At one point I was worried about the fact that USB does have the benefit of being locked to the FPGA clock, whereas SPDIF is entirely asynchronous. So this was a potential concern; indeed before Hugo 2 USB sounded slightly brighter than optical - was this brightness due to more transparency (because the clock is common) or because RF noise on optical is lower because it's perfectly isolated (and so warmer sounding due to lower noise floor modulation).I could not tell from listening tests which was the explanation. If USB was better, than that meant I had to improve the DPLL; but if the difference is down to RF, then I had to improve the USB isolation. But when developing the DPLL I compared a buffered system (local clock only) to the DPLL (asynchronous input), and managed to hear no significant difference - but it did take 6 years to perfect the DPLL. But SQ has moved on enormously since the DPLL had been perfected, so perhaps I had to re-evaluate the DPLL performance - and maybe it was that which was causing the SQ differences. My job as a designer is to constantly re-asses and to find areas to improve performance - and sometimes it is obvious where improvements can be made (like WTA tap length) but sometimes you are existing in a sea of uncertainty; so one has to constantly question and re-evaluate.
But with Hugo 2 I had improved the USB inputs by adding ferrites on the ground (they always were on the VBUS). Also I had the benefit of a source (my MSI lap-top) that had optical and USB outputs. So you can imagine my complete surprise that with HP listening I could hear no difference at all between the USB and optical inputs. This meant several things - that the DPLL was indeed perfect sonically; and that listening via headphones and with a lap-top the USB isolation was now good enough. This means too that any changes one makes to the source, the warmer one is the better solution - even if it makes the system too warm (then you need to brighten it by making changes elsewhere). All too often I have experienced that with the so called USB or source improvements, only to discover that the "improvements" are only increasing RF noise and making it sound brighter and hence worse.
So no I do not need to improve the DPLL code at all...
