Low Pass Filter shifts forward in time

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Low Pass Filter shifts forward in time

Permanent link to this post Posted by krzysztofS » Fri Sep 30, 2016 11:13 pm

Audacity 2.1.1, Windows 7, 32bit
Audacity 2.1.2-1, Linux Mint 18, 64bit

Low Pass Filter shifts forward in time.

To reproduce this problem:
* Open Audacity
* Set Project Rate 48000Hz and Mono
* Generate Tone (sine wave 440 Hz)
* Zoom in (+)
* Mark half period between points where line crosses zero, that is about 1ms
* Effect -> [Plugins 1-15] -> Low Pass Filter (the lower of 2 identical items in version 2.1.1)
* Set cutoff frequency = 1000 Hz, rollof = 12 dB per octave and click OK.

The result consists of 3 distinct lines: a flat line at the level of zero, followed by a part of sinusoid shifted forward in time and at the end an instant jump to level zero. The bigger the rollof, the bigger the shift of the filtered sample. One could discuss what formula should be used in a filter. Nevertheless, if the input (here a sine wave) is symmetric versus reversion of the time than the output also must be symmetrical.

Why I need and use this filter:
Sometimes a human voice is distorted for about 20ms or more by a short, [quiet] yet unacceptable "smack".
On top of a vowel (~500Hz) there is an ugly sound (~3kHz). Low pass filter cleans that nearly perfectly. But it must not be applied to the entire recording because of noticeable distortion and partial loss of some consonants. The problem described above may require moving by hand the flat line from the begging to the end of every filtered area, which is a kind of madness.
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Re: Low Pass Filter shifts forward in time

Permanent link to this post Posted by kozikowski » Fri Sep 30, 2016 11:32 pm

Sometimes a human voice is distorted for about 20ms or more by a short, [quiet] yet unacceptable "smack".

This just cries out for a DeEsser and DeClicker.

viewtopic.php?p=245549#p245549

Apply DeEsser at the default settings and see if it doesn't help.

Koz
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Re: Low Pass Filter shifts forward in time

Permanent link to this post Posted by Robert J. H. » Sat Oct 01, 2016 2:24 am

krzysztofS wrote:Audacity 2.1.1, Windows 7, 32bit
Audacity 2.1.2-1, Linux Mint 18, 64bit

Low Pass Filter shifts forward in time.

To reproduce this problem:
* Open Audacity
* Set Project Rate 48000Hz and Mono
* Generate Tone (sine wave 440 Hz)
* Zoom in (+)
* Mark half period between points where line crosses zero, that is about 1ms
* Effect -> [Plugins 1-15] -> Low Pass Filter (the lower of 2 identical items in version 2.1.1)
* Set cutoff frequency = 1000 Hz, rollof = 12 dB per octave and click OK.

The result consists of 3 distinct lines: a flat line at the level of zero, followed by a part of sinusoid shifted forward in time and at the end an instant jump to level zero. The bigger the rollof, the bigger the shift of the filtered sample. One could discuss what formula should be used in a filter. Nevertheless, if the input (here a sine wave) is symmetric versus reversion of the time than the output also must be symmetrical.

Why I need and use this filter:
Sometimes a human voice is distorted for about 20ms or more by a short, [quiet] yet unacceptable "smack".
On top of a vowel (~500Hz) there is an ugly sound (~3kHz). Low pass filter cleans that nearly perfectly. But it must not be applied to the entire recording because of noticeable distortion and partial loss of some consonants. The problem described above may require moving by hand the flat line from the begging to the end of every filtered area, which is a kind of madness.

The Low pass filter is an infinite impulse response filter (IIR) and they have different group delays at different frequencies.
The tail produced is musical though. Imagine applying it to a snare drum where the tail follows the natural decay of the sound.
FIR (Finite Impulse Response) filters have a linear phase--the group delay is always the same. However, it would not fit to the mentioned snare hit.

One workaround is to apply the filter twice, once normal and once on the reversed sound (and reverse it again).
Cumbersome, I know and it has the doubled roll-off (dB/octave).
You can try my Spectral FIR filter. It is linear and has also a short cross fade at the boundaries.
The disadvantage is that you have to switch to spectral view and allow spectral editing (all in the track drop down menu).
SpectralEditFir.ny
(6.09 KiB) Downloaded 26 times

The cut off frequency has to be set in the spectral selection tool bar. It is the upper line of the rectangle and the rectangle itself defines the slope.
I've not used it lately because the Spectral Editing has undergone some changes--not necessarily to my liking. ;)

Robert
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