Content Decoder (MHEG)

Introduction

The Content Decoder is a UK term for the simplified MHEG-5 API that will deliver information services to the digital terrestrial viewer. It defines how the tv picture may be displayed with added textual information, either as an overlay, as an information screen with picture insert and how the display may be used for data services entirely. The API takes its commands from the MMI (Man Machine Interface - ie the remote control to you and me!) and provides the software to allow the viewer to select or 'navigate' between available services.

Object Carousel

A fundamental source for the content decoder is the transmission of data in the form of object carousels. When the viewer changes channels, it is necessary to update the receiver with data information pertinant to the new selection. Thus data is transmitted repeatedly in cycles or 'carousels', as they are called. The system adopted by UK DTT is the DAVIC DSM-CC object carousel (Data Storage Media Command and Control), which defines the transmission of data objects in 4096 byte sections.

The objects may be information for display on the screen, or they may be applications, software modules which carry out functions within the television receiver. These applications are stored in the receiver memory and the MHEG engine has to manage their storage and release within the receiver's memory constraints.

Writing to the Screen

An essential function of the content decoder is the assembling of objects to be displayed in the screen memory. In a computer, the variety of screen resolutions and user-controlled windows mean that the positioning of text and the overall appearance is indeterminate. The API for television is fundamentally different in that the elements of the scene have to be positioned precisely. In other ways, though, the mixing of text, still and moving pictures has close parallels with the computer, though the restricted memory and lack of disc storage mean that only a very few data types are specified.

Text and Hypertext

The UK content decoder specifies a special resident font, Tiresias, which has been designed in conjunction with the British Royal National Institute for the Blind, with the objective of achieving maximum legibility on the screen for all viewers, including the visually impaired. The font is defined in four sizes, heading, subtitle, body and footnote, and in plain and bold styles only. Both text and graphics make use of a 256 colour palette, though the requirements of anti-aliasing reduce the numbers that may be displayed simultaneously.

Hypertext links are defined, allowing the user to select a screen 'hotspot' and branch to new text, change channels, or carry out other application defined functions.

The Relationship to HTML

Hypertext Markup Language (HTML) is universally used on the internet and the massive volumes of expensively crafted content dictate that it should be possible to reuse previously created HTML over the television API. However, computer users will know all too well that HTML is evolving rapidly and does not have the required stability for television usage. MHEG-5 defines markup coding for text and hypertext objects which are functionally equivalent to a basic subset of HTML but coded differently to improve transmission efficiency. A simple translation process in the MHEG editor allows HTML files to be input to the information service.