Join Bobby Owsinski for an in-depth discussion in this video A look at AAC, the iTunes file format, part of Mastering for iTunes.
iTunes uses the advanced audio coding, or AAC file format as a standard for all the music in its store. Contrary to popular belief, it's not a proprietary format owned by Apple. In fact, it's part of the MP4 specification and generally delivers excellent quality files that are about 30% smaller than a standard MP3 of the same data rate. All new music destined for the iTunes store is now encoded at a constant bit rate of 256 kilobytes per second at a sample rate of 44.1 kHz.
The iTunes store discontinued selling 128 kilobytes-per-second music files in April of 2008. While the iTunes store does the encoding for you after the songs have been submitted. Here are some of the parameters of the AAC encoder that are available if you do your own encoding in iTunes. The settings can be found under the iTunes > Preferences > General, you click Import Settings, choose Import Using AAC Encoder, and then under Setting choose Custom.
The first setting is Stereo Bit Rate. This allows you to select the bit rate for your AAC encode. The highest quality setting for this format is 320 kilobytes per second. The next is Sample Rate where you select the sample rate you like to encode at. If you're using an encoder other than the one in iTunes, never use a higher sample rate than the rate used for the source. In other words, don't use 48 kHz if your source is 44.1 kHz. Doing so will make the file larger without getting anything in terms of quality.
The Channels pop-up menu enables you to choose how you want the music to play through speakers in Stereo or Mono. You can leave Channel set to Auto if you want the encoder to keep the same channel settings as the file you're encoding. Check the box next to Variable Bit Rate Encoding, this option keeps the file size down and increases the audio quality. VBR varies the number of bits used to store the music as a song gets more or less complex. High efficiency encoding tailors the AAC encode to provide a better sounding and smaller file when encoding at lower sample rates, so we probably won't choose it if we're encoding music.
The Optimize for voice option is meant for podcasters and filters the audio to favor the human voice, which is obviously not something you want for music. To make things really simple, it's best to select the highest bit rate in the Stereo Bit Rate pop-up menu and leave the other two pop-up menus set to Auto.