Low/high-pass phase shift

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Timar
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Low/high-pass phase shift

Post by Timar » Thu Jun 09, 2011 2:54 pm

The high and low pass filters in Audacity are not linear phase. Is this itentional (e.g. is there any situation where this could be desirable)? I wanted to apply a high pass to a short selection of a track to get rid of some rumbling noise but this introduced really nasty artifacts at the borders of the selection. I figured out that these artifacts came up because the high pass completely messes up the phase - you can even "see" the waveform shift a bit after applying the filter...

steve
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Re: Low/high-pass phase shift

Post by steve » Thu Jun 09, 2011 6:15 pm

The high-pass/low-pass filters are built into "Nyquist" and are based on biquad filters and respond in a similar way to analogue filters. FFT filters (such as is used in the Equalizer effect) should be able to produce the required filtering without phase shifts. FFT filtering is also available in Nyquist, but it is a lot more complicated than using the biquad filters.
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Timar
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Re: Low/high-pass phase shift

Post by Timar » Thu Jun 09, 2011 7:36 pm

OK, I see, thank you. I used the equalizer for that purpose and that did it. But now I miss the analogue warmth.. ;)

Btw. maybe it should be mentioned somewhere in the wiki that those Nyquist filters are not suitable for editing sections of a track

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Re: Low/high-pass phase shift

Post by steve » Thu Jun 09, 2011 7:48 pm

Timar wrote:Btw. maybe it should be mentioned somewhere in the wiki that those Nyquist filters are not suitable for editing sections of a track
I can see your point, but it would be rather unusual to apply a high/low pass filter part way through a note or word. If a high/low pass filter is used on just part of a track, it is far more common for the selection to start and end at near silence between notes. More common again is to apply the filter to the entire track. When used in either of these more common ways, the phase shift is unlikely to present a problem.

I think it's probably true to say that 'most' effects are liable to produce clicks if applied part way through a note.
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