MIDI 4+1: Arduino-powered polyphonic and monophonic MIDI to 4x CV/gate interface in 6HP

I made a polyphonic MIDI interface for Eurorack using Arduino and 3D-printing. I was inspired by CVpal and MIDI Thing, but I implemented a feature-set more focused on my needs.

Code, details & pics:

It has two different polyphonic allocation modes, and two split-keyboard modes with a monophonic algorithm on one side. I also added an additional “logic OR” gate output (useful for single-filter setups) and the ability to “lock” held voices using a button.

But at the end, the main challenge was building it using only 6HP!


this is immensely inspiring! In theory, even I could build this on a lego plate.

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Great, thanks for sharing this! I’ll build it in June.


This is very cool, thanks for sharing your work! I’d be all over it if I didn’t have plenty of conversion already.

Your whole github is very inspiring. Eager to build in-cv, or at least play with the patterns…

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Thanks for the feedback :slight_smile: @ldezem @sakul If you’ll build this or “In CV”, share the result or send me a message, I’m curious about your takes!!!

Here’s a quick demo of MIDI 4+1 I uploaded today, where I use it together with my alternative firmware for Stages in a 52 HP simple polyphonic setup:

Patch notes, from left to right:

  • MIDI 4+1 interface allocates notes polyphonically from the keyboard into four channels of CV/gate. The additional top-most gate output is the combined OR.
  • CVs are sent to Chord v2 in Unison Poly Mode: each oscillator is controlled independently by its own 1V/oct input.
  • Gates are sent to Stages, which runs a custom firmware (link above) that generates five identical envelopes.
  • Veils uses four of these envelopes to drive one VCA for each of the four voices coming from Chord’s individual outputs.
  • WMD SSF Pole-Zero filters the final mix from Veils; its cutoff is modulated using the fifth envelope from Stages, the one triggered by the OR-gate signal.