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                   J.C. López, A. Gil, J. J. Pazos, C. López, M. Ramos, R.F. Rodríguez
   Depto. de Enxeñería Telemática, ETSE de Telecomunicación, Campus Universitario s/n, University of Vigo (Spain)

The versatility of the XML language is one of its main strengths, and thus it is suitable for being used in a very wide
range of applications that entail structuring, exchanging and processing the information. This versatility has favored its
widespread acceptance, an important advantage for its introduction in the standardization processes. In this paper, this
versatility is described in three activity fields related to the digital TV: in the specification of the MHP standard, in the
specification of the metadata describing contents and in the scope of the implementation of applications.

XML, interactive digital TV, DVB, MHP, TV-Anytime.

In next years, the digital technology will imply a significant change into the television world, where the
spectator will go from being a passive individual to using the interactive facilities increasingly offered by the
providers. The digital technology enables the transmission of multimedia software applications to be
executed in a digital TV or in an analogical TV equipped with a Sep-Top Box (from now on, STB). These
applications can be synchronized with contents and provide interactivity to the user.
    Nowadays, the main challenge that arises in the transition from analogical to digital television is not the
resolution of technological problems, but achieving the economic viability of some projects whose magnitude
imply huge investments both in development and technology acquisition and in contents generation. To
achieve this aim, it seems necessary to standardize signals and equipment.
      In what concerns to the format of transmission of the information, the technology existent today is
uniform and based on MPEG-2. However, for conditional access systems, interactive television services and
Internet access, the solutions are proprietary, involving higher costs and equipment incompatibilities.
    In Europe, the most important standardization organism is the DVB (Digital Video Broadcasting)
consortium. In the field of the standardization of the user terminal, the DVB has recently published the MHP
specification (Multimedia Home Platform) (DVB consortium, 2001) that offers a solution for running
interactive applications and for offering Internet contents in the television receiver.
     The rest of the paper is organized as follows: Section 2 reviews the main characteristics of the MHP
standard, Section 3 describes the role of XML and, finally, Section 4 summarizes the conclusions.

MHP specifies a generic interface for the applications that will be executed in the decoder, providing an
abstract model for the access to information streams, files or hardware resources (López-Ardao et al., 2002).
The API provided to the applications by MHP is independent of the hardware of the receivers. We can
distinguish several layers for processing this information (figure 1): hardware resources and software
resources dependent of the hardware; system software independent of the hardware; and applications.
    From the point of view of the applications, this interface lies between them and the system software, and
it consists of a running context and a set of APIs. These APIs facilitate the development of portable
applications that can execute their life cycle and receive, display and send information regardless of the

IADIS International Conference WWW/Internet 2002

hardware platform. In order to offer an abstract environment to the applications attached to contents, the
MHP terminal was turned into a virtual machine from the perspective of the applications. That is to say, the
applications will be Java classes independent of the hardware. So, a MHP application (known as DVB-J
application or Xlet) is the Java bytecode of a program, which fulfills two main conditions:
  • It uses only the libraries and Java APIs defined in the MHP standard (generally adapted and derived
       from existing interfaces like Personal Java, Java Media Framework, Java TV, HAVi or DAVIC);
  • It generates and deals with a set of signals implementing a life cycle perfectly specified in the MHP
       standard, and thus enabling the application manager to control the applications easily.

       Figure 1. Architecture of a MHP receiver                  XML is an extremely suitable tool to help to achieve
   DVB -J           DVB -J                  Legacy application   the proposed goals due to its advantages regarding
  applicatio       applicatio     Data            Plugin
      n                n
                                                                 extensibility,    structuring   capacity,    platform
                           MHP API
                                                                 independence or widespread acceptance. It is not
                                                                 just that XML and MHP share fundamental aims; or
 Sun Java   HAVi     DAVIC      DVB                              that the technical advantages of XML are
   APIs     APIs                APIs    Transport   Aplication
                                        protocols   manager
                                                                 appropriate to solve the problems that emerge
       Java Virtual Machine                                      around MHP. Also, the fast expansion of XML
                                                                 establishes this technology in a privileged position
               Operating system and drivers
                                                                 to serve as vehicle of diffusion of any
                             Hardware                            standardization attempt, and especially for MHP.

XML can be used in three activity fields in the standardization of digital television with MHP: in the
specification of the MHP standard, in the specification of the metadata describing contents and in the scope
of the implementation of applications.

