Quick update for anyone who was reading along:
Bruce, who offered above to make short "regular DB25 (1:1 pinout) to TDIF" adapters to test this, made the first such adapter and sent it to me. THANK YOU AGAIN, BRUCE!!!
As described further above, I already had 24 channels wired between my DAW computer's audio interface(s) and my D8B's Tape I/O cards (DIO-8) via ADAT, and this has worked flawlessly for me.
Now I added a TDIF connection between the first such DIO-8 card in the D8B, and the first DIO-8 card in my HDR (while leaving all ADAT connections as they were). This is by using a "computer" DB25 cable (serial/parallel - just because that's the only spare DB25 cable I had) with the adapter Bruce made, to match the TDIF pin-out.
(of course a store-bought TDIF cable would also work... but they're surprisingly expensive, and I didn't want to spend that kind of money before I even know if this whole thing works like this or not).
With audio playing from both, the DAW and the HDR, I was now able to switch the first 8 "Tape Return" inputs on the D8B from what's coming in from the DAW on channels 1-8, to what's coming in from the HDR on channels 1-8, simply by changing the software setting on the D8B under Setup/Digital I/O from ADAT to TDIF (for the first DIO-8 card).
I didn't even have to stop the playback. The audio switched over seamlessly with no clicking or crackling, etc. AWESOME!
I couldn't hear any digital artifacts during playback over TDIF (or ADAT), either - it sounded perfectly clean to me!
Of course the DAW computer/interfaces, the D8B and the HDR were all clocked to an external Master Clock unit, so all their digital streams were running in sync.
It was quite the experience to have 8 tracks from one project coming out of the first 8 channels, and 16 more from a completely different project out of channels 9-17, haha! (I didn't transfer stems from the project in the DAW to the HDR just yet, but played back an old rehearsal room band recording on the HDR).
So, the proof of concept was successful! AWESOME!
There's a few things to consider, though:
To use the HDR with the D8B properly, you want the HDR to generate SMPTE and/or MTC (i.e. the stuff that is needed to make the red time code display on the D8B move). Since this won't be coming from the DAW if your intention is to use an HDR as your "tape machine" and leave the DAW computer off, the timecode needs to come from somewhere... and in this case, this would have to be the HDR.
So, if you usually have the HDR set to "Chase Timecode", then you need to turn this off, since the HDR can't simultaneously generate timecode AND sync to incoming timecode. You will also need to turn on "Generate SMPTE" and/or "Generate MTC" in the Sync settings on the HDR.
You have to revert these settings, if you want the HDR follow your DAW's timecode again, of course (not sure who else does this, but I have the HDR run along to the DAW, sometimes).
Another thing is the MIDI wiring for the sync. If you don't want to change the wiring when switching between DAW and HDR operation on the D8B, you'll have to use a MIDI splitter on the MIDI out of the D8B (sending the same signal back to the DAW for MMC control, and also to the HDR, so it knows when to start and stop as well when you push the transport buttons on the D8B), as well as a MIDI merger that funnels the MIDI Out from the HDR and the DAW together, before being sent to the MIDI in on the D8B.
The MIDI wiring didn't function as expected for me just yet, but that's just because I never labeled the cables and have to spend some time sorting that mess out... but if I connect a pair of MIDI cables between the DAW and D8B directly, it works as it should. Same thing, if I instead connect the MIDI cable pair between the HDR and the D8B.
So, I'm more than thrilled that this works, and can't wait to use it with all 24 channels hooked up between my HDR and D8B, in addition (but not used at the same time) to the 24 channels between my DAW and the D8B.
If you're wondering about the use-case, two things come to mind:
Usecase 1. I'm a computer/DAW guy, but find computer screens and mouse/keyboard operation to be an unmusical distraction during tracking (e.g. I'd rather give my full attention to the talent while recording, rather than having my head buried in a computer screen), and also during mixing, where I also mainly want to focus on what I hear, rather than what I see on my computer screens. So, I might just do the tracking via the HDR, leaving my DAW computer off (although I'm sure I'll be missing the semi-automatic comping options modern DAWs have to offer, that I frequently utilize during tracking, already), and afterwards transfer the tracks to my DAW, to get all current options for editing and improvements (plug-ins, pitch and timing correction for vocals, etc. etc.).
Still in the DAW, I'm thinking to then export stems (as if I would get the whole thing mixed by someone else), transfer them back to the HDR, turn the DAW computer off and then mix the entire project with only the D8B and HDR, together with my outboard gear.
I'm not sure yet how practical this is, but I intend to try how far I can get with this, while keeping the D8B and HDR monitors off... so I can focus fully on what I hear, with no visual distractions. (But menu-diving on the D8B might get a bit annoying with the screen off, of course... but at least my D8B/HDR screen (with KVM switch) is off to the side, and thus less distracting for me).
Old-school, just like people did it back in the day with their tape machines, analog mixers, and racks of FX/Dynamics processors
I'm already exited about trying this, even though I don't have all the wires to hook it all up, yet
Usecase 2: Any current HDR/D8B user, who isn't ready to make a hard switch to use a DAW in place of the HDR, could use this approach to get their feet wet on the DAW side, while being able to switch back to HDR use instantaneously without losing any time.... and while being able to continue working in a familiar fashion with the D8B... just with the audio coming from a DAW, instead. Basically, you get the best of both worlds (as long as you have a total of 6 DIO-8 cards for your D8B and HDR, as well as a DAW computer setup that gives you 3 pairs of ADAT I/O), and can use your studio's entire audio wiring just as it was, with the source of your multi-track audio just coming from somewhere else digitally
Almost as if Mackie meant it that way at the time
If you're considering to try this and need to find DIO-8 cards for it, make sure to find "V2" or "v2.1" versions of those cards, though (there's a sticker on the main chip that says so). Older versions of these cards had issues (was it a phase reverse? Or even a 16-bit only issue or something?)
Once I have all the TDIF cables/adapters I need, I think I'll write a step-by-step guide on how to do this, including all the necessary settings and some pictures, etc. But for now, I just wanted to share that the first test worked BEAUTIFULLY, and IMO, this gives the D8B a whole new setup possibility, that blurs the lines between a DAW-based studio and a multi-track "digital tape machine" based studio. With the D8B, you don't have to chose - you get to have both and can switch back and forth.
Yay, D8B!
