Definition: Descriptors are used to describe essence stored in or referenced by an MXF file.

Description: In an MXF file, stored essence is described by the top-level file package. The Essence Descriptors are used to uniquely identify the essence to be retrieved/decoded/displayed. The relationship between the descriptors in SMPTE 377M (File Format Specification), SMPTE 381M (MPEG mapping) and SMPTE 382M (audio mapping) are shown in the Figure 3.6 on the next page.

Figure 3.6: Relationship between the descriptors defined in SMPTE 377M

You can see how the arrangement of the descriptors is categorized to allow different generic sorts of essence to be described and specific extensions to be created as subclasses of the generic descriptors. There is a single descriptor associated with each file package. If there is more than one track in the package, then the descriptor will be a Multiple Descriptor, which in turn has a list of SubDescriptors up to one per track. The FileDescriptor::LinkedTrackID property of each descriptor is set to the TrackID of the track within the file package that it describes.

• The abstract superclass EssenceDescriptor is never used directly hence it is abstract. It is also the parent of all the descriptors hence it is a superclass.

• The GenericDescriptor adds the Locators property. This allows hints about the location of external essence to be buried within the file. More on this under Locators in this chapter.

• The FileDescriptor adds generic properties for essence which is (or has...