[PlanetCCRMA] usb audio interface performance

[Fernando Lopez-Lezcano]

On Fri, 2007-05-18 at 12:55 -0400, Hector Centeno wrote:
> Hi,
> 
> I'm using a M-Audio Transit USB audio interface with a laptop and I
> was wondering if the rtirq script is actually setting the right
> priority for it. Would booting the laptop with the interface connected
> or disconnected make a diference? Is there something that I should
> tweak in the script to make it favor the USB port? Also I noticed that
> using the stock Fedora kernel I get much better performance than with
> the RT kernel, so that's why I was wondering about the priorities.

Hmmm, don't really know about this. 
You can run "/etc/rc.d/init.d/rtirq status" to see what the current irq
priority is. You could also stop and restart rtirq (as root) with the
card connected and see if that reorders them differently. 

"Better performance" means what? Fewer xruns? Lower buffer sizes
possible?

> Also, is there any recommendation regarding buffer sizes in Jack for
> using USB interfaces? I remember reading somewhere that the periods
> should be 3, is this true?

USB soundcards work best with 3 periods and sampling rate = 48000. This
has to do with the fact that packets are send every millisecond
regardless of sampling rate (bad design) and that does not mesh with,
for example, a 44.1KHz sampling rate (not all periods return the same
number of samples or something like that). I seem to remember that the
problem has to do with the way alsa handles interrupts and is not
something that can be easily fixed. 

-- Fernando