SP/DIF, which clock source?

[dplus]

Hi,

I have a MOTU ultralite MK3 and a Sound Devices 702 digital 2 track recorder. They are connected by SPDIF, the SPdif output from my Motu goes directly to the SPdif input on the Sound Devices 702, and the SPdif output on my Sound Devices 702 goes directly to the SPdif input on my Motu Ultralite.

The SPdif output on my Sound Devices acts like a direct output. I use it to create another mix in Cuemix FX.

My question is: which clock source should I use if I use both the SPDIF OUTPUT and SPDIF INPUT at the same time on my Motu Ultralite?

The manual states (http://cdn-data.motu.com/manuals/firewi ... id_Win.pdf" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false; page 22) that if I play music on my motu, the connected spdif device will slave automatically to the motu, and if I record audio on the spdif input of my Motu, the Motu device has to slave itself to the Sound Devices 702.
But the problem is that I now use both SPDIF INPUT AND OUTPUT at the same time.

Please advise.

Patrick

[HCMarkus]

I believe it is gnerally recommended that one use the internal clock where the A to D resides... in your case the MOTU.

[mhschmieder]

Yes, that is correct. Even if you have a dedicated D/A unit, my recollection is that the clock on the A/D unit is recommended.

I have three units at the moment, but not a dedicated D/A (that might change soon, and I might even dump my dedicated A/D after evaluating it against my RME Fireface UFX that just arrived five minutes ago).

I use the Mytek AD96 as master clock via word clock cable connected to my (about to be sold) RME Fireface 800. The S/PDIF signal on its own was useless for driving clock; totally unreliable and glitchy.

As I also sometimes augment this setup with the Focusrite OctoPre mk II via ADAT, I always have to remember to set its front panel correctly, but I'm at the office so can't double-check that right now. I THINK I set it to use external clock, as I ALSO connect it to the RME Fireface 800 via word clock cable, meaning the Mytek drives the Focusrite clock via word clock throughput in the RME unit?

I'll try to remember to verify this setup when I get home tonight. It's easy to get confused about this stuff, but fortunately in this case, both Mytek and Focusrite provided ample documentation on their recommendations and the rationale.

And as I said, I'm pretty sure I remember reading that if you add a two-channel DA unit and attach it to your multi-channel ADDA unit, you set it up so that either a separate two-channel AD unit, separate master clock, or the multi-channel ADDA unit (depending on what gear you have) drives the clock on the two-channel DA unit. That is, I don't think two-channel DA units are very often used as master clock; though in some cases their internal clock may be used just for the two-channel DA itself.

I hope this didn't confuse you further.

[mhschmieder]

Looks like I might have gotten that backwards, as my cached notes say to use Focusrite OctoPre mk II's internal clock, but those might be out-of-date notes so I'll check the routing and my most recent 20-channel session notes when I replace my Fireface 800 with the newly-arrived UFX tomorrow morning.

[mhschmieder]

Sorry, I didn't have time to verify this yet as I was overwhelmed trying to get my new RME Fireface UFX properly integrated into my system, with very different cabling and routine from before.

I'm not sure why I would have connected the 8-channel ADAT interface to the Fireface 800 with a word clock cable unless it was being slaved indirectly via the Mytek though. I'll re-read the manual when I find it though -- I think it's still at the project studio. Focusrite may have said something about it being needed in order to lock the clock when it's being internally driven, to prevent the clock from being robbed?