introduction multimedia
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standards

Imagine what it would be like to live in a world without standards. You may get the experience when you travel around and find that there is a totally different socket for electricity in every place that you visit.

Now before we continue, you must realize that there are two types of standards: de facto market standards (enforced by sales politics) and committee standards (that are approved by some official organization). For the latter type of standards to become effective, they need consent of the majority of market players.

For multimedia on the web, we will discuss four standards.

standards

XML, the eXtensible Markup Language, is becoming widely accepted. It is being used to replace HTML, as well as a data exchange format for, for example, business-to-business transactions. XML is derived from SGML (Structured Generalized Markup Language) that has found many applications in document processing. As SGML, XML is a generic language, in that it allows for the specification of actual markup languages. Each of the other three standards mentioned allows for a syntactic encoding using XML.

MPEG-4 aims at providing "the standardized technological elements enabling the integration of production, distribution and content access paradigms of digital television, interactive graphics and multimedia",  [MPEG-4]. A preliminary version of the standard has been approved in 1999. Extensions in specific domains are still in progress.

SMIL, the Synchronized Multimedia Integration Language, has been proposed by the W3C "to enable the authoring of TV-like multimedia presentations, on the Web". The SMIL language is an easy to learn HTML-like language. SMIL presentations can be composed of streaming audio, streaming video, images, text or any other media type,  [SMIL]. SMIL-1 has become a W3C recommendation in 1998. SMIL-2 is at the moment of writing still in a draft stage.

RM3D, Rich Media 3D, is not a standard as MPEG-4 and SMIL, since it does currently not have any formal status. The RM3D working group arose out of the X3D working group, that addressed the encoding of VRML97 in XML. Since there were many disagreements on what should be the core of X3D and how extensions accomodating VRML97 and more should be dealt with, the RM3D working group was founded in 2000 to address the topics of extensibility and the integration with rich media, in particular video and digital television.

remarks

Now, from this description it may seem as if these groups work in total isolation from eachother. Fortunately, that is not true. MPEG-4, which is the most encompassing of these standards, allows for an encoding both in SMIL and X3D. The X3D and RM3D working groups, moreover, have advised the MPEG-4 commitee on how to integrate 3D scene description and human avatar animation in MPEG-4. And finally, there have been rather intense discussions between the SMIL and RM3D working groups on the timing model needed to control animation and dynamic properties of media objects.

MPEG-4

 The MPEG standards (in particular 1,2 and 3) have been a great success, as testified by the popularity of mp3 and DVD video.

Now, what can we expect from MPEG-4? Will MPEG-4 provide multimedia for our time, as claimed in  [Time]. The author, Rob Koenen, is senior consultant at the dutch KPN telecom research lab, active member of the MPEG-4 working group and editor of the MPEG-4 standard document.

"Perhaps the most immediate need for MPEG-4 is defensive. It supplies tools with which to create uniform (and top-quality) audio and video encoders on the Internet, preempting what may become an unmanageable tangle of proprietary formats."

Indeed, if we are looking for a general characterization it would be that MPEG-4 is primarily

MPEG-4


a toolbox of advanced compression algorithms for audiovisual information

and, moreover, one that is suitable for a variety of display devices and networks, including low bitrate mobile networks. MPEG-4 supports scalability on a variety of levels:

scalability

  • bitrate -- switching to lower bitrates
  • bandwidth -- dynamically discard data
  • encoder and decoder complexity -- signal quality
Dependent on network resources and platform capabilities, the 'right' level of signal quality can be determined by selecting the optimal codec, dynamically.

media objects

 It is fair to say that MPEG-4 is a rather ambitious standard. It aims at offering support for a great variety of audiovisual information, including still images, video, audio, text, (synthetic) talking heads and synthesized speech, synthetic graphics and 3D scenes, streamed data applied to media objects, and user interaction -- e.g. changes of viewpoint.

audiovisual information


  • still images, video, audio, text
  • (synthetic) talking heads and synthesized speech
  • synthetic graphics and 3D scenes
  • streamed data applied to media objects
  • user interaction -- e.g. changes of viewpoint

Let's give an example, taken from the MPEG-4 standard document.

example


Imagine, a talking figure standing next to a desk and a projection screen, explaining the contents of a video that is being projected on the screen, pointing at a globe that stands on the desk. The user that is watching that scene decides to change from viewpoint to get a better look at the globe ...

