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ISO/IEC DIS 29341-4-4 2010 Edition, October 15, 2010 INFORMATION TECHNOLOGY - UPNP DEVICE ARCHITECTURE - PART 4-4: AUDIO VIDEO DEVICE CONTROL PROTOCOL - LEVEL 2 - AUDIO VIDEO DATA STRUCTURES
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Description / Abstract:
Introduction
This document 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 must 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.