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I have some rare old music files on my drive that are compressed to 128 kbps in the mp3 format. Some of them are of digital origin, and some of them are vinyl rips.

Audio data compression affects music in a variety of ways. For a pretty in-depth explanation, this article by SOS Magazine may be helpful. Mentioned effects are:

  • Decreased Frequency Response
  • Lower Dynamic Range and Mutated Dynamics
  • Loss of Frequencies
  • Frequency Anomalies and Distortions, Swirlies
  • Pre- and Post-Echoes
  • Audio Content Timing Errors (Roughness & Double-speak)
  • Loss of Transients and Sound Details
  • Loss of Stereo Imaging and Panned Details
  • Phase shifts
  • Noise addition

I am not an expert in any of these effects, and there may be more effects, so I encourage you to read all about it.

I am however interested in learning if there are ways to try to "recover" or at least "improve" bitrate-compressed audio files. It is probably near-impossible to restore them to their original state. But can they be adequately recovered or at least improved?

If so, which methods are used? What kind of expertise would be needed? Are there universal tools or utilities that can help along the way?

Also, could an amateur musician (like me) use common mastering/mixing tools to at least improve the files a little using EQs and whatnot – or does the ‘shit in, shit out’ principle apply?

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GIGO [garbage in, garbage out] does apply...

...however - a good mix, master or especially restoration engineer could pull apparent newness out of a low quality file - that's essentially the same task as recovering a track from an old 78 RPM record, or remastering the Beatles albums from the original multitracks.

You work to eliminate the 'bad' & emphasise the 'good'. The tools required to do this are generally not cheap & probably to go into detail would be overkill as an answer to this particular question.

At minimum, you would need

  • massive upsampling, to give you room to breathe in the following operations. [Upsampling in itself will not improve the track in the slightest, it is just a preparatory stage]
  • noise reduction - to remove unwanted background interference from the track
  • phase alignment/correction - to correct potential "wrong imaging" in the stereo field
  • transient restoration - to put back the tiny sound at the beginning of, for instance, a drum strike, that can be lost at lower qualities.
  • EQ [equalisation - treble & bass on steroids]
  • 'soundfield restoration' - which is a very hazy concept & quite beyond my ability to describe in mathematical terms - but involves methods by which you can change the apparent distance, direction & even reflectivity of existing ambient sound on the original recording. Think of it as being able to change the reverb on a track where the reverb is already 'printed' to the stereo image.

A lot of this process is 'opinion-based' -
To do this for an actual official re-release you often have to work with the original artist or producer, or if you're really unlucky, 'the guy from the record company' :-(
You guess at what the fuller detail may be & use your suite of tools to attempt to create the picture you have in your head. It's not 'true' but [depending on the skill of the engineer] it's an 'improvement' on the currently-available version.

I put a very short extract of the remaster of Stevie Wonder's Superstition on Soundcloud [posted with permission]* a while ago to demonstrate the cleanup process for another SE question. Quality is not optimal & also it's less than 20s long [so I could get permission to post it] - a few bars of the original version, followed by a snippet of the multitrack remaster, just so you can hear the difference in ambient spacing & bassline clarity.

* I did have to work through the permissions system on SE when I first linked to this extract. SE Admin were happy that if Soundcloud police their own domain & don't take it down, then I have sufficient external permission to post links to it.

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Mp3 is a lossy compression format, the removal of frequencies is permanent. There are lossless files such as flac files which 'pack' the file down to a smaller file size that can then be 'unpacked'. But if they are all 128kbps Mp3 then your out of luck. All those effects you listed are symptoms of destructive compression. You could 'improve' them with mastering techniques but any improvement would be subjective and not true restoration.

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You could take a look at Zynaptiq Unchirp and Unfilter, but they are expensive options. It's not guaranteed to work but it's an option worth demoing. It really depends on how important these files are too you.

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