Wednesday, October 29, 2008

Closed Caption

It's been a year now since i purchased my TV set. I didn't care about the users manual or even took a glance at other buttons on it. Until last night... I was curious about the button labeled "Closed Caption" and I wonder what it would be for and what will be the effect if i press it. I tried to press it once and my TV set responded that the Closed Captioning is enabled. But i did not notice any changes on the screen until i tuned to "CS" channel and the program playing at the moment was "Prison Break". There were texts displaying at the bottom of the screen that looks just like a subtitle in a DVD video. The text was of color white over a black placeholder. Instantly, i knew that it was not part of the analog TV signal transmission because it was so crisp unlike the images on the TV program itself. So i quickly moved my ass to the desk to consult Dr. Google what is Closed Captioning means. After a few clicks, I found out that it is done by embedding and encoding digital data over a television signal and surprisingly the technology was already there for 15 years now. I feel so stupid... the technology was in place since 1993 but I only knew about it just yesterday. I should have taken advanced communication courses way back in college. Or, I should have bought a new TV set long time ago... I could have replaced my antique TV set. Anyway, here's the snippet of the article I've read:

Closed caption information is added to Line 21 of the Vertical Blanking Interval (VBI). It may be added to either or both the odd and even fields of the television signal. However, the primary language program related information appears in the odd fields. (It should be noted that caption industry people typically refer to the odd fields as "Field 1" and the even fields as "Field 2".) To assure adequate caption performance wherever a usable picture can be obtained, a low instantaneous data rate of 503 kilobits/second (32H) was chosen. Data is preceded by a seven-cycle sine wave similar to color burst (called the "Clock Run-In") and three "start bits" that are always "0", "0", and "1".

Two bytes of data, using seven bits, odd parity format, are possible on a given line. The rise time is controlled (2T) and the amplitude of data and clock run-in is 50IRE units. Using only Line 21, Field 1, of the VBI, a delivery rate of about 3600 characters or 500 words per minute is theoretically possible (depending upon the particular caption style, discussed later).

Tests conducted by PBS determined that the typical reading rate for captioning is about 125 words per minute. (Incidentally, spoken dialogue may exceed 200 words per minute.)

Particularly with the availability of Field 2, the data delivery capacity (or "data bandwidth") far exceeds the requirements of simple program related captioning in a single language. Therefore, the closed captioning system allows for additional "channels" of program related information to be included in the Line 21 data stream. In addition, multiple channels of non-program related information is possible.

Currently I have something in mind right now about making a better use of this technology... bad thing television is one-way.


  1. hehehe noon pa yan, una kong nadiskubre yan sa cable channel. saka sa mga american movies na yung tv nila may caption.

    astig kung makakagawa ka ng device that can scoop the captions only and feed it to the computer for storage. or if you could have the frequency so you could put your own caption to your neighbor's tv wahahaha. I think madali lang i jam yun frequency, hindi pa naman uso ang encryption nuon, at mahirap iencrypt yung ganyan kasi pag nadecode na yung encryption algo niya mahirap palitan yun mga tv sets.

  2. ahem ahem techie ...closed captioning ..i dunno wat 2 say ... so ill just say this ..get the word verification off ;)