Advanced Systems Format

Advanced Systems Format (formerly Advanced Streaming Format, Active Streaming Format) is Microsoft's proprietary digital audio/digital video container format, especially meant for streaming media. ASF is part of the Windows Media framework.

Overview and features

ASF is based on serialized objects which are essentially byte sequences identified by a GUID marker.

The format does not specify how (i.e. with which codec) the video or audio should be encoded; it just specifies the structure of the video/audio stream. This is similar to the function performed by the QuickTime, AVI, or Ogg container formats. One of the objectives of ASF was to support playback from digital media servers, HTTP servers, and local storage devices such as hard disk drives.

The most common filetypes contained within an ASF file are Windows Media Audio (WMA) and Windows Media Video (WMV). Note that the file extension abbreviations are different from the codecs which have the same name. Files containing only WMA audio can be named using a .WMA extension, and files of audio and video content may have the extension .WMV. Both may use the .ASF extension if desired.

ASF files can also contain objects representing metadata, such as the artist, title, album and genre for an audio track, or the director of a video track, much like the ID3 tags of MP3 files. It supports scalable media types and stream prioritization; as such, it is a format optimized for streaming.

The ASF container provides the framework for digital rights management in Windows Media Audio and Windows Media Video. An analysis of an older scheme used in WMA reveals that it is 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.

ASF container-based media is usually streamed on the internet either through the MMS protocol or the RTSP protocol.

Codecs

Although the ASF container format can technically include any codec, Microsoft's encoding tools (including Windows Media Encoder and Windows Movie Maker) produce ASF/WMA/WMV files using the DirectX Media Objects (DMO) framework. So far, third-party DMO-based codecs remain extremely rare.

License

The specification is downloadable from the Microsoft website ^[2] and the format can be implemented under a license from Microsoft that however does not allow distribution of sources and is not compatible with open source licenses. The author of the free software project VirtualDub reported that a Microsoft employee informed him that his software violated a Microsoft patent regarding ASF playback.^[3]

Certain error-correcting techniques related to ASF are patented in the United States (United States Patent 6,041,345 Levi, et al. March 21, 2000) by Microsoft.

See also

• Advanced Stream Redirector (ASX)
• Comparison of container formats

References

External links

• ASF overview
• MSDN Windows Media Formats
• Library of Congress analysis of ASF format sustainability
• ASF Container Format - v2.0 (free available but unused) and v1.0 (reconstructed)
• MSDN How To Embed Windows Media Player in a HTML Web Page (For Webmasters)
• Creating A Windows Media Custom Experience (For Webmasters)