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Old 18th June 2012, 13:14   #23
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Volume Leveling/ReplayGain
(Keeping your songs playing at close to the same overall volume)


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. Volume differences between songs have always existed, but digital mastering has taken this to new heights. Link below to an article on the "Loudness War".

A random play through a music collection can have significant volume changes with every other track played! Tired of manually adjusting volume while listening to a playlist? There [I]is a solution[I] 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. Music players, that support it, can use this "replay gain" information to automatically nudge the volume up or down as required.

To prevent clipping when applying an overall target dB level for the standard loudness, it's best to use a relatively low level than a high one. Analog clipping is a form of waveform distortion that occurs when an amplifier is over-driven and attempts to deliver an output voltage or current beyond its maximum capability. Along with distortion, clipping can damage a speaker's tweeter (the part that plays high frequencies) via overheating. In digital signal processing, clipping occurs when the signal is restricted by the range of a chosen representation for the signal. For example in a system using 16-bit signed integers, 32767 is the largest positive value that can be represented, and if during processing the amplitude of the signal is doubled, sample values of, for instance, 32000 should become 64000, but instead they are truncated to the maximum, 32767. This causes the relative differences in amplitude between different parts of the signal not being what they should be.

Clipping in the original file or when applying an overall target dB level most often occurs because of the DSP engineering techniques applied to the file.

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 or volume for a given track or album, and provides guidance for players to make the required overall 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.

Your eyes glazing over yet? Had enough of this technical stuff? Yes, move on the Part 2. No, search the WEB for more details. Here's a link to get you started.

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