3.1. In the MHP standard
First of all, we must underline the extensive use of XML in the mechanisms defined in the MHP standard
itself. There are m any situations in the specification regarding the exchange of information among entities
that are solved via the definition of XML applications that structure the data to send. For example:
1. Specification of permissions to access to the resources. The capacity of the STB to execute software
   received from the network implies the necessity of authenticating the downloaded software to guarantee
   the correct access to the decoder resources. In the MHP standard, the permissions given to an application
   are the result of the intersection among the permissions requested by the operator sending the application
   and the permissions specified by the STB user. This latter mechanism is implementation dependent.
   However, the first one is explicitly defined in the standard and implemented via XML files. The standard
   defines an XML application called Permission Request File and it specifies that a document of this type
   can be sent with each application for describing the requested permissions. This application includes the
   elements and attributes to request the access to resources like the file system, the life cycle of other
   applications or the return channel.
2. DVB-HTML: an application of XML. The high cost of the technologies needed to transform the
   television into an active platform demands the development of value-added services. One of these services
   is undoubtedly the access to Internet via television, considered by MHP as a high-priority objective (Gil et
   al., 2002). Also, the DVB always emphasized the necessity to provide a channel for structured and stylized
   information, like a “super-teletext”, that provides a versatile mechanism to display information to the
   DVB-J applications in a homogeneous and stable way. The subsequent work led to the adoption of XML
   as backbone of the proposed solution, and to the definition of an XML application, the DVB-HTML
   language, for implementing both mechanisms. The result was the specification of the DVB-HTML
   applications, consisting in a set of interrelated documents, written in DVB-HTML. The objective is the
   definition of a framework for broadcasting sets of related pages, making the access to the information

                                     USING XML IN THE STA NDARDIZATION OF DIGI TAL TV WITH MHP

   faster. DVB-HTML is an XML application defined by means of the selection of diverse modules of the
   XML Modularization standard, which bring together the distinctive characteristics of the television
   compared to the computer: smaller processing power and screen resolution, longer watching distance or
   worse ergonomics in the interaction (Gil et al., 2002).
3. The declaration and association of triggers. The DVB-HTML application model does not try to be
   limited to a static outline where the user downloads information structured in pages. MHP enables the
   operator to interact with the applications through sending synchronization messages to modify their life
   cycles. These messages, termed triggers, are sent via the broadcast channel and delivered to the DVB-
   HTML applications as DOM events that usually entail executing script code. This mechanism defines two
   applications of XML for a DVB-HTML application to declare the triggers to those it wants to subscribe. In
   the first of them, the Event Factory File, the creator of the application associates triggers to concrete
   elements in DVB-HTML documents, to be delivered through the propagation of DOM events expressly
   generated. The second one, the Event Linkage File, is used to declare all the Event Factory Files related to
   a concrete DVB-HTML application and to associate it with its respective documents.

3.2. In the specification of the metadata describing contents
A second field where XML is achieving excellent results is that of the contents characterization. This use of
XML was quickly identified in the scope of the WWW and it has been extensively exploited to improve
services for searching information in Internet. However, also the multimedia contents in the WWW have
continued to grow spectacularly in volume, and this type of information has harder difficulties for the
establishment of search criteria. More and more, users demand search mechanisms f r this type ofo
information starting from easily identifiable characteristics. Nowadays, the only mechanism that is achieving
good results is the association of metadata with these contents through XML applications.
    In this scope, it is necessary to highlight the role played by the MPEG-7 standard in the process of
identifying the multimedia objects of our interest and the properties associated with them: still images,
graphics, 3D models, audio streams, voice, video, etc. In the context of this standard, the Description
Definition Language (DDL) is defined. The DDL is, basically, the XML Schema 1.0 standard extended with
several elements that facilitate the characterization of multimedia objects and situations.
    Focusing on the concrete field of the digital television, the most important project in contents
characterization was promoted by the TV-Anytime forum (TV-Anytime, 2002), that tries to implement
mechanisms to describe contents and to refer to them regardless of their location and broadcast date. The
main goal is twofold: to separate the information that describes the contents from the information needed for
obtaining them, and to make the whole independent of the transport mechanism.
    The utilized tool is also XML. Their implementation starts from the DDL defined by MPEG-7 to create a
set of XML applications that describe all the important parameters to enable the search of contents. This
metadata covers traditional objectives (descriptive attributes of the contents, technical characteristics of their
representation formats, revisions and criticism by a third party or information about different copies of the
same content). Besides, it includes ambitious ideas about parameters that would permit to implement novel
services, among them: The identification of user characteristics (it would enable the search of the more
attractive contents and the carrying out of market researches that guide the future generation of contents); the
specification of characteristics of social groups (it would permit to make a search in the opposite direction,
contents could look for their audience and offer to it); and the characterization of the different parts of the
contents (it would enable the segmentation and indexation of the contents, thus making the way toward a
non-linear model of watching television).
    This application field of XML shows us one of the most prominent characteristics in this technology: its
extensibility. XML enables to build standards on others in a natural way: XML Schema is built on XML, the
DDL of MPEG-7 is built on XML Schema, TV-Anytime is built on DDL, and so forth.

