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 media players to normalize loudness for individual tracks or albums. This avoids the common problem of having to manually adjust volume levels between tracks when playing audio files from albums that have been mastered at different loudness levels.
Contents
- Operation
- Metadata
- Target loudness
- Track gain and album gain
- Alternatives
- Portable media players
- Android compatible players
- Scanners
- References
Although the de facto standard is now formally known as ReplayGain, it was originally known as Replay Gain and is sometimes abbreviated RG.
ReplayGain is now supported in a large number of media software and portable devices.
Operation
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 and peaks values 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 gain adjustments from inducing clipping in the playback device.
Metadata
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 replay 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.
Target loudness
The target loudness of ReplayGain utilities is 89 dB sound pressure level. The SPL reference comes from a SMPTE recommendation used to calibrate playback levels in movie theaters.
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.
Alternatives
Portable media players
Typical CD players and other legacy audio players do not support ReplayGain.
Android compatible players
Scanners
replaygain_track_gain/replaygain_track_peak tags and replaces the iTunNORM metadata tag value, which is used by iTunes software and iPod music players for Sound Check volume normalization.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.