Bypass callback/notification

I have a synth out for sale and found a small bug that I really don't know how to solve properly:

When the instrument is up and running in a DAW, I send a NoteOn midi message (holding a note on the keyboard). Before sending the NoteOff (releasing the note on the keyboard), I bypass the plug-in. When the plug-in is bypassed I release the keyboard (sending NoteOff). When I enable the plug-in again the note obviously continues playing since the NoteOff wasn't parsed in processBlock() because the instrument was bypassed at the time.

My current (seem-to-be-working) approach is to have a check the current time in processBlock() and if there's a gap greater than some value based on blockSize and samplerate I turn off all playing notes. This method is a really ugly hack and will most certainly turn back at me at some point/in some DAW/in some special-case that I cannot think of right now.

So, to the question: Is there any other way do get a notification/read some flag whether the instrument is bypassed by host so I instead could check for actual changes instead of assuming stuff based on gaps in the audio stream.

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Interesting. What plugin format are you using? Is that AudioUnit, VST2, ...?

How about AudioProcessor::processBlockBypassed() – all the wrappers call that when the plugin is bypassed.


Yes, I was about to post that as well, but then I checked and found out that at least for AudioUnit, this AudioProcessor::processBlockBypassed is only called for effects, not for synths. But this is a synth.

Hmm… that’s only in the AU wrapper… The host really should disable the bypass button for a synth anyway.


Well, Logic does offer you a bypass button for a synth. I believe other hosts do, too. Clicking that button is exactly what this post is about, isn't it?

Apparently, right now a JUCE synth plugin has no way of knowing when this button was clicked.

In RTAS and AAX we have the ability to disable the host’s bypass button. It looks like the VST wrapper will call processBlockBypassed() for a Synth… the only issue is AU… is there a reason why Jules didn’t call processBlockBypassed() for an AU synth? The default processBlockBypassed() is an empty function and the developer could check if it’s a synth in the method.


It's AU and VST2.

I've only tested the processBlockBypassed() with the AU version (and since it didn't got called I assumed it wouldn't in the VST either). Anyhow, there's still an issue on how to solve this when creating AU's then... 


When peeking into I found this:


               #if ! JucePlugin_IsSynth

                else if (ShouldBypassEffect())


                    juceFilter->processBlockBypassed (buffer, midiEvents);



I'll remove those precompiler conditionals and see what happens first thing monday...



Hi sundhage,

Removing the precompiler conditionals won't help you, because the code still would only be called if it is an effect and not a synth:

else if (ShouldBypassEffect()) ...

That ShouldBypassEffect() is AudioUnit code, not JUCE code, so we can't change anything there. Unfortunately there seems to be no equivalent of that function for a synth in the AudioUnit SDK...


Ah! Saw that too now...