Windows Media Video
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.wmvvideo/x-ms-wmvWindows Media Video (WMV) is a compressed video file format for several proprietary codecs developed by Microsoft. The original codec, known as WMV, was originally designed for Internet streaming applications, as a competitor to RealVideo. The other codecs, such as WMV Screen and WMV Image, cater for specialized content. Through standardization from the Society of Motion Picture and Television Engineers (SMPTE),[1][2] WMV has gained adoption for physical-delivery formats such as HD DVD and Blu-ray Disc[3][4].
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[edit] History
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In 2003, Microsoft drafted a video codec specification based on its WMV 9 codec and submitted it to SMPTE for standardization. The standard was officially approved in March 2006 as SMPTE 421M, better known as VC-1, thus making the WMV 9 codec an open but still proprietary standard. Since then, VC-1 has become one of the three mandatory video codecs for the BD-ROM specifications.[3][4]
[edit] Container format
A WMV file is in most circumstances encapsulated in the Advanced Systems Format (ASF) container format.[5] The file extension .WMV typically describes ASF files that use Windows Media Video codecs. The audio codec used in conjunction with Windows Media Video is typically some version of Windows Media Audio, or in rarer cases, the deprecated Sipro ACELP.net audio codec. Microsoft recommends that ASF files containing non-Windows Media codecs use the generic .ASF file extension.
Although WMV is generally packed into the ASF container format, it can also be put into the AVI or Matroska container format. The resulting files claim the .AVI, or .MKV file extensions, respectively. WMV can be stored in an AVI file when using the WMV 9 Video Compression Manager (VCM) codec implementation.[6] Another common way to store WMV in an AVI file is to use the VirtualDub encoder.
This container 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.
[edit] Codecs
[edit] Windows Media Video
Windows Media Video (WMV) is the most recognized codec within the WMV family. Usage of the term WMV often refers to this codec only. Its main competitors are MPEG-4 AVC, RealVideo, DivX, and Xvid. The first version of the codec, WMV 7, was introduced in 1999, and was built upon Microsoft's implementation of MPEG-4 Part 2.[7] Continued proprietary development led to newer versions of the codec, but the bit stream syntax was not frozen until WMV 9.[8] While all versions of WMV support variable bit rate, average bit rate, and constant bit rate, WMV 9 introduced several important features including native support for interlaced video, non-square pixels, and frame interpolation.[9] WMV 9 also introduced a new profile titled Windows Media Video 9 Professional,[10] which is activated automatically whenever the video resolution and bit rate exceed 300,000 pixels (eg., 512 × 586) and 1000 kbit/s (eg., 1001 kbit/s). It is targeted towards high-definition video content, at resolutions such as 720p and 1080p.
The Simple and Main profile levels in WMV 9 are compliant with the same profile levels in the VC-1 specification.[11] The Advanced Profile in VC-1 is implemented in a new WMV codec called Windows Media Video 9 Advanced Profile. It improves compressions efficiency for interlaced content and is made transport-independent, making it able to be encapsulated in an MPEG transport stream or RTP packet format. The codec is not compatible with previous WMV 9 codecs, however.[12]
WMV is a mandatory video codec for PlaysForSure-certified online stores and devices, as well as Portable Media Center devices. The Microsoft Zune, Xbox 360, Windows Mobile-powered devices with Windows Media Player, as well as many uncertified devices, support the codec.[13]WMV HD mandates the use of WMV 9 for its certification program, at quality levels specified by Microsoft.[14] WMV is also the only supported video codec for the Microsoft Silverlight platform.[15]
[edit] Windows Media Video Screen
Windows Media Video Screen (WMV Screen) is a screencast codec. It can capture live screen content, or convert video from third-party screen-capture programs into WMV 9 Screen files. It works best when the source material is mainly static and contains a small color palette.[16] Depending on the complexity of the source material, the codec may switch between lossy and lossless encoding to enhance compression efficiency.[16]
One of the uses for the codec is computer step-by-step demonstration videos. The first version of the codec was WMV 7 Screen, The second and current version, WMV 9 Screen, supports VBR encoding in addition to CBR.[16]
[edit] Windows Media Video Image
Windows Media Video Image (WMV Image) is a video slideshow codec. The codec works by applying timing, panning and transition effects to a series of images during playback.[17] The codec achieves a higher compression ratio and image quality than WMV 9 for still images as files encoded with WMV Image store static images rather than full-motion video.
Since the codec relies on the decoder (player) to generate video frames in real-time, playing WMV Image files even at moderate resolutions (eg., 30 frames per second at 1024 × 768 resolution) requires heavy computer processing. The latest version of the codec, WMV 9.1 Image, used by Photo Story 3, features additional transformation effects, but is not compatible with the original WMV 9 Image codec.[17]
Hardware support for WMV Image is available from Portable Media Centers, Windows Mobile-powered devices with Windows Media Player 10 Mobile.[13]
[edit] Video quality
Microsoft claims that WMV 9 provides a compression ratio that is two times better than MPEG-4 , and three times better than MPEG-2;[18] Microsoft also claims that WMV 9 is 15-50% better than WMV 8 in terms of compression efficiency.[18] One test report published in January 2005, however, showed that WMV 9 had worse compression efficiency than WMV 8.[19] Many 3rd party WMV compilers have had worse performance than Windows Movie Maker.
[edit] Players
Software that can play WMV files include Windows Media Player, The KMPlayer, PowerDVD, RealPlayer, MPlayer, VLC Media Player, Zoom Player and Media Player Classic. The Microsoft Zune media management software supports the WMV codec, but uses a Zune-specific variation of Windows Media DRM which is used by PlaysForSure. Many third-party players exist for various platforms such as Linux that use the FFmpeg implementation of the WMV codecs.
On the Macintosh platform, Microsoft released a PowerPC version of Windows Media Player for Mac OS X in 2003,[20] but further development of the software has ceased. Microsoft currently endorses the 3rd party Flip4Mac WMV, a QuickTime Component which allows Macintosh users to play WMV files in any player that uses the QuickTime framework.[21]
[edit] Encoders
Software that exports video in WMV format include Avid (PC Version), Windows Movie Maker, Windows Media Encoder, Microsoft Expression Encoder, Sorenson Squeeze[22], Sony Vegas Pro[23]. and Adobe Premiere Pro[22]. Programs that encode using the WMV Image codec include Windows Media Encoder and Photo Story. For Mac users who wish to convert WMV to MP4, iSquint is freeware that will do the job.
[edit] Digital rights management
While none of the WMV codecs themselves contain any digital rights management facilities, the ASF container format, in which a WMV stream may be encapsulated, can. Windows Media DRM, which can be used in conjunction with WMV, supports time-limited subscription video services such as those offered by CinemaNow[24]. Windows Media DRM, a component of PlaysForSure and Windows Media Connect, is supported on many modern portable video devices and streaming media clients such as the Xbox 360.
[edit] Criticism
WMV has been the subject of numerous complaints from users and the press. Users dislike the digital rights management system which is sometimes attached to WMV files[25] The loss of the ability to restore licenses for WMV files in the Windows Media Player 11 was not positively received.[25] In addition, the Microsoft Zune does not support the standard Windows Media DRM system, rendering protected WMV files unplayable.[26]
