Nigel Henry:
If your soundcard is hda intel, there is a known
problem, where
crackling occurs if the "Periods/Buffer is set to the default "2".
The cure is to open Qjackctl's setup, settings, and change it to
"3".
I wish people would stop repeating this obsolete info. Use a recent
alsa version instead, doesn't have to be bleeding edge, any version
from after beginning 2008 should do.
Wolfgang