[sdiy] Digital oscillators [was: Active VCO temp compensation]

[Richard Wentk]

On 22 Dec 2008, at 15:10, Scott Gravenhorst wrote:

> [C] Recently, I experimented with a technique which includes  
> generating arbitrary
> shaped waveforms naively at a sample rate many times higher than  
> the DAC rate.  The
> signal is passed through a brickwall filter, FIR in my case, and  
> then decimated to put
> the pitch in the correct range.  My FIR filter had 511 taps.  Alias  
> artifacts were
> inaudible (to my ears) up to fundamentals of about 5 KHz.  Near the  
> end of the range,
> I noticed some small inconsistency in output amplitude as frequency  
> is changed.

There may be some mileage in direct sine wave synthesis with  
intelligent band limiting. Effectively you'd generate a frequency  
domain representation of your target waveform and assemble it in real  
time, muting frequencies above a certain cut-off. This would be  
processor intensive but you could have arbitrary waveforms with  
arbitrary waveform transformations and lower pitches would have as  
much top end as they usually do.

The other advantage is that this would make it possible to add some  
movement to the upper harmonics. I tried some experiments in Csound  
and sounds with slightly randomised upper harmonics are livelier and  
more analogue-sounding than static digital waveforms.

If you add too much randomness the sound turns into an interesting  
semi-pitched kind of mush. But with digital control you can use that  
as a deliberate effect.

Richard