Wednesday, December 17, 2008

Why are digital TV signals delayed? - Wednesday, Dec. 17, 2008

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Wednesday, Dec. 17, 2008

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Why are digital TV signals delayed?

QI subscribe to satellite television. I recently upgraded my service on two of my televisions. They now receive high-definition programming. However, I have noticed something strange. The high-definition programming seems to be delayed five to eight seconds. I've noticed this when I have multiple sets on. Is this delay common? Do you know what causes it?
—Bob in Fruitland Park, FL, listening on WDBO 580 AM

AI'm sure many other readers have noticed this phenomenon, Bob. It is quite common. The digital delay is sometimes true for radio signals, too.

Television stations these days are broadcasting in both digital and analog. Digital delay occurs because it must be encoded by the broadcaster, then decoded at your home. Both ends cause a delay. Analog isn't encoded or decoded. So, it runs faster.

Most people don't notice this, because their TVs are all analog or digital. So why don't the broadcasters delay the analog and solve the problem? Analog will be phased out by most stations in February. So why put money into it?

In addition to decoding the digital signal, your television also buffers the video. It stores a set amount of frames before starting to play the video. (A buffer is an area of memory. Buffering is used when you watch my Video of the Day. As the video plays, a dot moves across the bottom of the video. An area ahead of the dot is filled, keeping the video playing. That is a buffer.)

Why do TVs buffer? First, it helps with interpolation. Some TVs insert frames in the video. They do this to reduce picture smearing, historically a problem with LCDs. In effect, they create additional video.

The TV takes information from the frames before and after the insert, and uses that to create the new frame. This process, which is essentially high-tech guesswork, is called interpolation. (Interpolation is the opposite of extrapolation. You learned that in calculus, right?)

Your television needs to see both frames bracketing the interpolated frame. Otherwise, it can't guess the content of the interpolated frame. Buffering aids in that process.

Still awake? Let's move on. I have more thrilling info on buffers!

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Video of the Day: The ancient world is filled with wonders. But would you believe in an ancient computer? This Greek invention is amazing.

Cool Site of the Day: National Geographic has some amazing photos. Now, you have the chance to appear in National Geographic magazine!

Digital Minute, on your radio: It's time for year-end lists! Google has just released 2008's top search terms. And they will surprise you!

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Kim :)

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