[sdiy] PPG/Prophet VS question

[Paul Maddox]

Tom,

> I know (or think I do - I've read) that the digital oscillators on  
> the PPG (and the similar oscillators on the Prophet VS) produce  
> aliasing as the frequency becomes higher.

sort of.

> The manual for the DSI Evolver notes the same thing about the  
> digital oscillators in that instrument, which is what you'd expect,  
> since they were based on the Prophet VS.
>
> The question is this: Why should they produce aliasing?

It's not frequency mirror aliasing, it's aliasing due to lack of  
samples.

> It was my understanding that all of these instruments use a variable  
> sample rate, and just play back the same small wavetable of samples  
> at higher speed to raise the pitch. If this is true, the  
> relationship between the sample rate and the output frequency would  
> remain constant. To put it another way, one wave cycle always takes  
> 128 samples (or 256 or whatever it is) so why does a higher  
> frequency cause problems?

:-)
Due to Jitter
In the PPG you have a 24bit accumulator for the pitch, the top 7 bits  
are used to play the wave from memory.
you also have upto a 20 bit control value for the pitch. most of the  
scale is held in the lower 16 bits, but as you get to the top, you  
have start changing values in the bits that are used to lookup the  
waveform. if you have an odd number in there, you'll start skipping  
some samples, every so often, resulting in "aliasing"

Paul