cliveb wrote:
I don't think anyone has yet actually described the technical difference
between MP3Gain and ReplayGain, so I'll try and give the details...
As I'm sure you all know, an MP3 file is a sequence of "frames". As
well as the audio data, each frame also contains a header, and one
attribute in the header tells the replay device at what level to output
the frame. MP3Gain updates this attribute in the header of every frame
in the file, in order to adjust the playback level up or down. Note
that the actual audio is not updated, just the instruction as to what
level it should be. Therefore it's a lossless adjustment. Recent
versions of MP3Gain also add a comment in the file's metadata to
indicate by what amount it has adjusted the level, so that it can be
undone.
Note that there is a subtle edge case where the MP3gain method is not,
in fact, lossless: if the frame was originally recorded with an extreme
gain value (either very close to to 0 or very close to 255), and the
calculated adjustment for the song would push the value outside the
0:255 range, then mp3gain will clip the value to 0 or 255, and is thus
lossy. The mp3gain app will warn in this case.
Practically speaking, this never happens -- at least with mp3s created
with LAME, the original gain on the frame is well away from the extremes.
For more info, see `mp3gain -? wrap` .
The (huge) benefit of the mp3gain method is that you don't need a player
that understands non-standard tags to play back at the right level --
the frame gain value that mp3gain adjusts is part of the original mp3
spec, and any mp3 player (software or hardware) will respect it.
- Marc
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