If you use an external A-D converter such as the Audient ASP880 (pictured) then it usually, though not always, makes sense to set it rather than your audio interface to be the clock master.
I love to read Hugh Robjohns’ stuff, even if the deep technical knowledge sometimes (often?) overwhelms me! I just read his article about the need for master clocks. My own setup is an Audient ASP880 eight-channel preamp connected via ADAT to an Avid HD Omni working with Pro Tools 12.4 HD Native. I was wondering if it was better to set the Audient as master for recording (and the Omni as slave) since the Audient does the A-D conversion, or does it make no difference? And, should I set the Omni as master again when I’m mixing? Thanks in advance.
Guido Jakobi
SOS Technical Editor Hugh Robjohns replies: I’m glad you enjoy reading our technical features, and thanks for the kind words! As you’ve gathered, the most critical element in any digital system is the A-D conversion, since that’s usually where the sound quality is really ‘set in stone’. Unless you’re sending a signal out through the D-A to, say, an analogue compressor and recording the result back through an A-D, then if you don’t like the sound of a particular D-A converter you can always change it later, but once a signal has been digitised by an A-D, you’re stuck with the data created at that point!
One of the most important factors in A-D quality is the stability of the sample clock, and I think it has been well established now that most converters deliver their best technical performance when running off their internal clock, rather than being slaved from an external one. So, if the Audient is handling the most important analogue inputs to your system, then I would agree with your thinking that running the ASP880 as the system master clock (and setting the Omni HD to operate as a clock slave from its ADAT input) would be the best technical approach, and the most likely configuration to give the highest possible conversion quality when recording.
As it happens, this arrangement would also be the easiest and most convenient way to wire up your specific system, too. And it’s also the most sensible arrangement if using the Audient’s A-D converter and the Omni HD’s D-A converter simultaneously to hook up external analogue processors, as the A-D stage is usually slightly more sensitive to clocking than the D-A one.
But when it comes to mixing, and assuming that you are using the D-As in the Omni HD for your monitoring feeds, then it would make sense from a technical perspective to switch the Omni HD to be the master clock. To use the Omni HD as the clock master, you’ll need to run a BNC-BNC cable from the Omni’s word-clock output to the ASP880’s word-clock input (with the 75Ω termination switched on). Set the Omni HD to run on its internal clock, and the ASP880 to accept the external clock input.
Of course, there’s nothing to stop you experimenting with alternative clocking arrangements to see if you prefer the sound of one configuration over another — but be aware that any sonic differences are likely to be extremely subtle!
In practice, I suspect you probably won’t notice any sound quality benefit from re-setting the Omni HD to be the clock master when mixing, and I’d be tempted to leave it in slave mode throughout, just for the convenience factor. Rest assured that either way, the clock master selection will have absolutely no affect on the quality of your digital mix files at all — it will only affect the performance of the D-A and, therefore, the local analogue monitoring.
Published March 2017
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