A feature utilized in television and video media that displays dialog and sound effects cues in text on the viewer's screen. Originally developed as an aid to the deaf and hearing impaired. In most cases, the captions are inserted in a special scan line in a video image. To view the captions, one must use an external decoder or a television with a built-in decoder. Changes in federal law require all televisions with 13-inch or larger screens sold in the US to be equipped with closed-captioning decoders.

Closed captioning can be utilized in two ways. In realtime captioning, a typist trained in stenography uses a special keyboard device to enter the text, dialog and cues. The typed stream uses a complex form of phonetics, and the typist can enter the text at speeds of up to 250 words per minute. The stream is sent to a computer, where it is converted to plain text and sent to an encoder for broadcast.

In offline captioning, the captions are added to recorded events in post-production. Doing so requires staff to enter the text, encode it to the media, and check it for accuracy.

A fellow named Gary Robson has designed a site with extensive FAQs and information about closed captioning, laws and requirements, methods, and equipment. You'll find it at http://www.robson.org/capfaq/.

Also handy for watching late night programs and not waking the family.

There's also a third type of closed captioning, live display, which is halfway between realtime and offline. The closed captioner has a videotape or an audiotape in advance and can create a complete closed captioning file from that, but the captions are sent out live and encoded into the signal as the show airs. Talk shows, newsmagazines, and soap operas are the types of programming that are most often captioned this way.

For the viewer at home, the rule of thumb is that if the captions are in the pop on style, it's offline; if they're rolling up a word at a time, it's realtime; if they're rolling up a line at a time, it's sometimes live display and sometimes offline. (In general, if the captions are leading the dialogue as they roll up, it's offline, and if they're trailing, it's live display.)

The captioning industry in the United States evolved such that, in most cases, the networks and program producers don't do their own captioning, but instead contract with separate companies to do the captioning for them. E! and HBO are the notable exceptions that have their own captioning departments.

The three largest television captioning companies are The Caption Center, headquartered in Boston, which is a division of PBS affiliate WGBH, the National Captioning Institute, headquartered in Washington, D.C., and VITAC, headquartered in Canonsburg, PA (near Pittsburgh), although all three also have offices in Los Angeles, which makes it much easier for the post-production companies to deliver videotapes. There are several other large captioning companies that mainly handle videotapes and DVDs, such as Captions, Inc., plus a bunch of smaller captioning companies, but if you're watching network TV in the U.S., it's a safe bet that the captions you're seeing are the work of one of the Big Three.

Each has its own style guide, and it's therefore quite possible to tell whose captions are whose without having to see their credit at the end of the show. It's easiest with the offline pop-on captions:

There are other individual quirks between the companies, such as how they denote sound effects, but the above are the easiest to quickly spot.

By the way, any misspellings you see in offline captioning are usually the fault of your TV reception. Most often, a glitch will cause two adjacent characters to drop out of the captions. The second most likely cause is that the captioner wasn't given a script and had to try looking up a piece of jargon or a place name on the Internet, or had to guess at the correct spelling of a new character's name.

Occasionally, some words will be missing from the captioning, or some phrases will be rewritten. If it's realtime captioning, that's most likely because the stenographer is running behind and is desperately trying to catch up. In offline captioning, it's more a matter of reading rate. In the early days of captioning, the thinking was that deaf people weren't able to read anywhere near as well as hearing people, so captions were kept to a very low reading rate. As people have gotten more and more used to reading closed captions, reading rates have increased steadily. Today, because many deaf viewers have said they'd rather have the opportunity to try to read every word, and because there are more hearing viewers than ever before, many captioners just cram everything into the captions without worrying about reading rate..

If the captions don't match the dialogue at all, that's almost definitely because there was some last minute editing done to the audio, and the captions couldn't be fixed in time.

I've been a captioner for one of the Big Three since 1997, first working in live display and then in offline. I've captioned so many different shows that, if you've ever watched TV with the captions on, you've probably seen my work.

Technical. Hot! You know it.

A brief overview of the Closed Captioning System implemented in the United States


The current method for encoding caption information was originally developed by the National Bureau of Standards to transmit time of day information, it was later expanded to include captioning. The Public Broadcasting System currently handles the task of transmitting of time of day information and devices such as VCR's may be programmed to listen for this time of day information and automatically adjust their clocks. Preliminary captioning service first began in March of 1980 with basic text capability that allowed a transcript to scroll continuously at the bottom of the screen, the Electronics Industries Association later defined a set of expanded capabilities under their standard EIA-608. EIA-608 is a reccomendation and implementations of closed captioning are not forced to adhere to it, however it has become the standard in the United States. In 1993 the name CaptionVision was adopted for this system.

