5.6 Declarative Data Essence

The major organization concerned with content formats is the SMPTE. It has been working for some time to standardize the integration of various related content standards, including derivatives of HTML, CSS, and DOM. The DDE Ad-Hoc Group of the D27 Technical Committee of SMPTE has developed SMPTE 363M, also known as DDE-1 [DDE]. It standardizes some aspects of integration of iTV standards that provide basic functionality embodying current practice. The ATVEF specification was the basis for the DDE-1 effort.

DDE-1 specifies HTML 4.0, on which XHTML 1.0 is based, as well as CSS Level 1. CSS Level 2 includes features such as new pseudo-elements (e.g., the first letter of a paragraph), and new pseudo-classes (e.g., a link that has been visited). In addition, CSS2 specifies new types of selectors, such as attribute selectors, that can identify markup elements by the presence of an attribute and/or attribute value, and parent/child selectors which can identify markup elements based on parent/child relationships.

DDE-1 specifies ECMAScript 2nd Edition. The ECMAScript 3rd Edition adds "switch" statements, try/catch exception handling, and native regular expression objects. The functionality of these regular expression objects is modeled after regular expressions in Perl 5.

DDE-1 specifies DOM Level 0. DOM-0 generally has the functionality of the object model common to both Microsoft Internet Explorer 3.0 and Netscape Navigator 3.0. This functionality is currently in widespread use on the Web today. In contrast, iTV standards typically require DOM-1 or DOM-2. DOM-2 includes the reading and mutation of almost all elements of the XHTML document, in particular scripts and style sheets. With DOM-2, changing information on a page can be accomplished by mutation, rather than by replacing an entire frame. This is particularly useful when the amount of information to be changed is small (e.g., a number in a table).

The DOM-0 event model is based on intrinsic events as specified in HTML 4.0 syntax. In contrast, the DOM-2 event model is more robust and its behavior more well defined. Although not technically part of the DOM, DDE-1 specifies native objects in support of current practice. These are the Window , Location , History , and Navigator objects.

DDE-1 specifies two types of interactive content delivery: Transport A and Transport B. With Transport A, only triggers are delivered with the broadcast stream. Other primary content is actively acquired by the receiver by means of a return channel, typically, an Internet connection. With Transport B, all content, including triggers, is delivered using a Unidirectional HTTP (UHTTP) broadcast stream. A bidirectional return channel may optionally be present for transactions or general Web browsing. DDE-1 specifies the binding of the UHTTP to IP Multicast, as well as the binding for both Transport A and Transport B to the NTSC video standard using VBI. Further, DDE-1 specifies binding for both Transport A and Transport B to the PAL/SECAM system.

Broadcasters of iTV applications need to be able to initiate content for a segment of content, whether part of a commercial or a program. DDE-1 specifies a simple iTV application initiation and termination capability. The broadcaster may initiate applications using a trigger with name and URL that points to a different application than the current application. The launch of that application could be done either automatically or with viewer selection. Application termination is achieved in DDE-1 using a trigger with script that initiates a navigation to tv: .

In DDE-1, triggers consist of a required URL, the required tve attribute which identifies the version of DDE-1 content to which the trigger conforms, optional name-value attributes ( name , expires , script ), and depending on the means of delivery, a checksum. Each trigger delivers a single signal (i.e., a single script). The target of the script is the trigger's URL, i.e., the root of the application.