If you do Windows Media Video 9 or Windows Media Audio 9 encoding, you **need** the Windows Media Format 9.5 SDK.
Although it isn’t documented anywhere that I know of, the SDK includes these very important updates:
* The new [Windows Media Video 9 Advanced Profile](http://www.microsoft.com/windows/windowsmedia/9series/codecs/video.aspx#WindowsMediaVideo9AdvancedProfile “Learn more about the WMV9 AP codec”) video codec (which implements the Advanced mode of the proposed [VC-1](http://www.microsoft.com/windows/windowsmedia/forpros/events/NAB2005/VC-1.aspx “Learn more about VC-1”) standard)
* Performance and quality improvements for the standard Windows Media Video 9 codec
* Performance improvements for Windows Media Audio 9
You can get them at the link below.
* [Windows Media Encoder 9](http://www.microsoft.com/windows/windowsmedia/9series/encoder/ “Get Windows Media Encoder 9”)
* [Windows Media Format 9.5 SDK](http://download.microsoft.com/download/9/f/d/9fdfb288-b4bf-45fa-959c-1cc6d909aa92/wmformat95sdk.exe “Get the Windows Media Format 9.5 SDK”)