Danux
21st February 2006, 05:27
I did a bit of Googling, found several mentions of people trying to clean audio that was clipped at the source, but no real solutions. Thought for posterity I'd post the solution I arrived at. The result has less fidelity, just like bad video which is filtered comes out less-than-perfect, but it is easier on the ears than the original clipped track IMHO.
First, I ripped out the audio track as a WAV using Virtualdub ("File"->"Save WAV..."), then opened the WAV with a program called Audacity.
In Audacity:
1> Normalize the audio, with the defaults. Click "Effects"->"Normalize".
2> Perform a minimal "Noise Removal". To do this, find a place in the clip where there are no "real" sounds. just hiss/buzz. I usually use the very beginning or very end of the track. Use the magnification buttons to zoom in on the sound wave. Click and drag over the portion that is just hiss / background noise, you won't need a lot, just a wee sample. Now, from the menu, select "Effect" -> "Noise Removal". From the popup, select "Get Noise Profile".
Now select the entire track with ctrl-A, and again select "Effect"->"Noise Removal", only this time, drag the bar in step2 all the way to the left (Less), then click the "Remove Noise" button. A progress bar will indicate how much of the track has been denoised.
3> Now execute a Low Pass Filter. ctrl-a to select the whole track. From the menu, click "Effect"->"Low Pass Filter", and set the cutoff. For my files, I found 1009 to be a good cutoff frequency, try something around the 1000-mark. If you don't like the result, you can always undo it (ctrl-z) and try a different value.
4> At this point, the track is listenable for me, but if you are looking to emulate a higher degree of fidelity, you can open another Effect called "Equalization". None of the presets really cuts it for me, so what I do in this step is to make sure the "Predefined" section is set to "acoustic", then click the "Load Predefined Curve" button. Each of the little circles inthe resulting graph can be moved, so I adjust everything on the lower (left) two-thirds of the range to zero, then bump up the higher end of the range. My line kind of looks like this:
_________/---\_
You will have to be gentle with these adjustments though, or you'll simply re-introduce clipping.
5> A possible final step will be to Amplify. Click "Effects"->"Amplify", make sure the "Allow Clipping" box it *not* checked. The default values should make the track as loud as possible, while retaining its fidelity.
6> Go to "File"->"Export as WAV", and export the cleaned audio track as a WAV. You can use Virtualdub, under the "Audio"->"WAV audio" to select the track ,with your original AV file, to use the new audio track.
http://www.virtualdub.org
http://audacity.sourceforge.net/
The following link will list the "low-pass filter with Q"
http://audacity.sourceforge.net/download/nyquistplugins
Again, this caused me to lose some fidelity in the audio. It kinda sounds like you're playing the track through a blanket draped over a speaker, but all the clipping was smoothed over; it stopped hurting to use headphones <grin>.
HTH.
First, I ripped out the audio track as a WAV using Virtualdub ("File"->"Save WAV..."), then opened the WAV with a program called Audacity.
In Audacity:
1> Normalize the audio, with the defaults. Click "Effects"->"Normalize".
2> Perform a minimal "Noise Removal". To do this, find a place in the clip where there are no "real" sounds. just hiss/buzz. I usually use the very beginning or very end of the track. Use the magnification buttons to zoom in on the sound wave. Click and drag over the portion that is just hiss / background noise, you won't need a lot, just a wee sample. Now, from the menu, select "Effect" -> "Noise Removal". From the popup, select "Get Noise Profile".
Now select the entire track with ctrl-A, and again select "Effect"->"Noise Removal", only this time, drag the bar in step2 all the way to the left (Less), then click the "Remove Noise" button. A progress bar will indicate how much of the track has been denoised.
3> Now execute a Low Pass Filter. ctrl-a to select the whole track. From the menu, click "Effect"->"Low Pass Filter", and set the cutoff. For my files, I found 1009 to be a good cutoff frequency, try something around the 1000-mark. If you don't like the result, you can always undo it (ctrl-z) and try a different value.
4> At this point, the track is listenable for me, but if you are looking to emulate a higher degree of fidelity, you can open another Effect called "Equalization". None of the presets really cuts it for me, so what I do in this step is to make sure the "Predefined" section is set to "acoustic", then click the "Load Predefined Curve" button. Each of the little circles inthe resulting graph can be moved, so I adjust everything on the lower (left) two-thirds of the range to zero, then bump up the higher end of the range. My line kind of looks like this:
_________/---\_
You will have to be gentle with these adjustments though, or you'll simply re-introduce clipping.
5> A possible final step will be to Amplify. Click "Effects"->"Amplify", make sure the "Allow Clipping" box it *not* checked. The default values should make the track as loud as possible, while retaining its fidelity.
6> Go to "File"->"Export as WAV", and export the cleaned audio track as a WAV. You can use Virtualdub, under the "Audio"->"WAV audio" to select the track ,with your original AV file, to use the new audio track.
http://www.virtualdub.org
http://audacity.sourceforge.net/
The following link will list the "low-pass filter with Q"
http://audacity.sourceforge.net/download/nyquistplugins
Again, this caused me to lose some fidelity in the audio. It kinda sounds like you're playing the track through a blanket draped over a speaker, but all the clipping was smoothed over; it stopped hurting to use headphones <grin>.
HTH.