[sdiy] PPG/Prophet VS question

[Tom Wiltshire]

Hi All,

I've got a question; something that I've only just realised I don't  
understand.

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. 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 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?

I'm sure there's something I'm missing, or some stick I've got by the  
sticky end, so if someone could fill me in, I'd be grateful.

Thanks,
Tom