Editing ReplayGain (section)

==Operation==
ReplayGain works by first performing a [[psychoacoustic]] analysis of an entire audio track or album to measure [[dBFS|peak level]] and perceived loudness. [[Equal-loudness contour]]s are used to compensate for frequency effects and statistical analysis is used to accommodate for effects related to time. The difference between the measured perceived loudness and the desired target loudness is calculated; this is considered the ideal replay [[Gain (electronics)|gain]] value. Typically, the replay gain and peak level values are then stored as [[metadata]] in the audio file. ReplayGain-capable audio players use the replay gain metadata 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 level metadata can be used to prevent gain adjustments from inducing [[Clipping (audio)|clipping]] in the playback device.<ref>{{citation |url=http://wiki.hydrogenaud.io/index.php?title=ReplayGain_specification#Clipping_prevention |title=ReplayGain specification |access-date=2011-04-15}}</ref>

===Metadata===
The original ReplayGain proposal specified an 8-[[byte]] field in the header of any file. Most implementations now use [[Tag (metadata)|tags]] for ReplayGain information. [[FLAC]] and Ogg Vorbis use the <code>REPLAYGAIN_*</code> [[Vorbis comment]] fields. MP3 files usually use [[ID3v2]].  Other formats such as [[Advanced Audio Coding|AAC]] and [[Windows Media Audio|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 is specified as the loudness of a stereo [[pink noise]] signal played back at 89&nbsp;dB [[sound pressure level]] or −14&nbsp;dB [[DBFS|relative to full scale]].<ref>{{citation |title=ReplayGain 1.0 specification |url=https://wiki.hydrogenaud.io/index.php?title=ReplayGain_specification#Reference_level |access-date=2023-09-17 |website=Hydrogenaudio Knowledgebase}}</ref> This is based on [[SMPTE]] recommendation RP&nbsp;200:2002, which specifies a similar method for calibrating playback levels in [[movie theaters]] using a reference level 6&nbsp;dB lower (83&nbsp;dB SPL, −20&nbsp;dBFS).{{#tag:ref|Although the original ReplayGain proposal used the 83&nbsp;dB SPL reference level unchanged, an early departure from the proposal to 89&nbsp;dB SPL was endorsed by its author.<ref>{{citation|url=http://www.hydrogenaud.io/forums/index.php?s=&showtopic=83397&view=findpost&p=721854 |title=Does Replay gain work differtly {{sic|nolink=y}} in Media monkey |publisher=Hydrogenaudio |date=2010-10-07 |access-date=2010-12-30}}</ref>|group="note"}}

=== 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.