Technical "Stuff" on Captioning
(Source: Gary Robson, Cheetah Systems)
What is this "line 21," anyway?
The vertical blanking interval (VBI) of a television signal is the part "between frames." If you have an older television with a vertical hold adjustment, and you tweak it until a black bar rolls across the screen, that black bar is the VBI.
The VBI consists of a number of "lines" of video. The 21st line has been allocated to closed-caption information. The method of encoding used in North America allows for two characters of information to be placed in each frame of video, and there are 30 frames in a second. This corresponds (roughly) to 60 characters per second, or about 600 words per minute. That sounds like a lot, until you realize that it also takes one frame to transmit a command (like "go to a new line of roll-up"), and position information on pop-on captions take more than one frame. Also, that 600 words per minute includes everything required for caption 1, caption 2, text 1, and text 2, because they´re all stored in the same place, so there are decided limitations on the capacity of Line 21 for captions.
Caption 3-4 and text 3-4 are stored in field 2/line 21, a concept which is probably beyond the scope of this FAQ.
What are field 1 and field 2?
Each video frame has two fields. Caption 1 & 2 and text 1 & 2 are stored in line 21 of the first field in each frame. The second field´s line 21 can be used for caption 3 & 4 and text 3 & 4.
Each field actually has an effective throughput of roughly 600 words per minute, so it is theoretically possible to achieve four simultaneous caption channels at speeds approaching 300 words per minute each. In reality, the commands and intermingled text channel information, along with XDS (extended data services) data cuts this significantly.
What characters are valid in Line 21 captioning?
The character set was designed for the United States, and really has very little beyond basic letters, numbers, and symbols. There are some accented letters (lower-case vowels with acute and circumflex accents, a and e with grave accents, c-cedilla, and enya), but not many. See the Line 21 Captioning Character Set for the full listing.
The new EIA-708 standard for DTV captioning has a dramatically enhanced character set. Of course, you´ll have to get one of the new digital TVs and watch DTV programming to get this capability.
Why is the first caption in a commercial sometimes paint-on?
A pop-on caption needs to be transmitted in its entirety to the decoder before it can be displayed. This can lead to delays of over a second at the beginning of a program or commercial before the first caption can appear. A paint-on caption, on the other hand, will begin displaying immediately, so the viewer starts getting caption information faster.
Also, television stations sometimes start commercials a few frames late. To be safe, and avoid having the beginning of the first caption chopped off, captioners will usually delay an additional 7 or 8 frames, which is yet another quarter of a second. This is even more reason to want the first caption to start to appear as soon as possible.
For a description of paint-on and pop-on captions, see "What caption styles are available?"
How do color captions work?
Only the foreground color (the color of the text) can be changed - not the color of the black box. The colors available to the captioner are white, red, blue, green, yellow, cyan, and magenta. There is currently no way to see colored captions with an external decoder. You must have a television with a built-in decoder chip to see colored captions. Since this capability has not been around very long, color captions are quite rare in North America.
The Teletext system in Europe has supported color captions (subtitles) for quite some time, and the colors are used extensively to identify speakers (e.g. what one person says will be in cyan, and what another says will be in magenta).
The new EIA-708 standard for DTV captioning will provide a great deal more color capability than either the current U.S. standard (EIA-608) or the Teletext standard. Of course, you´ll have to get one of the new digital TVs and watch DTV programming to get this capability.
What is XDS?
XDS (previously known as EDS) stands for Extended Data Services. It is a concept that has not yet been fully implemented. The idea is that field 2 can be used to carry information that is related to the content of the broadcast, but isn´t caption related. XDS information will be carried in "packets," containing such information as the time of day, station call letters, network, name of the current program, and so on. This means that TVs and VCRs will be able to set their internal clocks automatically, and that you will be able to look for programs by name or network rather than channel number.
This document is Copyright ©1995-98 by Gary D. Robson. It may be freely duplicated and distributed only in its entirety. No modifications may be made to this document or any of the files that comprise it, including removal of this paragraph, and no excerpts may be taken, without prior written permission from the author. If you distribute this document, you may not charge for it, or include it with anything else that you charge for without prior written permission of the author.