GarageBand 4 can export songs your iPod can't play
It's all because of its new 24-bit capabilities.
A feature new in GarageBand 4 is that it has 24-bit capability. This means that recording quality will be higher: if you have 24-bit audio hardware, GarageBand will now record at 24-bit. GarageBand will also play 24-bit recordings. It can also export songs as 24-bit AIFF.
Be warned, however, that a 24-bit AIFF, although its quality is higher, is also 50% larger than an otherwise identical 16-bit AIFF. So, GarageBand songs, and GarageBand AIFF exports, can take up much more room on your hard drive.
Also, an iPod cannot play a 24-bit AIFF; if you transfer such a file to your iPod, the iPod will simply skip it when you try to play it. Transferring an AIFF directly to an iPod is a rare thing to do, so it's no wonder that we've had only one notice of this problem (from reader Stephen). iTunes can re-encode the 24-bit AIFF to 16-bit, or (preferably) to Apple Lossless or a compressed format such as MP3, to save space on your iPod.
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