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Advanced Streaming Format (or ASF, later renamed into Advanced Systems Format) is Microsoft's proprietary digital audio/digital video wrapper, especially meant for streaming. ASF is part of the Windows Media framework.
The format does not specify how the video or audio should be encoded, but instead just specifies the structure of the video/audio stream. What this means is that ASF files can be encoded with basically any audio/video codec and still would be in ASF format. This is similar to the function performed by the QuickTime format or the Ogg format.
The ASF format is based on serialized objects which are essentially byte-sequences identified by a GUID marker.
The most common contents of an ASF file is:
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.
ASF files containing only WMA audio can be named with the file suffix .wma and files of only audio and video content may have the suffix .wmv. Both may use .asf as a suffix if desired.
ASF is often confused with Microsoft's own implementation of MPEG-4 video format (Windows Media Video), because most of the ASF streams are encoded using this technology.
The ASF structure (the container, not some codec algorithm) is patented in the United States (United States Patent 6,041,345 Levi, et al. March 21, 2000) by Microsoft and Microsoft has invoked its patent against VirtualDub, a free video conversion program, which wasn't even marketed. The reason why they invoked the patent was the control over the content which they have without other programs being able to read ASF. Interestingly, Apple's iTunes software now has the capability to convert WMA files to AAC [1] (http://www.apple.com/itunes/import.html), which appears to have been overlooked by Microsoft.
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