Understanding stereo and mid/side
To begin, let’s summarise mid/side, or M/S. Sum an audio signal to mono, and only information that’s present equally in the left and right channels will remain: that’s the sum or mid signal. Conversely, solo the side of a signal, and only stereo content – ie, information that’s different in the left and right channels – will remain, hence it’s known as the difference signal.
Soloing mid content is commonly referred to as ‘collapsing’ or ‘summing’ to mono. Play a stereo track through a mono speaker system and only the mid information (ie, the mono sum) will be heard; the additional stereo information can only be heard when listening on a two-channel stereo system. Many mobile phones, Bluetooth speakers and club soundsystems output only the mono signal, so the importance of mono-to-stereo compatibility should be obvious – all the important mix elements must translate perfectly to the mono mix. However, many listeners will play the same track through a stereo speaker system or stereo headphones, too, so there needs to be enough stereo content to impress them. Clearly, then, the solution is to strike a balance between the two.
When processing an individual mix element, mid/side processing can help you alter its mono or stereo content independently. An M/S utility plugin can be used to turn the side signal’s level up, for example, which will make the sound appear ‘wider’. And vice versa: lower the side signal’s volume to reduce perceived width. Alternatively, M/S equalisation allows you to boost or attenuate specific frequencies in either the mid or side signal. Master these tools and you’ll be able to manipulate stereo width like a mixing ninja – but avoid heavy-handed M/S processing to retain a natural sound.
Keeping all this in mind, you should hunt down a plugin that lets you independently solo a signal’s mid and side information. Voxengo MSED is an essential – get it for free from bit.
ly/voxengo_msed – but you can, of course, use any equivalent M/S utility plugin. Other free alternatives include Tek’it Audio’s freeware MSControl ( bit.ly/MSControl) and Brainworx’ bx_control ( bit.ly/bx_control). Your DAW may even come bundled with an adequate plugin, Ableton Live’s Utility being one such example.