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Filtering border artifacts, How to avoid artifact on low pass filter
Northpack
post Jun 9 2011, 14:13
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Edit: errr... wrong title, I meant high pass of course

Hi,

I have recording containing some low frequency rumbles in a few places. I want to filter that out by selectively applying a high pass to those selection. However, it seems impossible doing this in Audacity without introducing artifacts which sound worse than the rumble itself. Have a look (before / after):



Of course I have adjusted the selection to zero points. Do you have an idea what is happening here and how to avoid it?

This post has been edited by Northpack: Jun 9 2011, 14:30
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pdq
post Jun 9 2011, 14:30
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I have a suggestion. Apply a high-pass filter to select just the rumble. Then invert that and add it back to cancel the original rumble, but first fade the rumble track in and out as needed.
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Northpack
post Jun 9 2011, 15:03
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So this is just an ordinary cut-off artifact? But why does a low/high pass applied to a selection introduce such an artifact?

Thanks for you suggestion. Sounds good but it just doesn't work. The high pass seems to mess up the phase, so that the rumble only gets amplified sad.gif
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Northpack
post Jun 9 2011, 15:06
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QUOTE (Northpack @ Jun 9 2011, 15:03) *
...why does a low/high pass applied to a selection introduce such an artifact...
...high pass seems to mess up the phase...

Ehm... I guess I've already answered my own question blush.gif
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Alexey Lukin
post Jun 9 2011, 16:26
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Make sure that this "amplified" rumble is not just a visual artifact on a spectrogram. Attaching a pair of WAV files would be helpful.
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Dirk95100
post Jun 9 2011, 16:31
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QUOTE (pdq @ Jun 9 2011, 15:30) *
I have a suggestion. Apply a high-pass filter to select just the rumble. Then invert that and add it back to cancel the original rumble, but first fade the rumble track in and out as needed.

You mean lowpass filter instead.

Idea is to subtract the rumble from the track. You can do this by making a copy of the whole track, then lowpass the copy, invert it. And when the rumble is pressant in the original track, turn up the fader of the copied low passed track track.

This post has been edited by Dirk95100: Jun 9 2011, 16:33
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Northpack
post Jun 9 2011, 16:37
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QUOTE (Alexey Lukin @ Jun 9 2011, 16:26) *
Make sure that this "amplified" rumble is not just a visual artifact on a spectrogram. Attaching a pair of WAV files would be helpful.

It's a clearly audible popping sound that goes well above the filter frequency. No wonder though because if the phase gets shifted, the samples at the beginning and the end of the selection are not aligned anymore.

QUOTE (Dirk95100 @ Jun 9 2011, 16:31) *
You mean lowpass filter instead.

Idea is to subtract the rumble from the track. You can do this by making a copy of the whole track, then lowpass the copy, invert it and when the rumble is pressant turn up the fader of the copied track.

I do always confuse these terms too wink.gif
As I wrote above, I did exactly what pdq suggested but it doesn't work because the filters are not linear phase. I did the same with the equalizer however and it worked as expected. I just wonder why Audacity's filters are not of a linear phase type... that kind of provokes such trouble.
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benski
post Jun 9 2011, 17:01
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QUOTE (Northpack @ Jun 9 2011, 11:37) *
As I wrote above, I did exactly what pdq suggested but it doesn't work because the filters are not linear phase. I did the same with the equalizer however and it worked as expected. I just wonder why Audacity's filters are not of a linear phase type... that kind of provokes such trouble.


I'm sure I'll get slapped with a TOS #8 over this, but generally speaking, non-linear phase filter sound more "musical" and act more like their analog counter parts. I'm not here to make this claim strongly, but it is a prevalent opinion and explains why most pro audio apps use minimum-phase filters or some other non-linear variant. They're also easier to implement and faster (especially IIR filters), which might be a more likely reason.

However, I definitely see where they are useful for situations like this smile.gif You could instead try to have a small cross-fade at the start and end of the location that you will be filtering. This should smooth out the transient that the filter introduces.

This post has been edited by benski: Jun 9 2011, 17:02
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Paulhoff
post Jun 9 2011, 17:36
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For what it is worth.

Paul

smile.gif smile.gif smile.gif


--------------------
"Reality is merely an illusion, albeit a very persistent one." Albert Einstein
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Northpack
post Jun 10 2011, 14:36
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QUOTE (Paulhoff @ Jun 9 2011, 17:36) *
For what it is worth.

Yepp, that's exactly what I did rolleyes.gif
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