News Production & Post

Subtitling and closed caption management

3 January 2014
Subtitling and closed caption management

A subtitle can go through a long, uncertain journey before it arrives at its onscreen destination. Benoit Février (pictured), SVP at EVS Media, provides us with a roadmap.

As they aim to reach as wide an audience as possible, broadcasters and content owners are looking toward multi-region distribution. At the same time, legislation from the EU requiring television programmes to include closed captions and subtitling means this is now a key element of the content delivery process.  Ideally, this is something that broadcasters have to ensure is done right from the moment content is ingested – however, in today’s complex, multi-channel, multi-format production environments, this is sometimes easier said than done.

Throughout its lifecycle within a file-based workflow, subtitling has to be considered – whether it’s where to put the subtitle or what process it needs to go through. What format the original AV file is in as well as the type of encoder used to generate that file will also impact the level of complexity involved in processing it. In SD workflows, subtitles are usually transported in VBI inactive lines, whereas in HD they’re carried in the ANC data, requiring different processing.

In the production environment, subtitles are transported within the baseband SDI signal to the ancillary essence layer. They are then reconciled with the rest of the AV content at the playout stage. In the case of tape-to-file archive migration, the decision of whether to leave the subtitle within the video signal needs to be addressed. Similarly, users need to decide how practical this is in the long-term – due to dependence on the codec technology.

The winding path of a subtitle file

At the ingest stage there are two options when it comes to subtitling. They can either remain as a RAW VANC, then wrapped in an MXF S436M track before they progress further through the workflow. Or, they become a separate text file (eg, .SCC or .MCC). The MXF file wrapper has only recently been put forward as a viable option for handling timed text, but at the moment this isn’t widely adopted except within DCP workflows.

Technology developments from companies like EVS mean that users can handle many of these options for subtitling and closed caption management. They can keep ANC data – along with subtitles and closed caption – in the MXF file as a S436M track, or make use of a .SCC/.MCC sidecar file which is generated alongside the MXF AV file and follows it through the entire production process.

Once an AV file is ingested into the workflow, it becomes increasingly easy to lose track of the subtitling data contained within it. If you’ve assigned the subtitling data to remain as a RAW VANC file, which is then wrapped in an MXF S436M track, it can be difficult to locate. You have to first identify if a file actually has a S436M track and then look to see whether it carries subtitles. If it does, how do you make sure they’re in the right format, or if the subtitles are displayed correctly and are in sync?

Once converted to a text file, a subtitle follows a different path through the production workflow to one wrapped in an MXF S436M track. If the workflow involves MOV files rather than MXF, having a sidecar text file hitched to your main AV content, which follows it through the various steps in the production process, really makes a lot of sense. Although the AV and subtitling get re-united prior to playout, it’s essential to check that all subtitling/closed captions are synchronised with the MXF video and audio content.

What’s really needed is an easy way to decode the subtitle and closed caption data from the S436M track – or sidecar file – and check it’s burned in to the correct frame.

Talking about file conversion
Regardless of the format of a subtitling file and whatever its path through the workflow, one thing is certain: it will need to be converted at several points along the way. This will usually be from a subtitling text file to one wrapped in an MXF S436M track and then unwrapped again at the other end of the production chain. This wrapping/un-wrapping process is time consuming and inefficient.

With the right technology, users can bridge the gap between an MXF 436M VANC track and .SCC files at each step. Within a file-based workflow subtitling and closed captioning data is as crucial to the final output as audio and video. It’s key to maintain the integrity of audio and video throughout the production process, and the same holds true for subtitling and closed captioning.

The finished article
When it comes to outputting the finished content the accurate management of your subtitles and closed captioning data, along with your MXF production file makes the whole process a lot less complicated.

To ensure that subtitles and closed captions can be efficiently managed, tracked and synced, it’s important the entire subtitle management process has been fully integrated into the workflow design from the start. What causes headaches and needless complication is when this part of the workflow is treated as a cumbersome and complex afterthought.

www.evs.com

Similar stories
RELATED WHITEPAPERS