mp3gain — lossless mp3 normalizer
mp3gain [
options] [
infile] [
infile 2
...
]
This manual page documents briefly the
mp3gain command.
This manual page was written for the
Debian distribution because the
original program does not have a manual page.
mp3gain can analyze and adjust mp3 files so that they have the same
volume.
mp3gain does not just do peak normalization, as many normalizers do.
Instead, it does some statistical analysis to determine how loud the file
actually sounds to the human ear. Also, the changes
mp3gain makes are
completely lossless. There is no quality lost in the change because the
program adjusts the mp3 file directly, without decoding and re-encoding.
mp3gain optionally writes gain adjustments directly into the encoded
data. In this case, the adjustment works with all mp3 players, i.e. no support
for a special tag is required. This mode is activated by any of the options
-r,
-a,
-g, or
-l.
If none of the above options are given, the recommended gain change is instead
written to a special tag in the mp3 file. In this case, the adjustment only
works with mp3 players that support this tag. Some mp3 players refer to this
as ReplayGain. The tag is written either in APEv2 format (default) or in ID3v2
format (with
-s i). If you only want to print the recommended
gain change (and not modify the file at all) you may use the
-s s (skip
tag) option.
The method mp3gain uses to determine the desired volume is described at
www.replaygain.org (link to URL http://www.replaygain.org/) . See also
/usr/share/doc/mp3gain/README.method .
-
-? -h
- Show summary of options.
-
-g i
- apply gain i to mp3 without doing any analysis
-
-l 0 i
- apply gain i to channel 0 (left channel) of mp3
without doing any analysis (ONLY works for STEREO mp3s, not Joint Stereo
mp3s)
-
-l 1 i
- 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)
-
-m i
- modify suggested MP3 gain by integer i
-
-d n
- 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)
-
-s c
- only check stored tag info (no other processing)
-
-s d
- delete stored tag info (no other processing)
-
-s i
- use ID3v2 tag for gain information; if the file contained
gain data in APEv2 format, it is upgraded to ID3v2
-
-s a
- use APEv2 tag for gain information (default)
-
-s s
- skip (ignore) stored tag info (do not read or write
tags)
-
-s r
- 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
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.
The homepage of mp3gain is located at
http://mp3gain.sourceforge.net/ (link
to URL http://mp3gain.sourceforge.net/) .
This manual page was written by Stefan Fritsch
[email protected] for the
Debian system (but may be used by others). Permission is granted to
copy, distribute and/or modify this document under the terms of the GNU Lesser
General Public License, Version 2.1 or any later version published by the Free
Software Foundation.
On Debian systems, the complete text of the GNU Lesser General Public License
can be found in /usr/share/common-licenses/LGPL.