Windows Media Audio

Windows Media Audio (WMA) is a proprietary compressed audio file format developed by Microsoft. It was initially a competitor to the MP3 format, but with the introduction of Apple's iTunes Music Store, it has positioned itself as a competitor to the Advanced Audio Coding format used by Apple and is part of the Windows Media framework.

A large number of consumer devices, ranging from portable hand-held music players and handphones to set-top DVD players, support the playback of WMA files. WMA is second only to MP3 in popularity in terms of number of devices supported.

Files in this format can be played using Windows Media Player, Winamp (with certain limitations, DSP plugin support and DirectSound output is disabled using the default WMA plugin), Real Player, and many other alternative media players. The FFmpeg project has reverse-engineered and reimplemented the WMA format to allow its use on POSIX compliant operating systems such as Linux.

In November 2005, a new update was available for the PlayStation Portable (version 2.60) which allowed WMA files to be played on the console for the first time.

Origin

An initial reason for the development of WMA may have been that MP3 technology is patented and has to be licensed from Thomson SA for inclusion in the Microsoft Windows operating system.

Design

WMA is capable of VBR and CBR encoding in order to be marketed as a versatile format.

Windows Media Audio supports digital rights management using a combination of elliptic curve cryptography key exchange, DES block cipher, a custom block cipher, RC4 stream cipher and the SHA-1 hashing function.

A WMA file is almost always encapsulated in an Advanced Systems Format (ASF) file. The resulting file may have the filename suffix "wma" or "asf" with the "wma" suffix being used only if the file is strictly audio. The ASF file format specifies how metadata about the file is to be encoded, akin to the ID3 tags used by MP3 files.

Codecs and versions

WMA began being widely distributed starting with the WMA 7 codec. The version 9 WMA bundle includes three more codecs, including a voice codec, lossless codec and WMA 9 Pro codec. The last one is based on a completely different technology (highly superior to "ordinary" WMA in terms of quality and features) and thus WMA 9 Pro files are incompatible with older players.

The most current version of the format is Windows Media Audio 10.00 includes specific codecs for lossless, multi-channel surround sound and voice encoding in addition to the main lossy codec. Both constant and variable bit rate encoding are supported.

Sound quality

Microsoft claims that files in WMA format sound better than MP3 files at the same bitrate; however, a 2004 independent test^* conducted at a common bit rate showed it (at 128kbps) as inferior to AAC and Vorbis, roughly equivalent to LAME MP3, and superior to ATRAC3. Microsoft also claims that WMA files sound better than MP3 files at higher bitrates. Although claims about audio quality should be taken with caution, double blind tests have consistently failed to support Microsoft's claims about its superior quality.

Tests of the newer WMA Pro show that it is at least equivalent^* to Ogg Vorbis, AAC and Musepack at comparable bitrates. However, device and player support for WMA Pro is not as prevalent as WMA Standard. Generally speaking, WMA (without any other qualifiers) refers to WMA Standard.

Digital rights management

While the Windows Media Audio codec itself does not contain any digital rights management facilities, the Advanced Systems Format (ASF) container format which holds WMA streams does. Janus is the codename for the most recent version of Windows Media DRM, which is commonly used in conjunction with WMA. The DRM technology supports time-limited music such as those offered by unlimited download services, such as Napster,Yahoo! Music Unlimited and Virgin Digital. Janus DRM is incompatible with portable audio devices that only support up to WMA DRM Version 9. CD tracks ripped with Windows Media Player are "protected" (DRM-restricted) by default, though this setting can be changed.

External links

Audio codecs | Computer file formats | Digital audio | Microsoft