[LAU] open hw soundcard (was Re: status usb2 for sound)

fons at kokkinizita.net fons at kokkinizita.net
Fri Nov 13 17:29:35 EST 2009

On Fri, Nov 13, 2009 at 10:56:44PM +0100, Arnold Krille wrote:

> On Friday 13 November 2009 22:37:33 fons at kokkinizita.net wrote:
> > - If you want the soundcard to lock to a reference provided by
> >   the master (PC), all you have is the timing of Ethernet
> >   messages. For example the PC could send a message every
> >   millisecond.
> Is this possible? Despite the fact that some operating systems don't provide 
> that granularity, there is the outgoing network layers involved and the 
> incoming layers on the receivers side. I don't think these are both with a 
> delay constant enough to be usable as a timing source for a-d-conversion.

That remains to be seen. Jitter on the network timing would not
modify the average _frequency_ of such messages, so you still have
a frequency reference. But using it to generate a good sample clock
could be quite difficult.

Fact is that if the only link between the PC and soundcard is
the network cable then either the soundcard is free-running,
or it must use Ethernet messages as a reference somehow.

Assuming free-running, a number of such cards could still be
synced in sample rate if they have word clock in/out. But that
would not sync the sample blocks, so they would still have delay 
differences of an integer number of a samples.

My option would be to construct the cards that way, and use the
timing of the network messages only to align the sample blocks,
not to generate the sample clock itself. That also means the
issue appears only in a second stage, when the target is combine
multiple cards into a single device.



Io lo dico sempre: l'Italia è troppo stretta e lunga.

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