The EM Poll

All Is Not Lost

Oct 1, 2006 12:00 PM, By Vijith Assar

           

Heir to the Throne

Advanced Audio Coding (AAC) is generally viewed as a technically superior successor to MP3. It is also championed by the Moving Picture Experts Group (MPEG), which infused the new format with everything it learned about human audio perception from its experiences with MP3 over the last decade. AAC is the audio layer of the MPEG-4 standard, which means that the AAC audio stream will often be placed inside an MP4 container file. This file usually has a .m4a file extension as prescribed by MP4 naming standards (though a .mp4 extension would also be valid).

FIG. 2: iTunes will use AAC encoding unless you specify otherwise in the Preferences menu.

Apple Computer has been the single greatest force in encouraging adoption of AAC, enabling support in iPods and iTunes and relying on it for the iTunes Music Store (see Fig. 2). However, audio files purchased from the iTunes Music Store are wrapped in FairPlay digital rights management (DRM) technology and use a .m4p extension. These files are very different from standard MP4 files because they are locked and copy protected. AAC will probably be around for quite a while, as it has the weight of both Apple and MPEG behind it. It provides an excellent ratio of file size to sound quality and thus is a great choice if you are not concerned about widespread compatibility.

Locks on Your Windows

Windows Media Audio (WMA) is a proprietary audio format developed by Microsoft, which claimed it would provide audio quality comparable to that of MP3 at half the file size. That claim has since been debunked — standard WMA files are no better than vanilla MP3s. Although the Pro version has proven to be a powerful, efficient codec, even providing support for 24-bit multichannel audio, it hasn't gained widespread popularity due to a complete lack of hardware support. A lesser concern has been Microsoft's attitude toward non-Windows operating systems: there's no official support for WMA in Linux (though it can be accomplished using the Xine playback engine), and the notoriously unstable Windows Media Player for Mac was recently discontinued. (Microsoft is now instructing users to install a third-party QuickTime input plug-in to get WMA playback on the Mac.)

FIG. 3: Enabling copy protection in Windows Media Player will lock out unintended users.

Nevertheless, WMA has managed to spread considerably due to its strong implementation of DRM. MP3 is an inherently insecure format, as it was developed before the online media explosion. WMA took shape as content providers became increasingly concerned about security, and as a result it includes proprietary licensing technology that has proven to be useful for many online music stores. In fact, this technology is at the heart of the PlaysForSure program, a Microsoft-led initiative designed to ensure compatibility among online stores and playback devices. Secure WMA is most appropriate for large distribution outlets that sell protected files to end users, for content providers, or for those working with sensitive material and high-profile clients who are concerned about leaks.

It is possible, however, to create your own protected WMA files using Windows Media Player (see Fig. 3). But because the WMA scheme stores authentication licenses separately from the audio files (in contrast to the iTunes model, which will authorize or deauthorize the entire collection at once), you may find yourself locked out of your own media if you haven't backed up your licenses and your hard drive crashes. (Always keep your source files as unprotected PCM audio files so you can reencode if necessary.) Moreover, almost any playback mechanism that supports WMA also supports MP3, so all in all, there's not much benefit to using WMA format.

What's in a Name?

Ogg Vorbis emerged several years ago as a free, open-source alternative to MP3 and WMA. The format consists of two distinct parts: Vorbis, the audio-compression codec, and Ogg, the container. This is a potent combination: Vorbis delivers great performance at any bit rate, and Ogg includes a powerful and flexible metadata system. In addition, Ogg Vorbis is the only encoding method to promise eventual support for bit rate peeling, a function that would allow scaling of high-bit-rate files down to lower bit rates without the quality degradation that usually results from reencoding a previously encoded file. (So far it has failed to deliver on this promise, however.)

Ogg Vorbis has been largely marginalized by the commercial support thrown behind AAC by Apple and iTunes. On the other hand, because it is open source, using it does not require the payment of royalty fees, such as those charged by the Fraunhofer Group for using its patented MP3 format. As a result, Ogg Vorbis is often used for embedded applications like video games, and it's the format of choice for Wikipedia for the same reason. Further, the metadata capabilities of Ogg are more advanced than those of most other audio formats, making it especially suitable for someone maintaining an extremely large library of compressed audio.

(Anything but) Lame

In spite of the strong showing from newer formats, MP3 should remain a viable option for some time. That's mostly because the LAME encoder, an open-source project, incorporates advanced options that have kept the format competitive. First and foremost is LAME's ability to vary bit rate over the course of the file. This allows the encoder to adapt to the complexity of the audio stream being encoded in order to use more bits when they're needed and to lower bit rate when the signal is relatively simple.

By default, LAME uses constant bit rate (CBR) encoding, which applies the same amount of compression to the entire file. But by using the “ — abr” command when accessing LAME (for example, “ — abr 192”), you can enable average bit rate (ABR) encoding, which will vary the compression rate a modest amount as needed. Though constant bit rate encoding can sound fairly decent, ABR encoding will sound better than CBR at any given bit rate without increasing the size of the resulting file.

A step above ABR is fully variable bit rate encoding (VBR), which allows the encoding quality to fluctuate wildly according to the input file. This will sound even better than CBR or ABR encoding, again without impacting file size. VBR is enabled using the “ — vbr” switch (for example, “ — vbr 192”). Though some older hardware MP3 players had trouble decoding VBR files, VBR has now been around long enough that it is preferable in all but the most unusual situations. (One of the technically superior features of the Ogg Vorbis format is that it uses VBR encoding by default.) If your hardware has trouble with VBR files, it probably makes more sense to replace the hardware than to use poor encoding methods.