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