[PD] Combat aliasing!

[cyrille henry]

Pierre Massat a écrit :
> Wait, so aliasing occurs when the signal is sampled?
yes
> So if i have this patch :
> 
> [adc~]
> |
> |
> |
> [dac~]
> 
> and if the signal already contains frequencies above the Nyquist, i will 
> get aliasing?
yes, it will.
so usually, your sound card have an analog filter prior to digital convertion in order to reduce aliasing.

cyrille

> I generally use my electric guitar as the main audio 
> source, and i'm assuming that it has lots of harmonics beyond the 
> Nyquist frequency (especially when the strings are new), yet i never 
> noticed any distortion of any sort. I might have a bad ear... Or is it 
> just that the energy of the upper harmonics is too low for me to notice 
> when they cause aliasing?
> 
> 
> Pierre
> 
> 2010/4/1 Matteo Sisti Sette <matteosistisette at gmail.com 
> <mailto:matteosistisette at gmail.com>>
> 
>      > Correct, nothing played back at original sampling rate will alias.
> 
>     It _won't_ alias; it may already _have_ aliased when sampled in the
>     first place.
> 
>     Aliasing occurs when sampling.
> 
>     When you digitalize (ADC), you are sampling. When generating a
>     waveform mathematically, you are sampling the mathematical function
>     at the very moment you compute its value at discrete points.
> 
>     When you play back a signal at a different speed than the original,
>     you are _resampling_ it, that is, theorically, interpolating it and
>     then sampling it again, and it is the sampling stage, not the
>     interpolating one, that produces the aliasing.
> 
>     The interpolation, since it cannot be an ideal interpolation, may
>     introduce other noises or artifacts, not aliasing as far as I can see.
> 
> 
>     -- 
>     Matteo Sisti Sette
>     matteosistisette at gmail.com <mailto:matteosistisette at gmail.com>
>     http://www.matteosistisette.com
> 
> 
> 
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