[PD] Combat aliasing!

Mathieu Bouchard matju at artengine.ca
Thu Apr 1 02:59:26 CEST 2010

Derek Holzer wrote :

> I was thinking about this the other day.... is it possible to have aliasing 
> with Karplus-Strong? Because it's a delay line, nothing is being played back 
> at any higher rate than it was sampled at, so no aliasing should be possible. 
> Right?

The aliasing comes from a moiré (interference) between the frequency of the 
sampling and the frequency of whatever is going on. In the case of 
Karplus-Strong, the choice of the number of samples of delay is an integer, 
therefore the rounding of those values will be a kind of aliasing, and this 
aliasing will accumulate as the signal is fed back, because all the 
fractional-sample delays exceeding or missing are going to add up instead of 
being compensated by the usual counters.

If you antialias Karplus-Strong synthesisers, instead, the antialias will act 
as a lowpass that will attenuate much of the upper range of possible 
frequencies. For example, a delay by n samples and a half, is quite equivalent 
to how [rzero~ -1] acts as a kind of low-pass.

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| Mathieu Bouchard, Montréal, Québec. téléphone: +1.514.383.3801

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