How would you describe such a scene? How would you encode it? And how would you approach decoding and user interaction?

The solution lies in defining media objects and a suitable notion of composition of media objects.

media objects


  • media objects -- units of aural, visual or audiovisual content
  • composition -- to create compound media objects (audiovisual scene)
  • transport -- multiplex and synchronize data associated with media objects
  • interaction -- feedback from users' interaction with audiovisual scene
For 3D-scene description, MPEG-4 builds on concepts taken from VRML (Virtual Reality Modeling Language, discussed in chapter 7).

Composition, basically, amounts to building a scene graph, that is a tree-like structure that specifies the relationship between the various simple and compound media objects. Composition allows for placing media objects anywhere in a given coordinate system, applying transforms to change the appearance of a media object, applying streamed data to media objects, and modifying the users viewpoint.

composition


  • placing media objects anywhere in a given coordinate system
  • applying transforms to change the appearance of a media object
  • applying streamed data to media objects
  • modifying the users viewpoint

So, when we have a multimedia presentation or audiovisual scene, we need to get it accross some network and deliver it to the end-user, or as phrased in  [MPEG-4]:

transport


The data stream (Elementary Streams) that result from the coding process can be transmitted or stored separately and need to be composed so as to create the actual multimedia presentation at the receivers side.

At a system level, MPEG-4 offers the following functionalities to achieve this:

scenegraph


  • BIFS (Binary Format for Scenes) -- describes spatio-temporal arrangements of (media) objects in the scene
  • OD (Object Descriptor) -- defines the relationship between the elementary streams associated with an object
  • event routing -- to handle user interaction
In addition, MPEG-4 defines a set of functionalities For the delivery of streamed data, DMIF, which stands for

DMIF


Delivery Multimedia Integration Framework

that allows for transparent interaction with resources, irrespective of whether these are available from local storage, come from broadcast, or must be obtained from some remote site. Also transparency with respect to network type is supported. Quality of Service is only supoorted to the extent that it ispossible to indicate needs for bandwidth and transmission rate. It is however the responsability of the network provider to realize any of this.

authoring

What MPEG-4 offers may be summarized as follows

benefits


  • end-users -- interactive media accross all platforms and networks
  • providers -- transparent information for transport optimization
  • authors -- reusable content, protection and flexibility
In effect, although MPEG-4 is primarily concerned with efficient encoding and scalable transport and delivery, the object-based approach has also clear advantages from an authoring perspective.

One advantage is the possibility of reuse. For example, one and the same background can be reused for multiplepresentations or plays, so you could imagine that even an amateur game might be 'located' at the centre-court of Roland Garros or Wimbledon.

Another, perhaps not so obvious, advantage is that provisions have been made for

managing intellectual property

Of media objects.

And finally, media objects may potentially be annotated with meta-information to facilitate information retrieval.

syntax

In addition to the binary formats, MPEG-4 also specifies a syntactical format, called XMT, which stands for eXtensible MPEG-4 Textual format.

XMT


  • XMT contains a subset of X3D
  • SMIL is mapped (incompletely) to XMT
when discussing RM3D, we will further establish whatthe relations between, respectively MPEG-4, SMIL and RM3D are, and in particular where there is disagreement, for example with respect to the timing model underlying animations and the temporal control of media objects.

the press

 Now to conclude our discussion of MPEG-4, let's see what the press has to say about it.

www.eetimes.com/story/OEG20010220S0065


MPEG-4 is "a big standard," said Tim Schaaff, vice president of engineering for Apple Computer Inc.'s Interactive Media Group. "It's got tons of tools inside." Its success, he said, will depend on the industry's willingness to home in on a small subset, winnowing from a number of profiles and levels designed for streaming a slew of digital multimedia types -- audio, several types of video, still images, and 2-D and 3-D graphics.

Some may find it to ambitious.

unfocused ambition

"MPEG-4 is a very ambitious standard, but its biggest problem is that it wasn't focused on anything," said Didier LeGall, vice president for R&D and chief technology officer at chip house C-Cube Microsystems Inc. LeGall dismissed MPEG-4's vaunted object-based coding -- one of the technologies that sets it apart from earlier MPEG spins -- as "science fiction" and "nothing more than a gadget" at this point. "I haven't seen any content with objects that really makes sense," he said.

