[sdiy] Non-linear synthesis

[Tom Wiltshire]

Hello all,

I've recently started learning about digital synthesis. I have to  
admit that for years I've been an analogue-only kind of guy, and that  
I rather looked down my nose at digital synths. Probably a result of  
being a teenager in the 80's - digital synths really did sound awful  
back then. However, technology has moved on, and Antti  Huovilainen  
showed that you can even do a decent Moog filter digitally these days  
- something I'd never have thought possible. (http:// 
dafx04.na.infn.it/WebProc/Proc/P_061.pdf)

However, I'm starting simple, and playing with the dsPIC 30F4013 16- 
bit processor. The main limitation is the very limited memory for  
wavetables (16K), although time is always an issue too.

I've got three stages in my current digital oscillator design. It has  
a basic DDS oscillator, to which phase distortion (PD) is applied,  
under voltage control. The output from the PD oscillator is then  
passed through a waveshaper function, also under voltage control.  
This effectively lets me modify both the X and Y axes of my sound  
wave using arbitrary functions. Feeding envelopes or LFOs to these  
two CV inputs is good fun.

The PD stage seems pretty versatile. It'll produce some crazy filter- 
esque sweeps and squorks. However, PD is equivalent to FM limited to  
only integer ratios, so there are very closely related techniques  
that are much _more_ versatile. That said, FM has a definite sound.  
PD seems to do a lot of "Parp" and "Burp" noises suitable for brass  
perhaps, but doesn't really push the outer edges of the synthesis  
universe, which is where I want to be!

The waveshaper stage is singularly unexciting. I was expecting much  
more. Granted, I've only got four functions in it, but a couple of  
them are quite complex, with loops and bends like a Serge middle  
section. In fact, the function that sounds best is a soft-knee  
distortion, which produces a nice crunchy attack on notes when you  
feed an envelope to the control voltage input.

Does anyone have any experience with waveshapers or phase distortion  
synthesis to share? Any good waveshaper functions? Anything I should  
avoid? Any other non-linear synthesis techniques I need to know about  
or try? Things to investigate? Ideas?

I'll put some details of the project online at some point when it  
solidifies a bit more. At the moment it's still at the "breadboard  
spaghetti" stage.

Regards,
Tom