> Is it possible to make an analog filter where you can continuously
> vary the cutoff slope I am imagining a bandpass effect where the
> band gets continuously narrower and wider. Perhaps a voice is
> filtered through, at first it sounds human but you continuously dampen
> off all harmonics until it sounds like a sine wave.

You'd have to continuously determine the pitch to control the
filter's frequency cutoff and my filter knowledge no real world
filter would be steep enough to isolate just the fundamental and
eliminate other frequencies of a human voice once you have the
determine frequency of it.

Creating a filter slope variable is quite possible. These are the
commercial modules I know of.

The Serge Voltage Controlled Slope Filter (VCFS) has multimode
outputs. There seems to be some question if it varies from 6 to 12db.
I've been told it's done by manipulating the CV's/currents that
control the two ( ) filter stages but the person mentioning it didn't
fully understand how it worked. I've not tried one. Certainly not a
steep slope but supposedly dramatic enough especially when resonating
(needs to be manually patched since there is no built in loop)

For bandpass and in Euro there is the Cwejman VCEQ-3, which is a
voltage controlled 3 band parametric equalizer. Am I correct a
parametric does offer a change in slope rather than merely bandwidth
Comparing it to a parametric equalizer is probably a better starting
point than to a VCF. You have a switchable 16db boost or cut (not
equal to 16 db per octave slope). You can control all the parameters
(frequency, bandwidth, level) except the boot/cut switch via CV, very
much unlike a studio parametric that can't be modulated. What totally
gets me frustrated is Cwejman gives you just one mono in going to the
3 filters in series and a mono out of the last one. While the module
would need to be made wider for 6 more inputs it's simply frustrating
each band doesn't have it's own in and an out on an expensive module
like this. I'd love to have a mod that gave them their own ins and
outs, but it's made more complicated since Cwejman use SMT.

nick