Re-encoding - Is there such a thing?
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You can compare decoded raw bitstreams with tools like Audacity but imho it is unnecessary. Re-encoding with VBR (Variable bitrate) encoding would be the way to go. It will use lower bits when there is less sound information, and will use your maximum provided bitrate when it actually needs it. Also, I would strongly advise to migrate from mp3 to a better format unless you're hardware locked. 128 kbps opus is said to be transparent (almost indistinguishable from uncompressed form to 99% ears) while 80-96 kbps opus is somewhat equivalent to 320 kbps mp3. And the format is royalty-free with wide support from various OSes.
Never heard of Opus until now. If I can't throw something at a random player and have it "just work", it's not even a contender in my book. I'm reading that "[...] in many respects Ogg Opus is the successor to Ogg Vorbis". That name, I recognized, and I stopped reading there. You can have the most awesome format in the world, if the support isn't there, it's a non-starter. Re-encoding is never a good idea, and going down this path to me sounds like having to re-encode in a different format every couple of years for the sake of using the format-du-jour. No thanks.
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Someone's come to me with what I thought was a good question. He's got some 320kbps MP3s that were upconverted from files that originally were anywhere between 128kbps and 256kbps. X| Don't ask me why this was done. Someone must've thought introducing extra bits would magically improve the lower-res recording. He no longer has the original versions of the files. The question he asked me, and I had no answer for, is this: Is there software that can analyze the audio in a given file, and determine that it's something that does NOT require 320kbps and there would be "no loss" converting it back to 256 or 128kps, or whatever it was originally encoded from? His argument is that his library is now taking roughly 2x+ the amount of disk space it used to, with no benefit to be gained. Of course, he's already got some MP3s that *were* originally ripped at 320kbps, so he doesn't want to bulk-convert everything back to 256kbps or lower - only those that were at the lower resolution to begin with. Obviously this isn't "audio fingerprinting" like [MusicBrainz Picard](https://picard.musicbrainz.org/) can do. And I can't come up with the right keywords for googling.
No, there will not be any such software. In short, your friend cannot return to their original 128k quality files from what they have now. The process you're referring to is called transcoding. Whenever you transcode across bitrates of a lossy codec, you lose information because the quantitization of the samples is different. So, quality was lost in the 128k -> 320k trancoding. More quality will be lost doing another transcoding from 320k -> 128k. Maybe your friend is now at a financial point in their life where they can afford to replace the mangled files?
I live in Oregon, and I'm an engineer.
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No, there will not be any such software. In short, your friend cannot return to their original 128k quality files from what they have now. The process you're referring to is called transcoding. Whenever you transcode across bitrates of a lossy codec, you lose information because the quantitization of the samples is different. So, quality was lost in the 128k -> 320k trancoding. More quality will be lost doing another transcoding from 320k -> 128k. Maybe your friend is now at a financial point in their life where they can afford to replace the mangled files?
I live in Oregon, and I'm an engineer.
patbob wrote:
Maybe your friend is now at a financial point in their life where they can afford to replace the mangled files?
There's "affording" the time, and "affording" the money. Despite the old saying, I hope you're not suggesting both are interchangeable.