iTunes can sing to a variety of audio file formats. Most digital tracks imported into the iTunes database are compressed (or shrunken) so that the music doesn’t require a lot of space on your Mac. But when you compress your songs, you generally have a tradeoff between file size and sound quality. As you might imagine, larger files offer the finest sonic fidelity — at least in theory.
The best known of these compression schemes is MP3, a method in which files are squeezed to a reasonable size, even though the sound is perfectly acceptable to all but the most serious audiophiles. Apple prefers an alternate compression method. On Macs with QuickTime 6.2 or later, Apple uses a default encoding scheme known as MPEG-4 AAC (Advanced Audio Coding), a compression format that Apple claims is equal if not superior to MP3s encoded at the same or a slightly higher bit rate. (If you have an earlier version of QuickTime, MP3 is the default.)
The songs you purchase at the iTunes Store are also in the AAC format. According to Apple, the High Quality AAC setting produces files that take up less than 1MB for each minute of music. But iTunes also recognizes other file formats, among them: Apple Lossless, AIFF, and WAV. These last two flavors are uncompressed, so the music is of exceptional quality, but the files gobble up disk space. Apple Lossless is an audiophile format that matches AIFF and WAV in sound quality but takes up half the space.
If you’re inclined to mess with these file formats, visit iTunes Preferences, click the Advanced tab, and make your choice in the Importing section. You can set up the encoder to import using AAC, AIFF, Apple Lossless, MP3, or WAV, and also choose the stereo bit rate. In techie terms, 128 Kbps is the default.