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Windows Media Audio
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 that 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.〔(【引用サイトリンク】title=Analysis of the Microsoft Audio Codec )〕 ''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 (the regular WMA format is lossy).〔 ''WMA Voice'', targeted at voice content, applies compression using a range of low bit rates.〔
==Development history==
The first WMA codec was based on earlier work by Henrique Malvar and his team which was transferred to the Windows Media team at Microsoft. Malvar was a senior researcher and manager of the Signal Processing Group at Microsoft Research,〔(【引用サイトリンク】title=Press Kit: Henrique Malvar )〕 whose team worked on the ''MSAudio'' project. The first finalized codec was initially referred to as ''MSAudio 4.0''.〔(【引用サイトリンク】title=Microsoft Windows Media Technologies Gains Support for Downloadable Music from Top Music Sites, Independent Labels, Popular Bands And Innovative Developers )〕 It was later officially released as ''Windows Media Audio'',〔(【引用サイトリンク】title=Windows Media Technologies 4 Delivers Cutting-Edge CD-Quality Audio On the Internet )〕 as part of Windows Media Technologies 4.0. Microsoft claimed that WMA could produce files that were half the size of equivalent-quality MP3 files;〔(【引用サイトリンク】title=MS Windows Media Technologies Features )〕 Microsoft also claimed that WMA delivered "near CD-quality" audio at 64 kbit/s.〔 The former claim however was rejected by some audiophiles.〔(【引用サイトリンク】url=http://www.edn.com/index.asp?layout=article&articleid=CA46537 )〕 RealNetworks also challenged Microsoft's claims regarding WMA's superior audio quality compared to RealAudio.〔
Newer versions of WMA became available: ''Windows Media Audio 2'' in 1999, ''Windows Media Audio 7'' in 2000,〔(【引用サイトリンク】title=Microsoft Announces Windows Media Technologies 7 )〕 ''Windows Media Audio 8'' in 2001,〔(【引用サイトリンク】title=Microsoft Releases Windows Media Audio and Video 8 )〕 and ''Windows Media Audio 9'' in 2003.〔 Microsoft first announced its plans to license WMA technology to third parties in 1999.〔(【引用サイトリンク】title=Microsoft Wins Major ISV Support for Windows Media Technologies 4.0 )〕 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 that 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''.〔
All versions of WMA released since version 9.0 - namely 9.1, 9.2 and 10 - have been backwards compatible with the original v9 decoder and are therefore not considered separate codecs. The sole exception to this is the WMA 10 Professional codec whose Low Bit Rate (LBR) mode is only backwards compatible with the older WMA Professional decoders at half sampling rate (similar to how HE-AAC is backwards compatible with AAC-LC). Full fidelity decoding of WMA 10 Professional LBR bitstreams requires a WMA version 10 or newer decoder.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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