From Hydrogenaudio Knowledgebase
ReplayGain is the name of a technique invented to achieve the same perceived playback loudness of audio files. It defines an algorithm to measure the perceived loudness of audio data.
ReplayGain allows the loudness of each song within a collection of songs to be consistent. This is called 'Track Gain' (or 'Radio Gain' in earlier parlance). It also allows the loudness of a specific sub-collection (an "album") to be consistent with the rest of the collection, while allowing the dynamics from song to song on the album to remain intact. This is called 'Album Gain' (or 'Audiophile Gain' in earlier parlance). This is especially important when listening to classical music albums, because quiet tracks need to remain a certain degree quieter than the louder ones.
ReplayGain is different from peak normalization. Peak normalization merely ensures that the peak amplitude reaches a certain level. This does not ensure equal loudness. The ReplayGain technique measures the effective power of the waveform (i.e. the RMS power after applying an "equal loudness contour"), and then adjusts the amplitude of the waveform accordingly. The result is that Replay Gained waveforms are usually more uniformly amplified than peak-normalized waveforms.
 Target loudness
The target loudness of almost all ReplayGain utilities is 89 dB SPL (an early departure from the proposal, endorsed by its author) — the ReplayGain proposal and SMPTE recommendation are 6dB lower.
Audio is generally recorded such that the loudest sounds don't clip, but the use of ReplayGain can cause clipping if the average volume of a song is below the target level. That is, upon playback, the volume of a quiet song is increased, so the parts of the song with above-average loudness, especially in the bass frequencies, will exceed the limits of the format and will be distorted. Whether this distortion is audible depends on the sounds in question, and the listener's sensitivity.
Implementations deal with the risk of clipping in different ways. Some have a "pre-amp" feature which reduces (or boosts) the original audio's level by a certain amount before doing whatever is needed for ReplayGain. Some have a "prevent clipping" feature to reduce the amount of ReplayGain adjustment to whatever amount would keep clipping from occurring, based on peak info stored in the file's metadata (thus reducing the effectiveness of ReplayGain). Some recommend using a compressor/limiter DSP to prevent or reduce clipping, regardless of whether it was caused by ReplayGain.
There are different ReplayGain implementations, each with its own uses and strength. Most use metadata to indicate the level of the volume change that the player should make. Some modify the audio data itself, and optionally use metadata as well. There are advantages and disadvantages to both methods.
In the metadata method, information on both types of ReplayGain (Track Gain and Album Gain) can be stored. The volume-change information can be very precise. If audio data was also changed, the metadata can contain "undo" info. Not all audio players/decoders know how to read and use ReplayGain information stored in metadata. And there's no standard for where and how ReplayGain info is stored; each implementation uses different formats and puts the info in different locations.
In the audio data method, the file's actual audio data is modified so that its natural/default playback volume is at the target level. In this scenario, only one type of ReplayGain (Track Gain or Album Gain) can be applied. If no "undo" info is saved somewhere, it may not be possible to restore the original audio data. Limitations of the audio file format may prevent precise (finely tuned) gain adjustments with this method. For example, MP3 and AAC files can only be losslessly modified in 1.5 dB steps. Depending on the audio file format, the process may also be lossy in the sense that it could irreversibly push a signal above the format's maximum amplitude (resulting in clipping) or below the minimum (resulting in silence).
MP3Gain is an implementation of ReplayGain. It can be used to just analyze files & recommend changes or to also modify the gain. If modifying the gain, it always modifies the global gain fields in the MP3 audio data. It can add somewhat precise metadata, including undo info. The gain can be modified to any target dB, or it can be changed by a specified amount. For balance correction, user-specified changes can even be made on just one channel in simple L/R stereo-mode files (not joint stereo).
- Format: MP3
- Method: Audio + Meta (in APE tag), or Audio only
- APE tag fields (ASCII bytes):
MP3GAIN_MINMAX ###,###- minimum & maximum global gain values for this file. 3 digits, zero-padded if necessary.
MP3GAIN_ALBUM_MINMAX ###,###- minimum & maximum global gain values across a set of files scanned as an album. Optional.
MP3GAIN_UNDO +###,+###,N- the global gain adjustment to restore the original values in the left and right channels, respectively, followed by an indicator of whether to wrap at the extremes (
Wmeans yes). The adjustment values are 3 digits, zero-padded, preceded by a sign (
REPLAYGAIN_TRACK_GAIN +#.###### dB- The value is always 9 characters including the sign and decimal point. Examples:
REPLAYGAIN_TRACK_PEAK #.###### dB- The value is always 8 characters including the decimal point. Example:
REPLAYGAIN_ALBUM_GAIN +#.###### dB- The value is always 9 characters including the sign and decimal point. Optional.
REPLAYGAIN_ALBUM_PEAK #.###### dB- The value is always 8 characters including the decimal point. Optional.
- Limitations: Although the metadata, if written, contains precise adjustment & peak values, the audio data modifications are limited to 1.5dB steps and may become irreversible (however, that's a very rare condition; see the "mp3gain is NOT lossless" forum thread)
AACGain is a modified version of MP3Gain that works on both MP3 and AAC files.
