Audiovisual Core Structure

Title: Audiovisual Core Structure

Date version issued: 2023-02-24

Date created: 2013-10-23

Part of TDWG Standard: http://www.tdwg.org/standards/638

This version: http://rs.tdwg.org/ac/doc/structure/2023-02-24

Latest version: http://rs.tdwg.org/ac/doc/structure/

Previous version: http://rs.tdwg.org/ac/doc/structure/2021-10-05

Abstract: The Audiovisual Core is a set of vocabularies designed to represent metadata for biodiversity multimedia resources and collections. These vocabularies aim to represent information that will help to determine whether a particular resource or collection will be fit for some particular biodiversity science application before acquiring the media. Among others, the vocabularies address such concerns as the management of the media and collections, descriptions of their content, their taxonomic, geographic, and temporal coverage, and the appropriate ways to retrieve, attribute and reproduce them. This document contains material introductory to the Audiovisual Core Term List

Contributors: Robert A. Morris, Vijay Barve, Mihail Carausu, Vishwas Chavan, José Cuadra, Chris Freeland, Gregor Hagedorn, Patrick Leary, Dimitry Mozzherin, Annette Olson, Greg Riccardi, Ivan Teage, Steve Baskauf

Creator: GBIF/TDWG Multimedia Resources Task Group and Audiovisual Core Maintenance Group

Bibliographic citation: Audiovisual Core Maintenance Group. 2023. Audiovisual Core Structure. Biodiversity Information Standards (TDWG). http://rs.tdwg.org/ac/doc/structure/2023-02-24

1 Introduction

This documentation describes the structure of the TDWG Audiovisual Core Multimedia Resources Metadata Standard (Audiovisual Core, or simply AC).

If you are unfamiliar with the Audiovisual Core, please read the Audiovisual Core Introduction before reading this document. The introduction lays out why there is perceived a need for a biodiversity media resource metadata schema, and how the standard attempts to use existing metadata standards where possible.

For term details, see the Audiovisual Core Terms List document and for a more detailed guide to the use of Audiovisual Core, see the Audiovisual Core Guide document.

During development, Audiovisual core was colloquially known as MRTG, after its developers, the GBIF-TDWG Joint Multimedia Resources Metadata Task Group. Please see the Audiovisual Core Guide and also MRTG Development History for the development history in detail.

1.1 Status of the content of this document

Sections 2 through 4 of this document are normative except for example sections, which are labeled as non-normative.

1.2 RFC 2119 key words

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.

2 Terminology of this specification

There are many ways to organize metadata specifications, particularly as to the nomenclature of the constituents of the metadata. Note the following as they apply to the Audiovisual Core:

• A Multimedia Resource is anything that a provider identifies as belonging to one of the possible values of the AC Type term and optionally one or more of the Subtype term values. A mechanism is provided by which providers can supply a privately defined subtype that will not collide with the AC defined Subtype values.
• An AC record is a set of terms with any values conforming to this document, and which contain at least the four mandatory terms described in the Audiovisual Core Core Term List, and which describes a single multimedia resource (possibly including a Collection). One of these, the value of Identifier is a Globally Unique IDentifier (GUID), which may have been assigned to the resource by an external authority or by the provider of the metadata record.

In the Audiovisual Core Term List, every AC term has a term name following a table entry “Term:”, a URI, a plain text normative Definition, a recommended English Label, an optional Notes attribute. In addition, a term has an attribute telling whether it is mandatory and one telling whether it is repeatable.

AC metadata can describe either individual multimedia resources or collections of resources. A few, but not many, of the AC properties have different values for collections than for individual media. If no such distinction is mentioned, AC does not assume one.

Term Names for terms borrowed from other vocabularies are those in use for the corresponding term in those vocabularies. Term Names are intended principally for navigation in the AC documentation. Term Labels are suggestions for English labels in applications. They are recommendations only and are offered only in English, with the added expectation that they may clarify intended usage of the term. Communities may wish to promulgate recommendations for Labels in other languages, or even alternative English Labels for specialized audiences, e.g. school children. Labels MAY be used for navigation within the Term List, and are often used within the Term List itself when a term is mentioned within the documentation of another term. The Term List provides indices both by name and label.

URI’s for terms conform to the http URI scheme (see http://en.wikipedia.org/wiki/URI_scheme, http://www.w3.org/TR/uri-clarification, or http://www.ietf.org/rfc/rfc2396.txt). Informally, one may understand this as follows: an http URI has the syntax of an http URL, but there is no expectation that putting it in a web browser will result in any information being returned to the browser, and if there is, it may have no relevance. This conformance requirement applies only to the URIs that identify AC terms. A few AC terms permit values to be taken from another controlled vocabulary chosen by the user. In this case, those values may involve URIs conforming to a scheme given by that external vocabulary, and AC is silent on what that scheme is.

The Notes field of a term’s documentation points to further information, if any exists, about the term. In particular, for terms borrowed from other vocabularies, this field generally carries a link to the originating vocabulary’s documentation for that term.

3 Multiplicity and Cardinality

A number of terms are repeatable. How to implement repeatability in a given serialization is not defined by Audiovisual Core. The following section gives advice on some best practices in the context of repeatability.

The simplest case is a single repeatable term (e.g., dcterms:identifier). In representations based on an XML Schema that permits elements to be repeated such a term may simply be repeated (e.g. “...<dcterms:identifier>http://example.com/123</dcterms:identifier><dcterms:identifier>http://example.com</dcterms:identifier>...”). In serializations that do not easily lend themselves to repeatable elements (e.g. “flat” schemata with all elements occurring only a single time in an otherwise unstructured record) it is possible to define separators to support a list of values within a single element (e.g. “...<dcterms:identifier>http://example.com/123; http://example.com/456</dcterms:identifier>...”).

