Need help understanding quite a bit in the Pro Tools / interface / converter bounce process

[cashhewn]

Hi,

I only started down this rabbit hole because of a problem that cropped up seemingly out of nowhere: my usual bounce method from Pro Tools started intermittently producing glitchy files where the waveform looked very strange (like the same half of the wave form was maybe being recorded onto both stereo tracks) and there was full dBfs buzzing/really fast pulsing on top of the recognizable audio.

I'm in a hybrid setup, not in the box, and my usual method is to mix everything out of Pro Tools via my Focusrite converters through loads of outboard and an analog console and then capture the console's stereo bus output back into Pro Tools via an RME ADI-2 FS converter connected to the Focusrite via SPDIF. Finally, then to get my captured stereo master mix as an interleaved WAV file out of Pro Tools I will use the "bounce mix" function and as the "mix source" will select the output of my stereo track which has historically been the SPDIF outputs of the Focusrite converters.

That's all been working great until the above problem in the first paragraph started cropping up. However if I simply switch the output of the mix capture track to a pair of the Focusrite's analog outputs, and choose those as the "mix source", the problem goes away. Yet it's only an intermittent problem...

I'm sure there are useful observations on all of that, but here are my real questions:

-if I am using my RME ADI-2 FS to digitize the mix bus of my analog console back into Pro Tools onto a track, do I even need to "bounce mix" again, or could I just export the file (or even use the files already written to the project's audio folder, although those are mono left and right, not stereo interleaved)? What would be the "best quality" here or most sensible/easiest workflow?

-when I "bounce mix" and have to select a "mix source", what is selecting a pair of the converter's SPDIF outputs vs a pair of its analog outputs actually doing? Is it using different conversion chipsets? It's already a digital file, so sending it out of SPDIF is just re-writing the file somehow (?), yet sending it out of an analog pair as the "mix source" is round trip converting it again somehow?

I'm so ignorant here...

Thanks for your help!

[Hugh Robjohns]

...
if I simply switch the output of the mix capture track to a pair of the Focusrite's analog outputs, and choose those as the "mix source", the problem goes away. Yet it's only an intermittent problem...

Sounds like a clocking problem to me.

How are you synchronising the RME's A-D clock to the Focusrite/PT?

[cashhewn]

Thanks Hugh, good question: my understanding is that the RME ADI-2 FS is set to be the master clock source, and that one of the Focusrites receives this signal via RCA over SPDIF and then passes it via BNC cable to the other attached Focusrite. This has all worked for several years until last week without any changes I can think of…

[cashhewn]

-if I am using my RME ADI-2 FS to digitize the mix bus of my analog console back into Pro Tools onto a track, do I even need to "bounce mix" again, or could I just export the file (or even use the files already written to the project's audio folder, although those are mono left and right, not stereo interleaved)? What would be the "best quality" here or most sensible/easiest workflow?

I'm going to attempt to answer part of my own question here: I don't need to "bounce mix" again of the captured stereo output from the analog console in order to get it out of Pro Tools, I can just use the two mono left and right wav files that were created in the default Pro Tools session "audio" folder when I captured the mix. I can simply give those mono files to the mastering engineer.

This also avoids the entire issue of the OP, though I'm still left ignorant of what "bounce mix" is actually doing in this case where there is no audio processing going on and ostensibly no summing or anything else necessary...and I'm still not sure why selecting one "mix source" over another in the bounce process causes the OP issue.