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QLab tech support reproduced the error in QLab but reported that it doesn't happen in Ableton Live. They also discovered the following workaround:

We found that it's possible to persist the settings for a QLab cue with
an extra step. After configuring the effect settings as you'd like them, 
save them as a user preset for that AU. Then load that user preset, and 
the configuration will persist even after stopping and reloading a cue, 
and after saving, closing, and reopening the workspace.

That sounds like what I did in that one case, and it makes sense because (duh) saving a preset saves the current patch state. Figure 53 (QLab's maker) doesn't have the resources to investigate further, so can't provide any more insight. @timothyschoen, would it be helpful I entered this issue into plugdata's github or is that redundant?

@ddw_music said:

FWIW I can't get Audacity to use plugdata-fx -- I've enabled it in the plug-in manager, but it still doesn't show up in real-time effects for a track.

Weird. I got nuthin for you. I installed Audacity 18 months ago, maybe updated it once or twice, and just recently installed plugdata. Wasn't even aware of the plug-in manager.

Update: I decided to skip the QLab group and submit a support ticket. I figure it's paid software, so why not?

@dreamer said:

So the problem you see with Qlab is that a saved project does not retain the plugdata state? or was it that simply closing the plugdata window resets its state somehow?

Well, both, but I would expect the former to fail if the AU forgets its state before you can save in the host. In QLab and Audacity, plugdata reverts back to its initial state (no patch loaded) in the course of normal host operations, before I've attempted to exit. I think I even tried exiting QLab while the cue and effect were running (and was prompted to save), but I'd have to rent another day license and check (which I'll do once I learn a enough about Cardinal to construct a useful test).

Hmm, but hold on a second. @ddw_music - if I somehow managed to save once in QLab (see the first paragraph of my original post), then doesn't that mean the huge plug-in state size hypothesis can't be true?

@dreamer said:

It makes more sense if you create a bug report on the plugdata github repository, so that the issue can be tracked, discussed and resolved there.

You're probably right, but having fielded tons of bad bug reports as a former developer, I just want to wait to see if someone has insight or if better tests occur to me. For instance, I just learned that the Mac version of REAPER runs AUs, so I tested in REAPER and there were no issues. So to me it's a tossup whether I should report it to the QLab group or the plugdata issue list.

(I'm also certain that someone on the QLab group is going to respond "plugWHAT?!!!" )

Update: plugdata as Realtime Effect in Audacity 3.6.1 is also forgetful.

I can't tell if this is a QLab 5.4.4 or a plugdata 1.9.0 issue. I create an audio cue in QLab, and then add plugdata to its audio FX. In the FX edit window, I load a patch into plugdata and a yellow circle appears to the left of the audio cue, indicating that the cue is loaded and ready to be triggered. If I run the cue at this point, I hear my patch altering the audio of that cue. But if I ever hit esc or stop the cue, plugdata forgets which patch was loaded and reverts to its opening state (no patch loaded). I've gotten it to remember the patch and save to the QLab file exactly once, but I didn't notice what I had done differently. I tried editing and saving the patch, but there was no change.

Previously, in QLab 5.4.1, I had issues getting plugdata to use the most recent version of my patch. It kept reverting to an older version that it had cached. Both issues are consistent with the hypothesis that the plugdata AU is not persisting the patch when QLab thinks it should. I've not had this issue with other AUs, but I don't use AUs that much.

Never had this issue with the plugdata VST3. I don't see any mentions of QLab on plugdata's github, nor any mentions of plugdata in the QLab Google group.

Does anyone know how to coax the plugdata AU to remember the current patch?

Update: I just confirmed similar behavior in Audacity 3.3.3. Effect->Add Realtime Effects->Add effect->AudioUnit->plugdata-fx. Load a patch in the FX edit window, and then play. Everything's as expected. But if you close the FX edit window, then as soon as you stop playback, plugdata forgets the patch. MacOS 14.5

@manuels Oh thanks! I missed else/highpass~

It actually seems to be the same

Teach a man to fish: you know this because you looked at source code, or some other way?

but you also have to take the square root of the Q factor!

Oh look, and they take the cube root for vcf_hp6~ when they chain 3 filters. I wasn't aware that Q worked that way. Do you know what they are doing with the cutoff freq signal, [iem_cot4~]?

Edit: just built it and listened on the cue I used to run live in Pd--I would not have been able to tell the difference blindfolded. Thanks again.

I'm assuming that I'd have to build an approximation in plugdata, is that correct? Do you have any suggestions on how to begin? I started stacking up a bunch of [vcf~] but had that strong "I have no idea what I'm doing" feeling.

@earlyjp I had to do something vaguely similar for a show last summer. I loaded the file I wanted to playback into REAPER, and then added automation and/or MIDI in relation to the audio file--all DAWs excel at that kind of thing. I triggered REAPER playback using OSC, and had a Pd patch running as a VST3 in plugdata (or maybe Camomile) that was reading the automation/MIDI and doing stuff in response.

BTW, since learning that's it's easy to encode arbitrary Pd messages as sysex, I've been using REAPER a lot more to record, edit, and playback Pd message sequences. To me, with my limited patience and imagination, It's more pleasant than doing it in Pd alone, although it's also true that I have to write myself plenty of notes regarding how. things are set up.