ASF Files

ASF Files

When you use DirectShow or the Windows Media Format SDK to compress audio and video, the end result is an ASF file. ASF stands for Advanced Systems Format. Like AVI, ASF is a neutral container format that can be used to store any type of data. Also like AVI, ASF is an open specification. You can download the specification from the Microsoft Web site, and you can create ASF files using your own custom tools.

Naming Conventions in Windows Media

Microsoft s recommendations on ASF file name extensions are as follows: an ASF file that contains only audio streams and is compressed with one of the Windows Media Audio codecs should have the .WMA extension. A video-only or audio-video ASF file in which all the streams are compressed using Windows Media should have the .WMV extension. An ASF file that does not meet these requirements should have the .ASF extension.

The primary advantage of ASF over AVI is its packetized structure, which enables it to be streamed efficiently over networks. An ASF file can contain several streams with the same content encoded at various bit rates, which enables a client and a server to choose the rate that is most appropriate for a given network connection. ASF, as you might expect, also has excellent support for metadata.

Another big advantage of the ASF format over the AVI format is its support for a scheme for digital rights management (DRM). DRM is naturally a contentious issue in today s environment of rampant music copying. It enables content producers or distributors, such as record labels, to encrypt files so they cannot be played back or in some cases cannot even be copied to another machine without a license. The license contains a numerical key that enables a DRM-enabled player to decrypt the file at playback time. The license, which an end-user typically obtains separately from the media file itself, also contains conditions of use known as rights. The Microsoft DRM implementation enables you to specify such things as the number of times the file can be played (from one to infinity), whether it can be copied to a CD, and whether it can be copied to a portable device. The restrictiveness and cost (if any) of the license are up to the content owner or distributor; however, Microsoft requires that license issuers adhere to Microsoft s privacy policy in their license terms.

A multitude of DRM techniques are in use, although Microsoft s is the only one in wide use as of this writing. DRM is more politics and social engineering than technology because it enforces usage and copyright restrictions; ASF can support any DRM scheme, not just Microsoft s. (We won t cover DRM in this book; for more general information on Microsoft s DRM implementation, see the Windows Media site on Microsoft s Web site. For more detailed technical information, see the documentation for the Windows Media Format SDK and the Windows Media Rights Manager SDK.)

The ASF format is fully extensible; if you need a data type with your media stream that s not one of the six types defined by the SDK, you can add a custom stream to the ASF file that s capable of holding any stream of binary stream. Of course, if you add a custom stream type, you probably need to write your own DirectShow filter that can handle that stream. You can find more information on how to add custom streams to an ASF file in the Windows Media SDK documentation.