Windows Media Audio

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

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

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) 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.

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

The most current version of the format is Windows Media Audio 9.1 which includes specific codecs for lossless data compression, Surround sound and voice encoding in addition to the main lossy data compression codec. Both constant bit rate and variable bit rate encoding are supported.

=See also=

*Windows Media Video - the video counterpart *List of audio formats WMA Janus This is the most recent DRM format introduced by Microsoft. The main difference between this and regular WMA is the DRM technology. WMA Janus uses a new DRM technology that supports time limited music such as that offered by unlimited download services. Just be warned that Janus DRM is incompatible with portable audio devices that only support up to WMA DRM Version 9. WMA Janus is also known as WMA Version 10.

Just note that there are two variations of unlimited download services that use WMA DRM, usually known as Unlimited and To Go , such as Napster To Go . Both subscription types allow the user to download an unlimited number of tracks for a flat monthly rate, however the To Go subscription service allows users to transfer music to a Janus compatible player, where as the cheaper Unlimited subscription service limits downloaded music to the PC the music was downloaded on. Neither service allows the burning of music to CD and the music expires once the user stops paying for their subscription. For each subscription renewal of the To Go service, the customer must synchronise their Janus compatible player in order to update the update the player with the new subscription expiration date.

Finally, while DRM is seen as the main reason to stay away from legal music services by many consumers, in fact it is not as bad as some may think apart from the DRM used with unlimited download and To Go services. The main drawback comes in is when consumers want to take their music on the move, where the consumer must ensure the player they purchase matches their stores DRM system or vice versa.

At this time of writing, a very simple way to unlock purchased music is to burn it to CD and rip it back off in the preferred format, however this does not work for music downloaded as part of an unlimited download subscription. This method is often the only way to play music on an MP3-only player or other player that does not support the DRM system the shops uses. For example an iPod will not play music purchased from Napster, however if one burns their music to CD and rips it back off into MP3, these will play on an iPod, although this workaround may not work for long if Napster updates its copy-protection system.

=External links=

*[http://www.microsoft.com/windowsmedia/ Windows Media Homepage at Microsoft] *[http://www.rjamorim.com/test/ Some listening tests featuring Windows Media Audio at several bitrates] *[http://wiki.hydrogenaudio.org/index.phptitle=WMA WMA page in Hydrogenaudio Wiki] *[http://www.spinnaker.com/crypt/drm/freeme/Technical Description of the algorithm used for Windows Media encryption] *[http://www.EZDRM.com EZDRM - Microsoft Certified Digital Rights Management (DRM) Provider] *[http://www1.mplayerhq.hu/cgi-bin/cvsweb.cgi/ffmpeg/libavcodec/wmadec.crev=1.25&content-type=text/x-cvsweb-markup&cvsroot=FFMpeg WMA Decoder Source Code at FFMpeg s CVS]