Not bad for a digital filter, sure sounds better than most software ones I've tried. Probably on par with the Roland efforts in their older digital gear.
Yamaha calls em Time variant IIR (infinite impulse response) digital filters

I'm using the two 12db low pass filters together to form  24db filter. When used like this filter will self oscillate at above 85% resonance. Sounddiver allows you to link both together so one controls the other.

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A single Sawtooth "oscillator" created by 3 FM operators in series. 
Operator 1 modulated by OP2 with level 100 and feedback 7, which is modulated by OP3 with level 100 and feedback 7
Phase sync off for all operators

I also have random pitch at 2 - so every new key press is slightly different


Yamaha TG77 Filter demo

My YouTube channel

http://www.youtube.com/user/CoolColJ