SMIL, ASF and SMIL+Time
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SMIL is based on XML, and is thus a markup language. SMIL is a language, that is meant to schedule multimedia presentations where audio, video, text and graphics are combined in real-time. All the different media elements are referenced from the SMIL file, similar to the way an HTML page references its images, applets, and other elements. You can, for example, control the precise time a sentence is being shown, and make it coincide with a video clip or a soundclip, another good example could be a slide-show followed by some text or sound.
SMIL also allows you to set language choices for your presentation, enabling you to create the time line and layout just once, but serving an international audience (see the Tags for the Identification of Languages at http://www.ietf.org/rfc/rfc1766.txt)
Compared to older formats, such as AVI or MPEG, SMIL presentations lets text appear outside the presentation. This is particularly good for search engines, since they will be able to also index this data, and thus make it easier for users to find exactly what they need. The separation of text from video and audio also increases accessibility, as users with text-only browsers still are able to access parts of the content.
The first commercial player/browser of SMIL to arrive was RealNetworks' RealPlayer G2 (http://www.real.com/g2). RealNetworks has implemented a large subset of the SMIL 1.0 spec. in G2, but chose not to implement the interactive part of SMIL, that lets you create hyperlink media elements (e.g. a linked table of contents). Hopefully this will be implemented in future versions of the RealPlayer. As an authoring tool, Allaire's HomeSite 4.0 (http://www.allaire.com/Products/HomeSite) has built in support for some SMIL elements.
RTSP (RealTime Streaming Protocol) was built to understand time, or more specifically, time-stamps. RTSP also supports VCR-like functionality, and multicasting.
One of the best features of SMIL is perhaps, that since it is a text based language, you have the ability to create code on-the-fly using a database. SMIL is not intended to replace any of the existing multimedia environments out there. Rather it is a universal glue for joining all kinds of different formats and types of media in interesting, and more even more important, useful ways, using a vendor-neutral language.
As opposed to SMIL, that is based on a plain text-based markup language, ASF (Advanced Streaming Format) is a full-blown object-oriented programming language. ASF is supposed to replace the older AVI (Audio Video Interleave) format, and it is backed very strongly by Microsoft, but also by about 100 other companies including IBM.
An ASF file is based on a sequence of objects. While some objects, such as the header object that provides global information about the content in the file are required, others are not. The file format is extensible, as any object type can be extended by adding sub-objects, and servers that do not recognize the sub-object can safely ignore it, just as browsers safely ignore any HTML-tags that they do not understand.
One of the interesting aspects of ASF, is that is supports component download and prioritization. Component download is achieved by storing stream-specific information about playback components (for example, decompressers and renderers) in the file header. If the user does not have the needed version, the user can then be offered a download option, or it can even be done automatically, so that the user does not have to worry. The prioritization capabilities in ASF allows authors to establish prioritization among the various different data streams. For example, in a casting from a BB King concert, the sound might be more important than video, which in turn might be more important than the lyrics.
Lastly, ASF also supports the language capabilities that SMIL offers, thus allowing you to create truly international multimedia content.
The two main priorities in HTML+Time was to apply time attributes to any arbitrary HTML element, and to use these same attributes to provide a means of describing that same media element's integration with other multimedia elements in the presentation. This functionality includes specifying when a streaming media element is supposed to begin, or the length of its duration.
The trouble with SMIL that HTML+Time tries to fix, is that SMIL elements work in their own environment. HTML+Time takes this to the next level, by letting all the elements in a page interact with each other. As an example of this, HTML+Time lets you apply different timing to individual bullets in an HTML list, where SMIL does not. HTML+Time also lets Web developers specify how long elements in basic HTML pages should remain on a page, or be replaced by other elements - for instance in a succession of images.
One major advantage of HTML+Time, is that it has already been built into a browser, namely Microsoft Internet Explorer 5. The implementation in IE5 is experimental, but it lets you start working with HTML+Time right away. You can go right ahead, and add images, video and sounds to your HTML page, and using HTML+Time, you can synchronize them over a specified amount of time, by just using a few new attributes to existing HTML elements and a few new XML-based elements.
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