[LAU] USB period size? (WAS: tracking down xruns)

[Patrick Shirkey]

On 12/22/2009 05:47 PM, Ng Oon-Ee wrote:
> On Tue, 2009-12-22 at 15:44 +1100, Patrick Shirkey wrote:
>    
>> Are you running the usb device at 44100 or 48000? Ideally usb cards
>> should be run at 48000 with period sizes that divide perfectly into
>> 48000. i.e 100, 200, 400, etc...
>>      
> Could I know more about this, please? 48000 would be the 'generally'
> used (besides 96k) for USB sound cards I understand, but what's this
> about period sizes? I've always thought period sizes were of the series
> 8, 16, 32, 64, 128, 256...
>
>    


Normally, period sizes as you describe above are the right way to go but 
with usb devices the packet sizes are different to pci cards so you will 
see better performance with these strange period sizes. That's not to 
say the card won't work it's just that you will be transferring more 
packets with period sizes that are not exact multiples of the frame rate 
and putting the system under more stress.

At least that's what I understand from Takashi Iwai who wrote the first 
version of the alsa usb driver.

I have personally found that with a fast enough cpu and plenty of memory 
that I can run a usb-1.0 device at 64 periods and 44100 frame rate with 
-n3 and get 4 channels of i/o.


Cheers.


Patrick Shirkey
Boost Hardware Ltd