This page provides an overview of the application of MPEG-21 DIDL as structural and technical metadata.

DIDL document structure

A DIDL document consists out of a main Item (the compound object) at least underlying Item containing the bibliographic metadata.
In addition to the bibliographic metadata, the compound object MAY include zero or more so-called objects. Examples of such object are files containing the textual publication, underlying datasets or other supplementary material or a Jump off page - this is a humanreadble webpage containing descriptive information about the bibliografic work, usually generated by the repository software based on the metadata present. The object may also be a reference to an external websource (i.e. a resource not in the custody of the repository).

<didl:DIDL
  xmlns:didl="urn:mpeg:mpeg21:2002:02-DIDL-NS"
  xmlns:dii="urn:mpeg:mpeg21:2002:01-DII-NS"
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

  <!-- Compound Object -->
  <didl:Item>

      <!-- Persistent Identifier - MANDATORY-->
      <didl:Descriptor>
        (...)
      </didl:Descriptor>

      <!-- Descriptive Metadata -->
      <didl:Item>
        (...)
      </didl:Item>

      <!-- Object File -->
      <didl:Item>
        (...)
      </didl:Item>

      <!-- Jump off Page -->
      <didl:Item>
        (...)
      </didl:Item>

  </didl:Item>

</didl:DIDL>

Compound Object Item

A DIDL Compound Object Item consists out of one descriptors and a number of items. In this case the Descriptor contains a description of a persistent identifier.

<didl:Item>

  <!-- Persistent Identifier for Compound Object Item - MANDATORY-->
  <didl:Descriptor>
    <didl:Statement mimeType="application/xml">
      <dii:Identifier>urn:nbn:ui:99-1234567890</dii:Identifier>
    </didl:Statement>
  </didl:Descriptor>

  <didl:Item>
    (...)
  </didl:Item>


</didl:Item>

Item structure

A DIDL Item consists out of a number of descriptors. A Descriptor contains a descriptions of the digital object referenced in the Component. Examples of such descriptions are persistent identifiers, dates and accessrights. These expressions are included in statements with the 'application/xml' mimetype.

<didl:Item>

  <!-- Persistent Identifier for Object Item - OPTIONAL -->
  <didl:Descriptor>
    <didl:Statement mimeType="application/xml">
      <dii:Identifier>urn:nbn:ui:99-abcdefghijkl</dii:Identifier>
    </didl:Statement>
  </didl:Descriptor>

  <!-- Semantic Type -->
  <didl:Descriptor>
    <didl:Statement mimeType="application/xml">
      <rdf:type rdf:resource="info:eu-repo/semantics/DescriptiveMetadata"/>
    </didl:Statement>
  </didl:Descriptor>

  <!-- Modification Date -->
  <didl:Descriptor>
    <didl:Statement mimeType="application/xml">
      <dcterms:modified>2010-12-14T14:22:48</dcterms:modified>
    </didl:Statement>
  </didl:Descriptor>

  <!-- Access Rights-->
  <didl:Descriptor>
    <didl:Statement mimeType="application/xml">
      <dcterms:AccessRights>http://purl.org/eprints/accessRights/ OpenAccess</dcterms:AccessRights>
    </didl:Statement>
  </didl:Descriptor>


  <!-- Content -->
  <didl:Component>
    <didl:Resource mimetype="application/xml">
      (...)
    </didl:Resource>
  </didl:Component>

</didl:Item>

Descriptors

Persistent Identifiers

Persistent identifiers are durable, lasting formal identification numbers through which a partical piece of information such as publications, datasets or software may be retrieved, even when they have moved from their original location.
There are various persistent identifier solutions such as DOI or Handle, each with their own pros and cons. In the Netherlands, one open and free identifier that is used for digital academic object is location in the UI namespace of the National Bibliographic Number. The Konklijke Bibliotheek is the authority for the Dutch namespace. An organisation may request their own subnamespace at with the KB (the '99' following the 'ui' in the example below) after which the organisation may mint its own identifiers inside that namespace. Go to ??http://www.kb.nl/expertise/voor-bibliotheken/registration-agency-nbn to register your Persistent Identifier namespace.

(...)
<didl:Descriptor>
  <didl:Statement mimeType="application/xml">
    <dii:Identifier>urn:nbn:nl:ui:99-123456890</dii:Identifier>
  </didl:Statement>
</didl:Descriptor>
(...)

Object Type

As stated previously an object may include multiple underlying items. In order to distinguish between the various parts in a meaningful manner, a semantic type is included. These types are expressed using and rdf:type element. The values placed in the rdf:resource attribute are maintained in a controlled vocabulary situated in the info:eu-repo namespace.
For the current situation in HBO, the following three semantic types are RECOMMENDED:

This list may grow overtime based on community needs.

(...)
<didl:Descriptor>
  <didl:Statement mimeType="application/xml">
    <rdf:type rdf:resource="info:eu-repo/semantics/DescriptiveMetadata"/>
  </didl:Statement>
</didl:Descriptor>
(...)

Object date modified

Every individual Item may have its own modification date. This date is included inside a dcterms:modified element. Its use is OPTIONAL.

(...)
<didl:Descriptor>
  <didl:Statement mimeType="application/xml">
    <dcterms:modified>2010-12-14T14:22:48</dcterms:modified>
  </didl:Statement>
</didl:Descriptor>
(...)

Object Accessrights

It is desirable to state the access rights of the objects included. This enables users to assess the assessiblity of object in advance and reduces frustration at the user from hitting an obstacle while trying to access the material.

Accessrights are included in a accessRights element from the dcterms namespace inside a Statement. The values are maintained in a info:eu-repo vocabulary.

(...)
<didl:Descriptor>
  <didl:Statement mimeType="application/xml">
    <dcterms:AccessRights>info:eu-repo/semantics/OpenAccess </dcterms:AccessRights>
  </didl:Statement>
</didl:Descriptor>
(...)

Object Mimetype

The MimeType is specified inside the mimeType attirbute of the Resource Element. For an overview of available MimeTypes, please refer to the IANA registration of MIME Media Types.

(...)
<didl:Component>
  <!--MimeType contained in the MimeType attribute -->
  <didl:Resource mimeType="application/pdf" ref="http://myrepository.org/resource"/>
</didl:Component>
(...)

Object Location

Locations of objects are indicated by the ref attribute.

(...)
<didl:Component>
  <!-- Object URI contained in the Ref attribute -->
  <didl:Resource mimeType="application/pdf" ref="http://myrepository.org/resource"/>
</didl:Component>
 (...)