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Chapter 4. Digital Audio Formats > Introduction to Digital Audio

4.1. Introduction to Digital Audio

The era of modern digital audio began in the early 1980s. A new, small, shiny format called the audio compact disc, developed by Sony and Philips, began to appear in music stores alongside albums on tapes and vinyl records. Unlike analog tapes and LPs, audio CDs stored music in digital form, and produced a bright, clean sound with pristine clarity. (Some audiophiles still prefer the "warmer" sound of vinyl, not to mention the expansive canvas that records provided for detailed album artwork, but many have accepted the CD.)

1985 was a pivotal year for the CD. The format's popularity got a huge boost from its first big seller, Brothers in Arms by Dire Straits, and a variation on the audio CD technology called CD-ROM (Compact Disc, Read-Only Memory) edged into the computer market as a way to play multimedia files and interactive programs.

4.1.1. When CD Met PC

Over the years, a CD drive became a standard component of a computer. On most audio CDs, songs are stored in a format called CD-DA (Compact Disc, Digital Audio), which is essentially the same thing as AIFF format.

On a Windows PC, if you inspect the contents of a music CD, you see a screenful of names like "Track01.cda." These turn out to be nothing but 1 KB files that point to the hidden audio tracks, as shown in Figure 4-1. Mac OS X displays the audio tracks in all their hefty glory as AIFF files, right in the Finder window.

Figure 4-1. Left: Here's what a desktop window looks like for a music CD inserted into a Mac. It looks just like an MP3 playlist, except that these AIFF files are much larger. Your computer can play these high-quality files, but they eat up a lot of hard drive space.
Right: Audio files are more bashful when a disc is inserted into a Windows drive. The tracks on this Prince CD remain hidden behind tiny pointer files, and you can lure them out only with CD-extraction software.

Even if you can't see the audio files, you can still extract them from the CD with software. "Extracting audio tracks" may sound like an uncomfortable medical procedure, but it means copying them from the CD to your hard drive in a computer-readable format. You may also hear the term ripping CDs, which is the same thing.

And while you're digesting new-millennium terminology: once the music files are on your Mac or PC, you encode them into a compressed audio format like MP3 or AAC so that more music fits on a CD that you burn—or on a music player like the iPod.

4.1.2. 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 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 some explanation of each Podworthy format.

4.1.3. MP3

Suppose you copy a song from a Sheryl Crow CD directly onto your computer, where it takes up 47.3 MB of hard disk space. (This sort of audio extraction is quick on a Mac, somewhat harder in Windows; see page 84.) Sure, you could now play that song without the CD in your CD drive, but you'd also be out 47.3 megs of precious hard drive real estate (see Figure 4-1).

Now, say you put that Sheryl Crow CD in your computer and use your favorite encoding program to convert that song to an MP3 file. The resulting MP3 file still sounds really good, but only takes up about 4.8 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 on Interstate 70 with tunes to spare.

4.1.4. How It Works

MP3 files are so small because the 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 overly 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.

4.1.5. AAC

The Advanced Audio Coding format may be relatively new (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.

The Law of the Digital Music Land

Years before the iPod was even a twinkle in Steve Jobs's eye, the Recording Industry Association of America sued the makers of the first portable MP3 player. RIAA felt that the player-the soon-to-be-released Rio PMP300 from Diamond Multimedia-could serve as a tool for pirating copyrighted songs. Diamond eventually won the case. The court found that the Rio itself violated no federal music-piracy laws, paving the way for dozens more MP3 players to hit the market.

The Audio Home Recording Act of 1992 says that you can make copies of your own legally purchased music for your personal (noncommercial) use. Even though MP3 players didn't yet exist, the court ruled in the Rio case that consumers have the right to move the music they own onto a playback device—as long as it's for personal use.

The arrival of the MP3 format changed the way people listened to their music—and shared it. Before MP3, nobody thought much about copyright law when they dubbed a song mix to give to a friend or a romantic prospect. (After all, the exchange of the mix tape/CD has been an important part of the young American courting ritual for years.)

But the small size of MP3 files, mixed with the power of the Internet, ripped the lid off a 55-gallon drum of worms. With music being passed freely all over the world and CD sales dropping (maybe as a result of MP3 sharing, maybe because of unrelated factors), industry groups like RIAA have begun looking for new ways to block digital music copying. They're also taking a highly litigious interest in Internet services that make sharing MP3 files easy.

In 1999, a file sharing service called Napster got a lot of attention from music fans and lawyers alike. The Napster software, authored by a teenager named Shawn Fanning, let music lovers search out and share MP3 files with anybody else on the Internet running the same program–which turned out to be millions of people. Napster traffic became so heavy that at some universities, it actually created a drag on their networks, and resulted in a Napster ban.

While Napster gave people a chance to sample a wide range of music for free, it also sent RIAA on a legal mission to shut down the file sharing service. RIAA eventually prevailed, and in 2002, Napster went to that great Web site in the sky.

(Napster, in its original form, died that day. But in October 2003, it returned in the form of an iTunes Music Store wannabe where music shoppers could legally download tracks for less than a buck apiece. In a further effort to ape Apple, Napster 2.0, as this Windows-only, pay-to-play service was dubbed, even began selling its own iPod clone.)

Although the group's first attempts to squash other file sharing services like Grokster and KaZaA in 2003 were later rebuffed by the courts, RIAA is looking into new technology that can prevent CD tracks from being copied onto computers at all. RIAA has recently begun filing lawsuits against Internet service providers to find out the names of customers who use file sharing services or host music-swapping sites and suing the individuals directly.

Mind you, MP3 files themselves are not illegal. Ripping tracks from CDs that you've purchased over the years for playing on your iPod is legal. Downloading MP3s offered freely by bands and musicians just wanting to be heard is also legal.

Copying copyrighted music that you didn't pay for is not legal, and that's why you can be sure we haven't heard the last from the RIAA.

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 10,000 songs onto a 40 GB iPod.

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.)


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.

Real Networks, with its own online music store, released a program called Harmony in the summer of 2004 that can convert its WMA wares to an iPod–compatible format and can even wrestle the files onto an iPod without iTunes. Apple released an update for the iPod Photo in late 2004 that disabled this, however, and other members of the iPod family may once again be Real-unfriendly by the time you read this. Apple, of course, would prefer that you do your 99¢-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 www.apple.com/mpeg4/aac.


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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