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. It is part of the Windows Media framework. An initial reason for the development of WMA might have been that MP3 technology is patented and has to be licensed from Thomson for inclusion in the Microsoft Windows operating system.
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. 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 have 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, DES block cipher, a custom block cipher, RC4 stream cipher and the SHA-1 hashing function.
The most current version of the format is Windows Media Audio 9.1 which 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.