[sDIY](early) wavetable synthesis

[P.M. van Nugteren]

mbartkow at ET.PUT.Poznan.PL wrote:
> 
> > Is there a way to predict the frequencies of partials which originate
> > from aliasing in a non-interpolated wavetable?
> 
> When you sample any waveform, its spectrum is being replicated
> at each multiple of the sampling frequency. For example, if you
> sample a 1kHz sine using 8kHz sampling rate, the spectrum of the
> sampled signal contains the original 1kHz partial (and a comple-
> mentary -1kHz one) as well as all partials resulting from the
> n*8kHz-1kHz, n*8kHz+1kHz formula, e.g. 7kHz, 9kHz, 15kHz, 17kHz
> and so on.

Hmmm so there's more than the law of the Niquist frequency?
But actually I didn't mean sampling but the other way around, putting a
digitally generated wave through a DAC. Or this apply to this as well?
Could you tell me more about the sampling law you're describing there?

> When you sample a complex signal, its whole spectrum is replicated
> in the same way as in the example. Aliasing occurs, when the sampling
> rate is too low and the reflected spectrum overlaps with the high end
> of the base band.

But when is the samplingrate too low? The Niquist law say the sampleing
freq. should be at least twice as high as the frequency you're sampling.

> > If yes, what happens with these partials when the pitch of the
> > wavetabele is changed (by different methods used)?
> 
> All partials scale appropriatelly.
> If you read out the samples from the wavetable faster than
> originally sampled, the whole spectrum scales upwards, and
> more partials fall outside of the "hearing range".

Well that technique isn't used very much. Instead the numbers of samples
taken from a wavetable is reduced for a higher pitch or multiplied for a
lower pitch. What happens than with partials created by sampling?

Thank you, it's been more confusiong tough! But please go on.


Regards,

Paul van Nugteren