Thought it might be time to recount some recent listening that I had been meaning to do for a long time, and its bearing (and confusion) on jitter. I built up a big pile of kit recently, and can test many different configurations. Firstly I compared spdif, toslink, i-link and hdmi interfaces when playing CDs. The first two were quite similar, and the differences were more of character than quality. I knew that i-link would be a big step up, and so it was.
HDMI was the interesting one for me, as before now, I hadn't had an amp that accepted it. I heard people say it wasn't as good as spdif, but this didn't prepare me for how bad it was. It was very disappointing, as one connection would have been so convenient; but it won't do. Difficult to say what it was doing wrong, but it just sounded a mess, and easily worse than everything else. It was difficult to keep the differences in perspective, as many manufacturers and consumers presumably think that audio over HDMI is the best they can get, yet it was hard to think of anything good to say about, or even continue listening.
Then I tried playing Blu-ray LPCM soundtracks over HDMI, and compared analogue and down-converted spdif connections. That might seem like an unequal comparison, as LPCM was 24 bit/48kHz, but my experience above suggested that HDMI itself may be the quality bottleneck. Where spdif sounds better when carrying the very same 16/44.1 stereo audio bits, I can only assume that hdmi has too much noise, jitter or interference to recover the audio faithfully. And that would also apply to HD audio.
Well, no. LPCM over HDMI simply blew spdif out of the water. I was kind of hoping that spdif might have been close, and then I could justify selling the HDMI receiver that I didn't really want to keep. But I simply couldn't turn my back on the difference that was available for the want of an interface. While I willingly rubbished HDMI with stereo, I wouldn't do that with spdif here, as it still sounded fine in isolation. There wasn't much wrong, it was just that LPCM was just so much more transparent. I even hoped that my Sony BDP-S1 might have decent analogue outputs, and I compared them. While they were a bit better than spdif - smoother, sharper, cleaner and a bit brighter - the difference wasn't great, and the gap to HDMI was still considerable.
Since the HDMI amp also has an i-link input, I have a straightforward way forwards, but this has now asked many new questions about how the different digital audio interfaces work (or don't). Looks like it's not a simple subject.