In mixing live audio, everyone starts small, and that’s probably with a few channels mixed to the stereo bus, a couple of pre-fader aux sends of foldback, and a post-fader FX aux send, most likely on a small analogue mixer. As you progress and mix on bigger gigs and systems, you’ll add subgroups to your toolkit, stereo aux busses for in-ears, then eventually encounter a desk with matrix busses. So what are they and what are they for?
The matrix mixer started out in separate rack-mounted units, and was usually used to mix feeds from different desks and sources together. They were eventually built-in to large analogue desks to provide busses that you could mix other busses to – a ‘mix of mixes’. The first application was to take group busses e.g. vocal group, orchestral group, FX group, and give engineers the ability to send different levels of those groups, post the group fader, to different parts of the PA.
For example, mixing a musical on a traditional distributed PA system in a theatre that has a centre cluster, left/right, front fill, and under-balcony delays, you’d typically mix vocals and no orchestra to the centre cluster, orchestra and a little or no vocals to the left/right, vocals and no orchestra to the front fill, and everything to the under-balcony delays, but at a lower level than the left/right. Mixing busses to a matrix out is an elegant way to achieve this.
Matrices also are an audio circuit, and as such can have their own processing inserted or in-line (in an analogue system), or built-in to the bus in the digital realm. This is particularly useful for providing different EQ settings when the bus is fed to a different kind of loudspeaker than the left/right, or adding delays for time alignment.
Matrices are also really useful for separate stereo broadcast or record feeds, particularly when no one other bus is providing a suitable mix to send. In a lot of rooms, what’s going to the left/right isn’t always suitable for recording, as the engineer is mixing to take into account stage volume, room acoustics, and natural reverberation. When sending a matrix bus out to cameras or a stereo record, they give you the advantage of having total control over the master level, and being able to put compression and limiting in-line to prevent clipping.
As requirements for more and more busses in consoles have been driven by the adoption of in-ear-monitoring, matrix busses have now appeared on most digital desks. The Soundcraft Vi Series, for example, has 16 configurable matrices (that can run mono or stereo) that can take inputs from up to 24 sources, which can include channel direct outs and inputs from the patching system, as well as sub groups and auxes. They have full processing on the bus, including parametric EQ, dynamics, delay, and graphic EQ.