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Which filter artifacts can be eliminated without trade-off?, (in a universe where computing power comes free)
googlebot
post Apr 3 2012, 08:04
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I'm learning, please correct any of the following sentences, if the aren't correct:

  1. We cannot eliminate ringing, just choose where we want it (pre-, post-).
  2. We can minimize ripple by computing power (choice of windowing function) without quality trade-off.
  3. Aliasing in resamplers can eventually only be controlled by passband width and filter steepness (dependency to 1).


If 3 is true, why does the SoX offer a dedicated "allow aliasing" option besides bandwidth and steepness?
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bandpass
post Apr 3 2012, 09:17
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QUOTE (googlebot @ Apr 3 2012, 07:04) *
why does the SoX offer a dedicated "allow aliasing" option besides bandwidth and steepness?

SoX offers a bandwidth option (of which "steep" filter is one particular case) and with this, the stop-band (the point at which the selected attenuation begins) is automatically set to the (old or new as appropriate) nyquist frequency, so there will be no aliasing or imaging (at levels above the selected attenuation). Selecting "allow aliasing" repositions the the filter's stop band to begin above the nyquist.

From an audio perspective, I've not found aliasing particularly useful: if filtering at ~20kHz, one (or at least I) can hear neither the ringing nor the traded aliasing, so it makes no odds. If filtering in the audible band (e.g. resampling 16kHz -> 44.1kHz) then you're trading one audible artefact for another—it might be useful to do so depending on the audio content.

Two minor things that "allow aliasing" give are quicker filtering/conversion due to the shorter filter (but the effect is very small when FFT filtering), and "full" spectrograms that don't fade out at the top of the graph (so that must mean fuller-sounding audio, right? wink.gif ).

This post has been edited by bandpass: Apr 3 2012, 09:19
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