Reverb Engine Causes Distortion

[spessasus]

FluidSynth version

fluidsynth --version
ALSA lib pcm_dsnoop.c:567:(snd_pcm_dsnoop_open) unable to open slave
ALSA lib pcm_dmix.c:1000:(snd_pcm_dmix_open) unable to open slave
ALSA lib pcm.c:2722:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.rear
ALSA lib pcm.c:2722:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.center_lfe
ALSA lib pcm.c:2722:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.side
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
ALSA lib pcm_oss.c:404:(_snd_pcm_oss_open) Cannot open device /dev/dsp
ALSA lib pcm_oss.c:404:(_snd_pcm_oss_open) Cannot open device /dev/dsp
ALSA lib pcm_a52.c:1036:(_snd_pcm_a52_open) a52 is only for playback
ALSA lib confmisc.c:160:(snd_config_get_card) Invalid field card
ALSA lib pcm_usb_stream.c:481:(_snd_pcm_usb_stream_open) Invalid card 'card'
ALSA lib confmisc.c:160:(snd_config_get_card) Invalid field card
ALSA lib pcm_usb_stream.c:481:(_snd_pcm_usb_stream_open) Invalid card 'card'
ALSA lib pcm_dmix.c:1000:(snd_pcm_dmix_open) unable to open slave
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
FluidSynth runtime version 2.4.3
Copyright (C) 2000-2025 Peter Hanappe and others.
Distributed under the LGPL license.
SoundFont(R) is a registered trademark of Creative Technology Ltd.

FluidSynth executable version 2.4.3
Sample type=double

Describe the bug

In the attached soundfont, there's a single sample which causes a lot of distortion with the current reverb engine. I'm not sure if the FDN reverb itself is flawed, or are those just the stock settings. Give it a listen.

Expected behavior

Proper reverbation without a distortion. For example try Polyphone, BASSMIDI or spessasynth.

Also AFAIK Polyphone uses Freeverb, so given that it works fine, maybe fluid should go back to it?

Steps to reproduce

1. fluidsynth ~/Desktop/Fluid\ reverb\ issue.sf2
2. Grab a MIDI keyboard or VMPK and play some notes.

Fluid reverb issue.zip

[derselbst]

I think I hear what you mean. I'd call it "early reflections" - sounds indeed unnatural. Designing a good reverbator truely challenging and probably an unreachable goal for an opensource project like fluidsynth. The Lexicon224 reverbator that I have an implementation of has a similar issue. Will most likely be addressed by #1157 ...if I ever find the time. And ofc. only after having fixed all the other bugs ;)

Related to #1215.

[spessasus]

What about reverting back to Freeverb?
Polyphone does not have this issue and it uses freeverb

this would mean reverting #380 and that's it. I'm pretty sure that the parameters are the same. It could fix #1215 as well maybe?

[derselbst]

This would mean to drop all the work JJC has put into this. Also, freeverb causes a lot of ringing for higher frequencies. So no, that's definitely not an option.

[spessasus]

I see. Then what about convolution-based reverb, like spessasynth?

Pros:
-In theory could emulate any type of reverb. Hall, room, church, anything you desire. And there are a lot of free impulse responses available

Cons:
-All the current reverb params (roomsize, damping, etc.) are gone and replaced with impulse response. So a breaking change.