APEv2 tags

Replay Gain

Replay Gain is a proposed standard published in 2001 to normalize the perceived loudness of computer audio formats such as MP3 and Ogg Vorbis. It works on a track/album basis, and is now supported in a growing number of media players. Although the standard is formally known as "Replay Gain," it is also commonly known as "ReplayGain" or "replaygain." It is sometimes abbreviated "RG."

Theory

Replay Gain works by first performing a psychoacoustic analysis scan of the entire audio file to measure the perceived loudness and peak levels. The difference between the loudness and the target loudness (usually 89 dB SPL) is calculated; this is the gain value. Typically, the gain value and the peak value are then stored in the audio file as metadata, allowing Replay Gain–compliant audio players to automatically attenuate (or in some cases amplify) the output so that such files will play back at similar loudness to one another. This avoids the common problem of having to manually adjust volume levels when playing audio files from different albums that have been mastered at different levels. With lossy files, another benefit of Replay Gain scanning is that the peak information can also be used to prevent loud songs from clipping. Finally, should the audio at its original levels be desired (i.e. for burning back to hard copy), the metadata can simply be ignored.

Replay Gain implementations usually involve adding metadata to the audio without altering the original audio data. While the Replay Gain standard specifies an 8-byte field in the header of any file, many popular audio formats use tags for Replay Gain information. FLAC and Ogg Vorbis use the REPLAYGAIN_* comment fields. MP3 files usually use ID3v2 or APEv2 tags.

CD players and other legacy audio players do not support Replay Gain metadata. Nevertheless, some lossy audio formats, such as MP3, are structured in a way that they encode the volume of each compressed frame in a stream, and tools such as MP3Gain take advantage of this to change the volume of all frames in a stream, in a reversible way, without adding noise. Otherwise, a tool can amplify a PCM audio file and save the result to another PCM audio file; this is not perfectly reversible in most cases.

Track gain and Album gain

Replay Gain analysis can be performed on individual tracks so that all tracks will be of equal volume on playback. Analysis can also be performed on a per-album basis. In album gain analysis an additional peak value and gain value, which will be shared by the whole album, will be calculated. Using the album gain values will preserve the volume differences within an album.

On playback, assuming the metadata method is used, listeners may decide if they want all tracks to sound equally loud (track gain, formerly called radio gain) or if they want all albums to sound equally loud with different tracks having different loudness (album gain, formerly called audiophile gain). In album gain mode, when album gain data is missing, players should use track gain data instead.

Replay Gain–compliant audio players

Amarok for Linux: There is not native support, but a Replay Gain script is available for Amarok's script manager. As it is an external script, however, there will be a slight lag between the start of a track and the volume adjustment. This is particularly noticeable when a track starts with a peak loudness.
Audacious for Unix-like systems
Aqualung for Linux and Windows
Banshee for Linux
cmus for Unix-like systems
Exaile for Linux/GNOME
FLAC: The reference FLAC decoder can create a new copy with Replay Gain applied, through the undocumented option --apply-replaygain-which-is-not-lossless as of version 1.1.1
foobar2000 for Windows
J.River Media Center for Windows
Zortam Mp3 Media Studio for Windows
MediaMonkey for Windows
madplay
Mpg123 supported for only Xing/Lame/Info header
mpd for Unix-like systems
Muine music player for GNOME
Play for Mac OS X
Quod Libet for Unix-like systems
QuuxPlayer for Windows
Rhythmbox for GNOME (hidden gconf option on/off only track gain is assumed)
Rockbox firmware for various digital audio players
Squeezebox (2 and newer) hardware and accompanying SlimServer/SqueezeCenter software from Slim Devices
Vlc media player a multi platform media player
Winamp for Windows
Xbox Media Center for the Xbox
XMMS for Unix-like systems with X11 (supports Replay Gain for Vorbis; for MP3 files, a patched version of the xmms-mad plugin which only supports APEv2 is available.)
XMMS2 for Unix-like systems
JavaTunes for Windows, Linux and Mac OS X

Implementations of Replay Gain scanners

AACGain: Directly modifies original file and writes undo information as metadata.
FLAC and metaflac: Encoder can optionally generate metadata. Tagger generates metadata.
foobar2000: Generates metadata through included plugin for all supported tag formats. Can directly modify MP3 files.
iVolume: Replaces the iTunNORM metadata tag value (optionally on a per-album basis), which is used by iTunes software and iPod music players for Sound Check volume normalization.
LAME: Encoder writes metadata to LAME tag
MediaMonkey: Analyze Volume calculates RG values and writes them into the files as tags and into its library database
MP3Gain: (open source) generates metadata. Can directly modify original file and write undo information as metadata.
Zortam Mp3 Media Studio: Generates metadata
Rapid Evolution: Generates metadata
replaygain.exe: Generates metadata which is stored in the file header of Musepack files.
VorbisGain: Generates metadata.
WaveGain: Generates a new copy with Replay Gain applied.
Winamp: Generates metadata

Alternatives

Audio normalization: Peak amplitude is not a reliable indicator of loudness, so consequently peak normalization does not offer reliable normalization of perceived loudness. RMS normalization is a little more accurate, but care must be taken not to introduce clipping, either by guaranteeing appropriate headroom or by using hard or soft limiting. (Replay Gain itself is an elaboration on RMS normalization.)
Audio level compression: With audio level compression, volume may be altered on the fly on playback, but the dynamic range will be compressed. Although this is beneficial in keeping volume constant at all times, it is not always desirable.