[sdiy] Experiment with bandlimited ramps

[Seb Francis]

By the way, what's the use of generating these bandlimited ramps 
anyway?  Can you actually use them to make an oscillator?  Surely they 
are only good for 1 frequency/note.  Do you need to generate a waveform 
for every note in the bottom octave?  And then how do you handle pitch 
bend and FM, etc.?

I'm interested as one of the modes of my Digi-Mod will be a 
"Super-Oscillator" which will basically be lots of slightly detuned saws 
(or other waves) which can have their detune amount, oscillator count, 
stereo spread, etc. CV modulated.  So basically I need a cheap way to 
generate lots of digital oscillators.

I haven't started experiments yet (will be after I get back from 
holiday), but I have a few approaches in mind:

(1) Highly oversampled (e.g. 64x) trivial waveforms with (probably 
multiple) FIR low pass filter and decimator.

(2) Slightly oversampled (e.g. 2x) slew-rate limited trivial waveforms 
with FIR low pass filter and decimator.

(3) Use bandlimited steps to generate waveforms at the actual output 
sample rate.

The best quality is likely with (3), but not sure how many waveforms I 
can get away with before I run out of cycles in a dsPIC.
I expect (1) to also be quite processor intensive for lots of waveforms, 
and for the filtering/decimation process.
So (2), as suggested by Antti, looks attractive.  It really depends 
whether it sounds pleasing to my ears... :)

Seb