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iPod & iTunes: The Missing Manual, Fourth Edition by J.D. Biersdorfer

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Compressed Audio Formats

Up until a few years ago, the MP3 format was the only game in town for playing quality song files on your computer, whether downloaded from the Internet or taken from CDs. MP3 still dominates the Internet, but other formats—like Ogg Vorbis (an audio format favored by Linux fans and the open source software crowd; details at http://www.vorbis.com)—have dedicated fans, too.

Ogg Vorbis isn’t on the list of iPod-compatible formats, but many others are, including MP3, AAC, AIFF, Apple Lossless, and WAV. Here’s a brief explanation of each Podworthy format.


Suppose you copy a song from a Lena Horne CD directly onto your computer, where it takes up 35.3 MB of hard disk space. Sure, now you could play that song without the CD in your CD drive, but you’d also be out 35.3 megs of precious hard drive real estate.

Now, say you put that Lena Horne CD in your computer and use your favorite encoding program like iTunes to convert that song to an MP3 file. The resulting MP3 file still sounds really good, but it only takes up about 3.2 MB of space on your hard drive—about 10 percent of the original. Better yet, you can burn a lot of MP3 files onto a blank CD of your own—up to 11 hours of music on one disc, which is enough to get you from Philadelphia to Columbus, Ohio on I-70 with tunes to spare.

How it works

MP3 files are so small because the format’s compression algorithms use perceptual noise shaping, a method that mimics the ability of the human ear to hear certain sounds. Just as people can’t hear dog whistles, most recorded music contains frequencies that are too high for humans to hear; MP3 compression discards these sounds. Sounds that are blotted out by louder sounds are also cast aside. All of this space-saving by the compression format helps to make a smaller file without severely diminishing the overall sound quality of the music.

New portable MP3 player models come out all the time, but many people consider the iPod’s arrival in 2001 to be a defining moment in the history of MP3 hardware.


MP3 is short for MPEG Audio, Layer 3. MPEG stands for Moving Pictures Experts Group, the association of engineers that also defined the specifications for the DVD video format, among others.


The Advanced Audio Coding format may be less than a decade old (it became official in 1997), but it has a fine pedigree. Scientists at Dolby, Sony, Nokia, AT&T, and those busy folks at Fraunhofer, collaborated to come up with a method of squeezing multimedia files of the highest possible quality into the smallest possible space—at least small enough to fit through a modem line. During listening tests, many people couldn’t distinguish between a compressed high-quality AAC file and an original recording.

What’s so great about AAC on the iPod? For starters, the format can do the Big Sound/Small File Size trick even better than MP3. Because of its tighter compression technique, a song encoded in the AAC format sounds better (to most ears, anyway) and takes up less space on the computer than if it were encoded with the same quality settings as an MP3 file. Encoding your files in the AAC format is how Apple says you can stuff 15,000 songs onto a 60 GB iPod.


You can think of AAC as the Apple equivalent of WMA, the copy-protected Microsoft format used by all online music stores except Apple’s. For better or worse, the iPod doesn’t recognize copy-protected WMA files.

The AAC format can also be copy protected (unlike MP3), which is why Apple uses it on the iTunes Music Store (see Chapter 7). The record companies would never have permitted Apple to distribute their property without copy protection.


Real Networks, with its own online music store, has a media-player program called RealPlayer 10.5 that can be used to wiggle your home-ripped tracks and RealPlayer Music Store purchases onto your iPod. Apple, of course, has changed the iPod’s software at least once to block RealPlayer from nosing around its iPods and would prefer that you do your 99 cents-a-song downloading from its iTunes Music Store only (Chapter 7).

Because the iPod can play several different audio formats, you can have a mix of MP3 and AAC files on the device if you want to encode your future CD purchases with the newer format. If you want to read more technical specifications on AAC before deciding, Apple has a page on the format at http://www.apple.com/mpeg4.


AAC is the audio component of MPEG-4, a new video format that’s designed to get high-quality video compressed enough to travel over computer networks (even pokey old modem lines) and still look good onscreen.

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