Linear/Exp Filters, was Re[2]: VCOs as modulation sources

[Sean Costello]

At 06:12 PM 7/12/96 -0400, Eli Brandt wrote:

>FM of a self-oscillating filter is excellent.  My favorite 2600 patches
>were based on a two-osc FM chain onto the filter... all kinds of
>screaming sidebands, dense clouds of bleeps, and squashy rhythms.
>Totally irreproducible, of course.
>
>Has anybody analysed FM with a self-osc filter as carrier?  I bet it's
>different from standard FM, because of the ringiness in the filter's
>response to modulation.

Are there any existing designs for a 4-pole resonant filter that can be
controlled by both linear and exponential modulation?  My DX-7 days have got
me thinking about having a resonating filter in which the output can be
routed to modulate the filter cutoff in a linear fashion, while the pitch of
the resonance is controlled by a 1V/Oct signal.  If this works anything like
digital FM, the resulting waveform would become more and more sawtooth-like
with increasing modulation feedback, eventually becoming very noisy as the
sidebands modulate the sidebands.  Run the output of the filer through a
waveshaper (say, the square wave/full wave rectifier found in Klein's book)
and crazy sounds would hopefully result.  I can't say for certain if this
would work (it's hard to get sounds like this out of my ARP Axxe, although I
can get some real dirty filter FM with the use of several patch cords and a
fuzz box), but I would love to try it.  I have the feeling that my favorite
sounds right now are the result of filter modulation (the Micromoog stylings
of Jessamine, and the screetching cheetah/dog whistle drums of Aphex Twin),
and would like to pursue it in a more controlled manner.

Any input, suggestions, etc. appreciated.

Thanks,

Sean Costello