Overview and Features
ASF is based on serialized objects which are essentially byte sequences identified by a GUID marker.
The format does not specify how (i.e. with which codec) the video or audio should be encoded; it just specifies the structure of the video/audio stream. This is similar to the function performed by the QuickTime, AVI, or Ogg container formats. One of the objectives of ASF was to support playback from digital media servers, HTTP servers, and local storage devices such as hard disk drives.
The most common file types contained within an ASF file are Windows Media Audio (WMA) and Windows Media Video (WMV). Note that the file extension abbreviations are different from the codecs which have the same name. The current version of Windows Media Encoder produces files with the extension .WMA (and MIME-type 'audio/x-ms-wma') and .WMV (MIME-type 'video/x-ms-asf'). These files are identical to the old .ASF files but for their extension and MIME-type. According to Microsoft, the change in extensions was made to make it easier for an application to identify the content of a media file (see Q284094). Although this differentiation was introduced in 2003, to this date most webbrowsers (including the latest versions of Internet Explorer and Firefox) do not function as might be expected with the 'new' extensions: even when a .wma or .wmv file is specifically encoded for progressive download, browsers still proceed to completely download the file before playing it. This can, however, be easily remedied by simply renaming the .wma or .wmv file to the old extension .asf.
ASF files can also contain objects representing metadata, such as the artist, title, album and genre for an audio track, or the director of a video track, much like the ID3 tags of MP3 files. It supports scalable media types and stream prioritization; as such, it is a format optimized for streaming.
The ASF container provides the framework for digital rights management in Windows Media Audio and Windows Media Video. An analysis of an older scheme used in WMA reveals that it is using a combination of elliptic curve cryptography key exchange, DES block cipher, a custom block cipher, RC4 stream cipher and the SHA-1 hashing function.
ASF container-based media are sometimes still streamed on the internet either through the MMS protocol or the RTSP protocol. Mostly, however, they contain material encoded for 'progressive download', which can be distributed by any webserver and then offers the same advantages as streaming: the file starts playing as soon as a minimum number of bytes is received and the rest of the download continues in the background while one is watching or listening.
Read more about this topic: Advanced Systems Format
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