Are these devices analog ? For instance the Boss one ?
I have the Boss Book. There you can read that the AC-2 is analog and consists of several filters.
They mostly are analog filters and some carefully dosed clipping.
The AC2 schem is out there:
http://www.generalguitargadgets.com/boss/BossAC-2.gif
For a less complicated build with very impressive results, try Mark Hammer's Woody ( http://hammer.ampage.org/ )