But, then again, what it offers is clearly worthwhile.

coding


MPEG-4's chief features include highly efficient compression, error resilience, bandwidth scalability ranging from 5 kbits to 20 Mbits/second, network and transport-protocol independence, content security and object-based interactivity, or the ability to pluck a lone image -- say, the carrot Bugs Bunny is about to chomp -- out of a video scene and move it around independently.

And, not altogether unimportant, it may offer significant commercial benefits.

Broadband service providers, such as cable and DSL companies, are right behind wireless in sizing up MPEG-4, largely because its low bit rate could help them add channels in their broadband pipes while incorporating interactive features in the content. Possibilities include multiple video streams, clickable video, real-time 3-D animation and interactive advertising.

SMIL

SMIL is pronounced as smile. SMIL, the Synchronized Multimedia Integration Language, has been inspired by the Amsterdam Hypermedia Model (AHM). In fact, the dutch research group at CWI that developed the AHM actively participated in the SMIL 1.0 committee. Moreover, they have started a commercial spinoff to create an editor for SMIL, based on the editor they developed for CMIF. The name of the editor is GRINS. Get it?

As indicated before SMIL is intended to be used for

SMIL


TV-like multimedia presentations

The SMIL language is an XML application, resembling HTML. SMIL presentations can be written using a simple text-editor or any of the more advanced tools, such as GRINS. There is a variety of SMIL players. The most wellknown perhaps is the RealNetworks G8 players, that allows for incorporating RealAudio and RealVideo in SMIL presentations.

parallel and sequential


Authoring a SMIL presentation comes down, basically, to

name media components for text, images,audio and video with URLs, and to schedule their presentation either in parallel or in sequence.

Quoting the SMIL 2.0 working draft, we can characterize the SMIL presentation characteristics as follows:

presentation characteristics


  • The presentation is composed from several components that are accessible via URL's, e.g. files stored on a Web server.
  • The components have different media types, such as audio, video, image or text. The begin and end times of different components are specified relative to events in other media components. For example, in a slide show, a particular slide is displayed when the narrator in the audio starts talking about it.
  • Familiar looking control buttons such as stop, fast-forward and rewind allow the user to interrupt the presentation and to move forwards or backwards to another point in the presentation.
  • Additional functions are "random access", i.e. the presentation can be started anywhere, and "slow motion", i.e. the presentation is played slower than at its original speed.
  • The user can follow hyperlinks embedded in the presentation.
Where HTML has become successful as a means to write simple hypertext content, the SMIL language is meant to become a vehicle of choice for writing synchronized hypermedia. The working draft mentions a number of possible applications, for example a photoalbun with spoken comments, multimedia training courses, product demos with explanatory text, timed slide presentations, onlime music with controls.

applications


  • Photos taken with a digital camera can be coordinated with a commentary
  • Training courses can be devised integrating voice and images.
  • A Web site showing the items for sale, might show photos of the product range in turn on the screen, coupled with a voice talking about each as it appears.
  • Slide presentations on the Web written in HTML might be timed so that bullet points come up in sequence at specified time intervals, changing color as they become the focus of attention.
  • On-screen controls might be used to stop and start music.

As an example, let's consider an interactive news bulletin, where you have a choice between viewing a weather report or listening to some story about, for example, the decline of another technology stock. Here is how that could be written in SMIL:

example



   <par>
      <a href="#Story"> <img src="button1.jpg"/> </a>
      <a href="#Weather"> <img src="button2.jpg"/></a>
       <excl>
           <par id="Story" begin="0s">
             <video src="video1.mpg"/>
             <text src="captions.html"/>
           </par>
  
           <par id="Weather">
             <img src="weather.jpg"/>
             <audio src="weather-rpt.mp3"/>
           </par>
       </excl>
   </par>
Notice that there are two parallel (PAR) tags, and one exclusive (EXCL) tag. The exclusive tag has been introduced in SMIL 2.0 to allow for making an exclusive choice,so that only one of the items can be selected at a particular time. The SMIL 2.0 working draft defines a number of elements and attributes to control presentation, synchronization and interactivity, extending the functionality of SMIL 1.0.

