Inside AAF

Discover how this authoring format bridges tape-based media archives with a new generation of tapeless broadcast systems.
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Newsrooms like the BBC News Television Center shown here can benefit from asset management systems with AAF integration. Photo courtesy BBC Technology.

The Advanced Authoring Format (AAF) standard is the result of unprecedented cooperation and collaboration among major media companies such as BBC, CNN, Warner Bros., FOX and Ascent Media, as well as major manufacturers such as Avid, Adobe, BBC Technology, Digidesign, Microsoft, Nucoda, Quantel, SADiE, Snell & Wilcox, and Sony. This cooperation among vendors and users is the true strength of AAF. These organizations and others have developed and refined AAF in an open-source environment, freely licensed, without any cost for using the technology. The AAF Developers’ Toolkit and reference implementation includes utilities with hundreds of thousands of lines of source code.

AAF developed in parallel with other important metadata standards and offers comprehensive support for KLV and MXF metadata. The AAF Association and the Pro-MPEG Forum have agreed to a zero-divergence doctrine (ZDD) and continue working to ensure persistence of metadata integrity across both formats.

Now that major manufacturers are offering products compliant with AAF, the format has begun moving beyond the “early adopter” stage. Now, broadcasters can consider AAF interoperability when making purchase decisions.

From a technology perspective, the value of AAF lies in its abstraction of the editorial process. It separates editorial decisions from the media that are used as source material. AAF is founded on the concept that everything necessary to create a finished program can be represented as composition metadata, with references to the media essence.

AAF file components

An AAF file is an object-based file containing a collection of object-based data. These data include a file index that locates all the objects in the file, material objects that represent AAF metadata, a dictionary for defining metadata objects that extend the AAF data model, and possibly essence data, the actual encoded media. The low-level container format is called structured storage.


Table 1. An AAF can contain eight kinds of Mobs. Click here to see an enlarged diagram.

Inside the Mob

AAF uses an object-based data model to represent complex metadata relationships. The material metadata items are called material objects or Mobs. An AAF file typically contains a number of Mobs. Table 1 lists the eight kinds of Mobs that an AAF can contain. Figure 1 shows the components of an AaAF Mob. The Mobs reference one another and, in doing so, create a derivation chain, or Mob Chain. The derivation chain describes the relationships among edit decisions, clips, file source material and physical source material. Figure 2 shows such a derivation chain.

Mob IDs

Every Mob has a unique identifier, or Mob ID. The Mob ID is a universally unique identifier and enables indelible referencing of other media objects, both within files and externally. The Mob ID functions such as a license plate for every metadata object, so that each is easily and accurately identifiable.


Figure 1. The components of an AAF Mob include Mob ID and tracks (slots). Click here to see an enlarged diagram.

Mob derivation (Mob source chain)

Mob derivation enables edited programs to include references back to the primary sources. This is important in a world where content is repurposed repeatedly. News environments commonly reuse edited segments as source material for subsequent versions of the same story. When a news organization or department uses an edited sequence as source material, AAF maintains all the references to the original file or tape sources, avoiding generation loss. The elements of the previous edited version, including titles and graphics, remain editable.


Figure 2. The derivation chain describes the relationships among edit decisions, clips, file source material and physical source material. Click here to see an enlarged diagram.

AAF specifies the Mob source chain by referencing one material object from another. Figure 2 on page 53 shows the normal referencing between Mobs. An important benefit of the Mob derivation chain is that the AAF composition or clip can reference back to its primary source(s), including videotape, a film reel, audiotape or even a news-agency feed.

Compositions

AAF represents edit decisions as a composition and specifies them using a Composition Mob. A Composition Mob can reference another Composition Mob, a Sub-clip Composition Mob or a Master Mob as source materials for an edit. The metadata contained in a Composition Mob may include:

  • Edit-decision information, such as source and record time codes, source tracks, physical source names, comments, clip names and track names
  • Visual-effects information, such as dissolves, SMPTE wipes, 2-D DVE spatial-positioning and zooming effects, frame-repeat effects, motion and speed-change effects, flip and flop effects, and composite-layering effects
  • Audio data, such as clip- and track-based gain, stereo pan, fade in and out, symmetrical and asymmetrical crossfades, and MIDI data
  • Embedded media files, such as video and audio files, as well as non-AAF files, including scripts, logs, etc.
  • Sub-clip metadata that identifies a section of a clip, which a user interface thereafter treats as an additional clip

Clips


Figure 3. AAF composition and sub-clip composition Mobs reference Master Mobs. Click here to see an enlarged diagram.

In AAF, Master Mobs represent clips. The Master Mob does not contain the media essence, but instead describes and points to the media essence. This creates a powerful level of indirection, which is key to how AAF enables interoperability among a broad range of tools, from simple Web-based browsing and logging applications to high-definition conforming products.

AAF provides three methods of specifying the source material associated with a clip:

  • Reference to source material stored internally to the AAF file and physical source material
  • Reference to file source material stored externally to the AAF file and physical source material
  • Reference to physical source material only, such as videotape or film

Figure 3 shows the relationship between a segment in the Composition Mob and the Master Mob that it references. The composition identifies the tracks, offset and range of the clip. All other clip-related metadata remains an attribute of the clip. Changes to the clip metadata are available by reference from the composition.

File source material

In AAF, a Source Mob represents all source materials. Source materials that are file-based use a file Source Mob. The Source Mob does not contain essence; it represents the specific instance of essence. It is not unusual for multiple instantiations of media essence to exist. The essence associated with a file Source Mob can either be internal to the AAF file in an Essence Data object or external to the AAF file. If the essence is external, the file Source Mob specifies the kind of essence container file, and the location of the essence.

Tape source material

Tape Source Mobs represent tape source materials. The function of tape Source Mobs is to link compositions to physical source tapes. This is useful for many functions, including creating an EDL from an AAF composition, automating digitization of a composition from the source tapes, or managing tape assets based on usage metadata.

Implementing AAF

When implementing AAF, it is important to remember the true value of AAF: open sharing and discussion among leading manufacturers and users. AAF is an open, nonproprietary interchange format that is freely licensed, including source code. Developers must first clearly understand the problem that AAF addresses. AAF is, after all, a means to an end, not an end in itself. AAF has two main advantages: it allows products to bridge tape-based media archives with a new generation of tapeless broadcast systems, and it marginalizes tape through the production process, thus reducing cost. AAF is the antithesis of tape because it is inherently nonlinear.

No matter what technology a manufacturer considers, it still must face one significant interchange challenge. Whether it is AAF, MXF, a flavor of XML or any other intermediary format, each manufacturer must “map” its internal or native data models to the interchange data model. This is no small task. Ultimately, effective interchange among vendors requires communication and consensus. The AAF Association engineering committee meets regularly to work on interoperability issues.

With the publication of the AAF Edit Protocol, it’s never been easier to implement AAF. In the coming months, the AAF Association will develop a conformance program that will ensure the long-term viability of the money and effort that manufacturers and users invest in AAF. The AAF Association provides the AAF Toolkit free of charge on SourceForge.net. The modest cost of association membership supports tutorials, example utilities, the AAF specification and vendor support.

Ed McDermid is head of marketing at BBC Technology, North America.

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