knock sensors

[Steve=Ravet%Prj=Eng%PCPD=Hou at bangate.compaq.com]

I'd like to start a discussion of knock sensors.  So, I'll ask some questions:
How do they work?  Is there a simple aftermarket solution, and is it the kind 
of thing that someone could build themselves?  How would one implement one?

Here is something I came up with that is probably way to complex for the task:

Apply a strip of piezo-electric tape to the head or other suitable place.  
Piezo-electric tape picks up vibrations and outputs a small (tiny) 
corresponding voltage signal.  Amplify this, and analyze the signal.  A normal 
running engine should produce a characteristic frequency distribution.  One 
that is knocking should produce a different one (perhaps more high frequency?) 
Anyway, figure out where the difference is in the frequency range, and watch 
that particular band with an analog filter.

This should work, but seems complex, and also would probably differ greatly 
from engine to engine.  I would like to hear comments from people with 
knowledge of how commercial knock detectors work.  thanks!

--steve
sravet at bangate.compaq.com