Reproducing Conkers BFD's IIR Filter by derselbst at 3:51 PM EDT on October 11, 2017
I was curious about whether it's possible to reproduce the IIR filter used in ConkersBFD and also in Jet Force Gemini. Although we have a HLE of it (thanks to JoshW), it seems there hasn’t been any further research on that.

The effect of that filter can be heard in CBFD's RockSolid or in JFG's Spacestation (as wind effect at about 1:10).

From a listening it sounds like a band pass filter is used. Converting the N64 cseq format using Subdrags N64 MIDI Tool, one can see that the filter is controlled via MIDI CC33 and 34. CC34 controls the filter's center frequency, while CC33 seems to influence the filters passband width.

With SoundFont2 modulators being able to be mapped from any MIDI CC to any destination sound generator, I modified fluidsynth to implement an IIR bandpass filter, intending to reproduce the tunes as close as possible. Sourcecode to be found here.

Use this command to try yourself:

fluidsynth -F test.wav -o -o synth.midi-bank-select=mma -o synth.volenv=compliant CBFD.sf2 rocksolid.mid

After many hours of try and error, this is the best render of RockSolid I've got:

It turned out that solely using a bandpass filter the hearable passband becomes way too narrow, no matter how I used CC33 for setting the filters Q. Looking at the HLE of that IIR, 3 multiplications are done, while a pure bandpass IIR would only need 2. It's more likely that Rareware cascaded a lowpass and a highpass. After having done this it sounds fine and also works well for JFG. However I'm not sure if my implementation is equivalent to what Rareware did. My reverse engineering skills are limited. Any input on that welcome.

IIR implementation according to Robert Bristow-Johnsons AudioEQ cookbook.

CBFD's SoundFont

JFG's SoundFont


edited 4:51 AM EDT October 12, 2017

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