Addressing the portamento issue

[derselbst]

Related discussion

As discussed in #1495

Is your feature request related to a problem?

#705, #1232, #1311 and #1456 all point to one thing: Fluidsynth should have a 7bit portamento mode to work with older MIDI files.

Describe the solution you'd like

As discussed, a new tri-state setting (synth.porta-mode or synth.portamento-mode) is proposed:

• auto mode is the default and would use 7bit portamento time MSB, unless it sees an LSB event (on any midi channel), in which case it would switch the entire synth engine over to 14bit portamento time,
• xg-gs mode, which would stick to 7bit portamento time MSB, and
• linear mode, which would stick to 14 bit MSB+LSB portamento time.

Describe alternatives you've considered

TLDR, see #1495

Additional context

If a system reset event is received and the synth was in auto mode, it will reset / reinitialize auto mode to 7bit MSB mode.

Testfiles:

• 1TOWOW.mid
• Descent Game_08.mid
• Descent12.mid https://github.com/FluidSynth/fluidsynth/files/14798600/fluidsynth-bug-files.zip
• MegaMan6_MrX_XG.zip

[spessasus]

You have missed two additional test files I have provided.

portamento.zip

It seems that Descent12 is an exact copy of game08?

Also attaching my current time calculation implementation for reference.

[spessasus]

One more discover I made: the initial portamento control appears to be "unset" for XG, not 64 or 0. So the first note doesn't glide after a reset even if the portamento time is set above 0. Only notes after that glide from the previous one.