"HDMI" and "audio-only" don't seem to go together for me.
I am still having trouble understanding why one wouldn't use SPDIF for audio in any case.
Hmm. HDMI supports higher sample rates, higher bit depths, more channels, and more encoding schemes (PCM, DTS, Dolby etc.). And even provides a "free" video channel too.
As noted by AngrewFG and the link below, S/PDIF is limited:
http://en.wikipedia.org/wiki/SPDIFin terms of compression for multi-channel audio. If you're a stereo only guy like me, then that's not an issue.
The next question about HDMI over S/PDIF that I've looked at is jitter. Here's a link that sums up what I've reviewed:
http://www.avsforum.com/t/1377020/spdif-hdmi-and-jitter#post_21290576Some further comment with more detail here:
http://www.thewelltemperedcomputer.com/HW/Connect/HDMI_connect.htmSome views at the other end of the spectrum:
http://nwavguy.blogspot.com.au/2011/02/jitter-does-it-matter.htmlhttp://www.audiocraftersguild.com/AandE/npt.on.jitter2.htmhttp://davidkessner.wordpress.com/2011/10/27/a-pragmatic-approach-to-jitter-in-digital-audio/The post from David Kessner resonated (excuse the pun) with me by putting various jitter sources in context, e.g. speaker cone movement, head movement and the rest. I've got an HDMI audio setup and I can't immediately pick up a difference when it's run via USB or optical S/PDIF. I haven't done a critical test but I'm not that worried.
Ultimately, it comes down to the ears of the individual. There's a well referenced 2 part jitter test with sound samples here:
http://hddaudio.net/HDD_Audio/Articles/Entries/2009/12/19_Can_you_hear_jitter.htmlhttp://hddaudio.net/HDD_Audio/Articles/Entries/2009/12/19_Can_you_hear_jitter_Part_2.htmlI've done this or a similar test (it was a while ago) myself and I obviously don't have golden ears. I couldn't pick a difference. I don't think I did test 2, so maybe I should give that a go.
Bear in mind that the research on this extends back beyond the mainstream digital audio age. Refer this paper by the BBC:
http://downloads.bbc.co.uk/rd/pubs/reports/1974-11.pdf(Note the paper is 7 pages long. Every 2nd page is blank)
Some relevant discussion on that paper is located here:
http://hddaudio.net/viewtopic.php?id=42Leave you to chew over the myth, fact, fiction and conjecture. There's plenty out there covering all four aspects. Enjoy ..