How Audio Compression Works

You don't have to understand how audio compression works in order to use iTunes, but you might wonder how an MP3 or AAC file can be roughly one-tenth the size of an uncompressed audio file and still sound nearly the same.

MP3's origins go back to the 1980s, when researchers began exploring ways to reduce the storage requirements of digital audio. One of the standards that came from these efforts was MPEG (Moving Picture Experts Group) Audio Layer IIIMP3 for short.

AAC is a newer kid on the block. Conceptually, AAC and MP3 are very similar: both reduce the storage requirements of sound by "shaving off" audio information that our ears have trouble hearing. But scientists have learned a lot about audio compression and human hearing in the decades since MP3 was created, and AAC takes advantage of these break-throughs to provide better sound quality at smaller file sizes.

As for the Apple Lossless encoder, it doesn't "shave off" any audio information. Instead, it uses compression techniques that are similar to those of a file archiving program such as StuffIt. You don't save nearly as much space, but there's no loss in sound quality either.

Will your ears be able to tell the difference? Do some tests and find out. One thing is certain: your hard drive and your iPod will know the difference. Apple Lossless files require dramatically more storage space than AAC or MP3 files.

Where iTunes Stores Your Tunes

iTunes stores your music library in your Music folder. The fastest way to locate the Music folder is to choose Home from the Finder's Go menu.

You don't have to venture inside the Music folderindeed, you should always add and remove songs to and from your music library by using iTunes itself, not by dragging files into and out of the Music folder.

If you're curious, here's how the Music folder is organized. The Music folder contains another folder named iTunes, and inside this folder is a file named iTunes Library. This file contains a database of all the songs you've added to iTunes, as well as all the playlists you've created. But it doesn't contain the song files themselves; those files live in the folder named iTunes Music.

Note that you don't have to store your music in the Music folder. You might want to store it elsewhereon a portable FireWire hard drive, for example. To specify a different location for your music library, choose Preferences from the iTunes menu, click the Advanced button, and then specify the desired location. (For more iTunes library tips, see page 80.)

Note

If you've been using iTunes for a while, you may find older music library files in your iTunes folder. These files will be stashed in a folder named Previous iTunes Libraries. It's safe to delete these older library files.