FluidSynth version

2.4 (and master)

Describe the bug

Since #1345 was merged, there is now a strange audio distortion when the filter is modulated by the modulation envelope. I created a demo MIDI file and audio renderings that should hopefully make this quite evident: filter envelope noise.zip

Inside the zip file, you will find the following files:

• filter envelope noise.mid – A MIDI file that uses the GS preset 008:038 Acid Bass featured in GeneralUser GS 2.0.1. This preset uses high filter emphasis and has both the modulation envelope and modulation LFO modulating the filter.
• filter envelope noise-FluidSynth 2.4.flac – The above MIDI file rendered using the latest FluidSynth master (one commit ahead of 2.4) via FluidSynth Plugin in REAPER. You can hear a strange, grainy distortion throughout correlating to the filter being modulated by the modulation envelope.
• filter envelope noise-1345 reverted.flac – Same as above but with Remove interpolation of IIR filter coefficients #1345 reverted. You can hear no more distortion. Everything sounds as it should.
• filter envelope noise-Roland SC8820.flac – The above MIDI file rendered on Roland Sound Canvas VA (SC8820 model, reverb disabled). Filter popping with high Q and quick modulation is a known issue with IIR filters. I included this recording to show that FluidSynth handles filter popping much better than the Roland Sound Canvas, even without Remove interpolation of IIR filter coefficients #1345.

It is unfortunate that such a substantial change was made to FluidSynth's behavior so soon before a major new release. From my perspective, FluidSynth's filter behavior was far better than any other SoundFont synth I had tested, and now it's broken. I'm really upset that I didn't have the time to test this until now, otherwise I would have been able to say something sooner.