In certain cases pairs or tuples of properties are repeated. In Audiovisual Core this situation occurs, for example, in the following cases:

• The language-dependent metadata like title, description, etc. need to be associated with ac:metadataLanguage. One approach here is to use complete Audiovisual Core records together with the Metadata Language property; see there for further detail.
• The values of properties about a Service Access Point MUST remain associated with that Service Access Point even if there are multiple Service Access Points. See ac:hasServiceAccessPoint for further details.
• The terms dwc:scientificName and dwc:identificationQualifier MAY optionally be structured into pairs. (See the notes on dwc:identificationQualifier.)
• The terms Reviewer, being the name of an individual providing some expert review of a resource, and the review text itself in Reviewer Comments are desirable to store as pairs.

3.1 Structured serializations

Many serialization languages provide sufficiently structured forms to deal with repeated terms unambiguously. In XML, we might define a container element and use a nesting structure as in Section 3.1.1 Alternatively, in XML we may reference access points by identifier as in Section 3.1.2 Where such structures are impossible or undesirable, an alternative solution is to permit only one access point per container element, but to repeat the container element for a single media resource, as shown in section 3.1.3 This is similar to one of the options discussed for multilingual metadata (see Metadata Language).

Note: In the examples, for human-readability the literal valued terms dc:format and ac:variantLiteral were used. However, it is designated best practice to use the IRI valued terms dcterms:format and ac:variant with controlled IRI values from the controlled vocabulary for format and controlled vocabulary for variant. See the notes on dc:format and ac:variantLiteral for more information.

3.1.1 Nested XML structure example (non-normative)

<MEDIA_METADATA_CONTAINER>
  <dcterms:identifier>http//:example.com/pictures/thePicture.jpg</dcterms:identifier>
  ...
  <ac:hasServiceAccessPoint>
    <dc:format>image/jpeg</dc:format>
    <ac:accessURI>http://example.com/fullres/thePicture.jpg</ac:accessURI>
    ...
  </ac:hasServiceAccessPoint>
  <ac:hasServiceAccessPoint>
    ...
  </ac:hasServiceAccessPoint>
<MEDIA_METADATA_CONTAINER>

3.1.2 XML reference by identifier example (non-normative)

<MEDIA_METADATA_CONTAINER>
  <dcterms:identifier>http://example.com/pictures/thePicture.jpg</dcterms:identifier>
  ...
  <ac:hasServiceAccessPoint>http://example.com/pictures/thePicture.jpg#ac0001</ac:hasServiceAccessPoint>
  <ac:hasServiceAccessPoint>http://example.com/pictures/thePicture.jpg#ac0002</ac:hasServiceAccessPoint>
  <ac:ServiceAccessPoint id="http://example.com/pictures/thePicture.jpg#ac0001">
    <dc:format>image/jpeg</dc:format>
    <ac:accessURI>http://example.com/fullres/thePicture.jpg</ac:accessURI>
    ...
  </ac:ServiceAccessPoint>
  ...
<MEDIA_METADATA_CONTAINER>

3.1.3 Repeated container element XML example (non-normative)

<MEDIA_METADATA_CONTAINER>
  <dcterms:identifier>http//:example.com/pictures/thePicture.jpg</dcterms:identifier>
  <dcterms:title>A red beech leaf</dcterms:title>
  <dct:format>image/jpeg</dc:format>
  <ac:accessURI>http://example.com/fullres/thePicture.jpg</ac:accessURI>
  ...
<MEDIA_METADATA_CONTAINER>
<MEDIA_METADATA_CONTAINER>
  <dcterms:identifier>http://example.com/pictures/thePicture.jpg</dcterms:identifier>
  <dc:format>image/png</dc:format>
  <ac:accessURI>http://example.com/fullres/thePicture-hires.png</ac:accessURI>
  ...
<MEDIA_METADATA_CONTAINER>

3.2 Tabular serializations

The same data as in examples 3.1.1 through 3.1.3 can be serialized as a “flat” spreadsheet-like table.

In the example of Section 3.2.1, only the required identifier is repeated, but not the title field. Whether to repeat all fields or whether to provide all fields only in the first record, limiting later records to the identifier and the service access point properties, is left to specific implementations. In the example of Section 3.2.1, the ac:hasServiceAccessPoint property is suppressed as unnecessary.

3.2.1 Example of a table with each service access point in a separate row (non-normative)

Another approach (Section 3.2.2) also eliminates the need for the ac:hasServiceAccessPoint property when flattening the ac structure. It is based on introducing new terms exploiting values of the ac:variantLiteral: “Thumbnail”, “Trailer”, “Lower Quality”, “Medium Quality”, “Good Quality”, “Best Quality”, “Offline”, as prefixes for additional properties in a new namespace.

3.2.2 Example of a table with metadata for all service access points in the same row (non-normative)

Note: acf: (for “Audiovisual Core Flat”) is a made-up namespace. Communities of interest might mint such terms in order to use this kind of structure.

4 Lists of plain text values

Some AC terms permit values that are lists to be represented as plain text. The choice of how to separate list items is ultimately left to the implementers of AC. Typical usage is to choose a punctuation mark such as “,”, “;”, or “|”. In these cases a special escape syntax needs to be defined for cases in which the separator is part of the metadata value. Unfortunately, even for standard list formats like CSV, different software packages choose different escape methods, hindering interchange. In the absence of an implementation-specific choice we RECOMMEND to use “|” as separator and “\|” as an escaped vertical bar.