Windows Media Video

From Free net encyclopedia

Windows Media Video (WMV) is a generic name for the set of video codec technologies developed by Microsoft. It is part of the Windows Media framework. The codecs were originally developed as proprietary codecs for low-bitrate streaming applications. However, in 2003 Microsoft drafted a video codec specification based on its Windows Media Video version 9 codec and submitted it to SMPTE for standardization. The standard was officially approved in March 2006 as SMPTE 421M, thus making the Windows Media Video 9 codec no longer a proprietary technology. Earlier versions of the codec (7 and 8) are still considered proprietary as they fall outside the SMPTE 421M standard.

WMV is not built solely on Microsoft in-house technology. It is believed that WMV version 7 (WMV1) was built upon Microsoft's own non-standard version of MPEG-4 Part 2. However, as WMV version 9 has been standardized as an independent SMPTE standard (421M, also known as VC-1), it's reasonable to believe that WMV has sufficiently evolved in a different direction than MPEG-4 to be considered a unique codec in its own right. There are currently (April 2006) 16 companies in the VC-1 patent pool. Microsoft is also one of the members of the MPEG-4 AVC/H.264 patent pool.

The video stream is often combined with an audio stream of Windows Media Audio and encapsulated in Advanced Systems Format files, carrying the .wmv or .asf file extensions.

WMV files are played by players such as MPlayer or Windows Media Player, the latter being only available for Microsoft Windows and Macintosh systems. Many third-party players exist for various platforms such as Linux that use the FFmpeg implementation of the WMV codecs.

WMV is generally packed into an Advanced Systems Format (ASF) container. It can also be put into AVI or Matroska container formats. The resulting files may be named .avi if it is an AVI-contained file, or .wmv or .asf if it is an ASF file, or .mkv if it is an MKV file. WMV can be stored in an AVI file when encoding with the VirtualDub encoder and using the WMV9 VCM codec implementation. Microsoft's Windows Media Player for the Mac does not support all WMV encoded files since it supports only the ASF file container. More files can be played with Flip4Mac and Quicktime or MPlayer for MacOSX.

When encapsulated in ASF file format, WMV can support digital rights management facilities intended to protect intellectual property rights.

Besides being one of the most popular codecs for distributing video on the Internet, the codec is also used to distribute high definition video on standard DVDs in a format Microsoft has branded as WMV HD. This WMV HD content can be played back on computers or compatible DVD players.

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Codecs

Windows Media Video v7 (FourCC: WMV1)
Windows Media Video v7 Screen (FourCC: MSS1) - Optimized for low-bitrate screen captures
Windows Media Video v8 (FourCC: WMV2)
Windows Media Video 9 Screen (FourCC: MSS2) - Optimized for low-bitrate screen captures
Windows Media Video 9 Image (FourCC: WMVP) - Used in Microsoft Photo Story 1 and 2 products
Windows Media Video 9 Image v2 (FourCC: WVP2) - Used in Microsoft Photo Story 3 and newer products
Windows Media Video 9 (FourCC: WMV3) - Compliant with VC-1 Simple and Main Profile specifications
Windows Media Video 9 Advanced Profile (FourCC: WMVA) - Early AP implementation. VC-1 non-compliant. Deprecated.
Windows Media Video 9 Advanced Profile (FourCC: WVC1) - Compliant with VC-1 Advanced Profile specification.

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