Before discussing how the functionality proposed in the SMIL 2.0working draft may be realized, we might reflect on how to position SMIL with respect to the many other approaches to provide multimedia on the web. As other approaches we may think of flash, dynamic HTML (using javascript), or java applets. In the SMIL 2.0 working draft we read the following comment:

history


Experience from both the CD-ROM community and from the Web multimedia community suggested that it would be beneficial to adopt a declarative format for expressing media synchronization on the Web as an alternative and complementary approach to scripting languages.

Following a workshop in October 1996, W3C established a first working group on synchronized multimedia in March 1997. This group focused on the design of a declarative language and the work gave rise to SMIL 1.0 becoming a W3C Recommendation in June 1998.

In summary, SMIL 2.0 proposes a declarative format to describe the temporal behavior of a multimedia presentation, associate hyperlinks with media objects, describe the form of the presentation on a screen, and specify interactivity in multimedia presentations. Now,why such a fuzz about "declarative format"? Isn't scripting more exciting? And aren't the tools more powerful? Ok, ok. I don't want to go into that right now. Let's just consider a declarative format to be more elegant. Ok?

To support the functionality proposed for SMIL 2.0 the working draft lists a number of modules that specify the interfaces for accessing the attributes of the various elements. SMIL 2.0 offers modules for animation, content control, layout, linking, media objects, meta information, timing and synchronization, and transition effects.

SMIL 2.0 Modules


  • The Animation Modules
  • The Content Control Modules
  • The Layout Modules
  • The Linking Modules
  • The Media Object Modules
  • The Metainformation Module
  • The Structure Module
  • The Timing and Synchronization Module
  • The Time Manipulations Module
  • The Transition Effects Module

This modular approach allows to reuse SMIL syntax and semantics in other XML-based languages, in particular those that need to represent timing and synchronization. For example:

module-based reuse


  • SMIL modules could be used to provide lightweight multimedia functionality on mobile phones, and to integrate timing into profiles such as the WAP forum's WML language, or XHTML Basic.
  • SMIL timing, content control, and media objects could be used to coordinate broadcast and Web content in an enhanced-TV application.
  • SMIL Animation is being used to integrate animation into W3C's Scalable Vector Graphics language (SVG).
  • Several SMIL modules are being considered as part of a textual representation for MPEG4.
The SMIL 2.0 working draft is at the moment of writing being finalized. It specifies a number of language profiles topromote the reuse of SMIL modules. It also improves on the accessibility features of SMIL 1.0, which allows for, for example,, replacing captions by audio descriptions.

In conclusion, SMIL 2.0 is an interesting standard, for a number of reasons. For one, SMIL 2.0 has solid theoretical underpinnings in a well-understood, partly formalized, hypermedia model (AHM). Secondly, it proposes interesting functionality, with which authors can make nice applications. In the third place, it specifies a high level declarative format, which is both expressive and flexible. And finally, it is an open standard (as opposed to proprietary standard). So everybody can join in and produce players for it!

RM3D

The web started with simple HTML hypertext pages. After some time static images were allowed. Now, there is support for all kinds of user interaction, embedded multimedia and even synchronized hypermedia. But despite all the graphics and fancy animations, everything remains flat. Perhaps surprisingly, the need for a 3D web standard arose in the early days of the web. In 1994, the acronym VRML was coined by Tim Berners-Lee, to stand for Virtual Reality Markup Language. But, since 3D on the web is not about text but more about worlds, VRML came to stand for Virtual Reality Modeling Language. Since 1994, a lot of progress has been made.

www.web3d.org


  • VRML 1.0 -- static 3D worlds
  • VRML 2.0 or VRML97 -- dynamic behaviors
  • VRML200x -- extensions
  • X3D -- XML syntax
  • RM3D -- Rich Media in 3D
In 1997, VRML2 was accepted as a standard, offering rich means to create 3D worlds with dynamic behavior and user interaction. VRML97 (which is the same as VRML2) was, however, not the success it was expected to be, due to (among others) incompatibility between browsers, incomplete implementations of the standards, and high performance requirements.

As a consequence, the Web3D Consortium (formerly the VRML Consortium) broadened its focus, and started thinking about extensions or modifications of VRML97 and an XML version of VRML (X3D). Some among the X3D working group felt the need to rethink the premisses underlying VRML and started the Rich Media Working Group:

groups.yahoo.com/group/rm3d/


The Web3D Rich Media Working Group was formed to develop a Rich Media standard format (RM3D) for use in next-generation media devices. It is a highly active group with participants from a broad range of companies including 3Dlabs, ATI, Eyematic, OpenWorlds, Out of the Blue Design, Shout Interactive, Sony, Uma, and others.