- Format: MP3, AAC (with or without MP4 container)
- Method: Audio + Meta, or Audio only
- Limitations: Limited to 1.5dB steps mode, may become irreversible (same caveat as for MP3Gain)
- Method: Header (mp3infotag)
- Method: Header (similar to Meta data method)
- Notes: ReplayGain values are stored in the header and ReplayGain is part of the Musepack specifications; therefore any Musepack decoder that does not support ReplayGain can be considered broken.
- Format: (Ogg) Vorbis
- Method: Meta (in Vorbis comment)
- new compiles of VorbisGain at www.rarewares.org
- Note: Andavari has provided a very useful script to integrate VorbisGain, which is a CLI tool, into Windows Explorer. Please (Ogg) check this section.
 FLAC / METAFLAC
 WavPack / WVGAIN
- Format: waveform
- Method: Audio
- Limitations: Irreversible
- Custom implementation, not derived from the original MP3Gain one (but inspired from). As far as I know, all other implementations are directly derived from the MP3Gain (gain_analysis.c, which is GPL) source.
- Format: any that FFmpeg supports
- Method: Audio
- Limitations: Doesn't modify the files at all. Stores the value in own database. Used only for playback.
 foobar2000 ReplayGain scanner
- MP3: Values written to ID3v2 (default) or APEv2 tags. A separate function can be invoked to apply the tagged Track or Album Gain to the MP3 global gain fields (as MP3Gain does, but requiring tags first), and to rewrite the tags to account for the peak change and compensate for the difference from 89 dB. The 89 dB reference level for tags isn't configurable, but the reference level applied to the global gain fields is (it's under Preferences > Advanced > Tools > ReplayGain Scanner > Target MP3 alteration volume level).
- Musepack: Values written to header.
- (Ogg) Vorbis: Values written to Vorbis comment.
- WavPack: Values written to APEv2 tags.
- AAC: Values written to APEv2 tags.
- MP4: Uses its own iTunes-compatible tagging system (though iTunes does not support ReplayGain).
- FLAC: Values written to Vorbis comment.
- APE: Values written to APEv2 tags.
- Modules (MOD etc.): Optionally saved into APEv2 tags.
- In addition to tags, all ReplayGain values are also stored in MediaMonkey's MDB database
- Album/Audiophile ReplayGain not supported until v3.0 (Dec 2007); support during burning & ripping added in 3.1 (Jun 2009)
- Also capable of (irreversibly) changing the volume of MP3 tracks, similar to MP3Gain
- Support Album/Track Gain
 Players support
ReplayGain being present in the specs of FLAC, Musepack, and APE formats, any player that support those formats usually support ReplayGain.
The situation with MP3 is rather different, as it was not part of the MP3 specs. The APEv2 tags metadata implementation is somewhat becoming the de-facto standard.
- foobar2000 supports ReplayGain in all possible aspects.
- Winamp supports ReplayGain in album or track mode.
- MediaMonkey supports ReplayGain, with many configuration options.
- XMPlay recently implemented ReplayGain
...and probably others.
- For MP3, use the CVS version of the xmms-mad mp3 plugin (it's not yet released as binary, furthermore not available in distribs' versions for now. Meanwhile binaries are available here: custom binaries)
- And possibly others, since TagLib added support for APEv2 tags in MP3 files, players using this library (like amaroK and JuK) might support that kind of ReplayGain tags in the near future.
- Requires support to be enabled (via the appropriate python bindings and libraries) for the above formats. Does not support ReplayGain values stored in APEv2 tags in MP3s. ReplayGain values are stored in RVA2 id3v2.4 frames. See the Quod Libet RVA2 / ReplayGain notes.
- foobar2000-style TXXX frames in MP3s are also supported in the latest development releases.
- MPlayer. Mplayer support for ReplayGain is codec dependent.
- Codecs that are known to support ReplayGain: vorbis
- Because of this, you need to prioritize the codecs that support it, or choose it individually on the command line. To add it to the command line, add an -ac [codec] option after each file that you want to choose the codec for, or at the beginning to make it apply to all files listed. To prioritize the codecs by default, list them in a line in mplayer.conf:
 Portable devices
Rockbox supports ReplayGain (in album or track mode) for most formats, including WMA, MP1/2/3, AAC, ALAC, Musepack, Monkey's Audio, Wavpack, FLAC and Vorbis.
Note that ReplayGain is only supported when using the respective codec's native tagging format. For example: ReplayGain stored in APEv2 tags is not supported for MP3, rather ID3v2.x tags are expected.
Sandisk Sansa Fuze with firmware 1.02.26 and 2.02.26
Sandisk Sansa Clip+
The iPod features Soundcheck, which seems to produce roughly the same normalization gains as ReplayGain, but doesn't provide an Album Gain.
 See also
- ReplayGain at Wikipedia
- ReplayGain - A Proposed Standard, the original proposal, now out of date with respect to current practice
- ReplayGain using foobar2000 (how to use ReplayGain in Windows using foobar2000).
- ReplayGain in Linux (how to use ReplayGain in Linux using foobar2000 and Wine, or using metaflac or vorbisgain).