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which is your system timer resolution?

nb. lesser than 1000hz is not here recommended, although most distro supplied kernels have lousy values eg. 250hz or even 100hz, in an attempt to save some battery wattage for the laptops and mobile stuff which is pretty marginal if not b.s :)

did you try the hrtimer (high-resolution timer)? depending on your kernel version, it should be available through the snd-hrtimer alsa module. load it with

as root and try to select it on qtractor options.

take note that the slave pcm timers are only good enough when the audio pcm period is short as well (ie. 256x2 might not be enough) and given that you're using a firewire device, well, it kind of makes it into a gray area to me--there's no alsa firewire driver per se, so it might not get tied to a pcm interrupt timer at all (thus the lousy results you get) . last time i saw it plugs directly into jackd through its ffado backend and stack.

do you experience the same behavior with any other audio device you can get your hands to? any but firewire, i ask.

did you said it works fine on rosegarden?

one other question, which i forgot to ask in first place: what are in fact those "xruns" you're talking about, which tend to occur on each loop turn-around? are those notes being playing out of tempo or dropped out entirely? is there a tempo variation or is it actually jackd reporting "xruns" instead?

seeya