Posted Sunday, May 11, 2003

Strategies for Converting Your Music to AAC

On Apple's iTunes discussion boards, some people have asked about converting an existing MP3 music library to AAC format. Many users want to take advantage of AAC's audio-compression efficiencies to free up disk space on their Macs and their iPods.

It's an understandable goal, and iTunes makes it easy. But if you don't do it right, your music will suffer.

iTunes can convert your existing MP3s to AAC, but you'll lose quality in the process. That's because MP3 is a lossy format -- like the JPEG image format, it discards information in order to deliver smaller file sizes. (See pages 23-24 of "The Macintosh iLife" for a gorgeous diagram of how MP3 works.)

AAC is also a lossy format, and when an MP3 file is compressed with AAC, the lossiness is compounded.

How much will your music suffer? That depends on the bitrate at which it was originally encoded and the bitrate at which you re-encode it. A high-quality (say, 256kbps) MP3 encoded into AAC at 192kbps may compress reasonably well. A 128kbps MP3 probably won't, and the resulting AAC file may have a swirly, shortwave-radio quality to it.

Bottom line: If you want to take advantage of AAC's space savings, you'll get better results if you re-rip your original CDs instead of recompressing your existing MP3s.

iTunes has some smarts that make this re-ripping a little less laborious: If you re-rip a CD that iTunes already has in its library, iTunes tells you that the songs have already imported and asks if you want to import them again. Simply click the Replace Existing button. Thus, you don't have to rebuild your playlists, retype any song information, or even manually delete your old MP3s.

Oh, and when might you want to use iTunes to simply convert your existing MP3s? I'll have that answer in a future tip.