3.3. In the scope of the implementation of applications
Implementing tools of generation, analysis, processing and displaying of information is another important
field in which XML can be successfully applied. In this scope, our experience centers on using XML in the
specification of MPEG-2 transport streams (TS) according to DVB-MHP.
    In the project described in (GRIS, 2001), with the goal of software verification, we needed TS that
offered a representative range of the different structures of information defined in MHP. These TS were not

IADIS International Conference WWW/Internet 2002

available until the project was advanced (in late 2000). So, we decided to implement a simulator for
validation purposes. XML was used in this work to structure and store the configuration information for the
application to generate TS with different characteristics, ranging from the elements that describe the
components of the broadcast network to the elements of the available interactive applications.
    If we just describe the most important entities, we can cast the role of root element in our system to the
entity “broadcast network”. This element would be compounded of several TS that in turn would be made up
of different services (TV channels). In each TS there are two important entities that require special signaling
information: the programs (sequence of audiovisual events) and the applications with additional functionality
like interactivity. The specified information is not only made up of the components describing the entities,
but also of its dynamic behavior (for example, configuring an application as autostart or not).
    When using XML, we have identified several advantages that we can classify in three categories:
    Advantages inherent to XML: These advantages are widely described in the literature: ease to organize
a lot of complex information; legibility of the results, what permits to use the specifications for discussion
and diffusion of knowledge; extensibility, that enables to work with partial descriptions; etc.
    Advantages derived from the availability of public-domain software: Our work was made much
easier due to the possibility of using public-domain tools for creating and editing the great quantity of XML
documents written. But even mo re important it is the possibility of using public-domain software for the
analysis and validation of the XML documents. The advantages derived from the choice of Xerces (the
Apache project) are quite clear as for the reduction of the development time, the software reliability and the
possibility of reusing the same functionality in different tasks of the project.
    Advantages derived from our implementation: Actually, the developed XML application is
implemented through the combination of five autonomous XML applications. Splitting the configuration of
the TS into independent applications contributes the additional advantage of reusing specifications in other
tasks of the project. For example, the XML application characterizing the DVB-J applications is reused in the
tools for the automatic generation of these applications, which our research group is now working on.

Television is undoubtedly one of the sectors where the standards enjoy greater stability. And the
technological transformation from the analogical to the digital world must observe this principle.
    XML is an extremely suitable tool to help to achieve the proposed goals due to its advantages regarding
extensibility, structuring capacity or platform independence. Besides, the technological model chosen for the
implementation of the interactive digital TV (by both DVB and ATSC) is intimately related to Java and
nobody is surprised at the close connection between both technologies, Java and XML. Any standard
associated to XML has numerous support tools developed in Java.
    As described in this paper, the versatility of XML makes this technology appropriated for many tasks,
from the standardization of the infrastructure to the characterization of the information sent through it.
    With regard to the characterization of the information, the use of XML is still in the early stages. The
great number of channels in digital TV implies a considerable increase in the available information to the
subscriber, with the subsequent complexity in searching and selecting contents. In this scenario, the contents
characterization through metadata seems an inevitable way for implementing automatic mechanisms that help
the user to identify contents. Although TV-Anytime does not result in a definitive standard on this matter, the
ease provided by XML to define automatic transformations among the current formats, and those that can
appear in future, guarantees XML a privileged position in this scenario.

DVB Consortium, 2001. Multimedia Home Platform 1.1,
GRIS, 2001. University of Vigo. Sistema de recepción digital multimedia,
López-Ardao, J.C. et al., 2002. Experiences from implementing a MHP receiver. Proceedings of International
     Symposium on Video/Image Processing and Multimedia Communications. Zadar, Croatia.
TV-Anytime, 2002. The TV-Anytime Forum,
Gil, A. et al., 2002. Surfing the WEB on TV: the MHP approach. Proceedings of IEEE International Conference
     on Multimedia a nd Expo. Lausanne, Switzerland.


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