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System Architecture

SUMIX is designed to be a flexible audio mixing system. To progress the design the following system architectures are presented. They serve two purposes: they provide concrete examples to further discussion and analysis, and they provide a starting point for exploration of alternative structures and applications.

The SUMIX system comprises: four aux buses, four mix buses, mono PFL, stereo master buses (Left/Right). All buses are balanced to minimise interference both from coupling into the bus lines and from common impedances in the 0V rail (a well-known problem in the Eurorack modular synthesizer format).


A mono bus system treats the four buses as separate mono channels. Each channel can have different audio content, and can be placed in a different position in the stereo field.

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SUMIX System Diagram

CHANIN is the traditional channel strip, tailored for SUMIX. It follows the usual signal path: input preamp, optional EQ, pre- and post-auxes, PFL, main fader, mute, and bus assignment.

AUXIO handles a single AUX bus, providing an insert point, PFL, fader, mute, and bus assignment.

BUSIO performs a similar function to AUXIO but focussed on the specific duties of directing the four mono buses to the Left/Right master buses. Instead of a bus assignment following mute BUSIO provides a pan control.

There are two output blocks. STOUT is the master output module, providing a stereo fader and balanced output drivers for feeding balanced lines to external equipment (mixing desk, amplifier, etc). PHONS performs two functions: it provides the PFL function - switching headphone monitoring to whatever source(s) are feeding the PFL bus - and it provides a stereo headphone amplifier.

Finally, the EXTENDER allows for expanding the SUMIX buses across multiple systems/cases/cabinets/rooms/etc.


* Should AUXIO and/or BUSIO have their own EQ sections? Perhaps simpler hi/lo shelving?
* Where to add automation? Channel faders? Pans? Mutes?
* Level monitoring: could put small (say, four segment) VU meter on CHANIN, and larger ten-segment VU meters on STOUT.


A stereo bus system pairs the four mix buses as two stereo buses. Each stereo bus can have different audio content, with content positioned in the stereo field at the source (either channel input or aux I/O), with the BUSIO modules providing insert, level and mute only.

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SUMIX System Diagram

BUSIO takes one of a pair of buses, representing left and right stereo channels. It provides a stereo insert, but loses the pan control since that function is now controlled by the audio sources.

There is the potential for a stereo version of CHANIN for handling stereo material. Again, no pan required since this would be determined by the source. There is also the matter of supporting stereo EQ, which is difficult if the stereo position is not to be compromised.


Perhaps the ultimate application of SUMIX, we treat the four buses as one quadraphonic mixing bus.

19.05.2016. 23:07