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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 Media Foundation framework.

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.

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.

The most common media contained within an ASF file are Windows Media Audio (WMA) and Windows Media Video (WMV). The most common file extensions for ASF files are extension .WMA (audio-only files using Windows Media Audio) and .WMV (files containing video, using the Windows Media Audio and Video codecs).

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