GMLab ST4

GMLAB‑ST4 is a four‑port switchable MIDI Thru box open source product based on the Arduino Leonardo. It routes incoming MIDI data to any combination of four output ports, selected through illuminated buttons. The unit guarantees safe switching by completing full MIDI messages before changing ports and by using Note Memory to prevent stuck notes when ports are disabled mid‑performance.

GMLAB‑ST4 fully supports the following MIDI message types: Note On, Note Off, Control Change (including dedicated handling for CC64 Sustain), Program Change, Channel Pressure, and Pitch Bend. Output port 1 is mirrored to USB‑MIDI.

The device does not process SysEx, Real‑Time messages (Clock, Start, Stop, Active Sensing), Polyphonic Aftertouch, Song Select, Song Position, or any other message types not explicitly handled in the firmware. USB‑MIDI input is recognized but not routed to the output ports.

Do you want more from it? Like any other GMLab products, it's open source and you can re-code it or modify it at your will... Not skilled on coding? You can ask the help of you favourite AI agent: they can add almost everything you want to it!