During the advent of captioning the most common method for display was to use set-top decoder boxes (external) instead of the integrated chipset based decoders (built into televisions) which would eventually become the dominant implementation. The set-top decoders never sold very well, only reaching a saturation of about 350,000 devices perhaps. To help foster the pervasiveness of closed captioning on both the transmitting and recieving end Congress passed the Television Decoder Circuitry Act in 1990.

Technical Information

Although captioning was not forseen and planned for when the original NTSC video standard was developed, it has been possible to integrate the transmission of caption and other digital information into the signal while maintaining backwards compatability. It is the task of the recieving device (television/monitor/set-top box) to decode and render the text data into a readable format.

The current standard provides roughly 120 characters per second encoded in an NTSC video signal. To achieve this two bytes of data are encoded into line 21 of the vertical blanking interval of each field. The vertical blanking interval is (essentialy) the brief period of time between subsequent video fields, there are sixty fields per second which yields thirty frames per second (since two fields are used to display one frame). The alternating fields carry even and odd scan lines respectively. Line 21 of each video field carries seperate caption information, so there are captions 1 & 2 and text 1 & 2 in the first field, and captions 3 & 4 and text 3 & 4 in the second field. Although most characters may be transmitted with a single byte, some text as well as control codes require two bytes to be transmitted, this lowers the effective throughput of the system somewhat.

To help ensure that caption information is properly recieved, it was decided that the data should be encoded at a relatively low speed. The data is preceded by a seven cycle sine wave and three start bits (0,0,1). There are two bytes of data encoded on the scan line (line 21) comprised of seven data bits and one parity bit (odd parity). The rise time is controlled and there is an amplitude of 50 IRE units. Tests conducted by PBS indicated that people have a typical reading rate of 125 words per minute for captioning, this is well below the approximate maximum of 500 words per minute which leaves a good deal of excess bandwidth in the second video field for other types of data. Control codes (non-character information) require two bytes to be transmitted and are typically transmitted for two successive frames (called byte pair doubling) to ensure that they are recieved. In the future expect to see enhanced captioning capability emerge under the new standard EIA-708.

Extended Data Services (XDS or EDS) is a feature which is not entirely implemented yet but will allow the second video field to carry information related to the content of the current broadcast. This information will be encapsulated in a packet and provide details such as the time of day, station and network identification, and the name of the program. Another subset of data which resides in the second video field is standard EIA-744-A, subject of some contention and commonly known as the V-chip. This data provides rating information about the current broadcast which allows televisions equipt with a V-chip compliant decoder to filter programming based on rating information.

It is possible to obtain a copy of EIA-608 for $106 by writing to the follow address.

Electronic Industries Association
Engineering Department
2001 Pennsylvania Avenue, N.W.
Washington, D.C. 20006
202 457-4900

Display Capabilities

Captions may be presented in a few different ways
  • Pop-On : the text must be recieved in entirity before display, so there may be as much as a one second delay before being revealed, however it is found to be less distracting than other methods
  • Paint-On : from left to right
  • Roll-Up : from the bottom of the screen
It is possible to change the color of the text (but not the color of the background) to any of the following; white, red, blue, green, yellow, cyan, and magenta. It is also possible to render characters in italics or with an underline.

The following is an incomplete list of character and control codes (in hexadecimal)

