Windows Media Metafile


 * Not to be confused with Windows Metafile.

The Windows Media Metafile format, very often referred to as ASX, is a playlist format associated with Windows Media Player. Its name is sometimes given as "Advanced Stream Redirector", and also as ASF Redirector. The Windows Media Metafile format is very strongly associated with ASF, and in practice almost always points to variants of the ASF format. Windows Media Metafile is based on XML, although one source notes that it "doesn't requires [sic] to be a strict XML document", as it apparently specifies that tag and attribute names are case-insensitive and does not properly escape some strings. It additionally resembles the SMIL format, suggesting that one was based on the other. The two are nonetheless incompatible. Its most common use is to serve as an index of media files which are on remote servers instead of users' their own computers (Windows Media Playlist seems to have been more widely used when making playlists of local content); compare M3U.

Windows Media Metafile files may be converted from Windows Media Playlist files and to ISX. ISX files will be in turn converted back to Windows Media Metafile in the process of streaming media from a server to clients.

Names and Identifiers
Although "Windows Media Metafile" is this format's official name, it is hardly ever used outside of official sources, with the most prominent third-party users of that name being technical sources and several "file information" sites that have (presumably copying from one another) confused it with WMA (WMA files are often pointed to by Windows Media Metafile files, but are not part of them). ".asx" is one of three extensions, the other two being ".wax" and ".wxv"; the three of these point to .asf, .wma, and .wmv files, respectively, which are all ASF files, but with different extensions depending on their content (see the ASF page for more details). Probably because there seems to have been little indication of the real name in the user-facing information, when the format is discussed, it almost always seems to have taken on its most common extension as a name.

Windows Media Metafile files share the mimetype  with ASF. They also use the mimetype, presumably when pointing to .wma files. Some sources gives their mimetype as being  ; however, it seems to be in very rare use, and it would not be surprising if it was only an erroneous "correction" of.

Identification
Parsed as XML, Windows Media Metafile files have a root element with a (case-insensitive) name "asx" and an attribute "version".

Examples

 * The text in the image at the beginning of

Specifications

 * Specification from docs.microsoft.com
 * A copy from 2006

Links

 * Can't play asf files on Windows 10
 * Windows Media player 12 has problem streaming live ASX file
 * Convert .asx to mp3
 * Advanced Stream Redirector