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Reader SurveyA Metadata Dictionary
May 1, 2003 12:00 PM, By Bob Turner
Terms You Should Know When AV Meets IT
 At NAB, Apple showed off its XML interchange format, while Adobe demonstrated a Media Composer to AfterEffects interchange. |
Big changes have come to video production with the advent of computer technology. And the biggest may be that computer engineers speak a different language from video people. For example, “online” in the video industry means creating a finished program, but in the IT industry it means something connected to a system or server.
Other examples are the new terms “essence” and “metadata.” Essence defines those portions of the digital signal whose bits are the images (video, graphics, animation, text) and sound (natural sounds, music, sound effects).
Metadata or data about data are the bits of that stream that complement the essence media and may include such data as timecode, sync, blanking, and color burst information.
Metadata may also include all sorts of descriptive data, such as where the sources originated, how they are composited, where the sources are located on the server or library, the type of special effects incorporated in the edited sequence, how the program is to be distributed, interactive authoring information, frame rates and aspect ratios, how to play this video in various playback devices, digital rights management, etc.
Media asset management (or the IT term DAM for digital asset management) is one of the fastest areas of development in postproduction today. Both displayed products and discussions of media asset management were dominant aspects of the recent NAB convention. In order to provide better communication and understanding, here are a few of the terms commonly used in these discussions and by media asset software manufacturers.
Note that a few of these are far more important to engineers or code-writers and less relevant to those of us who use the equipment or software they develop. But since the terms and acronyms appear before us from time to time, it is good to know what they mean.
AAF (Advanced Authoring Format): This acronym is frequently used to describe the standardized metadata definitions that are used to exchange metadata between creative content work-stations.
In many ways, it is easy to think about AAF metadata as a super-EDL, but this metadata format can contain much more information than that description implies. Nevertheless, this open standard “format” has been created primarily for postproduction use. It is worth noting that the definition of AAF does provide for essence exchange as well as metadata exchange.
AAF is also the name of the professional organization responsible for development, technical support, and promotion of this technology, the AAF Association. The association's web site is www.aafassociation.org.
Data Essence: This is a new term that refers to the bits and bytes of new forms of content, such as interactive TV-specific content, Advanced Television Enhancement Forum (ATVEF) content (SMPTE 363M), closed captions, etc.
Essence Media or Essence Data: This is another means of saying “essence.” The trend today is to use the term “essence” without the “data” or “media” appendage. The term refers to the actual bits and bytes that represent the sound and picture. It is frequently (and incorrectly) used by IT folks to describe a cassette, DVD, or streaming file containing audio, video, and graphics elements.
KLV (key, length, and value): KLV is a data-encoding protocol (SMPTE 336M) that complies with International Standards Organization rules for Object Identifier data and SMPTE Universal Label (SMPTE 298M).
This is the “header” information in a metadata stream that will identify the data and which metadata dictionary of definitions should be used for the metadata that follows. KLV and UMIDs (Unique Material Identifiers) are the basic engineering building blocks that have been designed to make metadata easier to exchange between different media (such as tapes or files) and metadata standards.
MXF (Material eXchange Format): MXF is an object subset of AAF and is on the verge of becoming an SMPTE standard. MXF was designed for less complex (less vertically rich) metadata applications, such as news editing and video streaming from servers. Because of its flatter metadata structure, it is better suited to be used as a metadata wrapper within a video signal or a TCP/IP stream. It offers performance benefits over the more complex AAF file structure because of its streamable nature.
For postproduction, one of the most important points about MXF video and metadata is that MXF will seamlessly interoperate with AAF-based postproduction environments. The less extensive MXF metadata can be accepted in full by AAF-based workstations, and AAF metadata can be flattened out to become the sleeker MXF metadata.
Thanks to the zero-divergence policy of the AAF and MXF proponents, the formats are fully interoperable with one another. All MXF metadata is understood by AAF, but if some AAF-specific metadata is not defined within the MXF standard, the non-MXF compliant metadata will be filtered and flattened out when being encoded as MXF.
MXF DMS-1: The MXF development community has been working on a specific dialect for Descriptive Metadata, called MXF DMS-1, which is being designed to describe people, places, times, production billing, etc.
This is just one of a number of emerging dialects. Others include MPEG-7, EBU, and P/Meta. AAF can accommodate any of these dialects.
OMF, OMFI, or OMF Interchange: Open Media Framework Interchange was a media and metadata exchange solution developed by Avid Technology. It was offered to an untrusting industry as a method of exchanging media and metadata between third-party platforms.
It pre-dates AAF, which was developed to replace OMF with a solution that would be more acceptable to film, video, and audio manufacturers after Avid ceased OMF development efforts.
Ironically, more manufacturers adopted OMF as an interchange standard after the standard was abandoned than during the period when Avid was evangelizing and investing major R&D in its efforts. OMF to AAF utilities are being developed to assist in the transition to AAF as the industry-accepted metadata exchange standard.
UMID (Unique Material Identifier): UMID is a Society of Motion Picture and Television Engineers (SMPTE) standard (SMPTE 330M/RP205) for metadata. The basic UMID contains 32 bytes of unique identification information (12 bytes identifying it as UMID data, followed by length and identification values). The extended UMID has an additional 32 bytes of information that contain “signature information” (time and date of creation, longitude, latitude, and altitude, as well as country, organization, and user codes).
XML (eXtensable Mark-up Language): XML is a simple, very flexible text format derived from the International Standards Organization's Standard Generalized Markup Language or SGML (ISO 8879). It was developed for electronic publishing and web page information exchange.
At NAB, Apple promoted its XML interchange format as a method of exchanging metadata because it is easy to print and the printed material can be easily understood by users. Apple also promoted Automatic Duck's AAF plug-in that converts Apple's XML interchange metadata to AAF interchange format. XML can be and is used for all kinds of purposes inside and outside of the industry. Its strength is that it can be used by such a wide breadth of applications.
Adobe demonstrated Media Composer to After Effects interchange at the NAB AAF press conference. Adobe used a prototype of the AAF XML Schema developed with the input and assistance of MetaGlue. Also, the AAF Association announced that it is actively working on a public XML Schema for AAF, called AAF-X.
Bob Turner has edited video on nearly every piece of equipment ever made. He can be reached at bobturner@attbi.com.
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