One would think there are tons of simple FIR/IIR design aids out there... not so. I mean sure, there are all kinds of *labs, octaves and whatnot, some of them free, but I don't need a full-blown higher math computation tool (and most definitely I don't want to learn how to use them first).
All I need is something that will eat a0-a2,b0-b2 and spit out a nice magnitude vs frequency plot. Possibly with an option to switch to log scaling on f axis. Native Windows GUI please. Since this task requires complex numbers and I'm too lazy to code a GUI with pure Win32 (no RAD on my home PC), I figured I could modify kingshriek's script to output the data for gnuplot. Of course no python on Windows box either, so I'm using a nearby Linux for all the work. I realized I don't have gnuplot on my FC8 so I did "yum install gnuplot" - and yum asked me if I would like additional 67 packages with it, just above 35MB. Err... no, thanks.
Anyway - I've run some tests. Neill's filter is indeed a low-pass IIR but I still think it's not the correct one. The function that converts filter value to f coefficient is wrong. I'm having a hard time coming up with something that would both work and not requite floating point math functions... Q values other than zero do create a resonance but not like AICA does it. The biggest difference it that AICA creates resonance a bit before f0, so cutoff point is basically unaffected. Neill's filter has the gain peak all over f0 and ends up with a different curve altogether.
The figures in SEGA docs are (for once) pretty much what I got from frequency spectrum analysis. The FEG timing is wrong though, or so I belive - it's basically the same table as AEG decay/release but multiplied by 4. There's even an obvious copy-paste bug that messes up the last part completely. FEG counter is not only wider than AEG, it also goes all the way up to all ones, whereas AEG ends at 960.