Windows Media Video (WMV) is a video compression format for several proprietary codecs developed by Microsoft. The original video format, known as WMV, was originally designed for Internet streaming applications, as a competitor to RealVideo. The other formats, 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 9 has gained adoption for physical-delivery formats such as HD DVD and Blu-ray Disc.[3][4]
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 format an open standard. Since then, VC-1 has become one of the three mandatory video formats for the Blu-ray format, along with H.264 and MPEG-2.[3][4]
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.
The ASF 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.
Although WMV is generally packed into the ASF container format, it can also be put into the Matroska[6] or AVI container format. The resulting files have the .MKV and .AVI file extensions, respectively. One common way to store WMV in an AVI file is to use the WMV 9 Video Compression Manager (VCM) codec implementation.[7][8]
Video compression formats
Windows Media Video
Diagram illustrating the relative frame sizes of several common video resolutions targeted by Windows Media Video 9 Professional, starting with 480p.
Windows Media Video (WMV) is the most recognized video format within the WMV family. Usage of the term WMV often refers to the Microsoft Windows Media Video codec only. Its main competitors are MPEG-4 AVC, AVS, RealVideo, and MPEG-4 ASP. The first version of the codec, WMV 7, was introduced in 1999, and was built upon Microsoft's implementation of MPEG-4 Part 2.[9] Continued proprietary development led to newer versions of the codec, but the bit stream syntax was not frozen until WMV 9.[10] 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.[11] WMV 9 also introduced a new profile titled Windows Media Video 9 Professional,[12] which is activated automatically whenever the video resolution exceeds 300,000 pixels (e.g., 528x576, 640×480 or 768x432 and beyond) and the bitrate 1000 kbit/s[citation needed]. 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.[13] 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.[14]
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.[15] WMV HD mandates the use of WMV 9 for its certification program, at quality levels specified by Microsoft.[16] WMV used to be the only supported video codec for the Microsoft Silverlight platform, but H.264 codec is now also supported starting with version 3.[17]
Windows Media Video Screen
Windows Media Video Screen (WMV Screen) are video formats that specilise in screencast content. They can capture live screen content, or convert video from third-party screen-capture programs into WMV 9 Screen files. They work best when the source material is mainly static and contains a small color palette.[18]
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 version, WMV 9 Screen, supports VBR encoding in addition to CBR.[18] Additionally there is MSA1 (aka “MS ATC Screen codec” or “MSS3”) which is used in Live Meeting 2007. FourCCs for the formats are MSS1, MSS2 and MSA1.[19]
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.[20] 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 (e.g. 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.[20]
Hardware support for WMV Image is available from Portable Media Centers, Windows Mobile-powered devices with Windows Media Player 10 Mobile.[15]
Since no known domestic DVD player supports this format, users of Photo Story 3 wishing to generate material capable of being played in a DVD player will first have to convert to MPEG-2 before burning a DVD (average file sizes in MPEG-2 are 5 to 6 times the .wmv file).
Video quality
While Windows Media Video 7 and 8 codecs were comparable to MPEG-4 ASP in terms of visual quality, Windows Media Video 9 (especially the later SMPTE-compliant VC-1 implementations) is comparable to H.264 Main and High Profile. While WMV9/VC-1 does have certain advantages over H.264 such as support for non-square integer transforms, H.264 is more consistently able to yield better picture quality because it supports multiple reference frames, reference B-frames, context-adaptive binary arithmetic coding (CABAC) and stronger in-loop filtering. H.264's quality advantage is typically more evident at low bitrates.
Players
Software that can play WMV files include Windows Media Player, RealPlayer, MPlayer, The KMPlayer, Media Player Classic and VLC Media Player. 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,[21] 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, free of charge to view files but chargeable to convert formats.[22] The WMV installer is bundled with Microsoft Silverlight by default, installation without Silverlight can be accomplished with a "Custom" install. According to the Flip4Mac website, WMV files with DRM encryption are not compatible with the component.
Decoders / Transcoders
Linux users can rely on FFmpeg based software like MPlayer, MEncoder.
For Mac or Windows users who wish to convert non-DRM WMV to MP4, HandBrake is Free Software that will do the job.
Encoders
Software that exports video in WMV format include CyberLink PowerDirector, Avid (PC Version), Windows Movie Maker, Windows Media Encoder, Microsoft Expression Encoder, Sorenson Squeeze,[23] Sony Vegas Pro,[24] Adobe Premiere Pro, Adobe After Effects, Telestream Episode, Total video converter and Telestream FlipFactory.[23][25]
Programs that encode using the WMV Image codec include Windows Media Encoder and Photo Story.
Digital rights management
Main article: Windows Media DRM
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.[26] 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.[27] The loss of the ability to restore licenses for WMV files in the Windows Media Player 11 was not positively received.[27] In addition, the Microsoft Zune does not support the standard Windows Media DRM system, rendering protected WMV files unplayable.[28]
Versions
| Public Name | FourCC | Description |
|---|
| Microsoft MPEG-4 version 1 | MPG4 | Video for Windows-based codec. Non-standard MPEG-4 codec incompatible with the later standardized version of MPEG-4 Part 2. |
| Microsoft MPEG-4 version 2 | MP42 | VfW-based codec. Non-compliant with finalized MPEG-4 part 2 standard. |
| Microsoft MPEG-4 version 3 | MP43 | VfW-based codec. Non-compliant with finalized MPEG-4 part 2 standard. Eventually locked for encoding only with ASF files (build 3688 and earlier could also encode to AVI).[29] |
| Microsoft ISO MPEG-4 version 1 | MP4S | DirectX Media Objects (DMO)-based codec. MPEG-4 Simple Profile compliant. |
| Microsoft ISO MPEG-4 version 1.1 | M4S2 | MPEG-4 Advanced Simple Profile compliant.[30] |
| Windows Media Video 7 | WMV1 | DMO-based codec. |
| Windows Media Screen 7 | 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 8 | WMV2 | DMO-based codec. |
| Windows Media Video 9 | WMV3 | DMO-based codec. Video for Windows (VfW/VCM) version also available. [2] |
| 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. Deprecated as non-VC-1-compliant. |
| Windows Media Video 9 Advanced Profile | WVC1 | DMO-based codec. VC-1 compliant format. |
See also
- Codec – The technical term for compressor and decompressor
- VC-1 – The SMPTE codec standard based on WMV 9 that supports additional broadcast industry requirements
- WMV HD – The marketing name for high definition videos encoded using WMV 9 codecs
- Windows Media DRM – A digital rights management component of Windows Media that controls how content can be used
- Windows Media Audio – An audio file format and codec developed by Microsoft
- Photo Story - Microsoft slide-show movie maker generating wmv image sub-set
- JPEG XR / HD Photo – An image file format and codec developed by Microsoft
- Windows Movie Maker – A video editing tool included with the Microsoft Windows operating system that allows conversion of WMV format files into .avi (DV/AVI)
- MPlayer – A third-party, open source, cross-platform media player capable of playing many WMV files using FFmpeg
- FFmpeg – A third-party cross-platform free software codec library which partially implements WMV decoding and VC-1 decoding among other formats
- WMV Player – A third-party, commercial codec which allows viewing of WMV files in QuickTime for Mac OS X
- Flip4Mac – QuickTime component to play and encode Windows Media files in QuickTime
- Lossy data compression – Data compression with loss of information
- Lossless data compression – Data compression without loss of information
- Perbandingan -- video codecs
References
External links
