flat.

If your chosen plug-in doesn't offer built-in M/S encoding and decoding, however, all is not lost. One option is to use an M/S encode/decode plug-in before and after your chosen processing plug-ins. There are free plug-ins that will do this, including HOFA's 4u Meter, Fader & MS Pan and Massey's Massey Tools, though the latter is not yet available in AAX so won't work on Pro Tools 11. As for a paid-for option, there is the Brainworx bx_stereomaker, which will also create a mono-compatible 'stereo' version of a mono file, and then there are Mathew Lane's DrMS and the Waves S1 Imager, M/S Matrix and Shuffler, which can be used as an M/S decoder or to process L/R audio using M/S techniques.

Used on a stereo track, M/S encoder plug-ins typically output the Mid signal on the left channel and the Sides signal on the right. To process these signals separately on the same stereo track, you'll need a plug-in that can operate in Multi-Mono mode rather than as a stereo plug-in. For instance, the first screenshot shows a setup where I have used two instances of HOFA's 4u to encode and decode the stereo signal as M/S. Inbetween them I have placed a Trim plug-in in Multi-Mono mode, which I can use to mute or apply gain to the Mid and Sides signals independently. To apply different settings to the two channels of a Multi-Mono plug-in, you need to deselect the Link icon and then use the L/R drop-down menu below to select the left or right (Mid or Sides) channel. In the screenshot, I have also added an instance of the free HOFA 4u Goniometer & Korrelator, which comes in the same bundle as the 4u Meter, Fader & MS Pan plug-in,Screen 2: encoding and decoding can also be achieved using routing in the Pro Tools mixer. The Trim plug-in is handling polarity reversal where appropriate. so that I can visually monitor the stereo width of the signal.

Mixer Routing

This is often the easiest way to implement simple M/S processing, but for more flexibility, including the ability to take the Mid and Sides signals out as separate mono Auxes, you can also set up the Pro Tools mixer to perform M/S encoding and decoding in a variety of ways. Here's one way to set up M/S encoding:

  • Create a stereo audio track and label it 'Input Track'. You will put the audio files on this track you want to process. Route the output to a stereo bus and label it 'L & R Input'. Now, create three stereo Aux tracks.
  • Label the first Aux 'L + R', and set its input to pick up the Input Track's output bus — in the second screenshot, I've labelled this 'L & R Input'. Set the output of the 'L + R' Aux to an odd-numbered mono bus labelled 'Mid'. Note the Pan control will disappear because you are feeding a mono source. This bus now contains the Mid channel, ie. the sum of the left and right channels.
  • Label the second Aux track 'L – R' and set its input as you did with the first Aux track, to pick up the Input Track's output bus. Insert a Multi-Mono Trim plug-in, break the link and enable the Phase Invert on the right channel only. This creates the Sides channel, ie. the difference between the left and right channels — although we are combining the left and right channels, the phase inversion in one channel makes this a subtraction rather than an addition. Set the output of the 'L — R' Aux to the next even-numbered mono bus after the 'Mid' bus and label it 'Sides'.
  • At this point we have a pair of mono buses, one containing the Mid component of the signal, the other the Sides. We can use the third Aux track, which I've labelled 'processing', to pick up this pair of buses, and use Multi-Mono plug-ins to process the Mid and Sides signals separately. Set the input to pick up the two mono buses labelled Mid & Side and label the stereo buss 'Mid & Side'. Set the output to another stereo bus and label that 'M & S Processed'.

To decode your M/S-processed audio:

  • Create three mono Aux tracks. Label the first 'Mid to L & R' and set its input to pick up the left side of the 'M & S Processed' bus (you can name mono buses independently of the stereo bus to which they contribute, so you could name this 'MidProcessed'). Set the output to another stereo buss and label it 'L & R Processed'. This will take the Mid channel and route it equally to both left and right outputs.
  • Label the second mono Aux track 'Side to L', set the input to the right side of the 'M & S Processed' bus and re-label this 'Side Processed'. Insert a mono Trim plug-in, set its gain to -3dB and set the output of the track to the left side mono bus of the 'L & R Processed' bus. This will route the Sides channel in phase to the left output channel.
  • Label the third mono Aux track 'Side to R', insert a mono Trim plug-in, set its gain to -3dB and enable the phase invert button. Set the input to the 'Side Processed' mono bus as above, but set the output to the right side mono bus of the 'L & R Processed'. This will route the Sides channel to the right output channel in opposite polarity.
  • Finally, create a stereo Aux track, label it 'Output Track', insert a Trim plug-in and reduce the gain by 3dB. The reason for the gain reductions is to compensate for the fact that our decode/encode process has doubled the level of both the Mid and Sides components (the Pro Tools pan law is taking care of the 3dB reduction in the Mid channel).

Now you are ready to process any audio on the 'Input Track' track in M/S, using plug-ins you insert on the Processing track, and hear the results on the output track. Both of these sessions are available for download here if you want to try it for yourself. There are also plenty of other ways to set up M/S encoding and decoding in the Pro Tools mixer, and if you use sends as well as tracks, you can do so using fewer Auxes than described above.  

M/S Applications

Here are just a few of the situations in which M/S processing can be valuable when mixing or mastering:

  • Suppose you have a great-sounding vocal in the centre but stereo backing vocals are a little dull. Conventional stereo EQ would brighten the lead vocal and the backing vocals equally, but by putting a little high-end boost into the Sides channel only, you can brighten up the backing vocals without changing the lead vocal or anything else in the middle of the mix. There are many other circumstances where equalising the Sides channel independently can open up a mix or add 'air'.
  • Conversely, a high-pass filter in the Sides channel set to roll off low frequencies can help to focus things in the Mid channel. I usually do this when I record on location with my Sennheiser MKH40/30 M/S pair: all the wanted low frequencies are still in the Mid channel, and unwanted rumble is much less obvious.
  • If you have a problematic resonant frequency popping out, a notch filter in the Mid channel only can be more effective than notching the entire signal.
  • Try putting a compressor in the Sides channel and listen to what happens to the reverb. As the compressor kicks in on louder sections, the ambience will be less prominent. If you want a more subtle effect, try parallel compression. With a multi-band dynamics plug-in such as Blue Cat's MB5, you can focus the effect on a specific frequency region.
  • If you want to clean up the middle frequency range of a mix, try compressing the Mid channel of the feed going to the reverb. This can work particularly well when you're using a subgroup with its own dedicated reverb, for instance for drums. Again you may find that only light amounts of gain reductions will make all the difference.
  • You can put a reverb in the middle of the M/S chain and turn down the amount of the Mid signal going to the reverb so that only the difference signal excites the reverb.
  • Try using an exciter on the high frequencies of the Sides channel and use a low-end harmonic processor to treat just the Mid channel, keeping the low end focused in the centre.
Published January 2014

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