In particular:

RM3D


The Web3D Consortium initiative is fueled by a clear need for a standard high performance Rich Media format. Bringing together content creators with successful graphics hardware and software experts to define RM3D will ensure that the new standard addresses authoring and delivery of a new breed of interactive applications.

The working group is active in a number of areas including, for example, multitexturing and the integration of video and other streaming media in 3D worlds.

Among the driving forces in the RM3D group are Chris Marrin and Richter Rafey, both from Sony, that proposed Blendo, a rich media extension of VRML. Blendo has a strongly typed object model, which is much more strictly defined than the VRML object model, to support both declarative and programmatic extensions. It is interesting to note that the premisse underlying the Blendo proposal confirms (again) the primacy of the TV metaphor. That is to say, what Blendo intends to support are TV-like presentations which allow for user interaction such as the selection of items or playing a game. Target platforms for Blendo include graphic PCs, set-top boxes, and the Sony Playstation!

requirements

The focus of the RM3D working group is not syntax (as it is primarily for the X3D working group) but semantics, that is to enhance the VRML97 standard to effectively incorporate rich media. Let's look in more detail at the requirements as specified in the RM3Ddraft proposal.

requirements


  • rich media -- audio, video, images, 2D & 3D graphics (with support for temporal behavior, streaming and synchronisation)
  • applicability -- specific application areas, as determined by commercial needs and experience of working group members
The RM3D group aims at interoperability with other standards.

  • interoperability -- VRML97, X3D, MPEG-4, XML (DOM access)
In particular, an XML syntax is being defined in parallel (including interfaces for the DOM). And, there is mutual interest and exchange of ideas between the MPEG-4 and RM3D working group.

As mentioned before, the RM3D working group has a strong focus on defining an object model (that acts as a common model for the representation of objects and their capabilities) and suitable mechanisms for extensibility (allowing for the integration of new objects defined in Java or C++, and associated scripting primitives and declarative constructs).

Notice that extensibility also requires the definition of a declarative format, so that the content author need not bother with programmatic issues.

The RM3D proposal should result in effective 3D media presentations. So as additional requirements we may, following the working draft, mention: high-quality realtime rendering, for realtime interactive media experiences; platform adaptability, with query functions for programmatic behavior selection; predictable behavior, that is a well-defined order of execution; a high precision number systems, greater than single-precision IEEE floating point numbers; and minimal size, that is both download size and memory footprint.

Now, one may be tempted to ask how the RM3D proposals is related to the other standard proposals such as MPEG-4 and SMIL, discussed previously. Briefly put, paraphrased from one of Chris Marrin's messages on the RM3D mailing list

SMIL is closer to the author and RM3D is closer to the implementer.

MPEG-4, in this respect is even further away from the author since its chief focus is on compression and delivery across a network.

RM3D takes 3D scene description as a starting point and looks at pragmatic ways to integrate rich media. Since 3D is itself already computationally intensive, there are many issues thatarise in finding efficient implementations for the proposed solutions.

timing model

RM3D provides a declarative format formany interesting features, such as for example texturing objects with video. In comparison to VRML, RM3D is meant to provide more temporal control over time-based media objects and animations. However, there is strong disagreement among the working group members as to what time model the dynamic capabilities of RM3D should be based on. As we read in the working draft:

working draft


Since there are three vastly different proposals for this section (time model), the original <RM3D> 97 text is kept. Once the issues concerning time-dependent nodes are resolved, this section can be modified appropriately.

Now, what are the options? Each of the standards discussed to far provides us with a particular solution to timing. Summarizing, we have a time model based on a spring metaphor in MPEG-4, the notion of cascading time in SMIL (inspired by cascading stylesheets for HTML) and timing based on the routing of events in RM3D/VRML.

time model


  • MPEG-4 -- spring metaphor
  • SMIL -- cascading time
  • RM3D/VRML -- event routing

The MPEG-4 standard introduces the spring metaphor for dealing with temporal layout.

MPEG-4 -- spring metaphor


  • duration -- minimal, maximal, optimal
The spring metaphor amounts to the ability to shrink or stretch a media object within given bounds (minimum, maximum) to cope with, for example, network delays.

