MHEG-5 overview

SInce MHEG-5 is the widest MHEG standard in operation and is widely known we will highlight this MHEG standard.

A multimedia application can be conceived as a set of self-contained objects based on synchronization and spatial-temporal relationships of multiple media formats, structural composition, event-action associations, navigation, and user interaction capabilities. Controlling the playback of time-dependent contents, like streams of multiplexed audiovisual data requires specific support. These streams demand VCR control functions (play, pause, fast forward, and so on), as well as the capability to manage events generated during their presentation. For example, rendering text subtitles can be synchronized with timecode events generated during the playback of a stream. MHEG-5 represents an application, as a set of scenes, which contain objects common to all scenes. A scene supports the spatially and temporally coordinated presentation of audiovisual content consisting of graphics, bitmaps, text, and streams (based on the multiplex of audio and video components). Interaction can be performed via graphic elements like buttons, sliders, text entry boxes, and hypertext selections. Every scene, as well as an entire application, is a self-contained entity that can represent its localized behavior by links that are event-action associations. Events can be generated by users, expiration of timers, playback of streams, and other conditions within the RTE.

The global scope of MHEG-5 is to define the syntax and semantics of a set of object classes that can be used for interoperability of multimedia applications across minimal-resources platforms. The developed applications will reside on a server, and as portions of the application are needed, they will be downloaded to the client. In a broadcast environment, this download mechanism could rely, for instance, on cyclic rebroadcasting of all portions of the application. It is the responsibility of the client to have a runtime that interprets the application parts, presents the application to the user, and handles the local interaction with the user.

The major goals of MHEG-5 are:

• To provide a good standard framework for the development of client/server multimedia applications intended to run on a memory-constrained Client.
• To define a final-form coded representation for interchange of applications across platforms of different versions and brands.
• To provide the basis for concrete conformance levelling, guaranteeing that a conformant application will run on all conformant terminals.
• To allow the runtime engine on the Client to be small and easy to implement.
• To be free of strong constraints on the architecture of the Client.
• To allow the building of a wide range of applications. This means also providing access to external libraries. An application using external libraries will only be partly portable.
• To allow for application code that is guaranteed to be "safe" in the sense that it cannot harm other code in the Client, nor put the Client in an abnormal state.
• To allow automatic static analysis of (final-form) application code in order to help insure bug-free applications and minimize the debugging investment needed to get a robust application. Note that this analysis should be possible to implement independently of the authoring environment.
• To promote rapid application development by providing high-level primitives and provide a declarative paradigm for the application development.

The MHEG-5 model is object-oriented. The actions are methods targeted to objects from different classes to perform a specific behavior and include:

• preparation,
• activation,
• controlling the presentation,
• user interaction,
• getting the value of attributes,
• and so on.

To allow interoperability across heterogeneous systems, MHEG-5 specifies the precise encoding syntax. Two notations are possible: the ASN.1 notation, which MHEG-1 also adopts, and a textual notation as illustrated below:.

In a client-server architecture (Fig 23.2), MHEG-5 applications are stored on the server and downloaded to the terminal for the RTE to interpret. This model is not limited to storage and retrieval services. In the broadcast environment, for example, the set of channels transmitted on a broadcast network can be considered a virtual server, where the download mechanism relies on cyclic rebroadcast of all portions of an application.

MHEG Client-Server Interaction