Hi all,
here is the link to a IMO very fine introductory text on filters
and the like.
http://on1.zkm.de/zkm/Institute/Musik/ZKMusik/veroeffentlicht/PD_Filters
They also have some other interesting texts, most written by
Pierre Dutilleux, most in german:
http://on1.zkm.de/zkm/Institute/Musik/ZKMusik/Veroeffentlichung/
<- bottom of page
Most of you will already know the site, since they hosted the
Linux Audio Developers Conference.
-Björn Pfeiffer