DAVID Systems’ Video File Transfer system
May 1, 2009 12:00 PM, By Ingo Hahn
The German public broadcaster group ARD bases file transfers on MXF standards.
What are the advantages of this concept? A limited range of compression formats ensures that video quality does not deteriorate as a result of too many encoding and decoding steps. In most cases, the sending station can provide video content in its native format, i.e., at the best possible quality. This would not be possible with a system limited to just one single format. As the main priority is to provide new video items quickly and at native quality whenever possible, a range of format-related issues — such as MiniDV's unlocked audio/video signals — may need to be addressed by the receiving stations. The decision to recode the received material for further processing rests entirely with the receiving station.
Again, the underlying concept here is that VFT is exclusively responsible for the transport of the files. If any recoding is required before or after the transfer, this is performed at the discretion of the stations involved. To speed up the process, the replicator can be extended with a range of add-on functionalities to automatically recode content upon receipt, and to provide the recoded files to the processing systems in the required format.
File format and metadata
The VFT system is informed by the latest advances in format standardization and employs the MXF OP1A as its wrapping and transport format. OP1A specifies that only temporally connected portions within any given file may be transferred together. If there are sets of multiple items (items containing other items), these have to be transmitted as separate files. In ARD's program exchange system, shows are usually transmitted to the broadcast center in their entirety as single files. This method greatly simplifies the identification and cataloging of transferred items and files.
In the ARD VFT system, metadata and essences may also be transferred separately, as long as the metadata is transmitted before the essence. All metadata is transmitted using DAVID Systems' proprietary XML-based format. In principle, metadata can also be transferred directly as XML files. Replication ID numbers are assigned to uniquely associate the metadata to their corresponding essences.
As is specified in the German Television Production Guidelines, every video item's metadata contains obligatory fields such as “title,” “orderer,” “order ID,” “program type,” “source broadcaster,” technical data on audio/video, etc., and a range of optional fields. For items that are exchanged with the ARD broadcast center, the metadata must additionally contain the ARD broadcast center playback number. The VFT system automatically detects the video file's length and enters this in the mandatory field “item length.”
Metadata can be sent to the VFT system via several methods. When videotapes are exported via the system, the metadata needs to be entered manually during the ingest stage. The data entry user interface is based on the familiar tape card layout. (See Figure 2.) When video files are to be sent from other, nonlinear, systems there is a third metadata option: transfer of the XML file via a Web service. The replication process itself can similarly be triggered from a third-party system. Using a range of VFT helper applications, receiving stations can easily view and update the metadata of all video items received via the replicator.
The structural separation of metadata and essences facilitates tight integration and synchronization with the databases of nonlinear production and playout systems. If the metadata were bundled into the same file as the essence, on the other hand, the MXF file would need to be opened even for the most mundane tasks, such as forwarding the item to regional studios or content programmers.
Objectives
With the introduction of the new VFT system, ARD is pursuing several aims, such as:
- Faster exchanging of current items — reduced transfer times (faster than real time);
- Simpler workflows for exchanging items — less involvement of the station's scheduling, programming and planning staff; and
- No or minimum loss of image quality — recompression is avoided where possible.
Interestingly, the focus of these objectives shifted over the course of the project. For example, the working group's initial discussions predominantly revolved around transfer times. Other early topics included bandwidth issues, priority management for time-critical transfers, and the capability of accessing and processing files during their transfer. Indeed the VFT system has been designed to provide most of the requested capabilities, particularly faster-than-real-time transmission speeds; however, many of the original aims are now regarded as minor points by the system's users.
Initial concern about the potential degradation of video quality was dispelled through the adoption of the MXF format and the specification of the video transfer formats. The overwhelming majority of content sent out by ARD stations today is transmitted at native quality and without recompression of the in-house format. In most cases, the receiving stations do not need to perform any recoding either.
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