iPod & iTunes: The Missing Manual, Second Edition

Bit Rates

Bit rate may sound like one of those unbelievably geeky computer terms (which it is), but it plays a big role in how your music sounds when you snag a song from a CD and convert it to MP3 or AAC format. When it comes to sound quality, all digital audio files are not created equal.

The bit rate has to do with the number of bits (binary digits—tiny bits of computer data) used by one second of audio. The higher the number of bits listed, the greater the amount of data contained in the file, and the better the sound quality.

Tip

Eight bits make a byte. So why are audio files measured in kilobits (thousands of bits), and not the more familiar kilobytes?

Force of habit. Geeks measure size and storage capacity in bytes, but network speeds and data-transfer speeds have always been measured in bits. When you encode an MP3 file, the transfer and compression of the audio data into the new format is measured in kilobits.

Files encoded with lower bit rate settings—like 64 kilobits per second—don’t include as much audio information from the original sound file. They sound thin and tinny compared to a file encoded at, say, 160 kbps.

Just as you can’t compare megahertz ratings across different chip families (like Pentium III vs. Pentium 4), you can’t compare bit rates between AAC and MP3 files. A 128 kbps AAC file generally sounds much better than a 128 kbps MP3 file. In fact, tests by the group that developed the AAC standard found that a 96 kbps AAC file generally sounds better than a ...