Re: [Jack-Devel] extra latency compensation

Chris Caudle wrote:
> The difference in results between the PCI card and the USB interface, and
> the fact that the USB interface wasn't consistent from session to session,
> or even track to track within a session, makes me think that there is just
> something off in the USB stack.

In addition to the already mentioned USB queueing delay (up to 24 ms,
depending on the period size), USB also adds a somewhat random delay
between the playback and capture streams.  The USB host controller
drivers do not implement the support for scheduling streams at
a specific time, so the offset between both streams might be one
millisecond if the second stream start occurred one frame later, plus
any delays due to executing the code.

> I only have the one USB interface, I suppose my hardware could just be
> broken in some weird way that only affects latency, or a bug in the
> device firmware.  I would want to compare results with someone who has
> different USB hardware first, I guess.

It is not possible for the USB hardware or the device firmware to affect
the latency in any meaningful way; all these latencies can be explained
by known deficiencies in the Linux USB stack.


Regards,
Clemens

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