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

[Folderol]

On Fri, 13 Nov 2009 23:42:31 +0100
fons at kokkinizita.net wrote:

> On Fri, Nov 13, 2009 at 10:15:15PM +0000, Folderol wrote:
> 
> > The fact there is software out there that enables collaboration via the
> > internet demonstrates that it is do-able. We just?
> 
> Not really. None of these systems generate sample clocks from
> network messages. Local clocks are free-running and not synced.
> Signals are resampled to match the other end, and the quality
> loss resulting from doing that is accepted, as are the effects 
> of lossy compression. Such quality loss is not acceptable for a
> standalone audio interface.

Ah! I wasn't aware of that. It's not a point that is exactly made
obvious - I wonder why?

> > need to think out
> > how it can be done with as simple hardware as possible :)
> 
> Yes. See my previous post. IMHO, soundcards should be free
> running if only one is used, synced by word clock links 
> otherwise, and any network-derived timing used only to
> align the delays. That will be tricky enough.
> 
> Ciao,

OK. That makes sense. So our single multi-channel network soundcard
should also provide the timing for the computer (rather like an
internal soundcard does). This takes all, well most, of the precision
out of the computer, which is something I like the idea of. With the
appropriate network-jack driver (which would be hardware agnostic) it
can be used with just about any machine.

Looking up the AES spec quickly reveals 48kHz +/- 10ppm. If the
card's master oscillator has that kind of stability, it would surely
require only very tiny slow adjustments to keep two of them in sync.
Worst case would be them consistently drifting the maximum allowance in
opposite directions, which if my maths is up to scratch results in
about a second before they would actually get out of step by 1 cycle.

A quick google reveals 5ppm temp compensated oscillators with a a 7ppm
voltage controlled pull-in - no idea how much these cost though.

-- 
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.