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Topic: Restoring audio quality by differencing mono channels? (Read 1156 times) previous topic - next topic

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  • tJw
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Restoring audio quality by differencing mono channels?
I have a recording for which the master tape and safety copy have both been lost.

(un)fortunately, the only surviving version is a mediocre quality 128kbps mp3 made about 10 years ago. It's passable, but not great - there is audible compression artifacting, especially noticeable on things like cymbals, which sound rather watery.

This mp3 is dual mono. Due to an error on the engineer's part, none of the tracks were panned, and thus instead of being a stereo mix-down, the mp3 has two channels of the same mono content.

However, when I invert one of the two channels and sum them to mono, the result is not silence - there is a difference between the channels. Presumably the mp3 encoder treated each channel slightly differently.

See the attached FLAC file: this is the resultant signal, which seems to be composed entirely of digital compression artifacts.

My question to you is this: is there some way to use this resultant information to restore/improve the quality of this file? Some sort of averaging or differencing perhaps? Is this pie-in-the-sky thinking, or something that's theoretically possible? Has anybody ever attempted something like this?

  • saratoga
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Re: Restoring audio quality by differencing mono channels?
Reply #1
If they're different encodings you can average them. If they use the same encoder though I would expect both channels to differ only by some rounding error though.

  • DVDdoug
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Re: Restoring audio quality by differencing mono channels?
Reply #2
I don't have much hope, but...

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Some sort of averaging or differencing perhaps?
A stereo-to-mono conversion is averaging.*  Here are some instructions for doing that in Audacity.

Subtraction won't get you anywhere...    If you play around with the algebra, you'll find that starting with L & R, subtracting the difference will give you left-only or right-only.     

And that's worth a try too....   Maybe the left or right channel sounds better and making a mono file from the best channel will improve the sound?

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However, when I invert one of the two channels and sum them to mono, the result is not silence - there is a difference between the channels. Presumably the mp3 encoder treated each channel slightly differently.
Strange....  I wouldn't expect that.  I can't listen to that file 'cause I'm at work, but is there a chance that those are analog differences?   




*  You sum the left & right and then reduce the volume, or you reduce the volume before summing.
  • Last Edit: 28 September, 2016, 10:32:12 AM by DVDdoug

  • tJw
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Re: Restoring audio quality by differencing mono channels?
Reply #3
If they're different encodings you can average them. If they use the same encoder though I would expect both channels to differ only by some rounding error though.
It's not different encodings, it's a single file with two channels each containing the same content. So you think this is a lost cause?

Strange....  I wouldn't expect that.  I can't listen to that file 'cause I'm at work, but is there a chance that those are analog differences?
The DC offset of each channel was slightly different as a result of the botched original capture - the output changes slightly if I remove the DC bias before summing to mono.

And that's worth a try too....   Maybe the left or right channel sounds better and making a mono file from the best channel will improve the sound?
I have tried that, and A/B'd the channels against each other - whatever differences there may be are too subtle for me to detect. Neither is lacking in compression artifacts.

  • saratoga
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Re: Restoring audio quality by differencing mono channels?
Reply #4
If there is no meaningful difference between the channels, then yes, you have nothing to work with.  If there is some difference between them, you can average to reduce noise a little or just take the best channel. 

Not much else to it. 

  • tJw
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Re: Restoring audio quality by differencing mono channels?
Reply #5
How do I tell if the difference is 'meaningful', though? The FLAC file I posted leads me to believe that the differences are in some way significant. Is averaging the two channels my only option here?

  • DVDdoug
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Re: Restoring audio quality by differencing mono channels?
Reply #6
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How do I tell if the difference is 'meaningful', though?
By listening...   But you already said you're not hearing any difference between left & right.   And, we don't know why there's a difference between left & right, or what caused the difference, in this file that seems to be mono...

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The FLAC file I posted leads me to believe that the differences are in some way significant.
It's about 20dB down so that's not a huge difference.

...It's true that if subtraction gives you dead silence, that proves there is no difference in sound, but the sound of the difference isn't the same as the difference in the sound...   The easiest way to demonstrate that to take a copy of a recording and delay it by a few milliseconds (add a few milliseconds of silence to the beginning).    Obviously there's no difference in the sound because you can't hear a few milliseconds of delay without a reference, but the subtracted file would be loud (probably louder than the original) and comb-filtered.    Averaging (mixing) the delayed & non-delayed files will give you an equally-bad sounding file.

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Is averaging the two channels my only option here?
I don't know if that will make it better, worse, or make no difference.
  • Last Edit: 29 September, 2016, 11:36:27 AM by DVDdoug