Re: [LAD] FOSS Ethernet Soundcard

On Thu, 26 Nov 2009 06:36:07 +0100
torbenh <torbenh@email-addr-hidden> wrote:

> On Tue, Nov 24, 2009 at 08:34:53PM +0100, Karl Hammar wrote:
> > Florian Faber:
> > > Karl Hammar wrote:
> > > > [..ethernet transports..]
> > > > And we are missing an open protocol for this.
> > > What is wrong with netjack? It's made for point-to-point and very
> > > simple. You just have to solve the clock issue unless you want to lose
> > > bit transparency.
> >
> > Ohh, sorry, I got the impression it was not ready. I don't mind being
> > wrong in that.
>
> may i kindly ask what gave you that impression ?
> i admit that some jack versions in svn were broken for the LAN use-case
> because i am focussing on making it work for wireless and
> transcontinental internet connections.

I'm not sure why, but I had an impression of 'unreadiness' too :o

> but the lan use-case it pretty trivial and doesnt need all this deadline
> machinery.
>
> if you just want to build a network soundcard, and use jack on a
> computer with that, its ready for 3 years now !!!

The remaining issue then is how much additional latency does netjack
introduce? I think, eventually, this will turn out to be the most
critical issue.

> having more than one network soundcard, and keeping stuff in sync is
> more tricky. it requires that you can control the wordclock on that
> soundcard, and you need a special version of alsa_out.c
> which instead of resampling controls the wordclock frequency.

If there is no internal sound card, why is there any need for alsa?

Wordlock may not be as difficult as it first seems. A bit I originally
wrote to LAU...

> 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.

Even without this, I wonder what the audible effect on real music would
be of just dropping one sample per second. It might be worth doing some
experiments to find out.

It also occurs to me that although we want to lock frequency, phase
relationship between cards is a red herring. Signals from different
sources will not have any phase relationship, so why should the cards?

-- 
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.
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