scope:

This third part of MPEG-21 (ISO/IEC 21000-3), entitled Digital Item Identification (DII), specifies

How to uniquely identify Digital Items (and parts thereof);

- How to uniquely identify IP related to the Digital Items (and parts thereof), for example abstractions;

- How to uniquely identify Description Schemes;

- The relationship between Digital Items (and parts thereof) and existing identification systems. Annex C contains a list of relevant identification systems. This is not an exhaustive list and is subject to change over time;

- The relationship between Digital Items (and parts thereof) and relevant description schemes. Annex C contains a list of relevant description schemes. This is not an exhaustive list and is subject to change over time.

ISO/IEC 21000-3 specification does not specify:

- New identification systems for the content elements for which identification and description schemes already exist and are in use (e.g., ISO/IEC 21000-3 does not attempt to replace the ISRC, as defined in ISO 3901, for sound recordings;

- Normative description schemes for describing content.

Organisation of the Document

This specification contains five Clauses and four Annexes:

The remainder of this Clause 1 contains an overview of MPEG-21 Digital Items and the relation between parts 2 and 3 of ISO/IEC 21000. Clauses 2 and 3 contain normative references and a list of applicable terms and definitions.

Clause 4 specifies how to uniquely identify Digital Items, how to associate related identifiers with Digital Items, how to identify different types of Digital Items. Clause 0 then specifies how to associate metadata with Digital Items by using description scheme identifiers.

The normative Annex A contains criteria that need to be fulfilled by a registration authority that is required for the identification of Digital Items and their parts as specified in ISO/IEC 21000-3. Annex B contains an informative example of how to resolve a unique identifier to appropriate metadata. Annex C contains an informative list of existing identification schemes that can be used by this specification. Finally, Annex D contains information of patent statements relating to this specification.

Introduction to Digital Items

Within any system (such as MPEG-21) that proposes to facilitate a wide range of actions involving "Digital Items", there is a need for a very precise description for defining exactly what constitutes such an "item". Clearly there are many kinds of content, and probably just as many possible ways of describing it to reflect its context of use. This presents a strong challenge to lay out a powerful and flexible model for Digital Items which can accommodate the myriad forms that content can take (and the new forms it will assume in the future). Such a model is only truly useful if it yields a format that can be used to represent any Digital Items defined within the model unambiguously and communicate them, and information about them, successfully.

The Digital Item Declaration specification (ISO/IEC 21000-2) provides such flexibility for representing Digital Items.

Example of a Digital Item

This sub-clause provides a simple example of a Digital Item. More complex examples can be found in ISO/IEC 21000-2.

This example uses ISO/IEC 21000 to create an "MPEG-21 Music Album" comprising a series of resources:

- Three audio files (coded in MPEG-2 AAC1), representing the "tracks" that form the basis of the album;

- Two text files (in Unicode2), representing the lyrics to two of the tracks;

- Two images (in JPEG3), representing the cover photograph and other artwork of the album;

- A text file (in HTML4), representing the introductory text for the album.

The relationship between these resources and how they relate to the Digital Item itself is expressed in the Digital Item Declaration (DID5). The DID contains, besides the references to the resources, information about the item and/or parts thereof. These metadata elements are associated through DID mechanisms to the item/resources as shown in Figure 1 overleaf.

Relationship between Digital Item Declaration and Digital Item Identification

Identifiers covered by this specification can be associated with Digital Items, containers, components, and/or fragments thereof by including them in a specific place in the Digital Item Declaration. This place is the STATEMENT element. Examples of likely STATEMENTs include descriptive, control, revision tracking and/or identifying information.

Figure 2 overleaf shows this relationship. The shaded boxes are subject of this specification while the bold boxes are defined in ISO/IEC 21000-2.

Several elements within a Digital Item Declaration can have zero, one or more DESCRIPTORs (as specified in ISO/IEC 21000-2). Each DESCRIPTOR may contain one STATEMENT which can contain one identifier relating to the parent element of the STATEMENT. In Figure 2, the two statements shown are used to identify a Component (left hand side of the diagram) and an Item (right hand side of the diagram).