Understanding Windows Media File Formats

You’re probably already familiar with two standard Windows digital file formats, AVI and WAV. These are video and audio file formats, respectively, that provide the highest-quality playback when played from a CD or your computer’s hard disk. But these file formats are not suitable for streaming because they are often uncompressed or have little compression applied, and are simply too large to traverse common network bandwidths.

Windows Media Format is the digital media format used for streaming. It is also suitable for local playback from CD or DVD, your computer’s hard disk, set-top boxes, and portable devices. Windows Media Format is composed of Windows Media Audio and Video codecs, an optional integrated digital rights management (DRM) system, and a file container called Advanced Systems Format (ASF). It is a format that enables you to deliver audio and video data over a network while maintaining a quality level that is close to that of the content source.

Audio and video content is converted to Windows Media Format during the encoding process. Once encoded, these audio and video files or streams are indicated by the file name extensions listed in the table below.

Extension	File type	Description
.wma	Windows Media Audio	Files that include audio and can be played back using the Windows Media Audio codec.
.wmv	Windows Media Video	Files that include both audio and video (or video only) and can be played back using the Windows Media Audio and Windows Media Video codecs.
.asf	Advanced Systems Format	Content encoded with earlier versions of the Windows Media codecs. This contentcan be played back using both the Windows Media Audio and Windows Media Video codecs.

Understanding ASF

Advanced Systems Format (ASF) is an extensible file format designed primarily for storing synchronized digital media streams. The format supports data delivery over a wide variety of networks and protocols, while still proving suitable for local playback. ASF is the container format for Windows Media Audio and Windows Media Video-based content.

The ASF file container can store the following elements in a single file: audio, video, attributes (such as the file’s title and author), and index and script commands (such as URLs and closed captioning). The file container supports files as large as 17 million terabytes.

In previous versions of Windows Media, files containing audio and video were indicated by the .asf file name extension. Although the .wma and .wmv file name extensions are now used, it is still possible to stream .asf files or to change the .asf file name extension to .wma (for audio-only files) or .wmv (for video-only or audio and video files).

Some tools and services that were created for use with earlier versions of Windows Media Technologies require the .asf extension to accept the content. You can simply rename any .wma or .wmv file with the .asf extension to use them with those tools. (However, it is recommended that you use the most current tool or service, when possible.)