VIPromCom-2002,4th EURASIP - IEEE Region 8 International Symposium on
`Video I lmaae Processing and Multimedia Communications, 16-19 June 2002, Zadar. Croatia
`
`INTERNET-TV CONVERGENCE IN DVB-MHP1
`
`Albert0 Gil, JosC Pazos, Ana Fernhndez, Rebeca Diaz, Manuel Fernindez, Manuel Ramos
`
`ETSE Telecomunicaci6n7 University of Vigo, Vigo, Spain, ag;il@det.uvig;o.es
`
`-
`
`Abstract: In last decade, we have seen the first steps to the end of passive television. Digital
`technology is mature enough to enhance traditional TV (limited to content reproduction) with
`computing capability to run multimedia sofhvare integrating richer formats. One step further,
`Internet access through television is becoming a real possibility. The MHP standard is the
`first one that tries to define regulations in that sense. Here, we comment some technical
`aspects of the MHP solution.
`Key words: Digital Interactive Television, MHP, DW-HTML, Internet through TV
`
`1. INTRODUCTION
`
`Digital technology is driving television towards a new world full of amazing possibilities,
`where spectator is no longer limited to observe contents selected by the operator. More and
`more, new dynamic and interactive services are being introduced in everyday digital TV:
`complementary information to audio-visual contents, electronic program guides, selection of
`properties in configurable contents (language, camera angle or particularised advertisement),
`pay-per-view, etc.
`Broadcasting of a digital transport stream permits operators to mix traditional audio-visual
`contents with binary data, so making possible to deliver multimedia software applications to
`be executed in a digital TV or in a SetTop Box. These applications, synchronised with
`traditional contents, adapt themselves to spectator characteristics, implement interaction with
`users and provide return channels for communication with content providers. But a lot of
`decisions must be taken to design the actors of the system: the kind of code of the
`applications, the running context, the API’s supported in the SetTop Box, the life cycle of the
`applications, the signalling of the operator, etc. Here is where standards play a decisive role to
`achieve an open market, with compatible equipment, where users are not tied to a brand all
`along the broadcast chain.
`Today, the main regulator body in this field is the DVB (Digital Video Broadcasting)
`consortium, which dominates the digital TV market with its standards about terrestrial,
`satellite and cable transmission. Now, the DVB has published a new standard, MHP
`(Multimedia Home Platform) [ 11, to achieve compatibility in the computing platform that
`software applications (Xlets or DVB-J applications in MHP jargon) will find when they arrive
`at the digital TV set. This new standard defines a context for running applications (including a
`life cycle and signalling mechanisms) and a software interface (the MHP API) for these
`applications to access hardware resources.
`To achieve a compatible environment, Java technology is used. MHP applications are
`defined to be Java classes which implement some well-defined interfaces. In the SetTop Box,
`
`This work was supported by Spanish R&D Project number 1FD97-1195 (cofinanced with FEDER funds)
`
`Authorized licensed use limited to: Immanuel Freedman. Downloaded on July 07,2020 at 21:09:22 UTC from IEEE Xplore. Restrictions apply.
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`447
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`Roku EX1030
`U.S. Patent No. 10,334,311
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`

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`VIPromCom-2002,4th EURASIP - IEEE Region 8 International Symposium on
`Video / Image Processing and Multimedia Communications, 16-19 June 2002, Zadar, Croatia
`
`a Java Virtual Machine will detach downloaded applications from specific hardware and
`system software (see figure 1).
`
`application
`
`application
`
`I
`
`I
`
`m
`
`1 I Protocols
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`I
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`Hardware resources
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`I Manager
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`1
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`I
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`Fig. 1: MHP in the receiver stack.
`
`Standardisation is necessary, but not enough to achieve a successful integration of these
`new technologies in the TV market. Content providers demand the introduction of new value-
`added services which users are willing to pay for. Killer applications are needed for users to
`migrate quickly to this new scenery and for companies to recover their investments, And
`Internet, of course, fits well in this pattern: enhancing TV with Internet access is useful,
`feasible and glamorous. Besides, the DVB consortium considers it is necessary to provide
`digital TV with a “super teletext” system to present structured stylised information to users,
`and Internet technologies are the natural candidates.
`So, MHP pays special attention to the specification of all the necessary details to integrate
`in its computing model a mechanism to access to Internet or to browser over similar
`information. Basically, MHP adopts W3C standards as a starting point and defines some
`reductions and extensions of functionality to cope with the differences between a television
`and a computer. In the remain of the paper, we present some technical aspects of these
`adaptations.
`
`2. INTERNET ACCESS THROUGH TV: THE MHP APPROACH
`
`Several Internet technologies complement each other to achieve the amazing web pages we
`can see every day: several versions of the markup language, styling mechanisms,
`programmatic access to the internal data structures, applets, cookies, plugins, Flash
`animations, etc. Here, we only examine a couple of items, where two circumstances concur:
`the technology is used extensively and the differences with respect to the W3C standards are
`significant. In the following, some knowledge about W3C technologies is supposed.
