Is there a way to get fluidsynth to play in just intonation, or more generally select the intonation of this soft synth?

  • I don't know the answer to the more general question, so this is a comment rather than an answer: It's not possible to tune a keyboard in just intonation unless you limit the chords you'll be using. Suppose, for example, you're playing a piece in C major, and you want to have perfect fifths (3:2 ratio) in your C, F, G, and Dm chords. Once you've tuned these notes, you cannot have a just major third in your F chord, because A is at a ratio of 81:64 over F instead of 5:4. Or, if you do tune the A as a just major third above F, you cannot use the D chord because the fifth is terribly flat. – phoog Jun 2 '19 at 21:15

It looks like Fluidsynth just interprets MIDI data, so it is really up to who/what is providing this data to make tuning adjustments. One could design a MIDI in-out tuning "converter" that took in MIDI 12-TET notes from a simple controller and output those notes with the tuning adjusted to a specified 12-note scale.

You can't take arbitrary 12-TET input and "change the tuning" to just intonation. You would need to make some decisions about which justly-tuned notes you want to be using, and of course the scale changes based on the key of the piece. And if you have any modulations, you need to re-tune the entire scale. Not to mention, even if you are interpreting 12-TET music with JI, depending on the context of two different notes, they may result in two different correct tunings. So you will (likely) have more than 12 tones to work with after translating to JI.

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    I have seen a VST plugin that does exactly what you are describing - it loads a scale pattern and then pitch-bends different notes so they fall into the correct intonation. This method is hacky and has a lot of problems unfortunately. – Darren Ringer Mar 8 '15 at 20:23
  • You have to retune even if you don't modulate, if you want to have both a just major third in the IV chord and just perfect fifths in I, IV, V, and ii (or V/V). It would be far easier and more practical to use an actual historical (non-equal) temperament. – phoog Jun 2 '19 at 21:09

I have somewhere on my computer an OpenMusic patch that does just that. It’s far from perfect, as I just wanted to play a little bit with OM, which I hadn’t use in a long time. I could make it available if you’re interested.

Here’s how it works: each note is sent to a different MIDI channel — yep, that means you use most channels just for a single instrument. Each channel’s pitch wheel is set slightly differently in order to achieve whatever temperament you’ve set the patch too.

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