Filters

[Ian Fritz]

Ben Stuyts wrote:

> 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.


Hal Chamberlin's book shows how to do this. First, you make a diagram of
the (analog) filter's topology, using integrators, amplifiers and
summers.  Then for the digital implementation you just need to realize
that a digital integrator is implemented by summing an input signal with
a delayed version of itself: y(n) = x(n) + y(n-1). A state variable
filter requires five additions and three multiplications per time step.
Six lines of BASIC.

  Ian