ReplayGain

ReplayGain is a method, published by David Robinson in 2001, to normalize the perceived loudness of audio in computer audio formats such as MP3 and Ogg Vorbis. It works on a track/ album basis, and is now supported in a growing number of media players including iTunes, VLC,  and Winamp. Replay Gain works by analyzing an audio track 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. The target loudness of almost all Replay Gain utilities is 89 dB.

Usually, the gain value and the peak value are then stored as metadata in the audio file, allowing Replay Gain-capable audio players to automatically attenuate or amplify the signal such that tracks play at a similar loudness level. This avoids the common problem of having to manually adjust volume levels when playing audio files from albums that have been mastered at different levels. Should the original levels of audio be desired (e.g., for burning back to hard copy), the metadata can simply be ignored.

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