The SMIL standard is based on a model that allows for propagating durations and time manipulations in a hierarchy of media elements. Therefore it may be referred to as a cascading modelof time.

SMIL -- cascading time


  • time container -- speed, accelerate, decelerate, reverse, synchronize
Media objects, in SMIL, are stored in some sort of container of which the timing properties can be manipulated. 


  <seq speed="2.0">
     <video src="movie1.mpg" dur="10s"/>
     <video src="movie2.mpg" dur="10s"/>
     <img src="img1.jpg" begin="2s" dur="10s">
                 <animateMotion from="-100,0" to="0,0" dur="10s"/>
     </img>
     <video src="movie4.mpg" dur="10s"/>
  </seq>
In the example above,we see that the speed is set to 2.0, which will affect the pacing of each of the individual media elements belonging to that (sequential) group. The duration of each of the elements is specified in relation to the parent container. In addition, SMIL offers the possibility to synchronize media objects to control, for example, the end time of parallel media objects.

VRML97's capabilities for timing rely primarily on the existence of a TimeSensor thatsends out time events that may be routed to other objects.

RM3D/VRML -- event routing


  • TimeSensor -- isActive, start, end, cycleTime, fraction, loop
When a TimeSensor starts to emit time events, it also sends out an event notifying other objects that it has become active. Dependent on itsso-called cycleTime, it sends out the fraction it covered since it started. This fraction may be send to one of the standard interpolators or a script so that some value can be set, such as for example the orientation, dependent on the fraction of the time intercal that has passed. When the TimeSensor is made to loop, this is done repeatedly. Although time in VRML is absolute, the frequency with which fraction events are emitted depends on the implementation and processor speed.

Lacking consensus about a better model, this model has provisionally been adopted, with some modifications, for RM3D. Nevertheless, the SMIL cascading time model has raised an interest in the RM3D working group, to the extent that Chris Marrin remarked (in the mailing list) "we could go to school here". One possibility for RM3D would be to introduce time containers that allow for a temporal transform of their children nodes, in a similar way as grouping containers allow for spatial transforms of their children nodes. However, that would amount to a dual hierarchy, one to control (spatial) rendering and one to control temporal characteristics. Merging the two hierarchies, as is (implicitly) the case in SMIL, might not be such a good idea, since the rendering and timing semantics of the objects involved might be radically different. An interesting problem, indeed, but there seems to be no easy solution.

research directions -- meta standards

All these standards! Wouldn't it be nice to have one single standard that encompasses them all? No, it would not! Simply, because such a standard is inconceivable, unless you take some proprietary standard or a particular platform as the defacto standard (which is the way some people look at the Microsoft win32 platform, ignoring the differences between 95/98/NT/2000/XP/...). In fact, there is a standard that acts as a glue between the various standards for multimedia, namely XML. XML allows for the interchange of data between various multimedia applications, that is the transformation of one encoding into another one. But this is only syntax. What about the semantics?

Both with regard to delivery and presentation the MPEG-4 proposal makes an attempt to delineate chunks of core fuctionality that may be shared between applications. With regard to presentation, SMIL may serve as an example. SMIL applications themselves already (re)use functionality from the basic set of XML-related technologies, for example to access the document structure through the DOM (Document Object Model). In addition, SMIL defines components that it may potentially share with other applications. For example, SMIL shares its animation facilities with SVG (the Scalable Vector Graphics format recommended by the Web Consortium).

The issue in sharing is, obviously, how to relate constructs in the syntax to their operational support. When it is possible to define a common base of operational support for a variety of multimedia applications we would approach our desired meta standard, it seems. A partial solution to this problem has been proposed in the now almost forgotten HyTime standard for time-based hypermedia. HyTime introduces the notion of architectural forms as a means to express the operational support needed for the interpretation of particular encodings, such as for example synchronization or navigation over bi-directional links. Apart from a base module, HyTime compliant architectures may include a units measurement module, a module for dealing with location addresses, a module to support hyperlinks, a scheduling module and a rendition module.

To conclude, wouldn't it be wonderful if, for example, animation support could be shared between rich media X3D and SMIL? Yes, it would! But as you may remember from the discussion on the timing models used by the various standards, there is still to much divergence to make this a realoistic option.
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eliens@cs.vu.nl

draft version 1 (16/5/2003)