There are many reasons to want to achieve a full, wide sounding mix. A mix that is perceivably fuller and wider will sound “better” to the listener. Creating a mix that sounds more present but with a more sonically interesting texture can add interest to the different sections of a song or enhance the arrangement of a track.
A wide sounding mix can be achieved through a variety of panning and stereo imaging techniques, including one secret weapon of the accomplished Engineer: Mid/Side processing. Mid/Side is a highly effective way of making adjustments to the spacialization of a mix or master.
The Mid channel is the center of a stereo image. When the Mid channel is boosted, the listener perceives a more centered (mono) sound to the audio.
The Side channel is the edges of a stereo image. When the Side channel is boosted, the listener perceives a more spacious (wider) sound to the audio.
Tonal or dynamic adjustments, such as Equalization (EQ), Dynamics, Reverb, etc., are traditionally done in mono or stereo. Any processing that is applied would affect the entire mono / stereo signal of the audio track.
An audio tool (such as an EQ) that supports M/S processing creates two separate processes, one for the Mid channel and one for the Side channel. The concept of Mid/Side Processing comes from a mic technique patented by Alan Blumlein in 1934. The original idea was to recreate how the human ears hear a stereo image. Mid/Side originally came to be used effectively as a recording technique to enhance ‘space’ before stereo playback existed.
The basic setup for recording in Mid/Side makes use of one cardioid microphone (mid) and one bidirectional (figure-eight) microphone (sides).
Listening to the mid microphone signal will offer a mono image. When adding the side signal, the stereo image changes. The louder the side channel is, the wider the audio is perceived. This is due to the phase correlation between the two mics. It’s an interesting sounding recording technique, particularly when capturing drum sounds in a room.
To be able to process in Mid/Side the audio needs to have been recorded in Mid/Side or the signal could be encoded for processing. In the latter case, the signal is decoded after processing back into the conventional L+R format for playback. There are a few plug-ins available that will perform realtime Mid/Side encoding. However there are ways of doing this without plugins.
Mid/Side can be a very pleasing sound but does not always translate to a better result for a mix. It's important to be aware of the effect settings are having on the mix by listening closely. Very gentle Mid-Side processing is generally favorable, making it possible to avoid extreme settings that can cause phase shifts or imbalance issues.
Here are a few tips and tricks for using Mid/Side audio processing effectively.
Mixing with Mid/Side Processing;
If a track has multiple guitar parts, route them through a bus, using Mid/Side processing on the guitar bus. Automate the Mid/Side tool to boost the volume of the side channel during a chorus, or other section of the track. This makes the guitars sound bigger without adjusting panning, and as a result the section sounds more impactful.
Likewise, a slight volume boost to the side channel on drum overheads can enhance the room sound, or a slight boost to the mid channel might enhance the snare drum and rack toms.
On any particular instrument recorded in stereo, a high frequency EQ boost on just the side channel makes the ‘wider’ elements sound brighter. A Baxandall filter or a high shelf filter work best. This helps to add clarity to a reverb, without muddying up the signal too much.
Mastering with Mid/Side Processing;
If a mix sounds muddy, try reducing low frequencies in the side channel with a low shelf filter. This might be useful, for instance, to surgically EQ the mud out of hard-panned guitars while preserving the vocal and kick drum in the center of the mix.
If the mastering compressor is struggling to reduce dynamic range without a perceived narrowing or squashing of the signal, use a Mid/Side compressor to apply less compression to the side channel than the mid channel. Heavy energy in the center of a mix, where the kick, snare, bass sit, can cause a compressor to kick in, which actually squashes the wider, more ambient and spatial elements in the mix. This technique helps avoid that problem.
A dry acoustic mix can be warmed up with Mid/Side reverb. Add reverb to the mid channel, but filter out some of the low end on the wet, reverberant signal to avoid muddying the kick drum and bass. On the side channel, add 2-4% more reverb than on the mid channel, with no filtering necessary.
In this video, iZotope’s Education Director, Jonathan Wyner, gives an introduction to mid/side processing for mastering, a technique that can make vocals pop, control high frequency sounds, and help you achieve a wider mix.
M/S mixing or mastering isn't something that you should do just because you can, however it is something that can add more depth and flavour to a mix. You can completely destroy a mix by doing this incorrectly; by simply working on panning and levelling can have a huge impact but when you only have access to stems (especially during mastering) there are a lot more options you can have by utilising these techniques to access elements within a stereo mix.