Deciding on a file format
Mar 1, 2009 12:00 PM, By Peter Thomas
Figure 1. Core processes in a TV facility
Click to enlarge
As TV broadcasters migrate to file-based workflows, selecting the right file format is becoming important. A file format should support all core processes in television without the need for time-consuming transformations, have a minimum number of transcodings outside of core processes and have a credible future.
Core processes include file import and signal ingest, post production, playout and delivery, and archiving. (See Figure 1.) File import typically requires transcoding to the house format. File-based delivery, such as file exchange or distribution via Web, IPTV, VOD or mobile, usually involves at least a transcoding step for delivery format creation.
A standard archive format optimizes those core processes. The selection of such a format is a holistic approach that should not to be dominated by one department or group but be considered as a cross-enterprise business decision.
Archiving
File formats have two facets — the encoding format and the wrapper format. For use in archiving, an encoding format must:
- have a standard encoding scheme;
- be widely supported in the industry;
- be compatible with the products used for in-house core processes; and
- be supported by transcoders.
For SD, popular encoding formats are D10 and DV-DIF. For HD, no common denominator has yet emerged. Organizations may have to use different encoding formats for different business processes, but should strive to avoid transcoding wherever possible in order to avoid generation losses and latencies.
The wrapper format must:
- be open and well-documented;
- be widely supported in the industry;
- support partial restore;
- support play while record;
- have little overhead compared with the payload;
- include well-documented mappings for the selected encoding formats; and
- support embedded technical metadata.
Suitable wrapper formats for TV archiving are MXF OP1a and QuickTime.
File exchange
For file exchange with external partners, the file format has to meet the specifications as agreed upon in the respective service contract. The wrapper format should also allow embedding descriptive metadata, as you may want to embed subsets of the available metadata as contractually required. Hence, external file delivery typically includes transcoding to the required file format and embedding of metadata.
For file exchanges within your organization, try to avoid any encoding format changes, as they are time-consuming and introduce generation losses. Wrapper changes are less critical.
Selecting the right wrapper format
Until recently, the obvious choice for the wrapper format was MXF OP1a. There are MXF-enabled products available to support all core processes, and sufficient interoperability between those products has been achieved. Some products use MXF OP Atom, but the rewrap can be performed easily during file transfer.
However, one product has successfully entered the market that changes this picture — Apple Final Cut Pro (FCP). FCP does not natively support MXF. Instead, it uses QuickTime — a wrapper format developed by Apple and widely accepted in the IT industry. That means that MXF-wrapped material has to either be rewrapped before being delivered to FCP, or a separate QuickTime reference file has to be created. Content created on FCP requires rewrap from QuickTime to MXF before it can be used in an MXF environment.
Hence, facilities that are using FCP as the predominant editing platform may want to consider QuickTime as house wrapper archiving format. A prerequisite is that QuickTime can be used across ingest, production, playout and archiving, thus avoiding rewrapping entirely. If this is not possible, MXF continues to be the best choice.
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