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## DOEP (Daily Open-Ended Puzzle) (intermittent): The speed of a crawl

During a speech a couple of weeks ago, I characterized the crawl on the bottom of CNN as “news delivered at 4 mph.” I made up the speed, but it seemed like a reasonable approximation, since it seems to go at about walking speed.

This morning I was watching a news channel on the little TV in our bedroom: It took about four seconds to go across a screen about 15″ wide. If I were watching it on, say, a 60″ wide TV, it would have taken four seconds to cover four times the distance and thus would be traveling four times as fast. If it were a 4 mile wide screen, it’d be travelling at a mile per second.

So, how fast does a news crawl (if a news crawl could crawl news)? And why doesn’t it look faster on a big set?

I know it’s so elementary that it’s embarrassing, but I’m sick, ok? Slightly feverish. Really. [Tags: ]

### 8 Responses to “DOEP (Daily Open-Ended Puzzle) (intermittent): The speed of a crawl”

1. Perhaps if you think of the movement as an illusion, recalling the story of the incandescent light bulb in your lamp flickering at 60 cycles per second and seeming to transmit a continuous stream of light…
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2. It depends on how close you’re sitting to the tv, rather than the size of the screen. While time intervals seem relatively constant, the perception of size and distance is more variable.

Think about this. When you’re on an airliner going to Blogapalooza 2007 on the left coast, you might be going 600 mph, but since you’re at 30,000 ft, the vast fruited plain of flyover country is just perceptibly moving underneath you. Imagine what the experience would be like if you flew at 500 ft instead? It would seem like you were in the middle of a Philip Glass/Koyaanisqatsi sequence.

You can see this effect with the zipper signs on the buildings in Times Square. If you’re standing on the sidewalk below looking up trying to read the words, it looks like they’re moving at a frenetic pace. But from one-half block away, they are easily readable.

I think this is why BASE jumping (freefalling from a building or fjord or El Capitan) is such a rush. Falling at 120 mph from a plane, there’s nothing around you to give you a sense of speed. But, if have a rock face or a building facade 30 feet away from you, if must give you a terrific sensation of speed.

3. But to answer the question about mathematically representing the text crawl rate independent of the screen size, I think Frank is on the right path.

You can look at this as periodic motion, ie, if the text takes 4 seconds to move across the entire screen, that’s 4 sec/screen, or take the inverse, 1 screen/4 sec., or 0.25 Hz. The refresh rate on your PC monitor uses Hz.

Getting back to apparent speed viewed at a distance from my first comment, if you set up your 4-mile-wide TV so that the text refreshes at 0.25 Hz, than take a helicopter and climb to an altitude so that the screen occupies the same field of view as your 15″ screen in your bedroom, I bet the text will look like it’s moving about the same rate, or 4 mph, even though it’s actually moving at 3600 mph.

But I think your audience can relate better to a description of text moving at about the pace of a leisurely walk, ie, 4 mph, rather than saying it has a refresh rate of 0.25 Hz.

4. very very nice blog thansk your informations… mr silici

5. It is all relevant to angular, rather than linear, velocity. One assumes the screen subtends the same visual angle regardless of size; if not, then it is like Bill said, dependent on viewing distance. Get right next to the 60″ screen and watch those pixels fly by!

6. It’s a matter of pixel size and relativity. A large screen showing the same content as a small screen will have pixels (the basic unit of color information) of much larger size. Those larger pixels will be advancing at the same rate as small pixels, but coverning a much larger distance with each movement. As CG noted, your distance to the screen determines your perception of the relative size of each pixel, and the relative speed at which each pixel travels.

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