[LAU] usb and jack

Clemens Ladisch clemens at ladisch.de
Mon Jan 25 08:57:00 EST 2010

Martin Homuth-Rosemann wrote:
> don't know where I found this on the net that setting the buffer size 
> for usb audio a multiple of 1 ms duration, e.g. for 48000 kHz p=48 will 
> allow very low latencies. I tried this today with the latest rt kernel 
> and the cheap burr brown usb codec pcm2902 used in my behringer uca202:
> options snd_usb_audio nrpacks=1

You don't need this option; the driver was fixed to use sensible
values years ago.

> /usr/bin/jackd -R -P79 -dalsa -dhw:1,0 -r48000 -p48 -n2 -Xseq
> It runs smoothly on a centrino laptop (pentium M, cpu freq stepping 
> between 0.6 and 1.6 GHz).
> Maybe some of the jack and usb gurus (Paul?, Clemens?) can confirm the 
> -p48 -n2 setting.

For most USB audio devices, using period sizes that are an integer
multiple to the USB packet size (1 ms) will definitely help.  Some
devices like the SB Audigy 2 NX or UA-101 do not synchronize their
sample clock to the USB clock (and this has nothing to do with USB
2.0), and in those cases, the period size does not matter.

(I did not know that Jack now supports lengths that are not powers
of two.)

Best regards,

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