ReplayGain is a proposed standard published by David Robinson in 2001 to measure the perceived loudness of audio in computer audio formats such as MP3 and Ogg Vorbis. It allows players to normalize loudness for individual tracks or albums. This avoids the common problem of having manually to adjust volume levels between tracks when playing audio files from albums that have been mastered at different loudness levels.
Although the standard is now formally known as ReplayGain, it was originally known as Replay Gain and is sometimes abbreviated RG.
ReplayGain works by first performing a psychoacoustic analysis of an entire audio track or album to measure peak levels and perceived loudness. The difference between the measured perceived loudness and the desired target loudness is calculated; this is considered the ideal replay gain value. Typically, the gain value and the peak value are then stored as metadata in the audio file, allowing ReplayGain-capable audio players to automatically attenuate or amplify the signal on a per track or per album basis such that tracks or albums play at a similar loudness level. The peak information can be used to prevent loud songs from clipping.
The original ReplayGain proposal specified an 8-byte field in the header of any file. Most implementations now use tags for ReplayGain information. FLAC and Ogg Vorbis use the
REPLAYGAIN_* Vorbis comment fields. MP3 files usually use ID3v2. Other formats such as AAC and WMA use their native tag formats with a specially formatted tag entry listing the track's gain and peak loudness.
ReplayGain utilities usually add metadata to the audio files without altering the original audio data. Alternatively, a tool can amplify or attenuate the data itself and save the result to another, gain-adjusted audio file; this is not perfectly reversible in most cases. 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 for directly applying the gain adjustment to MP3 files, adding undo information so that the process is reversible.
A more common means of specifying a reference level is relative to a full-scale signal. ReplayGain nominally plays at -14 dB relative to full-scale leaving 14 dB of headroom for reproduction of dynamic material. In contrast, the SMPTE RP 200:2002, on which the ReplayGain reference was originally based, recommends 20 dB of headroom. The more recent EBU Recommendation R 128 suggests 23 dB.
Track-gain and album-gain
ReplayGain 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, is calculated. Using the album-gain values during playback will preserve the volume differences among tracks on an album.
On playback, listeners may decide if they want all tracks to sound equally loud or if they want all albums to sound equally loud with different tracks having different loudness. In album-gain mode, when album-gain data is missing, players should use track-gain data instead.
- 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. (ReplayGain itself is an elaboration on RMS normalization.)
- 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.
- Sound Check is a proprietary Apple Inc. technology similar in function to ReplayGain. It is available in iTunes and on the iPod.
- Standard measurement algorithms for broadcast loudness monitoring applications have recently been developed by the International Telecommunication Union (ITU-R BS.1770) and the European Broadcasting Union (R128).
- AIMP2 & AIMP3 for Windows
- Amarok for KDE: Native ReplayGain support was added in Amarok 2.1. No native support is available for Amarok 1, but a ReplayGain 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 (music player) for Linux and Windows
- Banshee for Linux
- Clementine for Windows, Mac OS X and Linux
- cmus for Unix-like systems
- Cue Broadcast Audio Player for Windows (Freeware)
- DeaDBeeF for GNU/Linux, FreeBSD, and OpenSolaris
- Exaile for Linux/GNOME
- FLAC: The reference FLAC decoder can create a new copy with ReplayGain applied, through the undocumented option
--apply-replaygain-which-is-not-losslessas of version 1.1.1
- foobar2000 for Windows
- GMusicBrowser for Linux/GNOME
- hunisPRO automation system for Windows
- J.River Media Center for Windows
- JavaTunes for Windows, Linux and Mac OS X
- Zortam Mp3 Media Studio for Windows
- MediaMonkey for Windows
- Mixxx digital DJ software, v1.9.0 and up.
- Mpg123 supported for only Xing/Lame/Info header
- MPD for Unix-like systems
- Muine music player for GNOME
- MusicBee for Windows
- Play for Mac OS X
- Pocket Player for Windows Mobile, through the use of ReplayGain DSP plugin, by Conduits
- ProppFrexx ONAIR for Windows
- Quod Libet for Unix-like systems
- QuuxPlayer for Windows
- RadioBOSS windows radio automation software 
- Rhythmbox for GNOME (through the ReplayGain plug-in)
- Songbird: for Windows and Mac OS X
- SoX Cross-platform (Windows, Linux, Solaris, Mac OS X, et al.)
- Squeezebox (2 and newer) hardware and accompanying SlimServer/SqueezeCenter software from Slim Devices
- VLC media player a multi platform media player
- Winamp for Windows
- XMMS for Unix-like systems with X11 (supports ReplayGain for Vorbis; for MP3 files, a patched version of the xmms-mad plugin which only supports APEv2 is available.)
- XMMS2 for Unix-like systems
- XMPlay for Windows
Portable media players
- All devices with a working Rockbox port
- Sandisk Sansa Fuze and Sansa Clip+
- iPod through other programs that convert ReplayGain data to the Apple proprietary Sound Check format (e.g. iPod Manager for foobar2000, other alternatives elsewhere on this page)
Typical CD players and other legacy audio players do not support ReplayGain.
Android compatible devices
- DeaDBeeF for Android
- GoneMAD Music Player for Android
- Neutron Music Player for Android
- MyTunes for Android
- PowerAMP for Android
- Vanilla Music 
- Winamp PRO for Android
- XenoAmp Music Player 
- beaTunes: Writes the standard
replaygain_track_gain/replaygain_track_peaktags and replaces the
iTunNORMmetadata tag value, which is used by iTunes software and iPod music players for Sound Check volume normalization.
- Ex Falso: Included plugin scans files on a per-album base, writes the standard tags into metadata.
- FLAC and metaflac: Encoder can optionally generate metadata. Tagger generates metadata.
- foobar2000: Generates metadata through included plugin using EBU R128 (but at old 89dB levels) for all supported tag formats.
- iVolume: Replaces the
iTunNORMmetadata 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.
- QuuxPlayer for Windows: calculates gain values and saves them in its library database; optionally writes ReplayGain tags to files with ID3v2 tags.
- Rapid Evolution: Generates metadata
- Winamp: Generates metadata
- rgain: Generates metadata
- Although the original proposal specified a 83 dB SPL reference, an early departure from the proposal to 89 dB SPL was endorsed by its author.
- Specifically SMPTE RP 200:2002 recommends an 83 dB SPL plaback level for pink noise recorded at -20 dB with respect to a full-scale sine wave. ReplayGain uses -14dB headroom and therefore has a reference 6dB higher than the SMPTE spec.
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