It is not, as a gazillion idiots like to write, "Apple's proprietary format". It's an industry standard, supported by pretty much every modern mobile phone and most "MP3 players". What is exclusive to the iPod (and the abomination known as ROKR) is Apple's FairPlay DRM, but ripping your own audio into high-grade AAC does not encumber files in any way.
How it Works
- Protected AAC files have the extension: .m4p -- ripped AAC files are .m4a
- Unlimited CD Burning of Protected AACs
- Only the iPod and Apple's iTunes, and it seems Quicktime-based apps currently allow playing of these Protected AAC's.
- Up to three computers (at one time) can be authorized to play Puchased AAC's. Deauthorizing your computer and reauthorzing new computers is relatively simple (Note: uses the iTunes "Authorize/Deauthorize Computer" menu, no details as to the transport mechanism and PKI employed)
- Playlists containing any Protected AAC's can only be burned 10 times. You must change the list manually before you can burn again. Tech Note
- Burning a Protected AAC to a CD strips all encoding and DRM. That CD can then be used as any CD song is used. The quality of the song on the CD is identical to the AAC version. However, then ripping the song into MP3 or AAC will result in loss of some quality. While ripping a song into any lossy compression format will result in loss of quality -- recompressing these previously compressed songs may exaggerate the quality loss. Your results will vary depending on the exact piece of audio. Anecdotal evidence suggests re-ripping into AAC yields better quality than re-ripping into MP3.
- Transcoding from Protected AAC to MP3/AIFF from iTunes is prohibited by iTunes.
- If you're listening to a shared library or playlist, iTunes skips any purchased music in the list (if the computer is not authorized to play the music). To listen to a purchased song in a shared library or playlist, you need to double-click the song. If your computer is not authorized to play songs purchased by the person who is sharing the song, you'll need to enter that person's Apple Account ID and password to hear the song. Tech Note
- According to Apple: iTunes will only play AAC files that are created by iTunes or downloaded from the Music Store. "Other AAC files that you find on the Internet or elsewhere will not play in iTunes." However, Anecdotal evidence does not support this. Users have reported being able to play AAC files encoded outside of iTunes. Tech Note
- AACs you rip from CD yourself (via iTunes) have no restrictions.
- Authorization/Deauthorization appears to be based on a central server model... as Apple claims that "Initializing the drive will not deauthorize the computer. If you will be initializing the drive, deauthorize the computer first, then initialize the drive." Tech Note