What Is Sound Check in iTunes and iPods?
Sound Check in iTunes and iPods is the same as Winamp’s Replay Gain feature. It attempts to ensure that all songs in your iTunes library play at roughly the same volume level. While the algorithm is not perfect, I do find when I use sound check, that I mustadjust the volume far less when the next song plays. Sound check raises the volume on very quiet songs when listening in either iTunes or on your iPod, and lowers it on very loud songs so that they all play at approximately the same loudness.
Sound check however, is NOT audio compression or automatic volume. Sound check determines the overall volume of the song before it is actually played; not during play as is done in audio compression. Then Sounc Check sets the volume level accordingly, just once; unlike compression, which makes constant readjustments of the volume or audio gain during play.
Sound Check does not readjust the volume in response to loud or soft peaks in the song; meaning that even with sound check turned on, you’ll still have very loud and very soft parts in the song. But the average volume level of all your music, when using sound check, should be more constant than it is with Sound Check turned off, and loud passages won’t blast you as much as they would without Sound Check.
I think the Sound Check feature works throughout the iTunes library. In fact, when you import new music and you have Sound Check enabled, iTunes figures out at import time what the appropriate volume for the song should be, and then records that value in its database. You can observe these Sound Check calculations being done, immediately after importing a large bunch of songs, as messages appear in the iTunes status window; something about analyzing volume levels or other text to that effect.
Sound Check is not just for when you’re burning CDs. In fact, I doubt that it has any affect on burned tracks from your library. Articles I’ve read suggest that Sound Check adjustments are not applied to the actual track data. But I haven’t played much with this part of Sound Check to know for sure.
Also as noted earlier, Sound Check differs from audio compression. However, Sound Check DOES employ a limiter component that prevents the loudest peaks in your music from distorting. Limiters are like compression except that they function only on the loudest parts of the music (those parts that would trigger clipping), where compressors work on most all of it, including the softer parts – how much of the music compressors affect is determined by adjustments on the compressor. However, any internal adjustments to Sound Check in iTunes are preset and cannot be changed by the user.
For more information on Sound Check, see the following URLs:
- http://www.macworld.com/article/44344/2005/04/soundcheckyou.html
- http://support.apple.com/kb/HT2425
- http://ipod.about.com/od/itunes/g/soundcheck_def.htm
- http://ask.metafilter.com/33334/iTunes-Sound-Levels-for-Burning-a-CD
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Tags: iTunes Software