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alsa and jack latency



Hi all,

Trying to reduce the latency of jack on my ibook G4, i realised i can go
down to 8.7ms using the oss mode, which is really good. But alsa won't
accept running with less than 22ms and click on most X request:

$ jackd -R -doss -r44100 -p128 -n3 -w16
works perfectly, 128*3/44100=8.7ms latency, hardly any xrun on heavy load

$ jackd -R -doss -r44100 -p256 -n2 -w16
works fine, 11.6ms latency

$ jackd -R -dalsa -r44100 -p128 -n3 -S
runs but no clients can connect

$ jackd -R -dalsa -r44100 -p256 -n2 -S
fails saying ...
ALSA: cannot set number of periods to 2 for capture
ALSA: cannot configure capture channel

$ jackd -R -dalsa -r44100 -p256 -n4 -S
would run with 23.2ms latency, but xruns on most X queries

It looks to me that this is related to the pmac driver. I have similar
problems with snd-usb-audio and an edirol usb, which will only let jack
run in oss mode. 

I wonder what prevents the pmac module to do so while the oss emulation
layer does it very well. I am running 2.6.12-rc4, but had this problem
with earlier 2.6 kernels. Any thoughts?

Many thanks, Paul

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