Re: [LAU] [OT reply to recived OT ; )] RME analog out of phase to ADAT connected Behringer analog out

On Tue, 2011-06-14 at 10:37 -0400, Paul Davis wrote:
> On Tue, Jun 14, 2011 at 10:03 AM, Ralf Mardorf
> <ralf.mardorf@email-addr-hidden-dsl.net> wrote:
> > On Tue, 2011-06-14 at 08:22 -0400, Paul Davis wrote:
> >> On Tue, Jun 14, 2011 at 8:11 AM, Ralf Mardorf
> >> <ralf.mardorf@email-addr-hidden-dsl.net> wrote:
> >>
> >> > And as you could read I found an issue. At Thomann (a German dealer)
> >> > they claimed there are no sync issues, what I couldn't believe. Doing
> >> > this test I found out, that there are bad phases.
> >>
> >> You DIDN'T SYNC ANYTHING. Or at least, your description doesn't
> >> mention any kind of sync.
> >
> > That might be correct. The optical ADAT connection can't be used for
> > sync?
> >
> >> And if you had tried to sync, you would have
> >> found that you CANNOT sync two A/D converters unless they are the
> >> exact same model (so that their pre-A/D buffer is the same size).
> >
> > I suspect the RME using other, better converters than the Behringer
> > does. This is why I expect that a completely perfect sync for the phases
> > might be impossible. Thomann (the dealer) claimed I'm mistaken ;), sync
> > is possible.
>
> you're really deeply confused. let me try to illustrate (the diagram
> is somewhat simplified to try to make the point more clear. the actual
> reality is a bit more complex in terms of where the clock is actually
> used, and so on). the section marked "buffer" refers, typically, to
> some RAM into which a relatively large (say, 32 or more) samples can
> be stored "in front of" the D/A converter.
>
> digital samples-> buffer-> D/A converter digital-side>internal
> buffer->D/A converter analog side
> ^
> |
> |
> sample clock
>
> the sample lock says "get another sample", so the device will fetch
> another sample at precisely the same time (to within a picosecond or
> so) of other devices using the same clock (if any). this is important.
>
> but ... the way each D/A converter circuit works is independent once
> it has the next sample value available. the amount of buffering inside
> the D/A converter is variable from converter to converter, as is the
> actual conversion process itself. So lets put 2 of them together:
>
> digital samples-> buffer-> D/A converter digital-side>internal
> buffer->D/A converter analog side
> ^
> |
> |
> shared sample clock
> |
> |
> v
> digital samples-> buffer-> D/A converter digital-side>internal
> buffer->D/A converter analog side
>
> it makes no different whether the sample clock is delivered from an
> internal clock crystal, the ADAT cable, word clock via BNC or any
> other method. once the samples are delivered to each D/A converter,
> there is no "sync" anymore.
>
> this has nothing to with RME, and everything to do with the basics of
> digital audio and digital-to-analog convertion.

This was my understanding before I bought my RME and Behringer ADAT ;),
than I was confused regarding to different claims :D, ok, you confirm my
knowledge as correct, I had before I was confused :).

So - the ADAT cable is enough for sync, but of cause
   - it can't perform magic to solve the issue you described

Thank you,

Ralf

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Received on Tue Jun 14 20:15:03 2011