This Python package provides modules to read, write and calculate Replay Gain as well as 2 scripts that utilize these modules to do Replay Gain.
Replay Gain  is a proposed standard (and has been for some time -- but it's widely accepted) that's designed to solve the problem of varying volumes between different audio files. I won't lay it all out for you here, go read it yourself.
- Python 2.6 -- http://python.org/
- Mutagen -- http://code.google.com/p/mutagen/
- GStreamer 0.10 -- http://gstreamer.org/
- pygst 0.10 -- http://gstreamer.freedesktop.org/modules/gst-python.html
Just install it like any other Python package: Unpack, then (as root/with sudo)
# python setup.py install
This is a program like, say, vorbisgain or mp3gain, the difference being that instead of supporting a mere one format, it supports several:
- Ogg Vorbis (or probably anything you can put into an Ogg container)
Basic usage is simple:
$ replaygain AUDIOFILE1 AUDIOFILE2 ...
There are some options; see 'em by running
$ replaygain --help
This program is designed to apply Replay Gain to whole music collections, plus the ability to simply add new files, run collectiongain and have it replay-gain those files without asking twice.
To use it, simply run
$ collectiongain PATH_TO_MUSIC
and re-run it whenever you add new files. Run
$ collectiongain --help
to see possible options.
If, however, you want to find out how exactly collectiongain works, read on (but be warned: It's long, boring, technical, incomprehensible and awesome).
collectiongain runs in two phases: The file collecting phase and the actual run.
Prior to analyzing any audio data, collectiongain gathers all audio files in the directory and determines a so-called album ID for each from the file's tags:
If the file contains a Musicbrainz album ID, that is used.
Otherwise, if the file contains an album tag, it is joined with either
- an albumartist tag, if that exists,
- or the artist tag
- or nothing if neither tag exists.
The resulting artist-album combination is the album ID for that file.
If the file doesn't contain a Musicbrainz album ID or an album tag, it is presumed to be a single track without album; it will only get track gain, no album gain.
Since this step takes a relatively long time, the album IDs are cached between several runs of collectiongain. If a file was modified or a new file was added, the album ID will be (re-)calculated for that file only. The program will also cache an educated guess as to whether a file was already processed and had Replay Gain added -- if collectiongain thinks so, that file will totally ignored for the actual run. This flag is set whenever the file is processed in the actual run phase (save for dry runs, which you can enable with the --dry-run switch) and is cleared whenever a file was changed. You can disable these assumptions with the --ignore-cache switch; in that case, the program will actually physically check every file in your collection for Replay Gain data.
For the actual run, collectiongain will simply look at all files that have survived the cleansing described above; for files that don't contain Replay Gain information, collectiongain will calculate it and write it to the files (use the --force flag to calculate gain even if the file already has gain data). Here comes the big moment of the album ID: files that have the same album ID are considered to be one album (duh) for the calculation of album gain. If only one file of an album is missing gain information, the whole album will be recalculated to make sure the data is up-to-date.
Proper Replay Gain support for MP3 files is a bit of a mess: on the one hand, there is the mp3gain application  which was relatively widely used (I don't know if it still is) -- it directly modifies the audio data which has the advantage that it works with pretty much any player, but it also means you have to decide ahead of time whether you want track gain or album gain. Besides, it's just not very elegant. On the other hand, there are at least two commonly used ways to store proper Replay Gain information in ID3v2 tags .
Now, in general you don't have to worry about this when using this package: by default, replaygain and collectiongain will read and write Replay Gain information in the two most commonly used formats. However, if for whatever reason you need more control over the MP3 Replay Gain information, you can use the --mp3-format option (supported by both programs) to change the behaviour. Possible choices with this switch are:
- replaygain.org (alias: fb2k) Replay Gain information is stored in ID3v2 TXXX frames. This format is specified on the replaygain.org website as the recommended format for MP3 files. Notably, this format is also used by the foobar2000 music player for Windows .
- legacy (alias: ql) Replay Gain information is stored in ID3v2.4 RVA2 frames. This format is described as "legacy" by replaygain.org; however, it is still the primary format for at least the Quod Libet music player  and possibly others. It should be noted that this format does not support volume adjustments of more than 64 dB: if the calculated gain value is smaller than -64 dB or greater than or equal to +64 dB, it is clamped to these limit values.
- default This is the default implementation used by both replaygain and collectiongain. When writing Replay Gain data, both the replaygain.org as well as the legacy format are written. As for reading, if a file contains data in both formats, both data sets are read and then compared. If they match up, that Replay Gain information is returned for the file. However, if they don't match, no Replay Gain data is returned to signal that this file does not contain valid (read: consistent) Replay Gain information.