Windows Media Video Totally Explained

Everything about Wmv totally explained

Windows 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), WMV has gained adoption for physical-delivery formats such as HD DVD and Blu-ray Disc.

History

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 and HD DVD-ROM specifications. 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. 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.

Codecs

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. Continued proprietary development led to newer versions of the codec, but the bit stream syntax wasn't frozen until WMV 9. 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. WMV 9 also introduced a new profile titled Windows Media Video 9 Professional, 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. 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 isn't compatible with previous WMV 9 codecs, however.
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. WMV HD mandates the use of WMV 9 for its certification program, at quality levels specified by Microsoft. WMV is also the only supported video codec for the Microsoft Silverlight platform.

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. Depending on the complexity of the source material, the codec may switch between lossy and lossless encoding to enhance compression efficiency. 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 isn't compatible with the original WMV 9 Image codec. Microsoft also claims that WMV 9 is 15-50% better than WMV 8 in terms of compression efficiency.

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, 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.

Encoders

Software that exports video in WMV format include Avid (PC Version), Windows Movie Maker, Windows Media Encoder, Microsoft Expression Encoder, Sorenson Squeeze, Sony Vegas Pro. and Adobe Premiere Pro. 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.

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. 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.

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 The loss of the ability to restore licenses for WMV files in the Windows Media Player 11 wasn't positively received.

Versions

Public Name	FourCC	Description
Microsoft MPEG-4 Video Codec V1	MPG4	Video for Windows-based codec. Non-standard MPEG-4 codec incompatible with the later standardized version of MPEG-4.
Microsoft MPEG-4 Video Codec V2	MP42	Video for Windows-based codec. Non-standard MPEG-4 codec incompatible with the later standardized version of MPEG-4.
Microsoft MPEG-4 Video Codec V3	MP43	Video for Windows-based codec. Non-standard MPEG-4 codec incompatible with the later standardized version of MPEG-4. This version however can only encode to ASF files, it can't be used to encode AVI files without hacking.
Microsoft ISO MPEG-4 Video Codec V1	MP4S	DirectX Media Objects (DMO)-based codec. MPEG-4 Simple Profile compliant.
Microsoft ISO MPEG-4 Video Codec V1.1	M4S2
Windows Media Video V7	WMV1	DMO-based codec.
Windows Media Screen V7	MSS1	DMO-based codec. Optimized for low-bitrate sequential screen captures or screencasts. Deprecated in favor of Windows Media 9 Screen codec.
Windows Media Video V8	WMV2	DMO-based codec.
Windows Media Video 9	WMV3	DMO-based codec. Video for Windows (VfW/VCM) version also available.
Windows Media Video 9 Screen	MSS2	DMO-based codec. Optimized for low-bitrate sequential screen captures or screencasts.
Windows Media Video 9.1 Image	WMVP	DMO-based codec. Optimized for encoding video from sequential bitmap images. Used, for instance, by Photo Story.
Windows Media Video 9.1 Image V2	WVP2	DMO-based codec. Optimized for encoding video from sequential bitmap images. Used, for instance, by Photo Story.
Windows Media Video 9 Advanced Profile	WMVA	DMO-based codec. Video for Windows (VfW/VCM) version also available. Deprecated as VC-1 non-compliant.
Windows Media Video 9 Advanced Profile	WVC1	DMO-based codec. VC-1 compliant format.

Further Information

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