Hazards of changing non-Real-time synth.* settings at runtime

[jimhen3ry]

There are potential pitfalls when changing synth.* settings that are not marked “Real-time” in the Settings Reference (Synthesizer settings) while a synth instance is already instantiated.

We ran into this with a user setting of the FluidSynth sample rate in the Miditzer software. Changing the sample rate changed the pitch of the output. We eventually figured out that we were changing synth.sample-rate and restarting only the audio driver, not the synth. The result was that the synth continued to generate audio at the originally configured rate, while the audio driver/stream configuration used the updated synth.sample-rate setting, creating a mismatch between the synth rate and the OS rate expectation. This lead to a pitch error.

What, if anything, should be done to lessen the chance of this type of user error? Possibilities:

• Add documentation emphasizing that non-Real-time synth.* settings (e.g. synth.sample-rate) should not be changed while a synth is instantiated.
• Recommend guarding fluid_settings_set* functions with fluid_settings_is_realtime and a user maintained flag for synth_status.
• Modify the fluid_settings_set* functions to enforce setting non-Real-time synth.* settings only when a synth is not instantiated (return FLUID_FAILED).