Thomas Huber wrote:
> 
> Hello,
> 
> I'd like to have a bandpass where I can increase the
> Q until it starts self-oscillation, just like
> the moogvcf, but with bandpass characteristics instead
> of the lowpass. (And, just for the completeness, the
> same with a highpass characteristics).

How about using a RESON filter, with the bandwidth selected as a very
small fraction of the cutoff frequency (for very high Q), and normalized
to 1? It won't self-oscillate, but it will ring for a good long time
when excited by a signal.

As far as converting the moogvcf to a true bandpass filter, I don't
think it can be done. In the analog realm, cascade-style filters were
never used for bandpass - a state-variable filter was the filter
normally used (Csound might have a Chamberlin style state-variable
filter; if not, I might try and code one soon).  A Moog highpass would
be possible, but it probably wouldn't self oscillate; highpass filters
of the Moog topology generally don't.  The only oscillating highpass
filter I know of was the Sallen-Key highpass used in the Korg MS-20, and
I don't know of any digital equivalent of a Sallen-Key filter.

Does the moogvcf opcode self-oscillate? I am under the impression that
no digital filter self-oscillates without an initial source of
excitation, but I could be wrong.

As far as making Goa-Trance, don't get hung up on the fact that none of
the Csound filters sound as good as a 101 filter (well, the moogvcf
sounds pretty Rolandy).  If one filter doesn't cut it, try 2, or 5, or
20. Lots of great sounds can be made with several reson filters in
parallel.

Sean Costello