20 space
21 ! exclamation mark
22 " quotation mark
23 # number (pound) sign
24 $ dollar sign
25 % percent sign
26 & ampersand
27 ' apostrophe
28 ( open parenthesis
29 ) close parenthesis
2a á * lower-case a, acute accent
2b + plus sign
2c , comma
2d - hyphen (minus sign)
2e . period
2f / slash
30 0 zero
31 1 one
32 2 two
33 3 three
34 4 four
35 5 five
36 6 six
37 7 seven
38 8 eight
39 9 nine
3a : colon
3b ; semicolon
3c greater-than sign
3f ? question mark
40 @ at sign
41 A upper-case A
42 B upper-case B
43 C upper-case C
44 D upper-case D
45 E upper-case E
46 F upper-case F
47 G upper-case G
48 H upper-case H
49 I upper-case I
4a J upper-case J
4b K upper-case K
4c L upper-case L
4d M upper-case M
4e N upper-case N
4f O upper-case O
50 P upper-case P
51 Q upper-case Q
52 R upper-case R
53 S upper-case S
54 T upper-case T
55 U upper-case U
56 V upper-case V
57 W upper-case W
58 X upper-case X
59 Y upper-case Y
5a Z upper-case Z
5b open square bracket
5c é * lower-case e, acute accent
close square bracket
5e í * lower-case i, acute accent
5f ó * lower-case o, acute accent
60 ú * lower-case u, acute accent
61 a lower-case a
62 b lower-case b
63 c lower-case c
64 d lower-case d
65 e lower-case e
66 f lower-case f
67 g lower-case g
68 h lower-case h
69 i lower-case i
6a j lower-case j
6b k lower-case k
6c l lower-case l
6d m lower-case m
6e n lower-case n
6f o lower-case o
70 p lower-case p
71 q lower-case q
72 r lower-case r
73 s lower-case s
74 t lower-case t
75 u lower-case u
76 v lower-case v
77 w lower-case w
78 x lower-case x
79 y lower-case y
7a z lower-case z
7b ç * lower-case c with cedilla
7c ÷ * division symbol
7d Ñ * upper-case enya (N-tilde)
7e ñ * lower-case enya (n-tilde)
7f * solid block

Two-byte character codes
11,30 ® * registered trademark symbol
11,31 ° * degree sign
11,32 ½ * 1/2 symbol
11,33 ¿ * inverted (opening) question mark
11,34 TM * trademark symbol
11,35 ¢ * cents symbol
11,36 £ * pounds sterling
11,37 music note * music note
11,38 à * lower-case a, grave accent
11,39 * transparent space
11,3a è * lower-case e, grave accent
11,3b â * lower-case a, circumflex accent
11,3c ê * lower-case e, circumflex accent
11,3d î * lower-case i, circumflex accent
11,3e ô * lower-case o, circumflex accent
11,3f û * lower-case u, circumflex accent
14,29 Paint On style of display
14,25 / 14,26 / 14,27 Roll-Up styles of display
11,20 white
11,22 green
11,24 blue
11,28 red
11,26 cyan
11,2A yellow
11,2C magenta
11,2E italics
14,28 flashing
7 underline

If I can offer you one piece of advice about how to watch TV, it's this:

Watch With Closed Captioning On!

I have had a number of different roommates in the past, so I know that people who aren't accustomed usually find it annoying at first. But let me tell you, it's worth it.

One of the reasons is alluded to in this node, but it just scratches the surface. I have been regularly watching TV with captions on since 1994, and here are some of the things that I've noticed.

Offline captions are often inconsistent with the audible dialog

Not in an interference-induced typo kind of way either. Many times I have been watching a sitcom and when the punchline arrived, the spoken line was completely different from the captioned one. Usually, the captioned one tends to be racier or more outrageous. My hypothesis is that last minute rewrites are not uncommon, but that offline captions are developed from original scripts. This happens particularly often with The Simpsons. An example1:

Episode 250/BABF20: A Tale of Two Springfields

When Homer fails to blow himself up at a town meeting...

Homer: Nice wiring, Bart.
Bart: It worked on the test corpse.
Bart: It worked on the test goat.

Dialogue and sound effects are sometimes inaudible

This occurs less in sitcoms and more in drama and mystery shows. Sometimes dialogue is mumbled, sometimes it's not spoken at all. I personally think it's interesting to experience the story as the screenwriter indented.

You learn more

Mainly the spellings of obscure words, but sometimes other stuff too. Before I watched closed captioning, I didn't know that thing was called a raspberry. You're still going to have to study for your GRE, but I like trivia almost as much as minutiae.

You miss less

Living with roommates, I have often tried to watch TV while others in the room were being really loud. However, I'm not antisocial, and with closed captioning on, I won't miss a line of Law & Order.

It's humorous

This mainly occurs when the reception is poor, or when captions are being done in realtime (e.g. a live sporting event). When captions get messed up, the results are often highly amusing. As I understand it, one of the reasons is that realtime captions are entered using a machine similar to court stenographer's. The entry system is phonetic rather than alphabetic, and a computer is responsible for converting the output to normal text. A small error usually results in the computer selecting words which are phonetically similar, but utterly nonsensical2. This often happens with proper names as well.

1I'll add more when I remember them, or when people tell me
2I'll put up some examples the next time I notice them
Sources: http://www.tvtome.com/Simpsons/index.html

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