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Q: improving sound of old .mp3 files by filtering artifacts?
post Jan 3 2012, 16:33
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Before shouting «no, you cannot know what has been deleted»: this is not about knowing, it is about guessing better than one could once upon a time in the previous millennium.

Back in the 90's, you could way more often than today single out an mp3 file by its artifacts (for given bitrate in suitable range [*]). Of course we could not know without the original whether a certain signal is an encoding artifact or is part of the music, but back in those days we could fairly often make an educated guess by listening -- doing «ABX except without the A and B».

So, the last 15 years have brought forth lossy encoders which are better at avoiding audible artifacts: Not only do we know better what to avoid, we also know better how to avoid it. And in principle, it might very well be possible to use this improved knowledge to (i) detect in an old mp3 file «what would have been avoided today», and (ii) filter it out. Not to achieve the impossible certainty that this part of the signal was not in the original source, but to achieve fewer audible nuissances more often than not.

Q: Has this been attempted, successfully or not?

(Any such tool could be easily testable if one has an old encoder and a lossless source and a few good ears.)

[*] I presume that http://listening-tests.hydrogenaudio.org/s...8-1/results.htm should keep TOS#8 moderation off my back.
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post Jan 14 2012, 03:48
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I think one very crude way to do this could be to just use a lowpass filter. Indeed, some of the most annoying MP3 artifacts come from poorly-encoded high frequencies. You could add a lowpass filter at, say, 7 kHz and remove all the high-frequency content, and your audio might sound less artifacted. However, this assumes there aren't also artifacts in the lower frequencies (which is often the case with poor encodes). And you'd also be able to tell very apparently that the audio had been lowpassed, it'd sound like something recorded in the 1950s. Whether or not that sounds better than leaving the artifacted high freqs intact is entirely subjective.

I don't know much about the theoretical side of audio processing, FFTs and such, but it seems like removing MP3 artifacts would be very difficult, if not impossible, and the results would not be that great. Doesn't sound like something that would be worth someone's time to develop.

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