April 30, 2009 by admin
It is important to distinguish between the technical terms “Closed-Caption” and “Sub-Text” or “Subtitling.”
The term “closed” in closed-caption indicates that not all viewers see the captions—only those who choose to decode or activate them. This is distinguished from “Sub-Text Captions” (sometimes called “burned-in” or “hardcoded” captions), which are visible to all viewers.
For the techno-speak enabled: For all types of NTSC programming, captions are “encoded” into Line 21 of the vertical blanking interval – a part of the TV picture that sits just above the visible portion and is usually unseen. For ATSC (digital television) programming, three streams are encoded in the video: two are backward compatible Line 21 captions, and the third is a set of up to 63 additional caption streams encoded in EIA-708 format.
For those techno-challenged: What the above paragraph means is that special programming standards exist which rigidly define what closed-captioning is capable of doing and how it will appear to the viewer. If we develop closed-captioning for a video, it will be subject to those limitations. The standard for closed-captioning was written decades ago and consequently has some limitations that are today unnecessary.
With Sub-Text or “Subtitling” the sky is the limit as far as how the text will appear to the viewer. The trade-off is that the viewer cannot interactively turn off the subtitling since it is burned or hard-coded into the video. For this reason the closed-caption standard was not incorporated into the Blu-ray standard.
Here is an enumeration of some of the advantages Sub-Text facilitates relative to closed-captions:
- We can use a proportional spaced font which results in much more efficiency and improves readability.
- The font color can be defined whereas with closed-captioning it is simply white on a black background.
- The characters can have drop-shadow to improve readability.
- Older television decoder electronics had no font decenders (applicable to letters such as j, g, p, etc.) making it wise to use all capitals. This limitation is not applicable with sub-text.