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8. About Music Formats


Just jump right in!

No matter how easy to use a piece of software is, there are some trivialities of computer technology that just can't be escaped. One of these bits of esoterica is the plethora of digital music file formats you'll encounter while using iTunes. Ideally, and for the most common paths of use, file formats are little more than a curiosity you never have to deal with yourself; but the moment you try to do anything at all advanced with your music, you'll find yourself surrounded by what seems an alphabet soup of acronyms and labels, lurking malevolently just under iTunes' polished surface. It pays to know what each of these formats is all about and how to deal effectively with it.

The format of a digital music file refers to the specific structure of the data within it. All digital music files follow essentially the same idea: a series of numbers that describe the pitch and intensity of the sound waveform at each particular instant in the audio stream, all adding up to a familiar musical sound when it's interpreted by software such as iTunes at normal playback speed. Different kinds of files differ, though, in exactly what form those numbers take. Some music formats support compression (the ability to reduce the file size by discarding relatively unimportant sound information, or by indexing small repeated fragments of sound instead of encoding them all directly—as well as other, ingeniously mathematical methods). Some formats support Digital Rights Management (DRM), enforcing copy protection. Some forms of digital audio files encode stereo data differently from others, resulting in a somewhat different sound quality. These subtle but important differences are what make some formats more suitable for certain tasks than others, and explain why we have to deal with so many different file formats in the world of digital music.


Format— The specific internal data structure of a digital music file.

Compression— Fitting more music into a fewer number of bytes by discarding unimportant musical data or indexing repeated patterns.

The following audio file formats are either supported directly by iTunes or destined to be a part of your life as you work with iTunes. Except where noted, the customary filename extension—the three or four letters after the final period (.) in a file's name—is the same as the acronym of the format listed.

  • CDDA (Compact Disc Digital Audio)— An uncompressed, true stereo digital audio stream made up of a simple series of audio samples. This is the standard format in which every commercial audio CD is encoded and the format in which iTunes burns audio CDs. The CDDA format is essentially interchangeable with AIFF and WAV formats, and is almost never used directly on computers.

  • AIFF (Audio Interchange File Format)— An uncompressed audio stream format developed by Apple and popularized as the default sound format of the Macintosh platform. Still the preferred format for raw audio editing on the Mac. AIFF files can be converted without loss of quality to WAV format and back.

  • WAV (Windows WAVeform file)— An uncompressed audio stream format developed by Microsoft and IBM; equivalent to AIFF in its supported features and file size, WAV is the default raw audio format for Windows. WAV files can be converted without loss of quality to AIFF format and back.

  • MP3 (MPEG-1 Audio Layer 3)— Developed in the early 1990s as part of the MPEG video/audio compression specification, MP3 is the first audio format to have brought the file size of individual song tracks down to the point where they could be easily transferred across the Internet while still sounding nearly true to the original CD-quality sound from which they were derived. Encoding technology for MP3 files is subject to patents held by Thomson Consumer Electronics and the Fraunhofer Institute, who contributed to the format's original design. MP3 is a lossy compression format, meaning that any music converted to MP3 format cannot be flawlessly converted back to its original format; some music data is inevitably lost. The benefit is that an MP3 file usually achieves about a 12:1 compression ratio over the uncompressed source, depending on the bit rate selected during encoding.


Lossy— A compression or conversion process in which some sound information or resolution is irretrievably lost, as when encoding a CD to MP3 format. The converse is lossless compression, meaning a reversible process in which no information is lost.

Lossless— A compression or conversion process where perfect sound quality from the original source is preserved. Lossless compression doesn't achieve such small file sizes as lossy compression, but it's reversible.

Bit rate— The number of bits per second consumed by an audio stream; the bit rate can be constant or variable, and if constant can be used to calculate how much disk space a song file will take up.

MP3's compression is achieved through a number of techniques, including the discarding of superfluous (inaudible) audio data and the application of psychoacoustics. Stereo information is usually done in “joint stereo” mode, meaning that instead of two separate tracks for the left and right channels, most information is recorded as a “mono” track and a separate channel records the separation from left to right, which saves on file size. Also, Variable Bit Rate (VBR) encoding is frequently employed to encode lower-complexity sections of music at a lower bit rate, saving space without sacrificing audio quality.

MP3 files have no Digital Rights Management (DRM) technology built in, meaning that there is no way for a copyright holder to control or track the spread of an MP3 version of a song. MP3 files do, however, have info tags—also known as ID3 tags—that allow the user to embed a wide variety of organizational information into the file's text headers. This information does not interfere with the audio stream at all, but gives software such as iTunes the ability to organize MP3 files with much better control and flexibility than with filenames alone.

