Windows Media Audio (WMA) is a proprietary compressed audio file format developed by Microsoft. It was initially intended to be a competitor to the popular MP3 format, though in terms of popularity of WMA files versus mp3 files, this never came close to occuring. With the introduction of Apple's iTunes Music Store, WMA has positioned itself as a competitor to the Advanced Audio Coding format used by Apple and is part of Microsoft's 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

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, CBR and lossless audio encoding in order to be marketed as a versatile format.

Windows Media Audio can optionally support 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 often encapsulated in an Advanced Systems Format (ASF) file. The resulting file may have the extension "wma" or "asf" with the "wma" extension 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 lossy compression based codec and has now reached version 9. Microsoft's WMA bundle also includes three more codecs, a Windows Media Audio 9 Voice codec, Windows Media Audio 9 Lossless codec and Windows Media Audio 10 Professional codec (earlier known as WMA 9 Pro). The WMA 10 Pro codec is based on a completely different technology (superior to "ordinary" WMA in terms of quality and features) and thus WMA 10 Pro files are incompatible with older players. WMA 10 Pro supports 96 KHz 24-bit audio as well as 5.1/7.1 multi-channel audio.

Windows Media Player 11 is the latest version of Microsoft's media player, more information here

Players

Apart from Windows Media Player, WMA files can be played using MPlayer, Winamp (with certain limitations — DSP plugin support and DirectSound output is disabled using the default WMA plugin), RealPlayer, and many other 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, and RealNetworks has announced plans to support playing non-DRMed WMA files in RealPlayer for Linux[1].

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.

Sound quality

Initially Microsoft claimed that files in WMA format sounded better than MP3 files at the same bitrate; Microsoft also claimed that WMA files sounded better than MP3 files at higher bitrates. However, double blind listening tests with other lossy audio codecs have consistently failed to support Microsoft's claims about its superior quality. Indeed, the first independent test (2004/05) with WMA standard encoder provided by the Windows Media 9, conducted at 128 kb/s, showed that WMA was roughly equivalent to MP3 encoded with LAME encoder, inferior to AAC and Vorbis, and superior to ATRAC3 (software version).

Some conclusions made by recent listening tests:
At 128kb/s the most recent large scale test (2006/01) shows a four way tie between aoTuV Vorbis, LAME-encoded MP3, WMA Pro and iTunes (QuickTime) AAC, with each codec essentially transparent (sounds identical to the original music file). However, device and player support for WMA Pro is not as prevalent as WMA Standard. Generally speaking, WMA (without any other qualifiers) refers to the WMA Standard.
At mid-low bitrates (64 kb/s or more, less than 128 kb/s), latest private tests (80 kb/s (2005/07), 96 kb/s) (2005/08) show that WMA has a lower quality than the lossy audio codecs AAC (HE and LC) and Vorbis, a roughly equivalent quality than MP3, and a better quality than MPC. However, it must be remembered that these tests are only individual tests and not collective tests.
At low bitrates (less than 64 kb/s), a collective independent test targetting 32kb/s (2004/07) demonstrated that WMA is clearly superior to MP3 (produced by LAME), but not better than modern competitive lossy formats.

Many of these results, however, are difficult to keep up-to-date due to the ever-evolving nature of the codecs.

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, in which a WMA stream may be encapsulated, can. Janus is the codename for a recent version of Windows Media DRM, which is sometimes 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 most portable audio devices since they typically 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.