[sdiy] PPG/Prophet VS question

[Scott Nordlund]

The Evolver at least is fixed rate.  All the DSP stuff (digital oscillators, delays, etc.) is going at 48 kHz.  I'm not completely sure but I believe the PPG synths are also fixed rate, but higher frequency, maybe 200-300 kHz (the early digital Casio machines did this as well).  Not sure about the VS.

And if I'm not completely wrong, even variable sample rate systems can alias to some extent (jitter) if the DAC rates are derived from a fixed master clock (rather than a VCO).  

(...someone please correct me if I'm wrong)

> To: synth-diy at dropmix.xs4all.nl
> From: tom at electricdruid.net
> Date: Fri, 24 Apr 2009 16:46:34 +0100
> Subject: [sdiy] PPG/Prophet VS question
> 
> 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
> 
> 
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