HD Captioning - Overview

With CaptionSync you can get the Closed Captioning formats required for HD Captioning. In this article, we provide an overview of the several formats and captioning methods available.

The EIA-608 standard (or line-21, or CEA-608) is employed in Closed Captioning for NTSC broadcasts, using true broadcast outputs such as SCC, CAP, or ASC files. With the development of digital television, a new standard was created for closed captioning: EIA-708 (or CEA-708). Several methods were then developed to provide closed captioning for digital television: using a TTML format with an encoder (software or hardware); or using a broadcast format with an encoder capable of writing it into the 708 field. Below we talk about these several methods.

Using a TTML format for HD Captioning:

Our .smptett.xml format is compliant with SMPTE ST 2052-1, the standard governing SMPTE-TT, and supports "708"-style captions, being suitable for broadcasters looking for 708 or HD captioning. It also supports vertical and horizontal positioning. It can be used by multiple programs and hardware devices to encode captions for digital television. These programs and hardware devices are usually fairly expensive tools, but examples include PixelTools products, CaptionMaker / MacCaption , Blackmagic HyperDeck, AJA products, Harmonic products, or Digital Rapids products.

Using Broadcast formats for HD Captioning:

Broadcast formats can be used for HD Closed Captioning and injected into MPEG-2 video streams using a 708 wrapper (compatibility bit). Some programs can take the 608 data (like the SCC format), and write it into a 708 field so that it will appear for HD TVs. In many cases, software that uses this method is less expensive than the options presented above. Examples include Apple's Compressor, Harmonic's ProMedia Carbon, Final Cut Pro, Squeeze Pro, or Encoding.com (use the DFXP format, and select cea-708 as Mux Type). You can download a MPEG-2 Program Stream file encoded with captions, at the end of Compressor's tutorial.

Note that being able to take 608 data and write it into a 708 field so that it will appear for HD TVs is a function of the encoding/authoring software, not of the caption files.