I had a similar issue with exported WAV’s having ‘robot machine gun’ sounds in them. The root cause was the RT priority that jackd was running at; it was set by default to 10; bumping this to 72 (2 less than the 74 that the ICE1712 is at) fixed the robot machine guns (what an exported XRUN, with a small frames and buffers setting, sounds like), and no more XRUNs reported by JAMin. Did find that the lower the latency setting, the slower the export through JAMin; with a 128/2 setting, it took over twice as long to export as with 256/2. (See also the ‘Got Jack?’ thread, specificially http://ardour.org/node/2421#comment-10181 and the one right above it.)
Playback through JAMin wasn’t an issue; export was, and it didn’t matter how high I set the thing (finally got a mostly clean export with 2048/4; which just caused the fewer numbers of XRUNs, when they did occur, to cause a much more noticeable repeat in the audio). The rt kernel was installed and JACK setup to be realtime (using the default priority); /etc/security/limits.conf was set up (by automation of the PlanetCCRMA jack-audio-connection-kit RPM’s); everything else was Just Right (and, again, worked fine for the most part in ‘normal’ playback/recording mode. Just the export mode had issues).
Speaking of export mode, getting ready to file a bug report about the export mode causing a JACK problem; putting together as much information as I can before filing it, and, for that matter, trying to determine whether to file it in the ardour tracker or the jack tracker.
The root problem is that exporting in ardour causes JACK to go into a weird mode, and nothing will play or record until I exit ardour and restart JACK. The message output box in qjackctl, with verbose ON, ends with the following text:
Jack: JackClient::SetupDriverSync driver sem in flush mode
Jack: JackExternalClient::ClientNotify ref = 4 name = jamin notify = 8
Jack: JackExternalClient::ClientNotify ref = 5 name = ardour notify = 8
Jack: Create non RT thread
Cannot set scheduling priority for RT thread res = 22 err = No such file or directory
Cannot start thread
Jack: fPollTable i = 1 fd = 13
Jack: fPollTable i = 2 fd = 14
This occurs whether JAMin is loaded or not, and regardless of how simple or complex the session is.
Current versions: Ardour 2.8.2, Fedora version ardour-2.8.2-2.fc11.i586;
JACK version: jack-audio-connection-kit-1.9.2-2.fc11.ccrma.i586
RT kernel: kernel-rtPAE-22.214.171.124-1.rt23.2.fc11.ccrma.i686
Probably a JACK issue, though, which is why I haven’t yet filed a full bug report, as I am still trying to figure out how to determine if it’s Ardour or JACK causing this. Maybe JACK 1.9.3 fixes this; don’t know.