Information for MHEG-5 (Digital Teletext)

Posted on August 30, 2009
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MHEG-5 is part of the ISO/IEC 13522-5 Multimedia and Hypermedia Experts Group (MHEG) international standard used for “interactive services” in UK digital terrestrial television. These include:

General information services (sometimes known as “digital teletext”)
Programme specific information services (giving more details about the topics covered in the current or recently broadcast programme).
Simple games
Static displays (such as placeholders which are transmitted when a channel is not broadcasting, or for radio stations)
Services in which the view can interact with video and audio.

MHEG-5 is a language for describing multimedia presentations as a collection of objects.

MHEG-5 includes:

Visible objects such as Bitmaps, Text boxes, Rectangles, Buttons and Sliders.
Collections of objects for displaying as part of a multimedia application (Group objects).
Links – objects that trigger on certain conditions and perform a sequence of actions.
Programs, either resident in the engine, interchanged (downloaded) or remote (running on a server).
Variables storing integer, boolean, string, object or content references.

MHEG-5 is object orientated

An object hierarchy is defined, with objects inheriting attributes of their base class. To display a presentation, an MHEG-5 Application is started. This must load a Scene before anything can be displayed. Applications and Scenes include lists of Items (other MHEG-5 objects) called Ingredients. Ingredients can belong to the Visible class, meaning that they are displayed on screen or can be non-visible, such as Links, Programs or Variables.

MHEG-5 Events, Actions and Links

Events are things that can make something happen. Actions are things that can happen. Links bind Events to Actions by specifing when this Event happens, perform these Actions.

MHEG-5 defines two different event types:

Asynchronous events – an event that is raised by an entity outside the engine, such as user interaction, timer firing and content loading.
Synchronous events – an event that is raised as a direct result of an ElementaryAction being executed, usually indicating a change in state of an object.

ElementaryActions perform single operations, such as:

displaying and hiding Visible objects
moving Visible object
setting timers
setting and testing variables

An Action is a list of ElementaryActions.

A Link is an MHEG-5 object that waits for a specific Event to occur, then executes an Action. An example could be a Link that waits for the user to press the red button, and displays a menu when they do so.

Links implement dynamic behaviour within MHEG-5 Applications.

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