That’s actually a really good question. You’re right, absolute accuracy has much less of an impact on sound quality than jitter performance. Put another way, I believe the discrepancy on middle C worldwide is around 60,000 PPM, so we’re working at accuracies well above that as it is. Being perfectly frank, given this personally I do not have a solid explanation for why an incredibly accurate long-term clock would provide a sound quality increase.
With that being said, we’ve had reports from customers over the years who have done tests with different reference clocks going into a Vivaldi Clock. Invariably from a few who (I believe) were doing blind tests, they all independently concluded that an atomic clock with a very high long-term accuracy offered an improvement in sound quality. Obviously one clock’s long term accuracy being higher than another’s shouldn’t in itself offer a substantial increase in sound quality given our low psychoacoustic sensitivity to long term accuracy (as a whole the human ear and brain is much more sensitive to short term timing errors than long term frequency drift caused by long term inaccuracy).
I must admit that running an atomic clock into a Vivaldi Clock isn’t something I have had the opportunity to try personally, but the consensus seems to be it does offer an improvement. I should note that unlike some other “does it doesn’t it” topics floating around the audiophile world, this one is whether the human ear is sensitive enough to hear the difference, not whether the difference exists.
A clock directly feeding a DAC has a very different requirement to a clock feeding another master clock though, so obviously the relevant specs in each and how they should be taken into consideration across the whole system will differ. Absolute accuracy into a DAC is a different ball game to absolute accuracy into a Master Clock.
I try to stay away from inter-manufacturer debates and instead discuss the technology and engineering principals. I would say though that we have been in the Master Clock game for several decades now, and go to some impressive lengths to get the best jitter performance possible. We also have pretty much free-reign to create an audio system which is the highest performance possible, and with that over three decades we have settled on the FPGA enhanced VCXO system, with some incredibly capable PLLs throughout the system. The whole Vivaldi system has been designed to sound as good as is possible, period, with all the trade-offs that entails (there’s no such thing as “compromise free” in engineering, engineering is compromise), as opposed to something that’s good on paper.
In short, no, dither won’t impact an external reference clock’s effect on a Vivaldi Clock. The key point it impacts is the DAC’s ability to lock to the incoming Word Clock signal from the attached Master Clock when the phase difference between the two is low (somewhere PLLs naturally exhibit a dead-zone). It’s one of the ways the dCS system as a whole minimises jitter, but operates only within the dCS ecosystem and wouldn’t be impacted by an external reference clock.