ReplayGain

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.

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.

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.

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.

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