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Standards for Interactive Television:
A Brief State of the Union
Tim Halle, WGBH
Interactive television standards are still in a state of flux, in terms of both development and implementation. However, an overview of the standards, where they came from, and where they are going is useful to create a context in which to think about developing interactive television applications.
Standardization efforts are taking place three areas: the declarative environment, the procedural environment and the emerging metadata standards for VOD and PVR technologies.
A declarative environment allows the designer to create content in a high level language (such as HTML) to be rendered by a graphics engine. In the case of interactive television, the graphics engine may be within the set top box. In a cable system where the set-top boxes lack sufficient computing power for this function, the graphics engine may instead reside within the head-end equipment. In this case, as a user navigates interactive content, the graphics engine passes custom screen render images back to their particular set-top.
In a procedural environment, applications are directly executed within the operating system of the set-top box. In the early days of interactive television, set top boxes were very low powered and it was found that writing a complete application was more efficient than supporting a browser. An application could contain just enough functionality to address its specific needs, while a browser would require overhead for any needs including those not required for a specific application.
Program Metadata has become increasingly important in ITV. Metadata is the "data about the data." It is a file, usually in XML format, that contains attributes of a given piece of content. Metadata is ubiquitous throughout any industry with the slightest use of digital technology, including the standards mentioned above. In this context we are only addressing program related metadata as it relates to describing a particular television program.
Most ITV standards incorporate two or more of these functionalities. For purposes of clarity we will address each sub-topic individually.
DECLARATIVE ENVIRONMENTS
Early ITV implementations of interactive television such as the Liberate "A" platform and OpenTV's "Device Mosaic" were based on standards developed by the W3C and the IETF. Much of this work survives in current standards. These organizations provide archives of technical information that are a valuable resource in developing ITV.
ATVEF
In 1998 a consortium of industry players released the first version of the Advanced Television Enhancement Forum (ATVEF) specification. ATVEF members included CableLabs; CNN; DirecTV; Discovery Communications (DCI); The Walt Disney Company; Intel Corporation; Microsoft Corporation/WebTV Networks; NBC Multimedia; Network Computer, Inc. (NCI); NDTC Technology; Public Broadcasting Service (PBS); Sony Corporation; Tribune Company and Warner Bros. This specification, while at times problematic, forms the basis of conversation about implementation of interactive television to this day.
The basic ATVEF spec calls for HTML 4.0 (Frameset Document Type Definition), CSS 1, and ECMAScript/DOM 0, (taken together are roughly equivalent to JavaScript 1.1). The binding section contains descriptions of line 21 triggers (ATVEF transport A), data encapsulation within the broadcast stream via UHTTP and retrieval of cached information via the LID tag (ATVEF-B).
While a very good first effort, this standard soon ran into problems. Implementation varied widely from manufacturer to manufacturer, in part due to the nebulous nature of the specification. Missing from ATVEF is a standard definition of both font size and screen size as well as a standardized method to determine initial focus on a given page. This situation is exacerbated by its having based format scripting on DOM 0 which is in itself an ill-defined standard.
DDE-1 and SMPTE 343M
Following in the footsteps of ATVEF was DDE-1, a joint effort between NIST's now defunct interactive television program and SMPTE to make the ATVEF Specification a SMPTE standard as well as clean up parts of the ATVEF specification. The basic browser functionality is now defined under SMPTE 343, "Declarative Data Essence," while transport related issues are covered in SMPTE 357M, "Declarative Data Essence, IP Multicast Encapsulation," SMPTE 364M, "Declarative Data Essence - Unidirectional Transport Protocol," and SMPTE Proposed Standard 361M, "NTSC IP and Trigger Binding to VBI."
ATSC DASE
While SMPTE 343M was under development, many of the same personnel developed ATSC Standard A/100: DTV Application Software Environment (DASE). This standard contains both a declarative environment, and a procedural environment (discussed later).
DASE is an incredibly well documented standard, running to hundreds of pages. The Declarative implementation consists of an XDML and XHTML subset which is functionally similar to HTML 4.0, although the language requirements are more stringent (and therefore more standardized). A guide to transcoding HTML to XDML is provided within the documentation, (A 100 pt 2 annex F). A wide variety of content types are supported such as CSS, ECMAScript, DOM, along with most common graphics audio and video formats. Also provided is a procedural application environment which allows for system calls from the declarative environment.
The downside to this standard is that it is directed towards the eventual implementation of ATSC digital terrestrial broadcast and is therefore biased towards unidirectional transmission of content via DTV datacast. While this is a perfectly reasonable bias in the context of this project, many application possibilities are lost by not including an IP back channel. However, given this standards relationship to SMPTE 343M, there is a reasonable possibility that this functionality may be incorporated into future releases.
DVB MHP
DVB-HTML is the declarative portion of the European MHP specification. Basic presentation elements (a subset of XHTML) and content type support are similar to DASE. Unlike DASE, DVB-HTML is built around the idea of an internet connection. Accordingly DVB-HTML includes support for most common web protocols such as HHTP, NNTP and SMTP. There are a variety of trigger methods described by DVB-MHP. Because this standard is built around the European television standards that do not include line 21 closed-captioning, we have been unable to test this functionality.
OCAP
The Open Cable Application Platform (OCAP), is the standard proposed by CableLabs, a research consortium of US MSOs. Based directly on DVB-HTML, the declarative environment of OCAP is basically a stripped down version of DVB-HTML without direct support for internet transport protocols, although some of these protocols are supported via with in the OCAP procedural environment. So what is meant by stripped down? While officially this standard is said be comparable to HTML 4.0 with CSS-2 and JavaScript 1.1, actual implementation is said by sources within OCAP to be HTML 3.2 with no scripting support.
