[LAU] Testing sync with 2 sound cards

[Kevin Cosgrove]

On 2 March 2008 at 10:23, Joe Hartley <jh at brainiac.com> wrote:

> What's the best way to ensure that the cards are actually in sync?
> I've recorded an hour of a metronome in Ardour with the signal split
> to an input on each card and saw no drift between the tracks.  I then
> played them back and heard no drift even after 30 minutes, but I have
> a feeling that this isn't necessarily a valid way of testing.

Your test method seems to be closest to what you'll do in actual use. 
In that light it seems pretty valid to me.

Another way to test the system is to record a sine wave, maybe 1kHz 
but other frequencies are good too.  When you're done recording, then 
run an FFT on the result and look for frequencies other than 1kHz.
If you use a large signal size, then you might end up seeing 
distortion products in the frequency spectrum.  Certainly you'll see 
some noise that's broadly distributed.  Some of that noise will be 
due to clock jitter.  Of that noise, some could be due to an increase 
in clock jitter when using an external sync versus using the internal 
clock.  You could do a comparison.

Anyway, I think your system is probably sound.  But, you could test 
it more if it makes you happy.

Do let us know what you find out please.

Thanks....
 
--
Kevin