  • AAC (Advanced Audio Coding)— The official successor to MP3, AAC (another lossy compression format) is the latest iteration of the audio specification in the MPEG standard, part of the MPEG-4 framework that underlies modern versions of QuickTime. Apple is currently the most visible company using AAC in its products, although it is far from being a “proprietary” format; the only thing proprietary about AAC as used in the iTunes Music Store is how the DRM scheme (known as FairPlay) is keyed to individual purchasers. Functionally, however, as far as any company that wants to interoperate with music purchased through iTunes, Apple's format is essentially closed.

    AAC files incorporate many advances over the earlier MP3 format, including as many as 48 distinct audio channels, a more dynamic form of stereo encoding, and a notably smaller file size for files encoded with the same subjective sound quality. This means that if you encode your CD collection in AAC format instead of MP3, you can save about 25% of your disk space (which, remember, also applies to the space available on your iPod). AAC supports all the same info tags as MP3 does.

    There are two flavors to AAC as used in iTunes: protected and unprotected. Protected AAC files are keyed to an individual purchaser's identity and cannot be opened on a given computer unless that computer has been authorized with the central iTunes' authorization servers; as many as five separate computers can be authorized at one time for a single purchaser account. (See 23 Authorize a Computer to Play Purchased Music for more information.) Unprotected AAC files are as freely portable and playable as MP3 files are; you can send an unprotected AAC file to anyone else with iTunes or any other software capable of reading AAC files, and the recipient can play it successfully. When you import music from your CD collection, iTunes creates the digital music files by default in unprotected AAC format.

    AAC files are identified by either a .m4a (MPEG-4 Audio) or .m4p (MPEG-4 Protected) filename extension, depending on whether or not the files are protected with DRM.

  • WMA (Windows Media Audio)— Microsoft's answer to MP3 (and later, to AAC as used in iTunes), WMA is a lossy compression format that is entirely proprietary and owned by Microsoft. Its capabilities are comparable to those of AAC—the audio quality for a given file size is considerably better than with MP3, and thus for the same audio quality you get significant file size savings. WMA also comes in both protected and unprotected flavors. iTunes for Windows can import unprotected WMA files by converting them to AAC on-the-fly.

    The DRM scheme in WMA files as sold through the online music stores that compete with the iTunes Music Store is very flexible and is implemented differently by various sellers. Some stores restrict copying and playback to a certain number of computers, like iTunes does. Others, like Napster's subscription service, enforce an expiration date beyond which a file cannot be opened; periodic authorization is required to extend the expiration date on these files. Still other services set a limit on the number of times a given song can be played before it becomes locked.

  • Apple Lossless— A format developed by Apple and released with iTunes 4.5 in April 2004, with the intention of supporting high-quality audio storage for professional musicians and audiophiles without requiring the full amount of disk space required by uncompressed AIFF, WAV, or CDDA data. Apple Lossless achieves compression of about 2:1 over the uncompressed source data by using techniques similar to those found in GIF or ZIP files, both compression formats that must by their nature be totally lossless. If you encode your music using Apple Lossless, expect to consume about 5 megabytes of disk space for every minute of music; but this music will be at true CD quality without even the minimal degradation of quality found in MP3 or AAC formats. Apple Lossless files are encapsulated in MPEG-4 wrappers, and thus have a .m4a filename extension. These files are not, however, AAC files.

  • MIDI (Musical Instrument Digital Interface)— Completely unlike all the preceding formats, MIDI is not a series of samples at all, but a synthesized music format. MIDI files are generally tiny compared to sampled music files such as AAC and MP3—only 20 to 50 kilobytes—because all they contain are lists of commands comparable to what you'd see on a piece of sheet music. MIDI files depend on a library of playback technology to interpret these commands, as an orchestra would read the sheet music in front of it; both Windows and Mac OS X can play MIDI files natively, but the playback quality of a MIDI file depends greatly on the quality of the synthesized instruments in the software you use. iTunes can add MIDI files to its library and play them using the QuickTime MIDI instruments, but these files cannot be transferred to the iPod. MIDI files generally have a .mid extension.

  • Internet Radio— A “stream” of audio data (usually in MP3 format) coming from a source on the Internet, Internet Radio data cannot be saved directly by iTunes, paused, rewound, or scanned using the scrub bar—it's a live stream to which you connect by specifying a web address to listen to. Favorite Internet Radio streams are added to your iTunes library as you listen to them so that you can return to them whenever you want, but (naturally) they cannot be transferred to your iPod. Internet Radio streams sometimes use downloaded “playlist” files to schedule the playback of tracks stored on the server. Refer to 37 Listen to an Internet Radio Station for more information.

iTunes gives you the ability to convert between most of these formats, if not bidirectionally, at least from each to a native format such as AAC or MP3. In the tasks in this chapter, you will see how to take advantage of the strengths of these different formats as you bring in your music from varying sources to consolidate it all into your digital iTunes Library.

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