File-based acquisition
Jun 1, 2009 12:00 PM, By Nigel Arnott
Acquisition formats should be a primary concern in file-based environments.
Data rates
In the article “Tapeless camcorders” in the January issue of Broadcast Engineering World, the editor opined, “Camera operators have come to expect a record duration of around two hours from a single cassette, again setting an expectation for different media.” That is an important point. We cannot force a reduction in efficiency just because we are moving toward a file-based approach. So bit rates have to be balanced against storage capacities to ensure that expectations of recording durations are met.
There are camcorders that support a number of codecs and bit rates, allowing the operator to choose the right tool for the job. The best possible acquisition quality is 10-bit 4:2:2 HD encoded using JPEG2000 to 100Mb/s.
As a simple rule of thumb, 100Mb/s is about 1MB per minute of storage. On a 65GB disk, that comfortably exceeds one hour of storage; likewise, with two relatively inexpensive 32GB professional CompactFlash cards, this benchmark can be achieved.
At this point, the issue moves away from one of bit rates to a question of storage. In the days of SD digital television, people got very blasé about file sizes and built workflows around ingesting everything and sorting it later. In HD, users need to be more careful.
A one-hour drama that has a 10:1 shooting ratio — not atypical — has 450GB of raw data, which is not in itself a huge problem; it is common for PCs or Macs used for editing to have local storage measured in terabytes. But you have to move that content in and out, usually across a network, which is also busy with other traffic.
If there is another device in the way to flip the content from one codec and/or wrapper to another, it can add another significant time penalty. News is the most time-sensitive content, and that can usually exist with a lower bit rate codec (although the need to transcode will still be an issue). But you might want to create a highlights package from a prestigious sporting event, in which case waiting for a format flip could be a big problem.
One solution is to move away from ingesting everything toward a preselection process. Offline editors could browse the lower-resolution versions that are created as a part of JPEG2000 encoding to browse for the best takes.
An important workflow boost is to use an editor that can access the camera material in its native format. Using the CineForm intermediate codec, for example, you can keep 10-bit 4:2:2 HD intact and import it straight into Final Cut Pro. The editor can accept a large number of codecs, including 100Mb/s JPEG2000, directly into the timeline.
These two solutions can then be combined, if the acquisition media can be connected directly to the editor. The content can then be browsed directly from the acquisition media with only the required takes pulled into the editor, and then transferred to a central archive as a background task as network traffic allows.
Conclusion
This article has focused on top-end HD acquisition and file-based workflows, because that is the most tested environment. HD production is about quality, so it's important to capture and deliver the best possible quality.
The same lessons apply to other applications. News, for example, may trade off some quality for smaller file sizes and editing speed. It is still critical, though, to ensure that the whole workflow is clear and without bottlenecks. File-based systems should make life easier, not more complicated.
There may be a case, for example, where a real-time HD-SDI transfer between two devices is actually quicker than passing a file through a format flipper. It may not be as “sexy” as a file-based workflow, but it might get the job done.
In planning for file-based environments, the acquisition formats have to be the primary consideration. This sets the quality standard for the rest of the network, and it should be maintained as far along the pipeline as possible. With today's technology, that bar can be set very high indeed, with 10-bit 4:2:2 HD and excellent compression algorithms now readily available in practical and affordable camcorders.
Nigel Arnott is marketing development manager for Infinity at Grass Valley.
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