MXF
Aug 1, 2006 8:00 AM, By John G S Ive
The rapid growth of networking and file transfers has become an important focus for the television industry. IT technology performance levels open up new options with radically changed workflows and collaborative working.
Today’s developments emanate from work by SMPTE and the EBU, and its report entitled “EBU/SMPTE Task Force for Harmonized Standards for the Exchange of Program Material as Bitstreams.” Real-time video and transfer mechanisms occupy the majority of the pages. However the following work was identified: “A Proposed Standard on an Interchange Storage Wrapper is required for interoperability between different equipment.” This was to become MXF.
A visit to NAB2006 demonstrated that MXF is now taken for granted by many companies and is the file format of choice for content exchange. MXF is becoming the SDI and BNC cable of the networked environment.
Key components
MXF is a wrapper that packages video and audio content together, with metadata describing and managing content. Typically, files will be transferred non-real-time over Ethernet networks (or other IP-based transport layer), but streaming also is possible.
On arrival, files may be deconstructed to proprietary formats for internal use — for example, separation of video and audio components. Conversely, for file server storage or archives, preserving the MXF structure may be the best solution. The word eXchange in MXF places the emphasis on files in transit and content storage.
MXF does what tape exchange has done for many years but in a much smarter and more efficient way. This is achieved by the comprehensive application of metadata going way beyond the capabilities of tape labels.
Metadata in an MXF file is categorized as structural metadata and descriptive metadata. Structural metadata concerns itself with how the file is assembled and its contents. This metadata is often created automatically with the file and enables recipient equipment to unwrap the contents (a packing list). Descriptive metadata is more likely to be of value to production staff and subsequent creative decisions.
The functionality of MXF files varies considerably. It is unreasonable to expect simple applications to be encumbered by the highest levels of complexity. Consequently, MXF offers operational patterns from a single video clip through to multiple video and audio tracks, with metadata describing the content and how the file should be read.
The simplest component, OP-Atom, holds just one element of continuous audio or video within the file and is the most basic element. This is particularly useful in the editing environment, where access to individual components is required.
Simple operation patterns (OP) predominate in the first implementations. The lowest level is “1a,” a single continuous video track together with audio and metadata, packaged as one file. (OP-Atom would required three.) Further operational patterns rise up through to OP-3c, combining multiple clips (file packages) and multiple playout scenarios (material packages). (See Figure 1.)
Figure 1. MXF operational patterns
Click image to enlarge.
Applications
For acquisition, MXF builds upon the nonlinear nature of solid state, optical or hard disc technology and adds metadata. Iif implemented at an early stage, significantly improves productivity in post-production.
For transmission, metadata provides valuable information for automation systems and can offer multiple versions from the one file (simple cuts-only edit lists). This reduces distribution costs, reduces errors and simplifies time-critical operations.
There are clear signs that tape is becoming a smaller percentage of production and distribution operations. One area proving resistant to this trend is archiving. Low cost, small size and high packing densities still represent benefits, but most importantly, data tape separates the storage technology from the content to be stored.
Video tape recorders rely on specific video formats; they dictate the compression strategy together with the resultant picture quality. Migration to a new format requires manual intervention together with decoding/re-encoding of the content and resultant quality loss.
Data tapes resolve these issues. Automated migration to new formats becomes a routine process with MXF files transferred transparently. The video format of the file is of no consequence. Additionally, partial restore, a powerful MXF feature, translates timecode references into byte offsets, enabling the efficient recovery of short clips as a subset of a much larger MXF files.
Acquisition
Much of the useful metadata is created and acquired at the planning stages and during image capture. Nonlinear camcorders from Sony, Panasonic, Grass Valley and Ikegami have implemented MXF. More recently, these formats have evolved into HD versions – the XDCAM-HD, P2 HD, Infinity and EditCam HD, respectively.
Post-production
In developing MXF, Pro-MPEG recognized the need to work together with the Advanced Authoring Format (AAF). AAF provides a comprehensive exchange of “work in progress” files between nonlinear editors and post-production tools. By working together, the two groups have maintained a high level of metadata commonality and interoperability.
So with acquisition devices producing MXF content, workflow improvements depend upon efficient nonlinear editor acceptance and processing of MXF files. Users have been frustrated by the slow progress in this area. The majority of nonlinear editor companies claim MXF support, but the level of functionality and workflow convenience has varied. Fortunately, many of these concerns are historical and now resolved.
MXF editor support has been achieved either through in-house developments or third-party integrators. Several companies have turned to MXF software specialists such as MOG Solutions, OpenCube Technologies and Metaglue. These small companies list some of the biggest industry names as clients.
Playout and distribution
MXF offers unique benefits for playout, few of which are currently implemented but hold great promise for the near-term future.
The use of servers with automation for program playout is commonplace. However, the ability of MXF to carry metadata can benefit both the playout process and the downstream consumer.
Storage and archive
As the role of tape recedes, the last bastion is long-term storage. The low cost, small size and removable media used by datatape storage devices remains attractive. The unique features of MXF are encouraging its adoption for long-term storage.
Videotape recorders constrain users to a specific mechanical recording format, bit rate, quality level, compression strategy, number of audio tracks and so on. Migrating from legacy video formats requires substantial manual intervention and quality loss due to transcoding. These limitations are eliminated with datatape.
Traditionalists could claim that with the file storage, the entire file must be read in order to view or playout a short clip. Videotape machines can respond to timecode “in” and “out” points and play only the desired section. MXF answers this concern with a feature known as “Partial Retrieve.” Timecode points are translated to byte offsets, enabling only the required part of the file to be recovered.
With videotape, labels and notes get lost and have to be entered manually multiple times. Archivists establish a separate database, maintain it and provide linkage to the content. Separate databases will continue to exist, but with MXF, the linkage of entries to stored files can be more efficient and securely achieved. Additionally, essential metadata can be embedded in the MXF file and travel with it.
John G S Ive is a consultant and technologist with Ivetech.
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