Nonlinear Oscillators?

[Sean Costello]

Dan Slater wrote:

> Hi Sean;
> 
>         I have done quite a bit of this type of stuff. I wrote an article on
> this topic that is coming out in the next issue of the Computer Music
> Journal that you might find interesting. The article is heavily oriented
> toward analog techniques and includes a number of chaotic patches for
> analog modular synthesizers.

Wow!  When does the next issue come out?  I should probably just get a
subscription (I always put off buying the issues, then get them a few
months after they come out).  By the time the issue is out, I should
have my Nord Modular; hopefully the sort of chaotic patches you describe
can be implemented (I fear that the Nord might err on the tame side, in
order to minimize aliasing).
 
>         An even simpler variation is just to take a VCO output and run it back
> into the FM input. This approach was used in the Yamaha DX-7 as a noise
> generator.

I know that Bernie Hutchins had a circuit that used this technique in
Electronotes - I believe that it was VCO Option 3 in the ENS-76 series -
but that it was discovered that it caused pitch bends that were
unsatisfactory, due to the DC component that developed in the feedback
signal. Still, this is what I had in mind for the TB-303 type circuit:
VCO with linear feedback FM, with feedback running through a non-linear
circuit.  It might be difficult to make the circuit simple and still
retain good pitch accuracy with high amounts of feedback; on the other
hand, who's to say what would sound good in the given application?   

Hmm...isn't Yamaha FM based on phase modulation, as opposed to strict
frequency modulation?  If so, are there any analog oscillator designs
that allow phase to be controlled directly?  Maybe the way to get these
sounds would be to have an allpass network of several stages, where the
phase shift frequency of the network is modulated by the output of the
network.  I always thought that this would be a good way to get a sitar
sound out of a guitar (actually, to simulate a sitar, probably
non-feedback phase modulation is called for, as the sitar spectrum has
tightly clustered harmonic peaks that closely resembles the spectrum
from non-feedback FM, while feedback FM generates a spectrum closer to a
sawtooth wave).  Running the output of the allpass network through a
nonlinear element, then into the modulation input, would also be a
useful experiment.

Also, has anyone tried filter FM, with the modulating signal being the
output of the filter that is run through a nonlinear element (full-wave
rectifier, clipper, suboctave divider, etc.)?  Both the allpass-FM and
filter-FM methods would allow any signal to be run through the circuit,
not just an oscillator.  I wouldn't think that either of these methods
would produce the octave-jumping effect I am after, but I could be very
much wrong.  

Later,

Sean Costello