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ASF Format Definition: What is the ASF File Format?



Background to the ASF format

Microsoft first developed the ASF format from 1995 to 1998. It was originally named the Advanced Streaming Format, but this has now been renamed to the Advanced Systems Format. It is a proprietary format (part of Microsoft's Windows Media framework)that has been specifically developed for streaming audio and video content via networks (and the Internet of course). It is a container format similar to others such as OGG, MP4 (MPEG-4 Part 14), and RM. This means that no specific codec has to be used and thus makes it a flexible system for compressing media and streaming it over the Internet.

What is the advantage of using ASF and how does it work?

The main advantage of using the ASF format is for streaming media over the Internet (and via other types of networks). Users don't have to wait for the whole file to be transferred as with some other streaming formats. In the case of digital music for example, a song can be played back almost in real-time as the data is transferred from the web server. This method is sometimes called progressive downloading and gives the best user experience because there is a minimal amount of time to wait -- typically only a few seconds is required for the playback buffer to be filled with a small amount of data. The protocols used for streaming ASF files can be either MMS or RTSP, but the former progressive download method is typically used.

Text can also be stored in the ASF container format as metadata which is useful for tagging digital music. It is similar to other metadata schemes (like ID3) and can be used for identifying a song's characteristics -- title, artist, genre, year, etc. The metadata information in an ASF file isn't limited in size either as with some other formats. There is also the option to use the metadata either for a complete file, or link the information to an individual stream -- this is technically known as stream-level attribution.

How do I identify a file that is in the ASF format?

Even though files can have the file extension .asf, there are also other filename endings used for this format. This can make identifying files encoded in the ASF format difficult to do unless you already know what to look for. To help you, here is a list of file extensions commonly used for the ASF format:
  • ASF
  • ASX (Advanced Stream Redirector)
  • WMA
  • WMV

Are files in the ASF format copy protected by DRM?

Files in the ASF format don't necessarily come with DRM copy protection, but this container format has the facility to support it. In the case of WMA and WMV files, encryption can be used to protect the audio and video data.

How do I play an ASF file?

There are quite a few software media players (mostly free) that support playback of files in the ASF format. Some examples are:
  • Windows Media Player
  • VLC Media Player
  • Media Player Classic
  • KMPlayer
  • MPlayer
  • Foobar2000
Also Known As: Active Streaming Format, Advanced Streaming Format
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