[PD] Combat aliasing!

Pierre Massat pimassat at gmail.com
Thu Apr 1 13:18:44 CEST 2010


Alright, i got it! Thanks everybody!

Pierre

2010/4/1 cyrille henry <ch at chnry.net>

>
>
> 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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>>
>>
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