ReplayGain is a proposed technical standard published by David Robinson in 2001 to measure and normalize 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 this de facto standard is now formally known as ReplayGain, it was originally known as Replay Gain and is sometimes abbreviated RG. ReplayGain is 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 level and perceived loudness. Equal-loudness contours 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 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 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