Sound glitches / distorts when main window changes out of instance or DAW

Hi fellow GP users, hope there’s a simple fix for this one.

When running a main GP window into either (a) an instance, or (b) into GarageBand, I am experiencing sound glitching when I’m outside a GP window. It typically happens after ~10 to 45 seconds. It sounds like CPU overload kind of glitching but CPU meters in GP don’t exceed 30% in the main GP and 10% in the instance. I can reproduce the glitch at times with only 17% and 1% CPU.

As soon as I choose the main GP window in MacOS as the ‘active window’ the glitch is instantly fixed and everything goes back to normal. It’s almost like it’s not allowing me to run GP ‘in the background.’

The setup is Midi keyboard into synth/key plugin such as Surge, bx_oberhausen, Pianoteq, etc… and then out of the first GP via Blackhole input. The Blackhole output is then the input into either GarageBand or another GP instance. These destinations then output to the sound card outputs.

The problem doesn’t happen when I use a single GP by itself. It seems to only happen if I run from the main boot up of GP into either (1) an instance of GP or (2) GarageBand.

I’m on GP3 currently, and the audio interface is an IK Multimedia iRigHD2. I checked the audio settings in GP and I’m using 44.1k when doing this with GarageBand, and also tried 44.1 and 96k and various buffer sizes (128 up to 256) when not using GarageBand (i.e. GP into GP instance, with audio settings set the same in both GPs).

Am I missing a setting config? Am I supposed to create an Aggregate Device with both the “iRig + Blackhole” and use these in GP instead of: routing from the first GP into an independent “Blackhole” and using the “Blackhole” as the source for the GP instance? If so, what is the reason for the need to do that? A synchronization issue?

Machine is a 2015 MacBook Pro Retina i7 2.5Ghz 16GB, plenty of SSD space, Catalina 10.15.7

Thanks for your help!

It sounds like a strange one, if you’re finding differences simply by selecting the window.
I use blackhole, but I have it in an aggregate device with my interface. So I use the same aggregate device as both input and output in both apps (taking note I use the correct ports).

When you use an Aggregate Device, you can choose either your interface or blackhole as the ‘Clock Source’ in the Mac’s Audio MIDI Setup. I don’t really understand the detail of this, but I have resolved a glitching issue before because my interface was not set as the source.

It’s at least worth trying the Aggregate Device.

The other thing will be the higher buffer would be more likely to reduce glitches.

Thanks for sharing your insight. Yes, I’ve tried Aggregating and it seems to solve the problem for now. I haven’t tested extensively yet though.

I’m trying to understand what was the problem with the previous flow and why it would glitch when setting the active window/app to be something other than the main GP window.

It seems to me that when Aggregating and configuring the clock source the audio data is forced to synchronize. Maybe when not doing this and making a window other than GP the active window the OS doesn’t maintain synchronization?

Is it possible the other program has a different audio configuration (e.g, even that clock source)?

That was what I was thinking, but I can’t figure out why it happens when using GP into a GP instance also, and for example changing from GP window to the GP instance window, or another window such as a Chrome browser window.