[LAU] recommendations for usb sound card?

[Paul Davis]

On Mon, 2008-03-10 at 19:53 +0000, James Stone wrote:
> On Mon, Mar 10, 2008 at 12:56:17PM -0500, Lee Revell wrote:
> > On Sun, Mar 9, 2008 at 3:15 AM, James Stone <jamesmstone at gmail.com> wrote:
> > > On Sat, Mar 08, 2008 at 07:03:30PM -0500, Lee Revell wrote:
> > >  > On Sat, Mar 8, 2008 at 2:58 AM, James Stone <jamesmstone at gmail.com> wrote:
> > >  > > Hi,
> > >  > >
> > >  > >  I am wanting to use the eeepc for a mobile sound generation unit,
> > >  > >  but unfortunately the gfx card and sound card share the same
> > >  > >  interrupt.
> > >  >
> > >  > This is not necessarily a showstopper.  At least in theory.  Do you
> > >  > have any empirical evidence that it's a problem for you?
> > >
> > >  xruns..
> > >
> > 
> > OK.  And you're sure the IRQ priorities were set correctly, realtime
> > kernel used, JACK in realtime mode, etc?
> > 
> 
> Yes. but IRQ tuning is not much use with the video on the same
> interrupt as the sound card. Basically whenever a widget moves,
> sound dropout occurs.

this is not how video display works. they are a lot like audio
interfaces actually. periodically, they interrupt the host CPU to say
"i'm about to start redrawing the screen from the framebuffer you last
told me about". that's about it really. they never redraw the screen at
any other time.

when your GUI toolkit decides to redraw a widget, it doesn't interact
with the h/w. all that happens is that eventually the new image of the
screen gets into the framebuffer, and on the next "VSYNC" cycle, the
video card will push it out to the actual display.

what is an issue is that the drivers and/or h/w associated with the
video card sometimes do not play fair with the PCI bus - they claim
owernship over it for too long while doing a redraw, and this *does*
lead to xruns at low latencies.

--p