From Rivendell Wiki
This is a package provided as part of the Repository here @ Tryphon. It's intended to automatically start JACK at boot, so your Rivendell daemons can also be started at boot. It mostly works apart from..
In trying to get this working on my Ubuntu Hardy system, I came across a problem with the realtime option in /etc/jackrc - this fails at system startup, and thus neither Jack nor Rivendell start. It works however once the chosen realtime user is logged in. Hmmn.
My solution (eventually, after about 6 hours of research!) was to add the following line to the PAM configuration for su in /etc/pam.d/su:
session required pam_limits.so
This forces the su command to apply the new realtime permissions when /etc/init.d/jack calls it to run jack as the specified user (who must of course be in the appropriate realtime user group).