iTunes' Sound Check and you

Thanks to feedback from forum visitors James and iBeetle and their reaction to yesterday’s iPod EQ and you, I’ve determined to use the next couple of Today@Playlist entries to explain some of the more confusing elements of the effects found in iTunes and on the iPod. Today we turn to Sound Check.

For those who haven’t explored the option, Sound Check is a feature introduced in iTunes 3 and iPod Software 1.2 Updater that attempts to balance the volume of the music in iTunes and on your iPod. The idea is that if you play a Chopin Nocturne and Zep’s “Dazed and Confused” in succession, you shouldn’t have to fiddle with the volume as both tracks will be at approximately the same volume. To keep tracks from distorting, Sound Check imposes a limiter when they near saturation.

That’s the theory, anyway. In reality, Sound Check balances volume on a per-track basis, which results in the quiet tracks on an album being approximately the same volume as the louder tracks. Not everyone cares for this effect. Ideally, Sound Check would leave these relative volumes alone and simply adjust volume settings between albums. It doesn’t.

However, using a different algorithm within the iTunes database (something called ReplayGain), volumes between albums can be compared and adjusted with third-party utilities. Manfred Schwind’s $9 iVolume (a utility that adjusts the volume of tracks to the same “perceived loudness”) takes this approach. (Regrettably, iVolume can’t adjust the protected AAC files sold by the iTunes Music Store.) The SoundCheck and ReplayGain adjustment settings are contained within the iTunes database and ID 3 tags so the original audio files aren’t altered (in other words, no gain adjustment is made to the audio waveforms).

To put Sound Check to use, simply open iTunes’ Preferences window, click the Audio tab, and enable the Sound Check option. When you switch on Sound Check, it analyzes your music library and then adjusts volumes accordingly. When you add new tracks to the library, they’re evaluated and adjusted in the background.

If you disable the Sound Check option, iTunes ignores the volume adjustments imposed by Sound Check (Sound Check information isn’t deleted, however, so if you switch Sound Check back on iTunes shouldn’t reevaluate the tracks).

As for the iPod, if you turn Sound Check on in the Settings screen, the iPod will play your tracks with the volume adjustments imposed by Sound Check in iTunes. Turn Sound Check off on the iPod, and the iPod will ignore the Sound Check settings and play the tracks at their original volume.

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