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Dvb Html A New Standard Part 3

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Mark Bolton

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User since: 07 Mar 2002

Articles written: 3

In Part 2 we looked at the DTD and Xhtml Module application for a DVB-HTML application. Now we have established the type of document and accessing the new module functions we can look at what we can do with the document. DVB-HTML includes additional controls for triggering functions during broadcast and also for integrating Video and specialist controls specific to Digital Television. There are also some alterations to the supported CSS level 2 functionality.

Media Type Support

Defining the media type in any web development situation is always a good idea, although current browsers can operate without knowing the media type this means that they have to either identify the media from it's filename extension or from it's content. This provides a slightly faster response although almost unmeasureable. For example

<a href=""e type="text/xhtml"> would tell the browser which DTD schema to use for opening the page filename.xhtml. The DVB-HTML specification for MHP User Agents (MHP Browser) states that alternative strategies should be used to determine the file type in case of incorrect identification or no Identification of the media type.

The DVB-HTML standard supports the following media types:

  • text/html - XML applications
  • application/xml - XML applications
  • text/css - Cascading Style Sheet
  • text/plain - Monomedia format for Text
  • text/dvb.utf8 - Monomedia format for Text
  • audio/mpeg - Monomedia format for Audio Clips or Audio
  • image/jpeg - JPEG images
  • image/png - PNG images
  • image/gif - GIF images
  • image/mpeg - MPEG 2 I-Frames
  • video/dvb.mpeg.drip - MPEG 2 Video drips
  • video/mpeg - MPEG Video
  • multipart/dvb.service - Multipart DVB Service (selects a DVB service)
  • application/dvb.pfr - Downloadable fonts
  • application/dvbj - Initial class file of an Xlet
  • text/emcascript - EMCAScript

There are limitations on the element usage of the media types these are detailed in the MHP1.1 specification (Beware it is 17Mb) at the DVB-MHP website

Cascading Style Sheets

The DVB-HTML standard supports CSS Level 2 W3C Recommendation, there are some additional extensions which have been added and some removed (Some Aural Styles do not appear to be supported!!!!!!! Accessibility !!!!!!!!!). The added ones are:

  • Opacity:

    <alphavalue>: The uniform opacity setting to be applied across an entire object. Any values outside the range 0,0 (fully transparent) to 1,0 (fully opaque) will be clamped to this range. If the object is a container element such as a <div>, then the effect is as if the contents of the <div> element were blended against the current background using a mask where the value of each pixel of the mask is <alphavalue>.
  • Compose-rule:

    <compose-rule> Added to perform Translucency effects.
  • Clip-Video:

    <clip-video> Enables definition of a rectangular-shaped portion of the video as a possible source for Object and Image DVB-HTML elements

    <object src="dvb://current" style="clip-video:rect (top, bottom, right, left) " />

    The units are specified in pels which specify the distance offset from the top or left edge of the display.
  • Remote Control Navigation:

    <nav-up, nav-down, nav-left, nav-right, nav-index, nav-first> enables remote control navigation of user menus.
  • Viewport:

    <viewport> adds the ability to be able to define the amount of the screen in which the content will be displayed, and allows the content to display not dependant upon the local conditions (i.e. resolution, pixel/aspect ratio interlaced or progressive)

The content of a valid page can be aimed at differing platforms (Computer Browser, DVB-TV) by the incorporation of a specific stylesheet for the platform detected. This brings the power of the internet to Television viewers but can also be used in the opposite direction bringing the power of television presentation to your computer. Bandwidth of course is currently a concern with presentation of television on your PC, however with digital transmission this bandwidth becomes much greater removing the requirement for telephone modems to receive data.


With the advent of complete digital television services will come a stream of new services to access the internet. One vision of the future is that televisions and computers will be able to access the digital content via local transmitters, with feedback (interaction) via a local receiver.

This means the bandwidth restrictions currently caused by having to send signals down either copper or fibre optic cabling will be removed. The first evolution of this technology is already starting to be installed around the world in the form of 3G and CDMA telephones where video phones and high bandwidth internet access become reality.

The internet and web developers have a large part to play in the future development of interactive television services. The step for those already creating web applications will not be too great, ECMAScript (Javascript, Jscript) is already used by possibly 90% of internet sites (Although some tweaking will be necessary for the DVB DOM which i have jet to look seriously at), compliant (Valid) XML is reasonably easy to construct, and with the newer computer browsers will display almost exactly the same in all browsers and on all platforms. Programming Xlet's will be a new challenge but there are already commercially available software packages for MHP application programming (No recommendations as I have not gotten my hands on them yet).

The only current problem will be testing your application website. This is not a future technology but there is still a shortage of platforms to support it. Some of the larger television manufacturers are offering development kits which consist of the software and a set-top box. It would be nice to see that when the technology reaches the consumers home there is already a lot of content available to them. In order for this to happen the web development community has to be ready and producing content ready for iDTV

As an engineer developing platforms for the viewing of this content I would appreciate your views, comments, questions on this technology, as I am currently studying the standard (writing articles as I start to understand it) This is the end of the current series. I shall however add to the series as my knowledge increases hopefully with some example code

Mark Bolton is an engineer working on the development of Hi-Definition-Television (HDTV), who also has an active interest in the development of the internet through his company Boltonmedia

Currently based in Shanghai, China and actively developing websites in both English and Chinese whilst designing and developing new methods of viewing these websites

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