AAF and Other File Formats

metadata cover a very large span of production-related information. Such meta- data can provide essential information concerning the title of the program or the details of the producer and can also offer detailed information on different editorial aspects of a given clip and on its position inside a complete production plan. It is important to stress that a number of pieces of descriptive metadata infor- mation can be generated automatically. For example, a camcorder or a camera can have circuits that generate GPS coordinates, have the name of the cameraperson and of the production company already programmed, produce instructions con- cerning particular shots, as well as a number of other details that will be extremely useful during postproduction. All these pieces of information could be generated as descriptive metadata in line with the adopted DMS-1 vocabulary. These meta- data will be stored in the MXF file with the captured essence and therefore be available throughout the whole life cycle of the essence.

There is another metadata expression connected with MXF files that deserves a couple of words—dark metadata. That expression describes all types of metadata that are not recognized by a given decoding application—they are “dark for it.” Such metadata could be nonstandard user-generated information that requires a specially developed decoder or new metadata standardized after the construction of the decoder in question or simply standard metadata that are irrelevant for a particular decoding application.

8.4 AAF and Other File Formats

AAF was developed by a group of manufacturers and users as a file format for multimedia authoring and postproduction. It was intended for the exchange of the essence and a very rich set of associated metadata. Originally the complex pro- duction process was based on the exchange of essence and part of the metadata on videocassettes. In that process, other metadata were occasionally exchanged as recordings made on separate supports and the nonlinear editing was almost an isolated island. The introduction of the concept of networked production imposed the development of new workflows where the metadata generated in the pre- production were already available during acquisition, where rich metadata flew from acquisition to different stages of a complex postproduction, and where the need to distribute the same content to different channels and different media was taken into account at a sufficiently early stage and so initiated the genera- tion of different versions for different outlets (see more details in Chapter 12). All these aspects required a completely new approach to the organization of the interchange of essence and metadata, an approach where these two sets of data would be encapsulated in a single container, where they would be part of a sin- gle file system. The result of this new approach was the development of the

AAF format that, in a way, benefited from and superseded several previous attempts to facilitate the exchange between different parts of a postproduction process.

The AAF format facilitates an easy exchange of essence and rich metadata between different platforms and applications. It is structured for the exchange of pieces of material, where the essence can be stored on a physical carrier (tape or film) or inside a file in a computer-based storage system. In addition, in the AAF system, metadata are always encapsulated in the AAF file while the essence can be in the same file or in an external file. In that case, as in the previously described situations, the metadata and the essence are linked by unique identifiers (e.g., UMID). When two postproduction tools share the same storage (as, for example, do all nonlinear editing units [NLEs] and graphics or special-effects tools that are connected to the same server), the AAF file contains only metadata while the essence is directly exchanged between the tools. That approach avoids the need to build large AAF files and speeds up the operation, but it also offers an added flexibility since the external files can be other AAF files, MXF files, or even different elementary streams of compressed video. When postproduction tools do not share the same storage, it is necessary to generate AAF files that contain both the essence and the associated metadata.

The AAF is particularly well adapted to all possible aspects of present-day, complex postproduction operations. For its part, the MXF file system is devel- oped as a subset of the AAF and adapted to the task of simple interchanges of intermediate, partially complete, and/or fully finished content. Both systems share the same data model, that is, they use an identical method for describing the organization of the essence, which ensures full interoperability of the two systems and makes them complementary rather than competing solutions, as represented in Figure 8.3.

In addition to the recently developed AAF, the broadcasting industry still uses a number of heritage file systems. Some of them, such as the open media framework (OMF) were used as a starting point for the development of the AAF. Others, like the general exchange format (GXF), were and still are widely used for the transport of finished material to the playout systems or for the trans- port of finished commercials or news clips. A number of these file formats still offer an adequate service inside a given limited field and remain in use in iso- lated “island” areas. However, none of them can be used for intercommunication between different islands or communication with systems that use the AAF. Such communication requires a neutral and advanced format that can be converted from and to any of these heritage systems with the minimum computational com- plexity. The MXF file appears to be such a neutral format that it can be successfully used for island intercommunication as well as for the communication between these islands and the AAF-based postproduction systems (see Figure 8.4).

8.4 AAF and Other File Formats 127 MXF Acquisition MXF Storage AAF Postproduction Storage MXF Simple edit. MXF Archive MXF Distribution

Figure 8.3 The relationship between the AAF and MXF formats.

MXF Island MXF OMF Island MXF MXF MXF MXF MXF GXF Island Complex Postproduction Area AAF

Figure 8.4 MXF as an interchange format.

There is another interesting file exchange format, although it lies somewhat outside the boundaries of television technology—the digital picture exchange (DPX) format. The DPX is developed for the specific needs of the film indus- try, where it fulfills the role of exchange format in the newly introduced area of the digital intermediate.

In the filmmaking process, scenes are shot on negative film in the studio or the field and then developed; a working copy is made and used for editing. When special effects have to be introduced, an additional optical print is made and sent to special-effects departments or independent houses where the effects are created in a complex and difficult way. The advent of electronic imagery and especially the introduction of computer-based image-generation and processing techniques displayed the enormous potential that such a technology could bring to the domain of special effects. However, the equipment for electronic special effects was initially based on television standards and was consequently inadequate for the film industry (insufficient resolution, interlaced scanning standard, etc.). To successfully introduce the digital intermediate process (where original camera neg- atives would be scanned into digital electronic form, processed in that form in the special-effects facility, and then returned to the form of a duplicate film negative enriched with all the desired effects), it was necessary to develop special tools and techniques. These new ways of working would respect the requirements of film and ensure a seamless connection between electronically processed scenes and scenes that went through a conventional optical filmmaking process only. To achieve that goal it was necessary to develop

• telecine machines that would progressively scan the film negative and generate digital video signals featuring the high level of resolution needed • a digital coding method that would overcome the negative consequences of

two cascaded lossy optoelectric and electro-optic conversions

• a new file format for the transport of film frames and the accompanying information

• equipment for generating special effects that would operate at very high definition

• film recorders for the reverse conversion from electronics to film

The adaptation of existing equipment for the handling of very high resolution for progressively scanned pictures proved to be within the capabilities of the avail- able technology. A new coding scheme was developed based on the encoding of different print-density values, thus ensuring minimum quality degradation in the cascaded conversion processes. The missing link was the electronic content trans- port mechanism that would permit an unrestricted exchange of material between the film originators and different facility houses. Since different facility houses unavoidably used equipment coming from different manufacturers, it was obvi- ously necessary to standardize the material exchange system. The standardization process was conducted by the SMPTE and resulted in a specific file format able to transport a large number of information fields organized in separate headers that would facilitate a rapid interpretation and processing of the encapsulated images.

8.4 AAF and Other File Formats 129

The images, generated through a telecine optoelectric conversion and a digital coding process, were transported on a frame-per-file basis. Therefore each DPX file contained a large amount of information (at the time of the definition of the DPX format, these added pieces of information were not yet called “metadata”) and one single film frame in its digital form, but with no restriction concerning the image format. The file supported different aspect ratios, pixel shapes, and spatial resolutions, provided that the information data section adequately described all these parameters.

The box-office success of the movies based on special effects prompted the expansion of electronic intermediate techniques and generalized the use of the DPX files in the film industry.

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