Filters

[Sean Costello]

Ian Fritz wrote:
> 
> 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.

The topology that Chamberlin gives for a state-variable filter is
considered to be pretty decent in the literature - Jon Dattoro wrote
about it recently in the JAES, and states that it may be the filter
design used in the Kurzweil synths.  We are learning about filters in my
computer music class right now, but without going into detail about the
topologies - not enough time (sigh).  

There have been several attempts to bring the Moog filter into the
digital realm, mostly based on the work of Timothy Stilson at CCRMA at
Stanford (check out
http://www-ccrma.stanford.edu/~stilti/papers/Welcome.html for his paper
on the subject).  Josep Comajuncosas ported the Stilson design over into
a Csound orchestra, and Hans Mikelson coded it into a Csound unit
generator.  It sounds very good, although not as dirty as a real Moog -
closer to a Roland SH-101 filter. Unfortunately, the Csound moogvcf unit
generator tends to blow up, at least when I use it (i.e. the output
suddenly jumps from an acceptable amplitude like 16000 or so, to values
like 50389823798379, infinity, and so on), so it may not be the best
example of how to code such a filter.  

Off I go to read F. Richard Moore's Elements of Computer Music, and/or
fall asleep.

Sean Costello