Cracks in sound on Ubuntu 12.04

Sorry for my english, i am french!
i try to use ardour on Ubuntu 12.04 32 bits with a M Audio Delta series 2496. When I just play the click and just launch light applications or change desktop, the click is not regular, and there are cracks in the sound. I have tried to change latency, i have the same problem. On the same computer, under windows7 with Cubase, I’ve no trouble. I have tried to disable real time.
The system monitor show that the processor is not saturate. I work with a Intel® Pentium® 4 CPU 2.80GHz × 2 with 2,9 Go RAM.
Does someone know where the problem come from?

Most likely you have not set up the computer for realtime performance. I should also note that Ubuntu in particular tends to be a bad distribution for realtime audio work as well.


I have done the procedure on .

It doesn’t change anything. I continue to try somes options in jack…