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, though in terms of popularity of MP3 , this never came close to occurring. 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.
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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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Design
9 R1 S+ g* u3 a( f/ DWMA is capable of VBR, CBR and lossless audio encoding in order to be marketed as a versatile format.
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1 z1 T1 ~  ]$ t' iWindows Media Audio 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.
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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.

Codecs and versions
$ C  H5 i! w  ?WMA began being widely distributed starting with the WMA 7 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 10 Pro codec is based on a completely different technology which is not only superior to ordinary WMA in terms of quality and features, but also scales really 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.
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Windows Media Player 11 is the latest version of Microsoft's media player. It adds low bitrate support for WMA Pro, support for ripping music to WMA Pro and updates the original WMA to version 9.2.

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1 U/ X: ^! K# v% d% u+ mApart 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 format 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[1]. 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.

Sound quality
; b" t; j5 t8 q: G! z6 HInitially 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 (2004/05) with WMA standard encoder provided by the Windows Media 9, conducted at 128 kb/s, showed that WMA was roughly equivalent to MP3 encoded with LAME encoder, inferior to AAC and Vorbis, and superior to ATRAC3 (software version). 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.
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  j0 |5 p# A9 ~  l0 ]" j" {Some conclusions made by recent listening tests:

) ?1 H; |2 h) r& k, [1 [2 m% B- x. EAt 128kb/s the most recent large scale test (2006/01) shows a four way tie between aoTuV Vorbis, LAME-encoded MP3, WMA 9 Pro and iTunes (QuickTime) AAC, with each codec essentially transparent (sounds identical to the original music file). 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 WMA Standard.
7 q  X1 F# ~( l8 ]6 W- @7 O  EAt mid-low bitrates (64 kb/s or more, less than 128 kb/s), latest private tests (80 kb/s (2005/07), 96 kb/s (2005/08)) show that WMA has a lower quality than the lossy audio codecs AAC (HE and LC) and Vorbis, a roughly equivalent quality than MP3, and a better quality than MPC. However, it must be remembered that these tests are only individual tests and not collective tests.
( h$ S- e* }& vAt low bitrates (less than 64 kb/s), a collective independent test targetting 32kb/s (2004/07) demonstrated that WMA is clearly superior to MP3 (produced by LAME), but not better than modern competitive lossy formats.
. K7 ]1 c# \# t' ?# C4 e5 KRecent 48 kbps independent listening test organized by Sebastian Mares and supported by Hydrogenaudio Forums included latest version of WMA 10 Pro, included in Vista OS. 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). 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.
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5 j2 s8 t4 @* d/ c! c: H- hDigital rights management
4 @" P# h) z7 t. C2 P) tWhile 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.

CD tracks ripped to WMA with Windows Media Player can be optionally "protected" (DRM-restricted) so it can only be used on a specific system. (This should not be enabled unless there is a specific reason to do so, such as for protecting content stored on a public use PC.) Versions of Windows Media Player older than version 9 had this "protection" on by default, although this setting could be easily disabled in the UI.

% q  ?5 u! X% @' xMicrosoft Zune introduced a variation of PlaysForSure DRM specific for use with the Zune Ecosystem. The Zune Software and Portable Player will not play DRM content outside of the Zune Marketplace. However, Zune Marketplace content should work with most WMA "PlaysForSure" DRM capable software and portables. This, however, is not officially supported by Microsoft.
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: D6 {7 A  a, U  Y0 n) ]( Y- q[ 本帖最後由 masonchung 於 2007-1-20 02:43 PM 編輯 ]