A Staff Report from the Straight Dope Science Advisory Board

How does closed captioning work?

December 23, 2003

Dear Straight Dope:

How does closed-captioning - the dialogue that appears on a television screen to assist the hearing-impaired in their quest to be as dumb-downed as the rest of us - work for live broadcasts? I understand how it would be no problem for a pre-recorded show, like a History Channel narrative or a prime-time sitcom. But when I watch local news or a sports broadcast, for example, I notice a considerable delay between hearing the words and seeing them appear on-screen. Even then, many words are misspelled, and sometimes words or even whole sentences are missing. Is this done with some kind of not-yet-quite-perfect voice-recognition technology, or is some poor schlep furiously typing away at a keyboard in some dark closet at the station? Such a person would receive my sympathy (not that there's any reward in that). The job seems second only to air traffic controllers for stress!

First, a little background. Closed captioning was first introduced in 1971. Originally, the National Standards Bureau (now known as NIST) wanted to embed a time stamp from the Boulder, Colorado atomic clock in television signals, but the idea was abandoned. The ABC network thought that a modification of the proposed scheme could be used to carry captions, i.e., a typed transcript of the show's audio, which hearing-impaired users could decode using a special set-top box. This was considered an improvement over the alternative, an inset picture of someone transcribing the audio using sign language, as was done from time to time. By 1980 CBS, ABC, NBC and PBS were all broadcasting at least some programming with closed captions. In 1990 the Television Decoder Circuitry Act required that all TV sets built for sale in the U.S. with screens 13 inches or larger have a built-in closed caption decoder. These days almost every program on TV has closed captions.

The closed caption system uses a part of the television sync signal (that odd-looking black bar that you see when the picture "rolls") called line 21. The black bar is necessary because the set needs to turn the electron gun(s) off and return the beam to the top left of the screen before it starts drawing the next frame. The brief period during which the screen is black is called the vertical blanking interval. Since the set isn't doing anything else during this time, a portion of the blanking interval can be used to carry information. In the NSB's original scheme, that would have been time data. In the captioning system, it's the captions.

So, they must have some pretty fast typists doing this, huh? Yes and no. The captions for prerecorded programming are prepared before the broadcast. The typists can take their time using a regular computer keyboard. They may pause, restart or even rewind the program they are transcribing to improve accuracy.

Live broadcasts, such as the nightly news, are a different story. Live closed captions are done in real time by a trained stenographer using a court reporter's machine modified for closed captioning. The machine permits the stenographer to transcribe the spoken word a syllable at a time. Accuracy can suffer depending on operator experience, the voice quality and enunciation of the speaker being transcribed, and the overall audio quality. A stenographer who starts to lag behind may skip words or even whole phrases in order to catch up. Miskeys often cause "garbage" characters to appear in the captions, and misunderstandings of what was said can lead to odd-looking spellings or turns of phrase, often with humorous results. FellowSDSTAFF er C. K. Dexter Haven says that during the 2000 presidential campaign he saw a caption describing George Bush as the "Republican canned bait."

Someday voice-recognition software may take over during live broadcasts, but currently such software is notoriously unreliable, particularly in situations involving multiple speakers with a wide range of voices, accents and inflections. For now, the job falls to skilled men and women working tirelessly behind the scenes to make TV enjoyable - or at least comprehensible - for millions of hearing-impaired viewers. My hat's off to them.

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