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`VIPromCom-2002,4th EURASIP - IEEE Region 8 International Symposium on
`Video / Image Processing and Multimedia Communications, 16-1 9 June 2002, Zadar, Croatia
`
`2.1 The language: DVB-HTML
`
`The core element of the MHP Internet access is the markup language the browser
`understands. Following W3C latest standards, DVB has chosen XHTML for this task, using
`the XHTML Modularization [2] standard to build their own language, DVB-HTML. This
`language is composed of a set of modules, selected from the whole set defined in [2], that
`bear in mind the critical differences between a computer and a TV set: computing power,
`screen resolution, observation distance, etc.
`The set of modules included is quite ambitious, leaving out very few and, generally,
`irrelevant or deprecated modules (complete tables, edit, legacy, server-side image maps, name
`identification and applet). Moreover, it substitutes the XHTML Intrinsic Events module for
`the more TV-oriented DVB Intrinsic Events module, where the set of events has been refined
`and adapted to the television environment.
`For traditional Java applets, the more general object module is adopted. This element also
`permits the integration of MHP applications in DVB-HTML documents (embedded Xlets).
`Special considerations for the attributes of the object element are defined, so the embedded
`Xlet can be identified and found in the transport stream. A new MIME type is introduced
`(applicationldvbj) to declare these embedded Xlets.
`With respect to contents, new information formats are introduced, as can be MPEG-2 I-
`frames and video drips. DVB-HTML specifies how to reference audio and video information
`being received in the MHP transport stream. Through a specific dvb locator, DVB-HTML
`documents can play audio and video belonging to a DVB service (a traditional television
`channel). Even more, MHP applications associated to a DVB service can be referenced and
`launched from a DVB-HTML document (if authorized by the application manager),
`enhancing interactive possibilities.
`DVB generalises the concept of DVB-HTML document to the DVB-HTML application,
`which consists in a set of DVB-HTML documents forming a directed graph (with transitions
`labelled by hyperlinks) and where an entry point and a boundary to specify reachability have
`been defined. So, in addition to offer an open mechanism to access the whole Internet, the
`DVB-HTML application model permits the service provider to send formatted and stylised
`data (several related DVB-HTML documents) in a packed format. Even more important, the
`downloading, decoding, rendering and interaction processes of DVB-HTML application
`(performed by a HTML actor - fig. 1) must follow a well-defined life cycle in the user agent,
`which can be commanded from the content provider through the signalling mechanisms in the
`digital transport stream as defined in the MPEG-2, DVB and MHP standards.
`For this latest task of representing documents specifically designed for DVB-HTML
`(probably started and commanded by operators and/or MHP applications) the proposed
`framework seems to be suitable. But browsing web pages in Internet is quite different. Out
`there, documents can be very complex and, often, not conformant: plugins, Flash animations,
`complex style sheets, incompatible scripts, wrong tags, etc. Real browsers use to implement a
`lot of complex code to cope with these problems, to make interpretations about author’s
`intention and to smooth defects to satisfy final observers (who, probably, do not know about
`author’s intentions). This correction task seems to be difficult to achieve in an environment
`scarce of resources, as TV is. So, there is a risk that final results will disappoint to users.
`The idea of commanded web pages (driven by operator signalling) is novel but user’s
`attitude is unpredictable. Will users accept changes in DVB-HTML applications not requested
`by them? Will not they be confused when this happen with stored applications? We think that
`the model must be tested and refined to assess what must be and not be permitted.
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`VIPromCom-2002,4th EURASIP - IEEE Region 8 International Symposium on
`Video I Image Processing and Multimedia Communications, 16-19 June 2002, Zadar, Croatia
`
`2.2 Decorating DVB-HTML documents: CSS2
`
`In order to get a proper stylised representation of the contents included in a DVB-HTML
`document, DVB shares the W3C strategy about separating contents and decoration guidelines.
`With that goal, the W3C CSS2 standard [3] has been adopted as a language to specify
`decoration rules needed to render DVB-HTML documents in a stylised way. However,
`several modifications have been defined to adapt this language to the TV environment:
`It is only mandatory to understand rules directed to two media devices (the @media
`directive): the media screen (for a computer screen), and the media dvb-tv (for a TV-
`like device). Last one is defined to differentiate devices with tv-like characteristics:
`low resolution and computing power, limited scrollability, reduced interfaces, etc.
`A new at-rule is introduced (@viewport) to integrate the MHP graphics reference
`model: coordinates of the viewport, size and position respect to the application
`window, resolution, etc. Some new pseudo-classes are defined to adapt to several
`parameters tv-related: device aspect (4:3,16:9 ...), resolution, etc.
`Lower levels of requirements are demanded to the local fonts resources: observation
`distance and device resolution make nonsense to demand greater detail in font types
`and sizes. Only support for Tiresias type, plain, with four large sizes is mandatory.
`A default style sheet for DVB-HTML is provided in the standard.
`Basically, the DVB proposal for CSS application to the TV environment is very similar
`(with respect to the set of mandatory rules that makes a browser conformant) to the later W3C
`CSS TV Profile [6] (working draft published in December, 2001). As they share a common
`goal over the same media, a quick convergence is expected.
