GVerb bug

Version 1.3.14-beta (Unicode)

There appears to be a bug in the GVerb effect (or possibly a general bug in the effects).

To reproduce:

Generate a click track, Click sound: tick.

Apply effect GVerb, using Reverb time of approx 0.8 seconds.

Apply effect GVerb again, using the same settings as before.

Listen to the result.

After approx. 12 seconds, there is a change in the character of the reverb sound.

GVerb is a “simulation” effect.
Unlike a natural acoustic reverb the echoes and reflections occur at precise times according to the settings.
In particular, the effect that you are noticing is cause by the “Early Reflection” level. This setting creates a short “echo”. During the first 12 seconds (approx.) the echoes will re-enforce each other before they drift out of synch. I don’t really think this is “bug” but rather just a limitation of the simplistic method that GVerb users to simulate reverberation.

(note: using version 2.0.0 now)

Thanks for the reply, Steve.

After further experimentation, I’m convinced this must be a bug.

Try generating the click track as before, but make the duration 3 minutes.
Set reverb time to 0.1 second. Apply one pass of the reverb (I realized that
two passes aren’t necessary to reproduce the issue.)

As before, after the first 12 seconds, the early-reflection sound abruptly changes from a
longer-sounding decay to a shorter-sounding decay and retains this latter
characteristic for the remainder of the clip. (You can even clearly see the difference
between the two sections of the clip by observing the waveform.)

It’s rather odd, don’t you think, that the first 12 seconds of reverb should sound
distinctly different from the remaining 2 minutes 48 seconds?

Regarding your explanation, why should echoes that occur at precise times ever
drift out of sync if they do, in fact, occur at fixed times?

For the attention of the Forum Team:
I’m experiencing a minor problem with this topic. It repeatedly shows up when I do “View unread posts”. I’m hoping that by adding a post of my own it will clear the condition. By all means follow up off-forum via PMs so that this topic doesn’t get “hijacked”
Post edited to confirm that the condition was cleared by adding a post of my own. But why did the topic become “stuck” in the first place?

After further experimentation I’m convinced that this is not an “Audacity” bug, though it probably is a bug in GVerb.

GVerb is not part of the Audacity code.
GVerb is a third party plug-in from Steve Harris that is bundled with Audacity.
The problem occurs exactly after sample number 524288 (which is 2^19).

As far as I’m aware the main reason that GVerb is bundled in Audacity (rather than a different reverb effect) is because it is an open source plug-in (compatible with the GPL v2 license that Audacity is licensed under) and it is cross-platform. I don’t personally think that it is a particularly good reverb and the default settings suck.

Thank you for your persistence with this. I’ll report it as a bug but I very much doubt that it’ll get fixed (at least not by the Audacity developers) as it is not part of the Audacity code. This bug may however encourage the developers to consider bundling a different (hopefully “better”) reverb plug-in with Audacity rather than GVerb.

I’ll also see if I can find somewhere to report the bug to Steve Harris.

From the GVerb ReadMe: Reverb implementation by Juhana Sadeharju (kouhia at nic.funet.fi). I (Steve Harris) ported it to LADSPA and did some testing. Please contact Juhana directly regarding any bugs you find.

See also Steve Harris’ LADSPA Plugin Docs, section GVerb (gverb, 1216) for some recommended settings.

I generally agree that GVerb is not the best reverb plugin that has ever been made, but it’s one of the very rare reverb plugins that are not based on proprietary plugin interface software (VST etc.) and can be used on Windows, Mac, and Linux for free without running into legal problems. That’s probably the main reason why the GVerb effect is distributed together with the Audacity Windows version.

From the German Audacity forum I can tell two no-cost VST reverb plugins that are known to work with Audacity_2.0 on Windows:

  • edgar

Which makes it a bit problematic as we don’t know if the bug is in the original (Juhana Sadeharju) source code, or in the LADSPA port (Steve Harris and Vaughan Johnson). I’ve spoken to Vaughan Johnson about GVerb previously (Vaughan did the work to port the plug-in to Windows) and he suggested that I contacted Steve Harris, which I didn’t do at the time as I don’t really like GVerb anyway. I have e-mailed Steve Harris about this bug and I’ll post if anything comes back.

I just remember that some years ago I had wrapped Roger’s code from the Nyquist Reverb Example into an Audacity Nyquist Reverb plugin. Maybe we can improve the code and replace the ugly GVerb effect from the Audacity Windows version.

  • edgar

I’ve also got some Nyquist reverb plug-ins including stereo reverbs (iirc Roger’s code is mono). None of my reverb plug-ins are “release ready” but I’ve used them myself when GVerb was not good enough.

Audacity Nyquist includes three reverb functions (nrev, jcrev, and prcrev) which are useful building blocks for reverb effects, though there is the usual problem that they are a lot slower (on Linux) than LADSPA or built-in effects.