MXF. Let’s clear up doubts

So far I guess you all have it pretty clear and you will wonder why an MXF generated by an XDCAM camera is not compatible with the one generated by a P2 if the MXF format was created to ensure compatibility. Well, the great flexibility of MXF allows different interpretations and applications of the standard by different manufacturers, and this is how the MXF generated by the products of each one are not compatible with each other. This has led to the implementation of a series of different physical versions to improve interoperability according to their applications. In this way, the so-called Operational Patterns are established and each one will have its own specifications under its own standard that will define the type of image/sound that contains the essence and structure of the metadata. Of these patterns, OP-1a and OP-Atom are the most frequently encountered.

The generic operational standard is OP-1a and was created as a replacement for videotape, where a single MXF file contains video, several audio channels and timecode. It is very simple and flexible but has many limitations in terms of how it has to be worked with. An example in this case would be the XDCAM format or the JVC cameras that record on cards, where each of their clips has a single file with video and audio.

OP-Atom is a very simple file format that can have only a single element at its core, either a video or audio track. Typically, the metadata linked to the media contained in MXF OP-Atom is in AAF or XML files. A typical example is the P2 format, where the video and audio tracks are wrapped in separate MXF-Atom files and the metadata associating them goes in a separate XML file.

The media files generated by AVID are also OP-Atom and their associated metadata is in AAF. This is the most commonly used in editing environments, where individual access to audiovisual components is required. In the case of AVID, these files have the particularity of incorporating non-standard MXF metadata used by AVID applications to index them and that can create incompatibility problems if they are exchanged with other systems different to AVID.

In the case of digital cinema, the MXFs that carry the image and audio are also OP-Atom. In this case they are restricted to the specific compression and color space for this function (JPEG 2000 and XYZ) in the case of video and to 16 PCM tracks in the case of audio, so their compatibility outside this environment is very limited. Its synchronization and metadata is the information contained in XML files.

We can then establish a group of operation patterns (OP) with various levels of complexity, where the OP-1a pattern is the lowest level. A single continuous track with video, audio and metadata packed in one file is what defines the OP-1a pattern. The most complex level within these patterns is OP-3c, with the combination of several clips (packed files) with several playlists combined. Here is a table with them.

The Advanced Media Workflow Association (AMWA) was created to lead the development and promotion of the use of standards and technologies that enable more efficient workflows for the use of networked media. Its current projects are advancing the use of AAF, BXF, MXF and XML formats in data file-based workflows. The association works closely with SMPTE and other standards bodies.

Among its projects there is one that refers to MXF files and defines a set of rules that limit the MXF specification for the adaptation of this format to different applications and workflows. These specifications can be summarized in the following list:

AS-02 – MXF with mastering versions. It was developed to support the storage and management of the components of a program in MXF, to allow versioning, multi-languages and delivery to different media outlets. It is a file package that has all the necessary video, audio and metadata elements to generate multiple versions of a product. Who should use it? Post-production environments, broadcasters and content distributors.

AS-03 – Program Delivery. This MXF specification is optimized for program distribution and direct broadcast from a video server. It is a single file that incorporates audio, video and metadata from a single program. Who should use it? Networks of broadcasters.

AS-10 – MXF for Production. The AS-10 application specification is aimed at establishing a common MXF file format for the entire production workflow, including in-camera recording, ingest to a server, editing, playout, digital distribution and archiving.

An example is the use of MPEG Long-GOP. The project includes the development of an application to validate files as an aid to quality control processes. Who should use it? Production and post-production, broadcasters and content distributors.

AS-11 – MXF for redistribution. This is an MXF file format for delivery of finished products from broadcast stations to program creators. AS-11 includes the functionality of AS-03 and is extended to include AVC-Intra 100, and includes support for the D-10 standard definition video standard with AES3 audio. AS-11 defines a basic minimum metadata set, and a metadata schema with program segmentation. Who should use it? Broadcasters and content distributors.

AS-12 – Commercial Delivery. AS-12 is a subset of MXF format files for delivery of finished commercials to television stations or broadcast networks. The specification provides a clapperboard and other metadata to associate with the audio and video essence. AS-12 requires that explicit identification of content be computer-assisted and that the playlist be controlled by the computer. Who should use it? Production and post-production, commercial distribution, broadcasters and cable networks.

The main characteristics of these MXFs are shown in the following table, AS-02 does not appear because its development process has not yet been definitively completed.