ISO/IEC 29341-20-4:2017 defines the layout of the AV Datastructure Template (AVDT) XML document. An AVDT document describes the format requirements and restrictions of various data structures used within the UPnP AV specifications. Although these data structures are defined very precisely in the appropriate service specification, in most cases, each data structure definition allows for a certain degree of variation in order to accommodate differences between individual devices.
The purpose of an AVDT document is to enable each device to describe (at run-time) its particular variation of these AV data structures. AVDT documents allow users of AV data structures (e.g. UPnP control points) to reduce the number of instances of those data structures that comply with the service specification but are not compatible with the device's particular capabilities. The ultimate goal of an AVDT document is to reduce those error conditions that are caused by control points creating instances of a data structure that exceed the static (known) capabilities of the device. Unfortunately, the AVDT mechanism will never eliminate all preventable error conditions, but it will help to reduce them by giving the client more information about the device's particular capabilities.
As described above, an AVDT document is a machine readable, implementation-specific variant of an AV data structure defined by one of the UPnP AV specifications. For a given device, each instance of that data structure shall conform to both the specification definition and the device's AVDT definition of that data structure.
Ironically, an AVDT document is both a more-restrictive and more-permissive variant of the specification definition. AVDT documents are more restrictive because they limit certain aspects of the data structure (e.g. such as the allowed values for each field) that are otherwise permitted by the specification definition. However, due to limitations of the AVDT constructs, it is simply not possible to express some of the more intricate requirements defined by the specification (e.g. subtle interdependencies between data structure fields). Consequently, instances of a data structure that comply with a given AVDT description may not fully comply with all of the requirements defined in the specification.
The types of data structures that can be described by an AVDT document represent a (non-hierarchitical) set of named property values. The set of allowed property names and their allowed values for a given data structure are defined by one of the UPnP AV specifications. Individual instances of these data structures are manifested via an XML document whose elements and attributes correspond to the set of named properties. In other words, within the XML document that corresponds to a given instance of a certain data structure, each XML element and attribute contains the value of a specific named property.
An AVDT document is conceptually similar to an XML schema in that both entities identify the XML elements and attributes that appear in any given document instance. Additionally, both AVDT documents and XML schemas identify the allowed values that are permitted for each element and/or attribute which corresponds to a specific property. However, unlike an XML schema, an AVDT document can also identify certain dependencies between two or more properties. For example, the set of allowed values of one property may depend on the actual value of another property. This type of interrelationship is difficult to represent using an XML schema. Hence, the AVDTdocument structure is needed.
In the various AV Architecture scenarios, sometimes there is a need to exchange device capabilities to ensure high level interoperability. In order to express the parameterized capability, an AV specification defines various templates for each purpose. A device uses the template and populates it with values to reflect its capabilities at run-time.
The AV Datastructure Template (AVDT) is a common structure to define various templates, which are called Datastructure. This is written in XML and each data structure uses a subset of the AVDT to meet the necessary requirement.