Addressing the portamento issue

[spessasus]

So,
#705, #1232, #1311 and #1456 all point to one thing: Fluidsynth should have a "quirky" portamento mode to work with older MIDI files.

Now what is a "quirky" mode? It's the mode that would work with the old GS & XG MIDI files, which seems to use only the MSB portion of the portamento time and is sort of "exponential."

Clues we have:

• @johnnovak already did some experimenting on the topic with a real SC-55
• BASSMIDI seems to work fine with the MIDIs that use portamento, though the only thing I found was this thread in which Ian is very vague about the way it has been implemented.

I understand that the current portamento mode is important and needs to stay, so something like a toggle (-o synth.portamento=quirky?) could work. The default would be standard which is the current behavior, and quirky would be this strange portamento time mode.

I'll try to look deeper into it as I want to implement that special portamento time in my synth as well, so I'll post any discoveries I make here.

[johnnovak]

@spessasus Probably Roland invented that weird curve, so probably all Sound Canvas modules use that. Then all other modules thst aimed to be somewhat GS standard compatible (e.g. the Yamaha MU series). So probably gs or roland-old is a better name that quirky which doesn't say much 😆

[spessasus]

That's fair, though some Yamaha XG MIDIs I have also seem to use the same portamento time logic as they work fine on BASSMIDI. That's why it would not be only GS but also XG? Or do they just share the logic?