wmv format
DESCRIPTION
A detailed introduction of WMV formatTRANSCRIPT
A detailed introduction for WMV format
1. Simple introduction
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 (迎合,顾及) 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
2. Container formats
A WMV file is in most circumstances(情况下) encapsulated (封装,压缩)
in the Advanced Systems Format (ASF) container format. 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 (抨击,反对)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.
3. Codecs
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 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 was not 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 (插入,窜改). 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(1280×720) and 1080p(1920×1080).
The Simple and Main profile levels in WMV 9 are compliant with(顺从,服从) the same profile levels in the VC-1 specification. The Advanced Profile in VC-1 is
implemented (应用的)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. WMV is a mandatory(命令的,强制性的) 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.
4. 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; Microsoft also claims that
WMV 9 is 15-50% better than WMV 8 in terms of compression efficiency. One test
report published in January 2005, however, showed that WMV 9 had worse
compression efficiency than WMV 8
5. 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