[LAU] Jack gives xruns with alsa driver, but dummy driver + alsa_in/out doesn't

Hi All, I'm having a very strange problem setting up Jack. I have a card which aplay -l lists as card 0: SB [HDA ATI SB], device 0: ALC892 Analog [ALC892 Analog] Subdevices: 1/1 Subdevice #0: subdevice #0 card 0: SB [HDA ATI SB], device 1: ALC892 Digital [ALC892 Digital] Subdevices: 1/1 Subdevice #0: subdevice #0 I'm only interested in the analog outputs. when I start up jackd (from qjackctrl), I am offered three different options for my sound interface: hw:0 HDA ATI SB hw:0,0 ALC892 Analog hw:0,1 ALC892 Digital (input devices are similar) However, when I start up jackd, I get a constant stream of xruns. This is true regardless of what combination of input and output hardware device I choose, as well as real-time settings and period size (I obviously haven't tested everything, but a large representative swath). On the other hand, if I choose plughw:0, jackd gives me a warning, but I can get zero xruns and no artifacts for as low as 128 frames/period. Similarly, if I start jack with the dummy driver, and then run alsa_in, I can get down to 32 frames/period with no artifacts nor xruns. What could be going on? Thanks, Jeremy