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Video's big secret

technofile  by al fasoldt

Columns and commentaries in a life-long dance with technology
 

Simple gray rule


Video's big secret: It plays a trick on our brains
 

By Al Fasoldt

Copyright © 1991, The Syracuse Newspapers

My friend John used to drive me crazy every time he dropped by to watch a movie on my big-screen TV.

I always thought the picture was great. The screen is 6 feet from corner to corner, and the colors are rich and bright. But John didn't care about the colors; he always wanted to read the credits.

And they were almost impossible to decipher. Unless the producer had made them big enough, the letters in the words—you know, the ones that tell who was the "gaffer'' and who was the "best boy'' (whatever THAT means)—were too fuzzy to read.

"Adjust your TV!'' John would say to me, every time.

"It's not my TV,'' I would tell him.

But I don't think John believed me. After all, the picture certainly seemed sharp enough when we looked at regular scenes. Why wouldn't it be sharp enough to let us read the tiny words on the screen?

The answer is part technical and part psychological.

The technical part is easy enough to explain. The TV system used in North America, Japan and a few other countries just isn't sharp enough to show fine detail.

The ability to show detail is called resolution. Camera lenses and telescopes are judged by their resolution, too. If the "resolving power'' isn't high enough, you won't be able to read the road signs in your vacation snapshots.

TV sets show images by drawing them line by line across the screen. If you get real close to the picture tube, you'll be able to see these scanning lines. There are only a few hundred of them in a typical broadcast picture.

If you change the way the TV works and add a lot more scanning lines, you get a picture with more detail. That's how it's done in some of the other systems now in use in other countries.

If you make at least twice as many scanning lines, you end up with the beginnings of high-definition TV. For true HDTV, you need a few more changes, such as purer colors and a wider screen, but the big thing is to boost the resolution. (This isn't the same as increasing the sharpness, by the way. We'll get to that another time.)

So why do our TV pictures still seem so good? Why did my friend John like what he saw until he tried to read the credits?

Here's where the psychology comes in. TV images are never steady. Televisions are actually stroboscopes, flashing their pictures at us 60 times a second. To make things a bit complicated, the full picture takes two flashes to be shown. In one flash, the TV set draws the odd-numbered scanning lines, and in the next flash, it draws the even ones.

This constant flashing plays a trick on our brains. The images last just long enough to give us a fleeting impression. If something is missing—if the image on the screen shows the long tresses of a soap opera star, for example, without showing each hair in detail—our brains flip past the missing information.

It's not clear what is really happening inside our brains. Sometimes it seems as if we were automatically filling in the missing detail. We do this sort of thing all the time—when reading, for example. If you look quickly at these letters—"abcdefghiklmnoprstuvwxyz''—you'll recognize the alphabet right away, but you probably won't notice that the "j'' and "q'' are missing.

I explained all this to John, but he wasn't satisfied.

"Maybe you need an even bigger screen,'' he said at one point. I knew he was kidding. After all, you can't get more scanning lines just by making the screen bigger.

"Or maybe I need less-discerning friends,'' I said.

He never came back. Didn't he know that I was kidding, too?


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