-?-h
Show summary of options.
-gi apply gain i to mp3 without doing any analysis
-l0i apply gain i to channel 0 (left channel) of mp3 without doing any analysis (ONLY works for
STEREO mp3s, not Joint Stereo mp3s)
-l1i apply gain i to channel 1 (right channel) of mp3 without doing any analysis (ONLY works for
STEREO mp3s, not Joint Stereo mp3s)
-r apply Track gain automatically (all files set to equal loudness)
-k automatically lower Track gain to not clip audio
-a apply Album gain automatically (files are all from the same album: a single gain change is
applied to all files, so their loudness relative to each other remains unchanged, but the
average album loudness is normalized)
-mi modify suggested MP3 gain by integer i-dn modify suggested dB gain by floating-point n-c ignore clipping warning when applying gain
-o output is a database-friendly tab-delimited list
-t mp3gain writes modified mp3 to temp file, then deletes original instead of modifying bytes in
original file (This is the default in Debian)
-T mp3gain modifies bytes in original file instead of writing to temp file.
-q Quiet mode: no status messages
-p Preserve original file timestamp
-x Only find max. amplitude of mp3
-f Force mp3gain to assume input file is an MPEG 2 Layer III file (i.e. don't check for mis-named
Layer I or Layer II files)
-sc only check stored tag info (no other processing)
-sd delete stored tag info (no other processing)
-si use ID3v2 tag for gain information; if the file contained gain data in APEv2 format, it is
upgraded to ID3v2
-sa use APEv2 tag for gain information (default)
-ss skip (ignore) stored tag info (do not read or write tags)
-sr force re-calculation (do not read tag info)
-u undo changes made by mp3gain (based on stored tag info)
-w "wrap" gain change if gain+change > 255 or gain+change < 0 (see below or use -?wrap switch for
a complete explanation)
-v Show version of program.
If you specify -r and -a, only the second one will work.
If you do not specify -c, the program will stop and ask before applying gain change to a file that might
clip
TheWRAPoption
Here's the problem: The "global gain" field that mp3gain adjusts is an 8-bit unsigned integer, so the
possible values are 0 to 255.
MOST mp3 files (in fact, ALL the mp3 files I've examined so far) don't go over 230. So there's plenty of
headroom on top-- you can increase the gain by 37dB (multiplying the amplitude by 76) without a problem.
The problem is at the bottom of the range. Some encoders create frames with 0 as the global gain for
silent frames. What happens when you _lower_ the global gain by 1? Well, in the past, mp3gain always
simply wrapped the result up to 255. That way, if you lowered the gain by any amount and then raised it
by the same amount, the mp3 would always be _exactly_ the same.
There are a few encoders out there, unfortunately, that create 0-gain frames with other audio data in the
frame. As long as the global gain is 0, you'll never hear the data. But if you lower the gain on such a
file, the global gain is suddenly _huge_. If you play this modified file, there might be a brief, very
loud blip.
So now the default behavior of mp3gain is to _not_ wrap gain changes. In other words,
1. If the gain change would make a frame's global gain drop below 0, then the global gain is set to 0.
2. If the gain change would make a frame's global gain grow above 255, then the global gain is set to
255.
3.
If a frame's global gain field is already 0, it is not changed, even if the gain change is a
positive number.
To use the original "wrapping" behavior, use the -w switch.
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