Introduction

Replaygain calculates the average perceived loudness of either a track (track gain) or an album (album gain) and uses this information to properly adjust the volume of tracks, that are mastered at different volume levels, during playback. This will make sure that you will have a similar volume levels on all of your tracks or albums without losing any dynamic range of your music. In the case of Album Gain, it will even make sure that more quiet tracks of an album will sound more quiet and tracks that where meant to be louder, louder. It shouldn't be confused with algorithms that adjust playback volume dynamically within a single track.

Implementations

There are different replaygain implementations, each with its own uses and strength. Most of them use meta data to indicate the level of the volume change (Volume is adjusted on playback; needs player/decoder support) others modify the Audio Data itself. Generally it is recommended to use an implementation which uses meta data and does not touch the audio itself.

Replaygain has significant advantages over normalizing. It allows the peak loudness of a song to be consistent over an entire collection of audio, much like normalizing (this is called 'Track Gain'). However, it also allows the peak loudness of an album to be consistent over a entire music collection, allowing the dynamics of album to remain (This is called 'Album Gain'). In a meta data based solution, information on both types of replaygain can be stored, and the desired playback effect can be switched back and forth in the appropriate player. However, if the audio is permanently modified, only one type of raplaygain can be chosen.

MP3Gain

Format: MP3

Method: Audio or Meta

In Meta mode, the tags are written in APEv2 tags.
In Audio Data Mode, the gain is applied to each frame's gain byte, and the process can be perfectly reverted.

Limitations: Limited to 1.5db steps in Audio Data mode

LAME

Format: MP3

Method: Header (mp3infotag)

Notes: Added during encoding; not supported by any player yet; Track Gain only

Replaygain

MPC replaygain

Format: MPC

Method: Header (similar to Meta data method. See "notes".)

Notes: Replaygain values are stored in the header and replaygain is part of the Musepack specifications; therefore any Musepack decoder that does not support replaygain can be considered broken.

Vorbisgain

Format: Ogg Vorbis

Method: Meta (in Vorbis comment)

FLAC/metaflac

FLAC

Format: FLAC

Method: Meta (in Vorbis comment)

foobar2000 replaygain scanner

Foobar2000

Format:

MP3: Values written to APEv2 or ID3v2 tags.
MPC: Values written to header as decribed under "replaygain".
Ogg Vorbis: In Vorbis comment.
AAC: Values written to APEv2 tags.
MP4: Uses its own itunes-compatible tagging system (though itunes does not support replaygain).
FLAC: In Vorbis comment.
APE: Values written to APEv2 tags.
WAV: Optionally saved into APEv2 tags; otherwise the foobar2000 database is used.
Modules (MOD etc.): Optionally saved into APEv2 tags; otherwise the foobar2000 database is used.

You can also choose to only have the replaygain values saved in the foobar2000 database and leave the files untouched.

All other formats are supported but the replaygain values are saved to the foobar2000 database.

Wavegain

Format: PCM Wave

Method: Audio

Limitations: Irreversible

Players support

Replaygain being present in the specs of flac, mpc and ape formats, any player that support those formats usually support replaygain.

The situation with MP3 is rather different, as it was not part of the mp3 specs. The APEv2 tags metadata implementation is somewhat becoming the de-facto standard.

Windows

Foobar2000 supports replaygain in all possible aspects. (it can also calculate replaygain)
Winamp. MP3 replaygain support with adequate plugins (e.g. Otachan's in_mpg123 - if your japanese is not that good, you can find details in this thread)
XMPlay recently implemented replaygain

and probably others.

Linux

XMMS. Reads replaygain from FLAC, MPC, ogg ..

For mp3, use the CVS version of the xmms-mad mp3 plugin (it's not yet released as binaries, furthermore not available in distribs' versions for now. Meanwhile binaries are available there : custom binaries)

and possibly others, since TagLib added support for APEv2 tags in mp3 files, players using this library (like amarok and JuK) might support that kind of replaygain tags in the near future.

Portable devices

Sadly, none that I know of. So re-encoding (or using mp3gain) to attenuate the audio-data is the only solution.

The iPod features Soundcheck, which seems to produce roughly the same normalization gains as replaygain, but doesnt provide an Album Gain.

Additional reading

Original replaygain website (this hasn't been updated since 2001, and everything there is outdated except for the description of the algorithm - but know that the 83dB SPL reference has since then be changed to 89 dB SPL ..)
audio engineer Bob Katz's many interesting pages (e.g. this one) is the best source to understand why replaygain is useful, and understand all that digital audio stuff. This actually was in part responsible for the design of replaygain by David Robinson in the first place, IIRC.