What happens if you run the AAC files through ReplayGain (with the AACGain add-on) after using iTunes' Sound Check?
This is a more complex question than you think it is, so I'll try to be thorough. First, it helps to understand what the general idea of volume leveling is.
The idea is that you scan each music file and determine some number which will represent the "volume" of that song. Then, you will use that number to adjust the volume of the song equally throughout the entire song, in order to reach a preset value.
So if I scan a song and determine that its volume level is 85dB, and my target level is 89dB, then I bump the volume of that song up by 4dB. If I do this to all songs, then they are all at the same volume, more or less.
There's two ways in which I can make that volume adjustment:
* At playback time, I can dynamically change my volume based on this number, or
* I can actually change the song file itself in some way.
Sound Check uses the first method. It works by scanning a song when you add it to iTunes. It then writes a tag into the song with the volume level that it finds. If Sound Check is then turned on in iTunes (or on the iPod), the volume is adjusted at the time that song is played. The actual song file is untouched.
ReplayGain is sort of a broad name for a methodology rather than a specific choice of volume adjustment technique. Some implementations of ReplayGain (like the one foobar 2000 uses) act in the same way iTunes does. They adjust the volume based on some tag at the time the song is played back.
Other techniques, like MP3Gain and AACGain, actually change the song file itself by adjusting specific values in the songs data. This has both benefits and drawbacks to it, in various ways, but the important point is that since the song itself is changed, that change affects any program playing that song.
If you MP3Gain or AACGain a file before it's ever added to iTunes, you won't have a problem. SoundCheck will work just fine. Basically, since the song is already adjusted, when iTunes scans it, it will simply read the song as louder than it was before you adjusted it.
But since iTunes writes a tag to the file with that scanned volume level, if you add the song to iTunes first and then later run it through MP3Gain or AACGain, Sound check won't work properly anymore.
Look at it like this sequence of events:
* I add a song to iTunes, which scans it and determines that it is at 83dB. So it needs a +6dB adjustment to get it to 89dB.
* I scan it with MP3/AACGain, which gets the same result, and then actually performs a +6db adjustment, bringing the song data to 89dB.
* Playing it in iTunes will then add a +6dB adjustment, bringing it to 95dB, which is too loud.
Basically, iTunes won't see the effects of MP3/AACGain's changes to the files, since it only scans the file once.
Realistically, Sound Check and ReplayGain perform the same function, and work identically. There's only two benefits to using ReplayGain:
* Slightly more accurate, but much slower, scanning method.
* ReplayGain includes the possibility of using "AlbumGain" which adjusts whole albums by identical amounts, preserving volume differences between songs on the same album.
Tests I've done show that ReplayGain is only
slightly more accurate than Sound Check is, so that's really not a good reason to use it instead of Sound Check. AlbumGain is nice, if you play whole albums a lot.
If you're careful, you can use both together, but realistically it's a real PITA. So I'd stick to one or the other.
Additional: AACGain does not support Apple Lossless files. Don't try it.
More additional: One thing I forgot to mention: Use of MP3Gain or AACGain can cause a file to clip, on any playback device. Sound Check seems to be able to cause clipping in iTunes, but
not on an iPod, as near as I can tell.
