Multilingual subtitle creation
Feb 1, 2010 12:00 PM, By Darren Forster
Successful subtitle insertion requires a hybrid approach.
Figure 1. Closed-captioning and subtitling solutions are being deployed on a myriad of platforms using various standards.
Select figure to enlarge.
A prominent feature of today's global media world is the increasingly important role played by subtitling. More broadcasters are achieving global media distribution through the use of tapeless post production, storage and playout, and relying on the latest subtitling technologies to make content accessible in regions where dubbing is too costly an alternative and only suitable for premium content such as blockbuster movies. Legislation creates an additional requirement for subtitling for the hearing impaired; although, all content providers (including advertisers) are keen to reach the widest audience possible, regardless of accessibility issues. With the proliferation of Internet-based video, lawmakers are starting to propose that streamed and downloaded content should also carry subtitles. And in public places, such as at airports, gyms and other high-traffic areas where sound is muted or where there are high ambient sound levels, subtitles are becoming more widely used. Overall, the demand for subtitling is at an all-time high. (See Figure 1.)
But there are key issues to overcome to achieve the successful and efficient playout of subtitles to an ever-growing array of platforms. There is a very diverse range of video and wrapper formats to be supported, and there are significant limitations with some playout technologies. Although wrapper formats such as Media eXchange Format (MXF) and QuickTime are sophisticated containers for transporting and storing media, broadcast equipment does not typically support the storage of multilanguage subtitles in a format suitable for repurposing.
Although there are several newer formats that promise to be the new all-encompassing standard for the future, the massive number of legacy files in the field means any system must cope well with the import, repurposing and export of a huge number of formats. Broadcasters and content producers have simply invested too much to discard legacy files, with the vast majority being proprietary and often very guarded, vendor-specific formats. So in any subtitling process, great care must be taken when crossconverting formats to ensure the preservation of metadata and other essential information about the subtitles.
Subtitle-processing software can take advantage of existing technologies to embed data, enabling multilingual subtitles in a single version of an asset. This can be achieved by using VBI or VANC tracks associated with the media asset. The embedded subtitle data can be converted downstream to a variety of formats, including burnt-in subtitles or DVB bitmap subtitles.
Wrapping for diversity and repurposing
Figure 2. Captions and subtitles can be bound to the program file at different stages in the workflow.
Select figure to enlarge.
Like any aspect of broadcasting, subtitling can be a slow, labor-intensive process if it's done with outdated methods. The key to using subtitling extensively and effectively is an efficient workflow that allows broadcasters to shorten the creation and playout cycle and keep costs at a manageable level. All broadcasters have different requirements and processes and need individually tailored workflow solutions. The subtitling component needs to be integrated into the broadcaster's overall solution, preferably during the initial design of the system. With the goal of supporting a multitude of output video formats, the focus is shifting away from the traditional production system and transmission chain toward digital asset management (DAM) systems.
To aid in format and resolution conversion for diverse distribution formats, content is increasingly stored as a single, common “mezzanine” format representing the highest-quality version, and all subsequent broadcast and streaming versions are derived from that. This can be wrapped as a universal format for easier exchange, and, to further aid repurposing, subtitle data should also be stored in a highly generalized form suitable for repurposing at transmission time.
There are two methodologies broadcasters employ when taking this approach. One relies on the creation of an “übersubtitle,” which has as much information as possible related to the subtitle and from which less sophisticated subtitles can be derived. A “mezzanine subtitle” will often rely on a professional subtitler to make informed choices for presentational aspects such as font, color, position and alignment information, drop shadow, and character edging — sometimes following prescriptive formats or house styles defined by the agency or broadcaster. Such “über” formats can support the media asset for its lifetime, allowing for elegant, effective and highly automated translation to various output distribution formats.
Alternatively, broadcasters may use a transcode method that relies on a lowest-common-denominator file (such as a WS Teletext or U.S.-style EIA-608-compliant captions) being created and then upconverted to the target format. It's a quick and easy approach, but it does not take advantage of the sophisticated options available within higher-end standards (such as DVB ETS-300-743, DVD bitmap or EIA-708).
Many broadcasters choose to adopt a hybrid of the two approaches, implementing some of the capabilities while limiting the overall time dedicated to creating the subtitle by constraining and automating some options. In any case, it is well worth noting that different standards offer varying levels of control and sophistication.
The good news is that with effective content management, the subtitle data contained in the media wrapper can enhance the asset metadata, providing a rich, searchable source of content-related information. This can further enhance efficiency for logging and research as well as enable monetization of content, thus allowing the subtitling process to deliver value throughout the future of the asset.
Early, late or live binding for delivery
Once the created subtitle file is signed off for delivery, it must be bound to the content to enable it to be presented to viewers when they watch the programming.
This binding can be done at one of three stages in the process:
- Early binding
The preprepared file is matched to the programming well ahead of transmission.
- Late binding
Late binding occurs near to airtime and is only possible thanks to faster-than-real-time encoding techniques.
- Live binding
Live binding is employed for either live content or preprepared content that only becomes available very close to airing, making it impossible to prepare subtitles in advance. (See Figure 2.)
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