If you have ever loaded a microtonal scale into an MPE synth and heard notes warble, drift, or land a quarter-tone off, you have run into one of the least documented problems in expressive playing: MPE and microtuning are both fighting over the same pitch-bend messages.

This is not a bug in either system. It is a consequence of how both were designed, and once you understand the mechanism the fixes are straightforward.

Two systems, one pitch parameter

MPE works by giving every note its own MIDI channel, so that per-note pitch bend, pressure and slide can be sent independently. Pitch bend on channel 3 affects only the note playing on channel 3. That is the whole trick.

MTS-ESP, ODDSound's tuning standard, works differently depending on the plugin. A synth with native MTS-ESP support asks a central master plugin "what frequency should note 60 be right now?" and tunes its oscillator directly. No MIDI messages involved. That version is clean and coexists with MPE perfectly.

The trouble starts with instruments that do not have native support and are retuned externally — via a MIDI effect that converts a scale into pitch-bend offsets, or via a legacy MTS SysEx path that the synth partially implements. Now your tuning system is emitting pitch bend on the same channels your controller is using for vibrato and glide. The two add together, and neither knows about the other.

The three failure modes

Warble. You hold a note, apply a slight left-right movement on your Seaboard or Continuum, and the pitch oscillates unpredictably. The tuning layer is re-asserting its offset while your gesture pushes against it. Classic symptom of two sources writing to the same parameter.

Range mismatch. MPE conventionally uses a ±48 semitone per-note bend range, while non-MPE pitch bend defaults to ±2. A tuning offset calculated for a ±2 range and applied to a synth expecting ±48 will be 24 times too small — enough to look "nearly right" and drive you mad. Always confirm that the controller, the tuning tool and the synth agree on the bend range.

Held-note drift. MTS-ESP distinguishes between note-on and dynamic behaviour: if the tuning changes while a note is sounding, the note either holds its original pitch or slides to the new one. With expressive controllers you tend to hold notes far longer and modulate them continuously, so dynamic mode can produce audible slides you did not play. If you are not actively automating scales, note-on is the safer default.

Setting it up properly

Start by checking whether your synth supports MTS-ESP natively. A growing number do — Surge XT has arguably the most thorough tuning implementation of any free synth, and its tuning guide is the best plain-language explanation of the subject anywhere. Melatonin's Sine Machine, Audio Damage instruments and many boutique plugins now ship with it too.

If it does, the rule is simple: let MTS-ESP handle scale, let MPE handle gesture. Disable any pitch-bend-based retuning layer entirely. They should never both be running.

If your synth does not support MTS-ESP, you have a choice. Either accept that microtuning and per-note bend will interfere and use one at a time, or switch to an instrument that does. There is no clever routing that makes two independent pitch-bend writers coexist.

One more setting worth hunting for: several instruments have a pitch-bend curve or smoothing option that reshapes incoming bend data. Anything nonlinear applied to a tuning offset will produce a wrong frequency. Turn curves off before you diagnose anything else — in many reported cases that single switch was the whole problem.

Why this is becoming more common

Two trends are colliding. Microtonal and just-intonation work has moved from academic curiosity to mainstream production, helped by MTS-ESP making it a one-plugin decision instead of a per-synth chore. At the same time, MPE controllers have gone from rare to ordinary.

The overlap — musicians who want both a 31-tone scale and per-note vibrato — used to be tiny. It is not tiny any more, and plugin developers are catching up. The good news is that the fix is almost always configuration rather than compromise: get the bend ranges aligned, pick native MTS-ESP support where you can, and the two systems stay out of each other's way completely.

Sources: ODDSound MTS-ESP, Surge Synth Team Tuning Guide, MTS-ESP library on GitHub