Giovanni Galvez, Creative Planet /
05.20.2014 03:10 PM
Repurposing TV Closed Caption Files for Internet Video Distribution
More than 48 million Americans have hearing loss and rely on closed captions to comprehend TV shows. Others whose native language is not English rely on closed captioning for improved comprehension and fluency.
Telestream Vantage transcoder automates the conversion process from TV captions to SMPTE-complaint Internet captions via watch folders.
To accommodate these groups and others, the U.S. Congress recently passed a new closed captioning law. The new law and FCC rules require broadcasters to caption Internet-distributed video files if the content was broadcast on TV with captions. A March 30, 2014, deadline by the FCC mandates that clips on the Internet must be captioned within 45 days of the programming’s television airing.

Repurposing television closed captioning files for the web offers the benefits of obeying the law and reaching a wider audience, but it also presents technical challenges. These challenges can be met more easily when those who must convert TV videos to the Internet have information about the best options.

File Conversion Challenges
Most captioning files for television are created using authoring software designed to work with TV signals in broadcast control rooms, not with Internet protocol (IP) video. As a result, TV caption files may be archived in text file formats that cannot be used with most Internet video players.

Closed caption files for the Internet, on the other hand, can include a variety of different file formats. For example, many developers use timed text markup language, or TTML XML (formerly known as DFXP), for Flash video players. However, Flash video players potentially can support a variety of caption file formats. YouTube’s Flash video player supports TTML, for example, but also supports SRT, SCC and WebVTT.

Some Internet caption files are very simple and support only basic features such as timing and line breaks. Other more advanced Internet caption files, such as QuickTime closed caption track WebVTT and SMPTE Timed Text 2052, offer extensive captioning options, including positioning and text animation.

Because of the differences in formats, and also in the content itself, conversion from web to TV can be complicated. In many cases, TV audio or video content may have to be corrected in the following ways prior to Internet deployment:

Segment breaks of five to ten seconds to accommodate commercials may have to be removed.

  • A video may require separating into parts or combining parts into one video.
  • Certain music and sounds prohibited by copyright laws from Internet distribution may need to be removed or replaced.
  • Internet distributors may require original unedited 23.98 fps video without commercials, rather than TV broadcast files at 29.97 fps. As a result, the TV closed caption file created for 29.97 fps will not match the 23.98 fps video timing.


  • In each case, modifying or deleting elements could desynchronize or delete the captioning data.
    Telestream Vantage transcoder automates the conversion process from TV captions to SMPTE-complaint Internet captions via watch folders.

     
    How to Edit A TV Caption File for the Internet 
    Preparing a TV caption file for Internet use involves:

  • Converting the caption data format from TV CEA-608 protocol to an Internet format, and
  • Modifying the text, timecode and positioning.

  • Some knowledgeable TV engineers manually edit TV closed caption data files to repurpose the content, but this can be difficult and time-consuming. Using software to automate and batch extract the closed caption data from pre-existing TV caption files is a better solution. In the same way that one video file format can be transcoded to another, automated workflows can accommodate transcoding from TV captioning files to Internet captioning file types.

    With suitable software, the video content provider can open and edit a variety of TV caption file formats, including legacy CAP and TXT files, as well as modern caption files such as the Scenarist SCC format. Broadcasters can then modify caption timing and text formatting. They can also eliminate phrases from the captions that have been edited out of the video and consolidate caption files from two-part TV shows into a single caption file.

    Automation and Editing Captions
    Although most video editing workflows now include automation, closed caption data editing is usually a manual process that must be done twice: the video must be edited, and then the captions must be edited to match.

    To eliminate this redundancy, advanced closed captioning software uses edit decision lists from video editing systems such as Adobe Premiere and Apple Final Cut Pro. Using this technology, closed caption files can be efficiently converted to Internet files in batches. Batch conversion can be accomplished using the video editing software, or it can be scripted using a command line interface.

    What About CEA-708 Caption Data?
    Some TV closed caption files include both CEA-708 and CEA-608 data. Even though their formatting, timing, text and positioning are typically identical, if conversion is required from one to the other, software can handle that process.

    Frame Rate Conversion of Closed Caption Data Files
    Unlike the standard frame rate of 29.97 or 59.94 fps for broadcast television in North America, Internet video delivery supports a variety of frame rates. Some Internet distributors require the 23.98 frame rate of film, which means the TV caption files will not match the frame rate of the Internet video. To meet this challenge, closed captioning software can convert the frame rate to 23.98, and also stretch and shrink the timecode as necessary.

    Live Closed Captioning
    Closed captioning of live television is typically done on the fly by a fast typist or speech recognition software, resulting in live captions that are a few seconds delayed. Although there are ways to stream the captions live to the Internet while simulcasting the captioned TV signal, most Internet video content is uploaded after the live video has aired on TV. Repurposing live captions for the Internet requires software that can capture, extract and modify captions very quickly to avoid having to start the captioning process from scratch. At a minimum, the delay of live TV captioning can be offset to better synchronize the captions to the Internet video.

    Mobile Devices and Closed Captioning
    Another Internet video market that must be considered is handheld mobile devices such as tablets and smartphones. iPad, iPod and iPhone iOS devices can decode advanced QuickTime closed caption tracks that have the same look and feel as TV closed captioning. Unfortunately, there is no standard caption file format or player application for all mobile devices, making compliance especially challenging.

    Some Internet video distributors have created custom video playback apps that host two files on the streaming server: the video file and an Internet caption file. Adding a companion Internet caption file to accompany the video is a viable solution using a custom app.

    Repurposing TV closed caption data files for Internet distribution is more challenging than simply converting one file format to another. Content creators must take careful steps to ensure Internet-captioned video matches the quality of what was delivered to TV viewers. When this is done, the delivered Internet video will comply with government requirements and also provide an excellent Internet viewer experience that reaches a much wider audience.

    Giovanni Galvez is a captioning expert at Telestream, which is headquartered in Nevada City, Calif.

     



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