Clearly, it would not be a good idea to embed the metadata in the essence file if every time a change is made to metadata the entire file has to be stored again, and version numbers updated to reflect changed file content. This could result in explosive growth in storage needs in some cases.
Our football example begs to have the descriptive metadata stored in a separate repository, perhaps a database, with a reference to the essence file to which it relates. This allows small changes to be made with no impact to storage of the essence. This is sometimes referred to as a sidecar file.
Conversely, it makes no sense to put the structural metadata in a sidecar, which might make decoding the essence more complicated. In fact, one critical piece of structural metadata is the unique identifier linking essence and metadata together, which of course must be stored with both.
One content identifier often used is the ubiquitous SMPTE Unique Material Identifier (UMID). Although these are not globally unique and registered in a global database of all content ever created (which would be difficult, if not impossible), UMIDs are easy to generate and manage. Because they are locally generated, a camera can create an identifier on a mountaintop. This allows metadata to be referenced to that UMID-identified content later.
I have become convinced in the last few years that metadata may actually be more critical than some of the essence to which it refers. One case is finding content. The structural metadata that identifies where essence is located cannot be corrupted. If it does, it may be at best difficult, and at worst impossible, to find the essence itself. A MAM database that allows content to be searched and retrieved must be carefully protected. If the location data is lost, essentially the essence itself becomes at best opaque.
The SMPTE/EBU Task Force related an additional concept, which is sometimes confusing to people in our industry, that of “wrapper.” The wrapper concept is based in IT techniques and quite literally is a layer of structural metadata that surrounds the bits of essence and metadata. It allows the decoding and transport of content (defined by the Task Force as essence plus metadata) in a way that can be parsed by any application with knowledge of the wrapper syntax.
A standard wrapper does not guarantee the ability to decode the essence — because the receiving decoder might not understand, for instance, MPEG-2, only DV compression — but it will allow the structural metadata and essence to be retrieved from the file in a standardized way. The most obvious examples in common use today are Material Exchange Format (MXF) and QuickTime, though dozens of others exist.
To recap, metadata is truly “bits about bits,” but it is much more, and critically important in our industry today.
—John Luff is a television technology consultant.