Tracking bandpass filter

[Harvey Devoe Thornburg]

now for something analog...
has anybody seen or built something like this?

http://www.electronicdevelopment.com/mod3800.htm

a tracking bandpass filter, enabled to extract frequency 
information from audio inputs through several different modes 
(adaptive filter or PLL (?))

and it has automatic gain control... this would be really cool for 
feedback loops.

--Harvey