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

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DateWed, 13 Apr 2011 22:57:40 -0500
From Chris Caudle <[hidden] at chriscaudle dot org>
To[hidden] at lists dot jackaudio dot org
Follow-UpStéphane Letz Re: [Jack-Devel] is this latency coming from ALSA, jack, or ardour?
Follow-UpClemens Ladisch Re: [Jack-Devel] is this latency coming from ALSA, jack, or ardour?
Follow-UpPaul Davis Re: [Jack-Devel] is this latency coming from ALSA, jack, or ardour?
On Wed, April 13, 2011 1:03 pm, Chris Caudle wrote:
> My hunch at the moment is ALSA, but that is why I
> am experimenting, to determine that empirically.

I think I have confirmed that the latency reported by ALSA to JACK is the
problem, but I would be interested to hear if anyone spots something I've
overlooked.

So my previous session bouncing from track to track analog out back to
analog in with my USB interface ended up with each track offset from the
previous by about 13ms.

Using my PCI Envy24 based card (M-Audio Audiophile2496) each track was
offset from the previous by some small fraction of a ms.  The total offset
from track 1 to track 10 was in the range of 0.5ms to 0.7ms.  Using a
recording of the click track, I could just hear slight doubling between
any pair of tracks.  On any more realistic scenario (i.e. not comparing
identical tracks to each other) I think it would probably be so close it
wouldn't matter, there would be more variation in a player's rhythm than
the offset, so you shouldn't be able to tell.

I think what that says is that the M-Audio PCI card is reporting a latency
figure back to JACK that is within some fraction of a millisecond to being
exact, and the USB interface is reporting back some bogus number that is
off by over 10ms.  That, or there is some additional latency in the USB
stack that the interface can't know about, and ALSA is not compensating
correctly and adjusting the number when it passes the information to JACK.
 I don't know how JACK gets the latency information for the analog
interfaces, so I'm not sure where the inaccuracy could come in for the USB
interface.  I think the short of it is I won't be using my USB interface
to overdub percussion, unless I'm prepared to do some manual shifting to
get everything lined back up.

It also brings up a question of digital interfaces.  If I use the digital
out from the card with external ADC and DAC, that is additional latency
that the card has no way to know about. Is there anyway to tell JACK (or
Ardour) about the additional latency so that it can be automatically
compensated like it is when the analog interfaces of the card are used?

-- 
Chris Caudle
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