Re: [Jack-Devel] is this latency coming from ALSA, jack, or ardour?

On Fri, April 15, 2011 7:54 am, Paul Davis wrote:
> Current versions of Ardour do not get the capture
> alignment correct in all cases. I am not sure whether
> yours is one of those cases.

I am running Ardour 3 build alpha 4.  Do you mean current as in alpha
builds, or current as in 2.8.11?

I don't know if you saw the original description I wrote from several days
ago, but I don't see how this case can get any simpler.
Basically route a track to a physical output, connect that physical output
back to a physical input, and route that input to another track.
Trying to simulate a perfect percussionist who hears a click track in the
headphones, and synchronizes playing exactly to the click track, then
another percussionist listens to the first track, and lays down another
track while keeping time perfectly to the first track heard in headphones.

I know that in reality you probably would not keep changing reference
track, everyone would be listening to the click.  And just to be clear, I
have no complaint now with A3 alpha 4 with jack 2 SVN and my PCI card
(envy24 based).  That works well.
A3 alpha4 with jack 1.9.6 seemed broken, and A3 alpha4 with jack2 SVN and
a USB interface still seems off.

Since I know what works now, I am trying to feel for the edges and see
where things break again.  Currently that seems to be with my USB
interface, but I am hopeful that if I get the proper use of jack_iodelay
figured out, that can be corrected as well.

> The error is not in ALSA.

I have not yet heard an alternative explanation for why there is a factor
of nearly 200x difference between the capture alignment of a PCI vs. USB
interface.  The PCI interface compensation resulted in tracks that lined
up to within 0.7ms after 10 bounces, implying track to track alignment
withing 0.07ms.  The USB interface resulted in tracks that were offset by
13ms (with jack2 SVN; was 130ms using jack 1.9.6).

-- 
Chris Caudle

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