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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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davelasker
post Jan 13 2013, 01:36
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I am the author of aacgain.

When you undo gain changes, the AAC data streams are restored to their original values. This means that your music will play identically to the way it did before using aacgain. There is no data loss associated with undo.

However, the MP4 container in which those AAC streams are embedded will not be identical to the original. Because of the way tags and other data are stored in the MP4 container, and the way the open source libraries I use to modify the container operate, some of the tags and other data inside the container may be reordered, and the amount of padding (unused space) in the file may increase or decrease.

There is no need to re-rip your music.

Hope that helps...

Dave
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