Firstly, thanks for taking the time to write such a comprehensive and informative explanation.
I guess my terminology was incorrect; "prone to" does suggest that the dac is the cause of the jitter/errors. You're saying DS is more sensitive to them. TBH, I wasn't sure which.
But your explanation does seem to suggest that maybe my severely limited understanding, of the basics, wasn't totally incorrect.
I don't know that anybody thinks that you can actually hear the jitter frequencies. But, I guess, what's in question are the effects of jitter that are audible.
This understanding seems to be changing as time goes by. Originally, it was totally mis-understood, unknown even! But over the years it seems that engineers are discovering that it's effects are ever more influential to the sound reproduced.
You seem to believe that jitter can relatively easily be kept so low as have no influence on the sound, even for the sensitive DS designs. I'm not so sure that other learned people would agree, today or in the future.
I've heard lots of different opinions about "what jitter sounds like".... My (informal) way of testing it was simply to compare the way a given DAC sounds by itself, and how it sounds when I feed the signal going to it through a "jitter remover" (one which I trust to, by the numbers, actually be reducing the jitter to very low numbers). What I tend to hear is that, with "higher jitter", wire brush cymbals tend to sound somewhat unnatural (more like a burst of steam than like metal on metal), and that sometimes sibilants sound odd (sort of emphasized but not necessarily louder). I hate going into details because these are clearly subjective, but I've asked other people their impressions of what was happening, and theirs seemed to agree with mine. I personally don't hear any significant difference with tracks that contain mostly vocals, or tracks with no strong high frequencies, and I find that only a few recordings seem to be "clear" enough to begin with that I notice the difference.
Unfortunately, measuring jitter directly is very difficult, and very few companies are set up to do so. (The oft-mentioned J-test doesn't actually measure jitter directly; it delivers a "torture test signal" to the input of the device, then looks for increases in the jitter spectrum at the output. In short, while it provides a reasonable inference about the device's susceptibility to jitter, it neither delivers a jittered signal to test with, nor measures actual jitter at the output.) Note that the output noise spectra caused by jitter are its "symptom", so measuring them
DOES give you an accurate picture of how much the sound quality of your output is being affected by jitter.... it just fails to provide detailed information about the error mechanism involved. (So "not seeing significant jitter spectra" really is sufficient proof that "jitter isn't a problem".)
As for "how easy it is to design for low jitter" - that depends on your design and production process. You need to start out with parts that have very low jitter, and a design that doesn't introduce too much extra jitter, then you have to build it in such a way that you preserve those benefits. As a very wide generality, assuming you start with a perfect signal, commercial S/PDIF interface chips can maintain jitter at or under 100 pS (that's a quote - I haven't measured it). An asynchronous USB interface can deliver lower jitter since, because the clocking is done by the receiving device, the jitter of the incoming signal really doesn't matter.
Unfortunately, designing for very low jitter tends to entail critical placement and spacing of circuit paths, which pretty well rules out any point-to-point wiring (you need to create a PCB design that works well and keep any interconnect wiring as short and as neat and consistent as possible). This makes it virtually impossible to "breadboard" a really low-jitter design. So, for a company designing a product, it tends to work out to "design the basic circuit, understanding that it won't work very well; once the basic design is good, design a board; send it out to be built; test it and see if it works like you thought; correct any errors; try again; repeat until you get it right" - which is a bit cumbersome for DIY.
I was merely referring to an example of two DACs I have listened to recently where there was a clear difference in sound, that's all.
