What Is an ASF File?

How to Open, Edit, and Convert ASF Files

ASF Files
ASF Files.

A file with the ASF file extension is an Advanced Systems Format file developed by Microsoft that's commonly used for streaming audio and video data. An ASF file can contain metadata too, like a title, author data, rating, description, etc.

The structure of the audio or video data is understood by an ASF file but it doesn't specify the encoding method. However, WMA and WMV are the two most common kinds of data that's stored in the ASF container, so ASF files are most often seen with one of those file extensions.

The ASF file format supports chapters and subtitles, and also stream prioritization and compression, which is what makes them ideal for streaming.

Note: ASF is also an acronym for Atmel Software Framework and a texting abbreviation that means "And So Forth."

How to Open an ASF File

You can play an ASF file with Windows Media Player, VLC, PotPlayer, Winamp, GOM Player, MediaPlayerLite, and probably several other free multimedia players.

Note: Be careful to avoid confusing an ASF and ASX file. The latter is a Microsoft ASF Redirector file that's just a playlist/shortcut to one or more ASF files (or some other media file). You can most likely open an ASX file like you would an ASF file since some multimedia players support the playlist format, but you can't treat the ASX file as an ASF; it's just a shortcut to the real ASF file.

How to Convert an ASF File

There are numerous applications that can convert an ASF file, including free video converter programs and free applications that can convert audio files.

Just open the ASF file in one of those applications and choose to convert the file to a new format.

For example, if you need your ASF file to be an MP4, WMV, MOV, or AVI file, consider using Any Video Converter or Avidemux.

Zamzar is one way to convert ASF to MP4 on a Mac or any other operating system.

Just upload your ASF file to Zamzar's website and choose to convert it to MP4 or any other supported format, like 3G2, 3GP, AAC, AC3, AVI, FLAC, FLV, MOV, MP3, MPG, OGG, WAV, WMV, etc.

More Information on ASF Files

ASF was formerly known as Active Streaming Format and Advanced Streaming Format.

Multiple independent or dependent audio/video streams can be included in an ASF file, including multiple bit rate streams, which is useful for networks with varying bandwidths. The file format can also store web page, scripts, and text streams.

There are three sections, or objects, that are contained within an ASF file:

  • Header: File size information, the number of streams that it has, error correction details, codecs, metadata, and other objects and and general information are stored in the file's header.
  • Data: The section holds the actual content that's to be streamed.
  • Simple Index: A time stamp, frame number, or presentation time is stored in the Simple Index object so that the ASF playback program can seek through the file.

When an ASF file is streamed over the internet, it does not need to be fully downloaded before it can be viewed. Instead, once a particular number of bytes have been downloaded (at least the header and one data object), the file can be streamed as the rest is downloaded in the background.

For example, if an AVI file is converted into ASF, the file can begin playing shortly after instead of having to wait for the whole file to download, like what's necessary for the AVI format.

Read Microsoft's overview of the ASF file format or the Advanced Systems Format Specification (it's a PDF file) for more information.

Still Can't Open Your File?

The first thing to check for if your file isn't opening with any of the programs mentioned above, is the file extension. Make sure it actually reads ".ASF" and not something similar. Some file formats use a file extension that's spelled a lot like ASF but that doesn't mean that the two are similar or that they work with the same software programs.

For example, AFS is a file extension for STAAD.foundation Project files that are created by Bentley Systems' STAAD Foundation Advanced CAD software version 6 and prior. Though the same file extensions letters are used, they have nothing to do with Microsoft's ASF file format.

The same is true for other file formats like Street Atlas USA Map files, Secure Audio files, SafeText files, and McAfee Fortress files. All of those file formats use the SAF file extension and belong to (mostly) discontinued software.