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Reversibility of aacgain
BrownNote80
post Jan 12 2013, 07:48
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In the past I have used mp3gain and aacgain on my library. I recently "undid" all of my files, in an attempt to return to original gain levels. It appears to have worked fine on all files, both mp3 and aac, and I have not observed any issues. However, I am very curious about how "different" the aac files are now compared to the original files. According to a description at the aacgain site (apologies for all-caps),
"AACGAIN IS NOT COMPLETELY REVERSABLE. THE UNDO OPTION IS INTENDED TO RESTORE A MUSIC FILE TO BE FUNCTIONALLY EQUIVALENT TO ITS ORIGINAL STATE, BUT IT WILL NOT BE BIT-FOR-BIT IDENTICAL TO THE ORIGINAL FILE."

My question is: what does the above mean? Would the "bit-for-bit differences" affect sound quality at all, or would it only be from a playability perspective (i.e. corrupted or unplayable files)? I'd just like to know what changes have been made. I do have backup files of the original aacs (as well as the cds), but I'd prefer to use the current "ungained" versions rather than re-import my backups if possible. Many thanks.
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Rescator
post Jan 12 2013, 12:05
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I'm purely speculating here but it probably stored a gain adjustment clue in a meta tag or similar, in dB maybe.
And now that it's adjusted back there may be rounding errors. If you are worried about clipping then that could be possible. (rounding errors can go both ways after all)
If any of the differences are audible, is something I seriously doubt.
Technically speaking you may (or may not) have "lost" the "bottom bit" of the audio.

In the future may I suggest ReplayGain or something compatible to that, as they only add a meta tag and do not alter that audio at all (it is instead done during playback).

Also, somewhat ironically. With any lossy encoding you can forget about bit accurate and instead embrace bit inaccuracy, rounding/quantization errors, frequency limiting and so on, after all that is what makes lossy encoding possible.
Lossy encoding intentionally "damages" the audio, and then approximately rebuild it when decoding.

But back to your question, personally I'd re-rip/re-encode my collection, but that's just me. Even if you did an ABX test I doubt you'd notice any difference, so don't worry smile.gif


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"Normality exist in the minds of others, not mine!" - Rescator
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