Dear Richard,
Thank you for your kind enquiry and your recent purchasing of a set of iFi units.
To answer your question:
"Most USB inputs convert USB to S/PDIF internally anyway. "
http://www.head-fi.org/t/645638/how-is-everyone-connecting-their-computer-to-their-dac/15#post_9057887
I remember you telling me about how bad SPDIF was. How is this done in my iDac? Are just doing USB to SPDIF?
We can confirm that iFi (using licensed technology from AMR) starts off with an Asynchronous USB connection. SPDIF is not used internally, but instead clocks and data are send separately to the dac chip.
The iDAC then adds JitterLess Lite which further reduces the jitter to negligible levels. Suffice to say, the asynchronous USB transmission design in the iDAC is second-to-none in terms of jitter. The iDAC has Jitter so low it cannot be measured!
SPDIF is an analogue transmission standard devised for Betamax digital audio transfers!
If you have the time, you may wish to take a look at this:
http://amr-audio.co.uk/html/dp777_tech-papers_spdif.html
It has only been adopted to carry a digital signal and was never great for the job, just as composite video connections are not good for high quality video.
If you have any further questions, please do not hesitate to ask.
Best regards
Vincent Luke - Markets
ifi-audio.com