What is Mid/Side Processing?

Since stereo first became an integral part of recorded music, producers, engineers, and musicians have been experimenting with left and right to achieve a more full, wide sounding mix. Simple techniques like using panning and mix automation are common, but there is another technique in the engineer’s toolkit that isn’t often explored: mid/side processing.

In this piece you’ll learn:

• What mid/side processing is
• How to use mid/side processing when mixing
• How to use mid/side processing when mastering
• How to determine when and when not to use Mid-Side processing

Start getting wider, more focused mixes by trying mid/side processing in your DAW with the Ozone mastering plug-in, also included in iZotope’s Music Production Suite. 

What is mid/side processing?

Mid/side processing takes a left/right stereo signal and breaks it up into two separate layers: Mid and Side. Mid/side processing is a highly effective way of making adjustments to the spacialization of a mix or master to make your mix sound full and wide. 

In your DAW, you most likely have both mono and stereo tracks. If you pan a track center, you perceive the sound as coming from the middle of the left and right speakers, as if there is a third speaker between them. 

You can think of all center panned elements as the Mid channel when using mid/side processing. If you boost the mid channel in your mix, the listener perceives elements panned center as louder and more present.

The Side channel is everything panned left or right in the stereo image and doesn’t include the center. Boosting this channel can make a mix sound wider from left to right.

Most plug-ins default to processing in traditional stereo, applying an effect to the entire stereo image without any separation. A plug-in with mid/side processing allows the engineer to split up the mid and sides, processing them differently and independently if desired.

How to use mid/side processing in mixing

The concept of mid/side comes from Alan Blumlein’s M/S mic’ing technique patented in 1934. This stereo mic’ing technique uses a cardioid and bidirectional microphone together to create a mid channel (cardioid) and the sides (bidirectional)

Technically, a signal must be recorded in M/S to use mid/side processing, but some plug-ins, like iZotope’s Ozone, have real-time mid/side encoding.

Note: Mid/side processing is subtle and should be used with care. I recommend listening on a good pair of headphones or monitors to hear the changes on these examples, and also when you experiment with these techniques yourself.

Let’s say you’re given a stereo drum loop to mix in with the rest of a multitrack. You can use  mid/side EQ in Ozone to separate the center from the sides and manipulate each sound independently. To recap, a mid/side EQ encodes a stereo signal into separate mono and stereo channels.

Here is the raw drum loop without any processing:

Using the Ozone EQ, change the mode from Stereo to mid/side.

Here is what the Mid and Side channels sound like solo’d:

To bring out the kick and snare in the Mid channel, two bands are boosted on the EQ.

On the Side, a high-pass filter is used to remove the low frequency of the kick from the sides:

The result has a more solid, deeper kick and more focused snare in the center channel:

How to use mid/side processing in mastering

Using mid/side processing in mastering is quite common. It can be useful for the engineer to split the signal into mid and side to have more independent control of the elements in a stereo mix.

If your master is lacking clarity or sounds muddy, try reducing the low/mid frequencies on the side channel with an EQ. This process will preserve the warmth in a center-panned lead vocal but reduce the muddiness in the other side-panned elements. You can also liven up a dry mix by adding a small amount of reverb on the side channel, avoiding the kick and bass in the center.

Here is a mix without any mid/side processing:

Using Ozone’s mid/side Encoding, both the EQ and compressors are split. For the Mid channel, a subtle EQ is used on the mid range of frequencies to remove some muddiness, and a compressor is used to bring the lead vocal more forward:

On the Side, a high pass filter is removing the sub bass, and a very light compressor is added for additional control:

The result is a more vocally-focused master, with less warmth on the sides:

For comparison, here are two other versions of the master: one that is even more center focused, and another where the sides are boosted:

When to and when NOT to use mid/side processing

Mid/side processing is best used in a mix or master that needs a bit more attention to how it presents in the stereo field. It can be useful to emphasize stereo-panned guitar layers, or bring a lead vocal out of the shadows in the center channel. Modern plug-ins also make it a lot easier for budding engineers to experiment with these techniques.

However, although this type of processing can feel very tempting, it isn’t necessarily useful for all mixes or masters. Mid/side processing is a powerful tool, but it isn’t foolproof. 

When using a plug-in to artificially separate mid and side, there will always be crossover into each of the channels. It’s almost impossible to completely separate your Mids from your Sides seamlessly. This means that even if your vocal is panned center and shows up in the Mid channel on your EQ, it doesn’t mean that there aren’t also elements of that voice present in the Sides. With this in mind, it’s important to use your ears and listen critically when applying this type of processing, and go easy on it at first, as the result is subtle but impactful.

You also want to be mindful of how manipulation of either Mid or Side will impact the way your mix is perceived. For example, if you want a super-wide mix and compress and boost the Sides, you might lose your lead vocal or any center-panned elements in the process. Make sure you can find the right balance so it doesn’t sway your mix too far in one direction. 

Start using mid/side processing in your productions

Mid/side processing isn’t a new concept, but modern plug-ins make it even easier for engineers to experiment with the technique. Don’t be afraid to see how this approach to mixing and mastering will impact your process and remember, always save previous versions of your sessions just in case! A little bit goes a long way.