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Windows Media Audio

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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, but has not yet received such popularity. However, with the introduction of WMA Pro and 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.

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[edit] Design

WMA is capable of Variable Bit Rate (VBR), Constant Bit Rate (CBR) and lossless audio encoding. Compression is based on the Modified discrete cosine transform. It 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.

[edit] Codecs and versions

The first version of the codec was Windows Media Audio 7, to synchronize the version number with Windows Media Player, (also known as WMA1 because of the FourCC). However, it began being widely used, especially in video, starting with the Windows Media 8 (WMA2) lossy compression based codec and has now reached version 9.2 (Windows Media Player 11). Microsoft's WMA bundle also includes three more codecs, a Windows Media Audio 9 Voice codec, Windows Media Audio 9.2 Lossless codec and Windows Media Audio 10 Professional codec (earlier known as WMA 9 Pro). The WMA 9 voice codec is optimized for low bitrate speech encoding. The WMA Lossless codec provides mathematically lossless compression. It compresses uncompressed PCM wave audio to a range of 206 to 411 MB, at bit rates of 470 to 940 kbit/s. Mathematically equivalent to the original audio track, WMA Lossless provides the highest quality WMA format for ripping audio CDs. It also supports streaming, multi-channel high-definition lossless audio. The WMA 10 Pro codec is based on a completely different compression algorithm which is not only superior to ordinary WMA in terms of quality, efficiency and features, but also scales quite well at low bitrates. However, the WMA Pro standard is often confused with the original WMA and thus is less popular. Also, the files are incompatible with older players and WMA Pro is yet to gain wide playback support in devices. WMA 10 Pro supports 96 kHz 24-bit audio as well as 5.1/7.1 discrete multi-channel audio.

Windows Media Player 11 is the latest version of Microsoft's media player. It includes the Windows Media Format 11 runtime which adds low bitrate support for WMA 10 Pro (as low as 48 kbit/s, 44 kHz CBR stereo, VBR stereo and 128 kbit/s, 44/48 kHz 16/24-bit CBR 5.1/7.1 multichannel audio) <ref> A "window" to the world of digital media: Interview with Amir Majidimehr, corporate vice president of Windows Media division </ref>, support for ripping music to WMA Pro and updates the original WMA codec to version 9.2.

[edit] 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 codecs (with the exception of WMA Pro) 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.<ref>Real to plug Windows media support into Linux (Stephen Shankland, CNET News.com, 17 August 2006]</ref> Microsoft's new Zune software, along with the new Zune portable player, supports WMA as well as a Zune Ecosystem specific variation of WMA DRM. In November of 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.

The Toshiba Gigabeat S portable media player supports most WMA formats.

[edit] 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 (May 2004) with WMA standard encoder provided by the Windows Media 9, conducted at 128 kbit/s, showed that WMA was roughly equivalent to MP3 encoded with LAME encoder, inferior to AAC and Vorbis, and superior to ATRAC3 (software version) (ATRAC3plus was not evaluated in this test). WMA 10 Pro, however, starting with Windows Media Player 11, is meant to compete against the popular AAC and low bitrate aacPlus, and is clearly superior to ordinary WMA.

Some conclusions made by recent listening tests:

  • At 128 kbit/s, the most recent large scale test (January 2006) shows a four-way tie between aoTuV Vorbis, LAME-encoded MP3, WMA 9 Pro and AAC (used by iTunes and QuickTime), with each codec sounding close to the uncompressed music file for most people. 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 original WMA Standard.
  • At mid-low bitrates (64 kbit/s or more, less than 128 kbit/s), latest private tests (80 kbit/s (July 2005), 96 kbit/s (August 2005) show that WMA has a lower quality than the lossy audio codecs AAC (HE and LC) and Vorbis, roughly equivalent quality to MP3, and better quality than MPC. However, it must be remembered that these tests are only individual tests and not collective tests, and also that WMA Pro is not tested.
  • At low bitrates (less than 64 kbit/s), a collective kbit/s/results.html independent test targetting 32 kbit/s (July 2004) demonstrated that WMA is clearly superior to MP3 (produced by LAME), but not better than modern competitive lossy formats.
  • Recent (December 2006) 48 kbit/s independent listening test organized by Sebastian Mares and supported by Hydrogenaudio Forums included latest version of WMA 10 Pro, included in Windows Vista. WMA 10 Pro has been proved to be statistically better than WMA 9.2, but it was ranked second in the test, being defeated (with statistical significance) by MPEG-4 HE-AAC implementation from Nero AG (Nero Digital Audio). This test however used CBR for WMA 10 Pro and VBR for the other audio codecs. Results are here: http://www.hydrogenaudio.org/forums/index.php?showtopic=50888

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

Though it is intended to compete with AAC, HE-AAC and other more popular codecs, WMA Pro has received very little support from hardware makers and cannot be played on players from key PlaysForSure partners such as Creative Technology.

[edit] 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 PlaysForSure DRM technology supports time-limited subscription music such as those offered by unlimited download services, such as MTV's URGE, Napster, Rhapsody, Yahoo! Music Unlimited and Virgin Digital. Janus PlaysForSure DRM is supported on many modern portable audio devices and some streaming media clients such as Roku SoundBridge and Xbox 360.


[edit] External links

This article was started using a Wikipedia Media Audio article
Retrieved from "http://en.audiolexic.org/wiki/Windows_Media_Audio"
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