Windows Media Audio (WMA) is an audio data compression technology developed by Microsoft. The name can be used to refer to its audio file format or its audio codecs. It is a proprietary technology which forms part of the Windows Media framework. WMA consists of four distinct codecs. The original WMA codec, known simply as WMA, was conceived as a competitor to the popular MP3 and RealAudio codecs.Today it is one of the most popular codecs, together with MP3 and MPEG-4 AAC. In 2003 it came second after MP3 in terms of standalone players supporting it. WMA Pro, a newer and more advanced codec, supports multichannel and high resolution audio. A lossless codec, WMA Lossless, compresses audio data without loss of audio fidelity.And WMA Voice, targeted at voice content, applies compression using a range of low bit rates.

The first WMA codec was based on the previous work from Henrique Malvar and his team.According to the published article, the technology was transferred over to the Windows Media team at Microsoft. Malvar was a senior researcher and manager of the Signal Processing Group at Microsoft Research, whose team worked on the project called MSAudio.The first finalized codec was at first referred as MSAudio 4.0. It was later officially released as Windows Media Audio,as part of Windows Media Technologies 4.0. Microsoft initially claimed that WMA delivers the same quality of MP3 at half the bit rate; Microsoft also claimed that WMA delivers "CD-quality" audio at 64 kbit/s.The former claim however was rejected by some audiophiles according to EDN. RealNetworks also challenged Microsoft's claims regarding WMA's superior audio quality compared to RealAudio.

Development history
Newer versions of WMA became available: Windows Media Audio 2 in 1999,Windows Media Audio 7 in 2000,Windows Media Audio 8 in 2001,and Windows Media Audio 9 in 2003. Microsoft first announced its plans to license WMA technology to third-parties in 1999. Although earlier versions of Windows Media Player played WMA files, support for WMA file creation was not added until the seventh version. In 2003, Microsoft released new audio codecs which were not compatible with the original WMA codec. These codecs were Windows Media Audio 9 Professional,Windows Media Audio 9 Lossless, and Windows Media Audio 9 Voice.

Container format

A WMA file is in most circumstances encapsulated, or contained, in the Advanced Systems Format (ASF) container format, featuring a single audio track in one of following codecs: WMA, WMA Pro, WMA Lossless, or WMA Voice. These codecs are technically distinct and mutually incompatible. The ASF container format specifies how metadata about the file is to be encoded, similar to the ID3 tags used by MP3 files. Metadata may include song name, track number, artist name, and also audio normalization values.This container can optionally support digital rights management (DRM) 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.