Foobar2000:ReplayGain Scanner:Alter File Content
This feature applies ReplayGain adjustment - or manual adjustment - directly to audio data on supported file formats.
No undo option - back your files up first!
There is no way to undo the adjustment. If you're not 100% confident in what you're about to do, please make backup copy of your files first. Additionally, a file will be left partially altered if the operation is cancelled halfway thru, with no way to recover.
Manually applying a reverse adjustment may not produce a file bit identical to the original file due to scalefactor clipping.
Standalone MP3 and MP2 files.
MP4/M4A files with AAC, MP3 or MP2 audio.
It is worth noting that this feature has been extensively tested and works with audio streams in MP4 video files.
Advantages over tag based ReplayGain
Files altered this way will play with their intended loudness in every software or hardware player, regardless of its ReplayGain support.
This is particularly useful for manipulating loudness of audio streams in MP4 video files, as video players rarely support ReplayGain.
How it works
Each compressed audio frame contains scalefactor fields that control the loudness of decoded audio. These fields can be manipulated without having to re-encode the entire payload. Hence, the adjustment is lossless - except for the possibility of adjusted scalefactors exceeding the finite allowed range and becoming clipped. The potential artifacts from this should not be audible - however, the adjustment should not be applied more than once to the same file.
Opus header gain
In foobar2000 v1.4, a similar feature to "alter file content" was introduced for Opus files. However, it does not alter the entire audio stream, it alters Opus header gain value - which every compliant decoder is required to respect. It does not suffer from any of the issues noted above and can be fully reversed.
Opus header gain adjustment is currently supported for plain Opus files as well as Opus in Matroska.