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Old 29th May 2012, 11:37   #1
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Volume Leveling and Winamp


Why Volume Leveling is Needed

Although music is encoded to a digital format with a clearly defined maximum peak amplitude, and although most recordings are normalized to utilize this peak amplitude, not all recordings sound equally loud. This is because once this peak amplitude is reached, perceived loudness can be further increased through signal-processing techniques such as dynamic range compression and equalization. Therefore, the loudness of a given album has more to do with the year of issue or the whim of the producer than anything else. Because of this, a random play through a music collection can have significant volume changes with every other track.

There is a solution to this annoyance: within each audio file, information can be stored about what volume change would be required to play each track or album at a standard loudness, and music players can use this "replay gain" information to automatically nudge the volume up or down as required.

What is ReplayGain

The ReplayGain specification is a standard which defines an appropriate reference level, explains a way of calculating and representing the ideal replay gain for a given track or album, and provides guidance for players to make the required volume adjustment during playback. The standard also specifies a means to prevent clipping when the calculated replay gain exceeds the limits of digital audio, and it describes how the replay gain information is stored within audio files.

The audio industry does not have a standard for playback system calibration, but in the movie industry a calibration standard has been defined by the Society of Motion Picture and Television Engineers (SMPTE). The standard states that a single channel pink noise signal with an RMS level of -20 dB relative to a full-scale sinusoid should be reproduced at 83 dB SPL.

ReplayGain adapts the SMPTE calibration concept for music playback. Under ReplayGain, audio is played so that its loudness matches the loudness of a pink noise signal with an RMS level of -14 dB relative to a full-scale sinusoid.

In ReplayGain implementations, the reference level is described in terms of the SMPTE SPL playback level. By the SMPTE definition, the 83 dB SPL reference corresponds to -20FS dB system headroom. The -14 dB headroom used by ReplayGain therefore corresponds to an 89 dB SPL playback level on a SMPTE calibrated system and so is said to be operating with an 89 dB reference level.

SMPTE cinema calibration calls for a single channel of pink noise reproduced through a single loudspeaker. In music applications, the ideal level of the music is actually the loudness when two speakers are in use. So, ReplayGain is calibrated to two channels of pink noise.

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