Bug report: Multi-mono AAX breaks Bypass button in strange way

There’s a bug with AAX plugins running as multi-mono. It’s a bit obscure, and it’s a tricky bug to tackle because it doesn’t appear consistently.

Here’s the deal: a JUCE AAX plugin, in multi-mono with the channels unlinked, and then with each channel put in bypass, and the session saved in that state, will re-open with the Bypass button broken. It will appear to be on, however, the plug-in will be processing audio on the bypassed channel, AND clicking on the Bypass button will have no effect (in other words, you can’t take it out of bypass).

Here is the GainPluginDemo plugin, on a 5.1 track. It is being fed pink noise at -20 dB on all channels. You can see that the plugin window’s Channel Selector is on the L channel, and that the channels are unlinked. Gain is set for 0.00 amplitude, but the channel is in Bypass. Yet, you can see in the metering that the first channel of the track has no level:

And here, clicking the Bypass button has no effect on the L channel - it looks like it’s stuck ON, even though it’s functionally OFF.

To reproduce: build the JUCE GainPluginDemo completely stock, with the only modification being enabling the AAX plugin format.

In macOS (Mojave tested so far), launch Pro Tools. Create a 5.1 track (this also works on stereo tracks), with pink noise feeding a multi-channel instance of the GainPluginDemo. Set it up as described above, with all channels bypassed, unlinked, and with one or more of the channels set for a Gain of 0. Important - you seemingly must add another plugin to the track, after the GainPluginDemo, for this bug to appear. I have a multi-mono instance of the stock EQIII 7-band.

You may also need to have at least one other track in the session for the bug to appear. As I said, it can be inconsistent in appearing, and so I haven’t been able to isolate all the factors.

Once it’s all set up, save the session, and re-open it. It should open looking like the screenshot above. If it doesn’t, try re-opening it a few times, sometimes it doesn’t happen.

I will try to attach a Pro Tools session file here to make it easier to reproduce.

OK, this seems to have worked. Here is a Pro Tools session file to save the headache of reproducing the setup:

Gain Plugin bug session 01.ptx.zip (18.5 KB)

I think I can see the issue. Could you try out this patch and check whether it resolves the problem for you?

potentialfix.patch (1.3 KB)

Yes, looking good here so far.

I have tried this patch on the GainPluginDemo, and on our own plugin that I was initially troubleshooting.


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Great, I’ll get that added to develop then.

Thanks again for reporting this issue. It should now be fixed on develop.

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