Bringing Yourself Up to Speed with AAC, MP3, and Digital Audio - What Is AAC? Should You Use It?
(Page 4 of 16 )
Advanced Audio Coding (AAC) is a codec for compressing and playing back digital audio. AAC was put together by a group of heavy hitters in the audio and digital-audio fields, including Fraunhofer IIS-A (the German company that developed MP3), Sony Corporation, Dolby Laboratories, AT&T, Nokia, and Lucent. AAC is newer than MP3 (which is discussed in the next section), is generally agreed to deliver better sound than MP3, and is more tightly controlled than MP3. It is one of the key audio components of the MPEG-4 specification, which covers digital audio and video.
NOTE: Some people refer to the Advanced Audio Codec instead of Advanced Audio Coding. The same abbreviation—AAC—applies. In any case, most people stick with the abbreviation rather than using the full name.
NOTE: The iTunes Music Store (discussed in Chapter 6) uses AAC for its songs, so if you buy songs from it, you won’t have any choice about using AAC. If necessary, you can convert the songs to other formats. For example, you might convert a song from AAC to MP3 so that you can use it with an MP3 player that can’t handle AAC.
MPEG-2 AAC and MPEG-4 AAC At the time of writing, AAC comes in two flavors: MPEG-2 AAC and MPEG-4 AAC. From the names, you’d hazard that MPEG-2 AAC is part of the MPEG-2 specification for digital audio and video, whereas MPEG-4 AAC is part of the MPEG-4 specification—and you’d be right.
MPEG-2 AAC is used for several purposes. It’s part of the specification for the DVD-Audio Recordable (DVD-AR) format and is used for some Internet-audio purposes, such as streaming and downloading audio.
MPEG-4 is a newer specification that includes more capabilities and delivers higher-quality sound than MPEG-2. Among other uses (such as in iTunes, QuickTime, and your iPod), MPEG-4 AAC is used as the general audio codec for 3G wireless terminals.
QuickTime 6 includes an AAC codec. If your Mac has QuickTime 6.2 or a later version installed, iTunes uses MPEG-4 AAC as its default encoding format. (If your Mac has an earlier version of QuickTime installed, iTunes uses MP3 as its default encoding format.)
AAC’s 48 Channels AAC can work with up to 48 full-frequency audio channels. This gives it a huge advantage over MP3, which can work with only two channels (in stereo) or a single channel (in mono).
If you’re used to listening to music in stereo (in other words, using two channels), 48 channels seems an absurd number. But typically, only a small subset of those channels would be used at the same time. For example, conventional surround-sound rigs use 5.1 or 7.1 setups, using six channels or eight channels, respectively. Other channels can be used for different languages, so that an AAC player can play a different vocal track for differently configured players. Still other tracks can be used for synchronizing and controlling the audio.
AAC Licensing Dolby Laboratories handles AAC licensing via its independent subsidiary Via Licensing Corporation (www.vialicensing.com). Via charges royalties on the sale of AAC encoders and decoders (either hardware or software), but there are no fees for distributing content in AAC format. So usually the manufacturers or developers of the encoders and decoders take care of the licensing payments and pass along the costs to the end users by including them in the cost of hardware and software decoders. That means you don’t need to worry about getting a license for using AAC—Apple has taken care of them for you in iTunes, QuickTime, and your iPod.
The cost of AAC licenses varies depending on the type of AAC (MPEG-4 AAC is a little more expensive than MPEG-2 AAC) and the number of channels used in the implementation. For example, stereo uses two channels (the left channel and the right channel), is widely used in consumer products, and comes at a very affordable cost. (Mono is even cheaper, but few people want to listen to mono sound when they can have stereo instead.) A high-end home-theater product using 7.1 surround sound uses eight channels and costs correspondingly more. Products classed as “professional” rather than “consumer”—for example, video-production equipment—cost more yet.
Advantages of AAC For music lovers, AAC offers higher music quality than MP3 at the same file sizes, or similar music quality at smaller file sizes. Apple reckons that 128 Kbps AAC files sound as good as 160 Kbps MP3 files—so you can either save a fair amount of space and enjoy the same quality or enjoy even higher quality at the same bitrate. Around 24 Kbps, AAC streams provide quite listenable sound, whereas MP3 streams sound quite rough. (Streaming is the method of transmission used by Internet radio, in which you can listen to a file as your computer downloads it.)
Small file sizes are especially welcome for streaming audio over slow connections, such as modem connections. AAC streamed around 56 Kbps sounds pretty good (though not perfect), while MP3 sounds a bit flawed.
The main advantage of AAC for the music industry is that the format supports digital rights management (DRM). This means that AAC files can be created in a protected format with custom limitations built in. For example, the song files you can buy from the iTunes Music Store are authorized to be played on up to three different computers at the same time. If you try to play a song on a computer that’s not authorized, the song won’t play.
NOTE: To tell if an AAC file is protected or not, choose File | Get Info and check the Kind readout on the Summary tab of the Song Information dialog box. If the file is protected, the Kind readout reads Protected AAC Audio File. If not, Kind reads AAC Audio File. Alternatively, check the file extension: the .m4p extension indicates a protected file, whereas the .m4a extension indicates an unprotected file.
Disadvantages of AAC AAC’s disadvantages are largely acceptable to most users of iTunes and the iPod:
- At the time of writing, AAC files aren’t widely used. One reason is that, because AAC is relatively new and hasn’t been very widely implemented, few AAC encoders and decoders are available. However, in iTunes (and QuickTime) and the iPod, Apple has provided AAC encoding and decoding for the Mac and for Windows. Because AAC is the default format for ripping (discussed in “Understand Ripping, Encoding, and ‘Copying’,” later in this chapter) in iTunes 4, its usage is growing rapidly.
- Encoding AAC files takes more processor cycles than encoding MP3 files. But as processors continue to increase in speed and power by the month if not by the week, this becomes less and less of a problem. For example, even relatively antiquated Macs (such as my PowerBook G3/333) and PCs (for example, a Celeron 600) have plenty of power to encode and decode AAC—they just do so more slowly than faster computers.
- For consumers, the largest potential disadvantage of AAC is the extent to which DRM can limit their use of the files. At the time of writing, Apple has delivered a relatively flexible implementation of DRM in the music sold by the iTunes Music Store. However, if Apple and the record companies tighten the licensing terms of the files in the future, consumers may have cause for concern. In this sense, AAC could act as a Trojan horse to wean customers off MP3 and onto AAC, then gradually lock them in to a format that the music industry can control.
 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.
Buy this book now. |
Next: What Is MP3? Should You Use It? >>
More Software Articles
More By McGraw-Hill/Osborne
/ 70 