PROCEDURAL ENVIRONMENTS
Early efforts to implement ITV on low powered set top boxes used a procedural environment because an application is a more efficient method of displaying ITV than a browser rendering declarative content (such as HTML). A number of proprietary systems such as Wink and Open TV still use this approach. Most of the current standardization efforts are centered around Sun Microsystems Java, which also forms the basis of commercial middleware produced by Liberate, Canal+ and others.
The basic principle behind JAVA is that it provides a consistent software platform (the Java Virtual Machine) regardless of hardware. Therefore such applications should, at least in theory, be transportable from platform to platform, solving the common transportability problem in ITV.
JavaTV is a collection of APIs which allow java applications a standardized method to communicate with various set-top-box hardware and television-centric functions. These include audio and video processing, conditional access, in-band and out-of-band data channels, service information data, tuner control for channel changing, and on-screen graphics control. In some cases these functions are handled by calls to an existing declarative environment.
The following procedural standards are all based on the Java VM, so at a high level they are quite similar. Implementation differences arise only at a code level outside the scope of this document.
DVB-MHP
The DVB-MHP Java environment, DVB-J, is widely installed on set top boxes in Europe. As such, it is the only procedural standard in wide use. Unlike OCAP and DASE, this standard supports an IP back channel in addition to a broadcast data carousel.
OCAP
This standard is a derivation of the DVB-MHP Java environment. The main deviations are around supporting implementation of ATSC DTV functionality and the differences between the NTSC and PAL television standards.
DASE
Because it is an element of a much larger specification, the DASE procedural environment is much more tightly integrated with its component parts. Because the broadcast community has already begun adoption of much of what is contained in the ATSC DTV specification, this standard has a significant leg up on the competing OCAP standard. Whether the MSOs will follow suit is an open question, although there have been a number of meetings around harmonization of these two specifications.
PROGRAM RELATED METADATA
Attributes described by program related metadata can include anything from simple EPG data (show name, actors, genre rating etc), to a complete index of different scenes and associated enhancement content with in-depth rights information detailing how the content package may be displayed, copied, or sold. In the context of ITV, metadata can enable anything from URL pointers for non-synchronous download of enhancement content, to incorporation of advanced searching and parsing of video content and descriptions of the enhancements themselves.
OCAP
OCAP VOD Metadata has been developed by CableLabs, a research consortium of US MSOs. It is oriented towards deployment of content within cable VOD or SVOD systems. In addition to metadata, this standard also defines content encoding specifications for VOD.
TV Anytime
The TV Anytime Forum is a collection of manufactures, primarily from the consumer electronics industry, working to define standards around consumer electronics devices with on board storage (such as PVRs), and the emerging home media center products. Standards have been under development in four areas: Business Models, Transport Interfaces and Content Referencing, Metadata and Rights Management and Protection.
MPEG (Moving Picture Experts Group)
MPEG-7, the "Multimedia Content Description Interface," is a standard developed by MPEG (Moving Picture Experts Group), the committee that developed the widely used MPEG-1, MPEG-2, and MPEG-4 standards. By far the most ambitious of these projects, MPEG-7 supports descriptions of all forms of content, including still pictures, graphics, 3D models, audio, speech, video, as well as information about how they are related and integrated into a complete pieces or "scenarios." Finally MPEG-7 specifies a method for extending its functionality via the Description Definition Language (DDL) which allows for the creation, extension and modification of various functions.
iTV Standards Committee
Initiated by the ITV developer Goldpocket Interactive, the iTV Production Standards Initiative focuses on defining server side implementation of ITV applications. It defines a common way of writing XML code to describe components of common enhanced television programming such as trivia questions, polls, interactive advertising, and leaderboards. Not surprisingly, this standard conforms closely to the needs of Goldpocket's technology.
Harmonization efforts: Standardizing the standards
It is important to note that there are a number of initiatives afoot to standardize these various standards into a single "meta" standard. Most notable among them is the December 2002 strategic agreement between ATSC and CableLabs. While all are hopeful that this will reach a successful conclusion, various vested interests may make such an arrangement impossible.
CableLabs are the research arm of the MSOs who are interested in functionality that enables them to create money generating services at the lowest cost. Accordingly they seek to create a standard which supports only the most minimal requirements of these services.
Conversely, ATSC has been extremely supportive of advanced services within digital terrestrial television. While there will undoubtedly be some sort of standardization involving reception and transmission of DTV content within cable television system, a situation enforced by the ongoing transition to digital television within the United States, the implications for ITV are unknown.
Another important standardization effort is the December 2002 memorandum of understanding between the MSOs and the consumer electronics manufacturers. This agreement goes into great depth on the subject of conditional access and copy protection. While discussion of advanced interactive services is left for later implementation, it is encouraging that it is at least mentioned.
There have been no official program-related metadata announcements to date. However it should be noted that the entities involved cite each other as normative references in their documentation, and in some cases as members of their organization, indicating some degree of crossover and cooperation. Many services enabled by program-related metadata, notably VOD and SVOD, have been proven to generate income for MSOs, making standardization and implementation of these standards a clear priority within the industry.
Revised Thursday, 27-Mar-2003 00:42:21 CST
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© 2000 - 2003 Local Enhancement Collaborative &  .
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