Sunday, June 18, 2017

The zero jitter synchronous DAC

Just to show a fully synchronous (as I think I like) DAC can be made to have zero jitter, along with the usual full information preservation of fully synchronous, consider the "memory dac."

In the worst case, it could simply store entire songs to memory, then play them out.  That's the proof that this is possible.  However an open line that just stays running digital audio for wouldn't want to wait until the next day to play your audio would you?  So, while it's possible to to have a simple spdif DAC with an absolutely perfect (or at least from the stanpoint of the 'better' DAC) clock, at minimum you have to change the clock speed in the dac to match the source over any reasonably long amount of time.

But it wouldn't be hard not to have it that hard.  Or just the obvious thing to do is keep a sort of clock quality whenever a digital SPDIF/AES line is locked up, a notion of how much the clock has moved around, so how much needs to be allowed for.

The quality analysis is used in figuring the size of a buffer needed to keep up with changes.  Then there may be some lag when any playback is started, because of allowances for small irregularities.

Now in this scheme, you can't have a perfectly on-time clock, you need to set the ultimate clock based on the analysis of the line as well.

And in worst cases, you may need to make slow changes to the clock rate to keep things going.  But that's very small changes, and very slow changes.

OK, so maybe not exactly zero jitter.  But it can be zero above some very low frequencies where it is even more inaudible than other frequencies.  (Remember, digital jitter being important in audio is just hype, it isn't really, though the hardware obsessed audiophile just wants to make everything perfect just because, I draw the line at two way digital interfaces because that's a loss of flexibility for me.)

Now this quality thing could be remembered, perhaps until you give some kind of reset, or when the cables are changed.  So after the first time out it has pretty good knowledge of the source clock.

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