Advanced Streaming Format

Advanced Streaming Format (or ASF, later renamed to Advanced Systems Format) is a proprietary digital container format from Microsoft, designed especially for streaming.

The format does not specify how it should be encoded; instead, it only specifies the structure of the video/audio stream. ASF files can be encoded with virtually any codec, while remaining in the ASF format. This function is similar to that carried out by QuickTime, AVI or Ogg formats.

The most common file types contained in an ASF file are Windows Media Audio (WMA) and Windows Media Video (WMV).

Files containing only WMA audio usually are named using the.wma extension, and files containing only WMV video usually have the.wmv extension; In any case, the file is still ASF format; both can use the.asf extension if desired.

This format is often confused with the Microsoft MPEG-4 (Windows Media Video) implementation, because most ASF streams or files are encoded using even this codec very often.