.ASX File Format – Syntax

Identifying a metafile

The first four characters of the file must be <ASX. The ASX element appears exactly once, it has no parent, and it is used in every metafile whatever the file’s extension. The element identifies the file as a metafile; it does not say what kind of media the file refers to.

The VERSION attribute is required and must come straight after the element name. The current syntax is version 3, written 3 or 3.0. Microsoft says the Player “supports all previous versions of metafile syntax”, but it does not document those earlier versions.

Tag rules

Extensions and media types

The extension is a hint about the content; the file’s syntax is the same in each case. Microsoft’s advice is to name the metafile after the media it points to.

The media types are conventions rather than registered types, and video/x-ms-asf is shared with ASF media files, so a server’s type does not settle what a file is. Check for the <ASX opening instead.

The element tree

Every element of version 3. The parent and child columns are Microsoft’s; an element found elsewhere is not defined by the format.

Microsoft’s page for LOGURL says it may be a child of ASX or ENTRY, but its pages for ASX and ENTRY do not list it among their children. The same is true of SKIN and ASX. Both are listed here by the page that defines them, and in practice both appear where shown.

Time values

The VALUE attribute of DURATION, PREVIEWDURATION and STARTTIME is written hh:mm:ss.fract, in hours, minutes, seconds and hundredths of a second. Leading fields may be left out.

URLs

An HREF holds a URL in any protocol the Player supports, usually http, https or the older mms (Microsoft Media Services). A local path such as c:\asfroot\Port na Pucai.wma also appears in Microsoft’s examples, as does a relative one. Inside a file pulled in with ENTRYREF, the ENTRYREF’s URL is the base for relative URLs.

Two special protocols address discs that allow it:

A REF can name still images (.gif, .jpg) and Flash files (.swf) as well as audio and video. These need a DURATION, or they have no length.

Scope and precedence

Several elements can appear at more than one level. The nearest one to the media wins.

The [Reference] form

Some servers answer with an INI-style text file instead of an ASX document: a line [Reference], then lines such as Ref1=http://example.com/stream. Microsoft’s metafile pages do not describe it, so this is not part of the format above, but players meet it. mpv, for one, recognises the exact [Reference] line and reads any key beginning Ref.

An example

<ASX VERSION="3.0" PREVIEWMODE="NO" BANNERBAR="AUTO">
  <!-- Show information -->
  <TITLE>Example Radio</TITLE>
  <AUTHOR>Example Broadcasting</AUTHOR>
  <COPYRIGHT>(c) Example Broadcasting</COPYRIGHT>
  <MOREINFO HREF="https://radio.example.com/" TARGET="_blank"/>
  <PARAM NAME="Encoding" VALUE="utf-8"/>

  <!-- A clip with two addresses and a start point -->
  <ENTRY CLIENTSKIP="NO">
    <TITLE>Station ident</TITLE>
    <REF HREF="mms://radio.example.com/ident.wma"/>
    <REF HREF="https://backup.example.com/ident.wma"/>
    <STARTTIME VALUE="0:05.0"/>
    <DURATION VALUE="0:20.0"/>
  </ENTRY>

  <!-- Two clips, played three times -->
  <REPEAT COUNT="3">
    <ENTRY><REF HREF="song1.wma"/></ENTRY>
    <ENTRYREF HREF="https://radio.example.com/more.asx"/>
  </REPEAT>
</ASX>