Does this apply to any implementation? In my understanding a low-pass filter with a quite round fall-off characteristic (thus with a low Q factor) will produce a rather large-band ringing, far from the ideal of a filter with infinite sharpness, which exclusively rings exactly at the filter frequency (in return infinitely).
How much, in your view, is the audio band affected by delayed decay from the finite filter sharpness in common DACs and CD players? Isn't exactly this discrepancy between ideal and real-world reconstruction filter (among the other mentioned issues) responsible for the effort you had to invest for Hugo and finally DAVE? («So the closer we get to this ideal filter, the closer we get to the original, and the more transparent the system becomes.»)
Like Sunya, I'm also concerned about the bandwidth limitation necessary before A/D conversion, as it introduces the same (still underestimated) flaws. Hopefully Davina will make for recordings that sound even more natural in the future.
I suppose the point I was getting across that for a DAC interpolation or reconstruction filter, I have never seen the ringing performance being a significant factor; by this I mean there were always other explanations for the differences in SQ I can hear whilst optimizing the SQ during listening tests on the WTA algorithm. In broad terms, getting the filter to be close to the ideal sinc response is what you are doing - but when optimizing the sound quality sometimes that will mean more ringing, sometimes less ringing - its just not a relevant factor. The reason for this is that an ideal filter will ring infinitely, but will recover the un-sampled bandwidth limited signal absolutely perfectly with no change to the signals ringing at all. If we want to evaluate a filters performance using ringing, then we need to use a bandwidth limited impulse response test signal. So the ringing performance using an illegal (from sampling theory) impulse response has no bearing on the SQ - except that the more ringing, then perhaps the closer it becomes to the ideal sinc filter. Certainly going for a filter that does not ring is a very bad idea - the ringing is absolutely essential as it is the multiple rings from all the past and future samples that when added up gives the correct transient values.
Just to give you an idea - I have designed a 8FS WTA filter, and a 16FS WTA filter with identical ringing performance - except the 16FS filter has of course double the output resolution - and when you see the two impulse responses after they have been further filtered to 2048 FS they look identical - but the two filters actually sound very different.
So the filter for the interpolation filter has to be a certain way with lots of ringing, if you want to recover the signal perfectly. But for the ADC the only requirement is that it is perfectly bandwidth limited, theory does not care how it is done. So I plan to just try different filters to define the aliasing problem (which I know for certain from listening tests is a major SQ issue) and then try differing ringing responses to see which type of filter sounds best. My current thinking is that ringing in an ADC will turn out to be unimportant; we just need it to be bandwidth limited by enough to eliminate aliasing, but
I too am worried about the effect of 22 kHz bandwidth limiting for redbook.
But until you do the work, and do lots of careful listening tests, you know nothing...
Rob