Multilingual subtitle creation
Feb 1, 2010 12:00 PM, By Darren Forster
Successful subtitle insertion requires a hybrid approach.
Figure 3. With traditional early binding too time- and labor-intensive, files are now either
sent for time-of-air transmission or are transcoded into a file-based video asset.
Select figure to enlarge.
The traditional method for subtitling preprepared content was early binding by creating a submaster tape with the subtitles encoded into the VBI space on the tape by inserting them into baseband video. Although this was an appropriate method when linear, single-channel broadcast TV was the only form of output (it is still possible to do subtitling this way), it is now outmoded because it's so time- and labor-intensive. Instead, files are now either sent for time-of-air transmission (live binding) or are transcoded into a file-based video asset (during early or late binding). (See Figure 3.)
Time-of-air transmission generally involves systems that integrate with the automated workflow of a master control facility, with the subtitle playout system approving files in advance, and then airing the correct file at the right time automatically, either with or without time code. The time-of-air system can also be used as a gatekeeper for real-time subtitling, with the system authenticating the subtitler and work slot prior to allowing pass-through to air. This system of checks is useful given the distributed and freelance nature of real-time subtitling.
A hybrid of ingest and time-of-air methods has become increasingly popular and results in ingest of the video asset whenever possible, with time-of-air playout where appropriate. The time-of-air subtitle system receives the playlist information directly from the automation system. Issues such as missing files, missing time code and media lacking encoded subtitles or metadata can be reflected directly back to the automation system to ensure master control staff take immediate remedial action. Where the automation playlist indicates that the video asset already contains subtitle data, the time-of-air subtitle system can check that it is complete and QC the subtitles, flagging any errors appropriately. The time-of-air subtitle system can also provide interfaces to other ancillary data signals and XDS information such as widescreen signaling, vChip parental controls, broadcast flag information, DRM controls such as CGMS-A data, DPI data and more.
Broadcast equipment technologies continue to roll out at a rapid pace, and it is important that ancillary data (such as subtitle data) is supported in all equipment in the transmission chain. Systems have to remain interoperable and ideally include APIs developed to assist in the exchange of data around the modern workflow.
The MXF format provides an open-standard wrapper for broadcast media including ancillary data, and many equipment vendors support it because it enables seamless workflows. MXF is an extremely flexible specification, and most broadcasters who use the standard design a profile or shim to remove some of the ambiguity from this comprehensive and far-reaching standard. Any profile used should define the way subtitling data is supported.
Adapting to moving targets through subtitle transcoding
When broadcasters want to generate multiple versions of the same content for alternative nonbroadcast distribution, such as for online video, distribution methods must employ a variety of subtitling technologies, all of which must be supported in a holistic way by the subtitling system.
Implementing a mezzanine video format and including the übersubtitle file, the subtitle data component can be transcoded appropriately at the same time as the video, ensuring the same or better quality as in the broadcast version. In addition to supporting different output distribution formats, workflows often need to provide for reversion outputs where the time of the video asset is manipulated, or it is split into different program segments. This reversioning process is often carried out on an NLE system and can effectively destroy the subtitling data, but modern transcoding solutions can avoid this by using the edit decision list from the NLE to bridge the original and final versions of the subtitle data.
Again, the embedded data concept plays an important part here because the mezzanine format can be either a rich or lowest-common-denominator format; but, it is vital that it is unbreakable to reduce risk and ensure that the subtitles reach the viewer correctly. The wrapper provides a means to store subtitle data in a way that makes it transparent to the video and allows multiple languages to be stored within the same asset. This further reduces risk and saves time and storage space through the use of a single-asset version.
By using the VBI or VANC tracks that are available in wrapper formats such as MXF rather than the video essence, subtitle data can be stored without having to be encoded into the video essence. In addition to the previously mentioned advantages, this also ensures that the video remains clean and is not potentially degraded through additional transcoding processes.
By implementing an integrated subtitle creation/repurposing, encoding and transmission solution, the subtitle data embedded in the media asset can then be seamlessly transcoded to the desired format at time-of-air, or even completely bypassed. Viewers will enjoy the highest quality of subtitles however they choose to consume their content, and content providers will reach the widest audience.
Darren Forster is chief technology officer at Softel.
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