`Content and styling separation through style sheets is a strong bet of W3C that is having
`difficulties to get wide acceptance. Most of web pages do not use style sheets for formatting,
`but a lot of elements and attributes marked as deprecated in W3C standards and classified in
`the Legacy module, not included in DVB-HTML. So, a great risk is taken of users not being
`satisfied with the results of the rendering process. Moreover, experiences with today browsers
`show that it is difficult to get the same appearance, mostly when user’s profiles, rich in
`configuration options, come into play.
`Last, but not least, cascading style sheets application has shown to be an intensive resource
`demanding process, driving to slow applications when such resources are not available.
`
`2.3 Giving access to the structure of documents: DOM Level 2 and ECMAScript
`
`MHP has adopted the W3C DOM Level 2 [4] and the ECMA ECMAScript [5] standards to
`offer programmatic access to the internal structure of the document, contents and decoration
`rules. Of course, as before, several reductions or extensions of functionality have been defined
`for adaptation to the TV scenery.
`The Core and Views modules of DOM Level 2 have been completely integrated. Some
`HTML DOM features have been omitted, for example, those related to the creation of a
`document from scratch (the interface only offers document modification). The events module
`defines mouse events as optional and includes new events related to the life cycle transitions.
`Moreover, it includes a new kind of event derived from synchronisation signals (triggers)
`received from the service operator. This mechanism permits user agents to implement an
`interaction and synchronisation channel with content providers. Last, with respect to DOM
`CSS2 module, MHP does not require to implement it, but it defines a simpler module to
`support basic access to CSS rules.
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`Authorized licensed use limited to: Immanuel Freedman. Downloaded on July 07,2020 at 21:09:22 UTC from IEEE Xplore. Restrictions apply.
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`VIPromCom-2002,4th EURASIP - IEEE Region 8 International Symposium on
`Video I lmaae Processina and Multimedia Communications, 16-19 June 2002, Zadar, Croatia
`
`All the interfaces defined in the previous paragraph must be offered as ECMAScript and
`Java interfaces. So, scripting code and embedded Nets (applets) have direct access to the
`internal structure of DVB-HTML documents, their contents and styling information. Besides,
`an interface must be offered to all MHP API’s, including intercommunication ECMAScript-
`Xlets, which permits the implementation of hybrid applications which divide functionality
`between scripts and Java code in Xlets. For this communication, and taking into account the
`differences in the data type systems of these languages, the standard defines extensively a
`number of casting rules to decide transformations between both data type systems.
`Resuming, DVB has adopted a complex model to support dynamic DVB-HTML pages.
`This is a powerful model but not for free. Computers can pay the price of complexity as
`resources are not scarce and web pages with ambitious objectives demand powerful
`technologies. But, what about TV? What do TV user’s really need? Or, more important, are
`they willing to pay for the effects that may be achieved with that complex framework?
`
`3. CONCLUSION
`
`DVB has made a great effort to define a complex framework that integrates W3C
`technologies for Internet access in the digital TV environment. The result, quite ambitious,
`seems to be suitable for rendering documents specifically designed for this media, So, a
`walled garden approach would guarantee a nice and well-structured information repository.
`But experience has shown that users do not like to be limited to islands in the Web (see WAP
`reduced deployment) and, when accessing the wider Internet, some doubts arise about the cost
`of the resources needed to implement it with a minimum efficiency. Is going the user to pay
`that cost? Is going the user to accept some kind of delay or difference in results? These are
`questions that should be carefully studied before building the specification.
`To start with, the whole process of making the standard has not follow the successful
`guidelines of Internet technologies: openness and incremental development. The MHP
`standard has been delivered to public once finished, in their final and complete shape. The
`result is a lack of feedback along the specification process, and greater difficulties to change
`the standard now, once different pieces have fit together.
`Nobody should forget that history is full of failed projects that proposed a lot of new
`amazing technical capabilities and were highly supported by industry, organizations or
`governments. May be market opinion was not carefully listened. Is this the case?
`
`REFERENCES
`[I] DVB Consortium. Multimedia Home Platform 1.1, httD:llwww.mhn.org July, 2001.
`[2] W3C Consortium. XHTML Modularization. htto:llwww. w3c.org, April, 2001.
`[3] W3C Consortium. CSS2 specification. http-llwww. w3c.org, May, 1998.
`[4] W3C Consortium. DOM Level 2. http:llwww. w3c.orc November, 2000.
`[5] ECMA. ECMAScript version 3. ht@:llwww. ecma.ch, December 1999.
`[6] W3C Consortium, “CSS TVprofile 1.0 ”, Working Draft, December, 2001.
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`Authorized licensed use limited to: Immanuel Freedman. Downloaded on July 07,2020 at 21:09:22 UTC from IEEE Xplore. Restrictions apply.
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`Authorized licensed use limited to: Immanuel Freedman. Downloaded on July 07,2020 at 21:09:22 UTC from IEEE Xplore. Restrictions apply.
`
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