Bringing Yourself Up to Speed with AAC, MP3, and Digital Audio - Choose an Appropriate Compression Rate, Bitrate, and Stereo Settings
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To get suitable audio quality, you must use an appropriate compression rate for the audio files you encode with iTunes. In the previous chapter, you saw how to set your encoding preferences. Here, you’ll learn what effect the different settings have.
iTunes’ default settings are to encode AAC files in stereo at the 128 Kbps bitrate using automatic sample-rate detection. iTunes calls those settings High Quality, and they deliver great results for most purposes. If they don’t suit you, you can choose custom AAC settings for the files you create. With AAC you can change the bitrate, the sample rate, and the channels.
iTunes’ MP3 encoder gives you more flexibility. The default settings for MP3 are to encode MP3 files in stereo at the 160 Kbps bitrate, using CBR and automatic sample-rate detection. iTunes calls those settings High Quality, and they deliver results almost as good as the High Quality settings with the AAC encoder, although they produce significantly larger files because the bitrate is higher.
For encoding MP3 files, iTunes also offers preset settings for Good Quality (128 Kbps) and Higher Quality (192 Kbps). Beyond these choices, you can choose the Custom setting and specify exactly the settings you want: bitrates from 16 Kbps to 230 Kbps, CBR or VBR, sample rate, channels, the stereo mode, whether to use Smart Encoding Adjustments, and whether to filter frequencies lower than 10 Hz.
Test the Sound Given by Different Bitrates to Find Which Is Best Choosing a compression rate for your music collection shouldn’t be a snap decision; making the wrong decision can cost you disk space (if you record at too high a bitrate), audio quality (too low a bitrate), and the time it takes to rip your entire collection again at the bitrate you should have chosen in the first place.
Ideally, you should rip a representative selection of the types of music you plan to listen to using your computer and your iPod. Encode several copies of each test track at different bitrates, and then listen to them over a period of several days to see which provides the best balance of file size and audio quality.
Make sure some of the songs test the different aspects of music that are important to you. For example, if your musical tastes lean to female vocalists, listen to plenty of those. If you prefer bass-heavy, bludgeoning rock, listen to that. If you go for classical music as well, add that to the mix.
This probably all sounds obvious enough—and it should be. But plenty of people don’t take the time to find out which bitrate is best for them and their music, although they find plenty of time to complain about the results.
 This is chapter three of How to Do Everything with Your iPod & iPod Mini, by Guy Hart-Davis (McGraw-Hill/Osborne, ISBN 0072254521, 2004). Check it out at your favorite bookstore today.
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