Re: [LAU] Testing sync with 2 sound cards

On Mon, Mar 3, 2008 at 1:53 PM, Kevin Cosgrove <kevinc@email-addr-hidden> wrote:
>
> On 3 March 2008 at 10:54, "Mark Knecht" <markknecht@email-addr-hidden> wrote:
>
> > I think this sounds like a better solution. I would personally look
> > for a way to treat the audio information as data, calculate the the
> > expected data from each card and then do a comparison of input and
> > output data mathematically.
>
> A cross-correlation between the captured data on a channel in
> one unit versus a channel in the other unit would be the right
> thing here. A value of 1 would be perfect. In practice you
> won't achieve that even between channels in the same unit. I
> suppose if the match across the two units were as good as the
> match across channels within one unit, then one could really
> consider the two Delta 1010s as one larger unit.
>

Yep, that's one way to do it.

>
> > In all likelihood, there are going to be small phase shift differences
> > between the cards.
>
> I don't know if I'd really expect drift. I'd expect jitter more than
> drift.
>
>
> > The inputs won't be perfectly in sync even if they
> > are coming from the same sine wave generator. I would probably doing
> > something like a standard deviation comparison
>
> Std dev of what? Frequency? Period?
>

I think cross correlation may be easier.

I was thinking of the data itself. If you are inputting a sine wave
you have an equation so you could build a perfect data set assuming
you really know the frequency. More practical would be to use the
master's receive data and then look at how much the slave is deviating
from that. I like the cross correlation idea better as it really
doesn't make any assumptions about what the data is but only that the
two should be identical.

>
> > between the input and
> > two outputs.
>
> How would you measure the input? It would have to be digitized, or
> one would have to trust it's frequency and spectral purity. Chances
> are that it will drift and have jitter too. To do a really good job
> at a test like this one would need a very good generator, or a test
> setup like those offered by Audio Precision <http://ap.com/>.
>

I agree.

>
> > If the data looks identical, within some some deviation
> > amount, then most likely everything is working OK. On the other hand
> > if there is a single byte getting dropped or added once in awhile then
> > it will show up doing this sort of analysis very quickly.
>
> Before going to all the trouble, one might look up the tolerance
> of crystal frequencies for the kind of crystal used in the Delta
> 1010. Using the possible frequency mismatch one could calculate
> how long it would take for the data to get out of alignment by
> one sample (either at 44.1kHz, 48kHz, or 96kHz). One could
> suspect that a PLL or DLL should force the frequencies to match
> at least 10X better than they would without a sync circuit. I'm
> guessing that a 1000X improvement is unrealizable.
>

Are these two cards using the same clock? I'm assuming that both cards
have the clocks physically linked as per Timo's paper on the subject.
If they aren't all bets are off as the two crystals will NEVER give
exactly the same sample rate.

>
> > If you don't want to do it in C or some programming language then you
> > can likely do it in Open Office or Excel.
>
> Or Gnumeric, or octave, or Maxima, or any of the other Open
> Source math programs.
>
> Anyway, it seems that the test already done is a pretty good
> indicator that the sync between the units is functional. Now
> it's just a question of how good the design is, which is what's
> left to measure. If it were me, and I also own two 1010s, I'd
> test the system by recording some music and seeing how good it
> sounded. Up to this point, my gear has never been the limiting
> factor; it's always been the musicians that limit the ultimate
> quality.
>
> Unfortunately, my system is just slightly too old in terms of
> JACK and kernel to synchronize my two units without patching and
> upgrading according to:
>
> http://www.sound-man.co.uk/linuxaudio/ice1712multi.html
>
> I'll be upgrading as soon as I get through a few recording
> projects that I'm working on.
>
> G'luck....
>
>
>
> --
> Kevin
>
>
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Received on Tue Mar 4 04:15:01 2008