Filters

[Ben Stuyts]

On Mon, 11 Jan 1999, Ethan Duni wrote:

> anyway, back to the point.. using an FFT based filter, while very cool, is
> probably overkill for doing analog synth modelling (that is what you were
> wanting to do, no?).  Making an FFT filter do modulable resonance and cutoff
> in real time (while really cool) would be a severe pain in the ass,
> especially compared to using "FIR" and "IIR" methods, which will probably be
> just as good for doing analog-synth style filtering (anyone have any
> opinions here?).

I think one of the bigger problems would be that you would like your control  
inputs to change things like the VCF cut-off frequency, amount of resonance,  
etc. Those parameters are not directly accessible with a FIR or IIR filter,  
and the coefficients of the filter terms are not easily calculated from the  
requested input. (Please correct me if I'm wrong though.)

I was wondering if a slightly different approach would work. Why not just  
model the analog circuit of a VCF in real time? Basically it's just R's and  
C's and some op-amps. You could take a SPICE-like approach, but then very much  
simplified.

I have done something like this a long time ago to simulate (in basic,  
duh...) a charging circuit for a stroboscope I built. It had a diode, R and C,  
and was fed using 220 VAC. I wanted to know the charging time of the C, but  
because of the AC input voltage I couldn't figure out how to compute it  
directly. So I just stepped (in 1 ms increments or so) through time, and  
calculated the currents and voltages. Worked great, and it is very easy to  
follow!

Having the power of a DSP at your hands, this could be easily done (I  
suppose) for a 44 kHz sampling rate.

> incidentally, i have a big design project coming up in the next couple of
> years for my degree.  what i've been thinking of doing is sort of along
> these lines.. I'm calling it a convolution-synth for lack of a better term;
[snip]

That was an interesting explanation you gave. Please keep us informed about  
your progress.

Best regards,
Ben