Skip to main content

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 ;)


Post a Comment


DIY Airsoft Chrono [PC-based]

Chrono or Chronograph, the term used by Airsofter and Paintball players to refer to the device that measures the muzzle velocity of firearms. This is also called MVMD, short for Muzzle Velocity Measuring Device by the British Army. Muzzle Velocity is the velocity of the bullet as it exits the barrel of the firearm.

A year ago I was an active Airsofter in Iligan who's more interested in modifying and improvising AEGs (Automatic Electric Gun) rather than playing airsoft. I have made my AEG susceptible to high current burns by using locally available MOSFETs. I have improved velocity by replacing cylinder gaskets and spring. I have increased the rounds per minute by altering the armature windings of the drive motor. And most of all, I have made my own cheap alternative PC-based Chrono.

I have posted an artivle before that details the development of my chrono but unfortunately the online forum was hacked/deleted by webmaster of due to some violation in the contents. So, I&…

Disable PrintScreen on C# without Keyboard Hooks

Yes, there is a simple solution to prevent grabbing information on your C# application screen with the keyboard's printscreen key without using keyboard hooks or calling COM interops. The solution makes use of Windows Forms Message Filter to trap keyboard events on your application window.

To trap keyboard events with Windows Message Filter, you need to implement the IMessageFilter interface and override the member PreFilterMessage(ref Message WM) method. This is the method called whenever a Form receives a keyboard or mouse event. You may want to read more about IMessageFilter.

The problem is that even though you have trapped the PrintScreen keypress event, the captured image will still persist to the clipboard. Therefore the simplest solution is to clear up the clipboard right after the PrintScreen is pressed on the keyboard.

This is how the overridden method will look like this:

publicbool PreFilterMessage(refMessage WM)
Keys kCode = (Keys)…

DIY Mic Shock Mount

For the past couple of weeks I was looking for a cheap microphone shock mount and the cheapest I saw that was available in my country was around US$160 so I decided to build a cheap one. I only spent less than $6 and a 2 hours of my not very precious time.Before I left office today, bought materials and tools to for my DIY shock mount which are a piece of 4”-diameter PVC pipe coupling ($0.50), a set of hair ties($0.40), set of stove bolts ($0.50), a set of coping saw frame and blades ($2.5).First, I decided to cut the PVC coupling so it would look like Rode SM3 shock mount’s frame.Then I used the section of the PVC coupling that was removed to be a attachment brace to the original mic holders base and bolted it to the base of the frame. I had to heat that part so could bend it to a desired angle.And then cut slots on the edges for the rubber suspensions (hair ties). Right after positioning the suspensions, i found a problem. The rubber bands slip every time i attempted to position the…