MadSci Network: Computer Science

Re: Why do Computer screens flicker on TV?

Date: Tue Sep 28 01:29:08 1999
Posted By: Matthew B. Weyerich, Technical Coordinator,ES&R Dept., CPI Corp.
Area of science: Computer Science
ID: 937917885.Cs


Thanks for a great question! (I particularly love this one because it 
involves something I have to work with every day!) I've been really slow 
answering this, and I apologize for it. My computer has been giving me fits 
for the last week! 

I could write you a really long, technical answer to this question, but I 
won't. (Well, I will, if you want to know more. Feel free to e-mail me.) 
Instead of boring you with the technical mumbo-jumbo, though, I thought I'd 
defer to another MadSci, who's answer is in the archives. I think he sums 
it up rather nicely. (This is fortunate: I really couldn't find any good 
links on the web...and I LOOKED!) Anyway...

"Subject: RE: Lines on monitors viewed on TV

Date: Mon Oct 7 14:09:04 1996
Posted by Keith Little
Position: Computer Science


Television pictures are created when the phosphorus-coated picture
tube (the screen) is struck with a beam from its internal electron
gun, which is then converted into visible light.  The image on the
screen is composed of 525 horizontal lines, and is painted from top
to bottom in what is called a "raster".  This raster is displayed
roughly 30 times per second to form moving images (like at the
cinema), and works because the persistence of your eyes is greater
than the scanning rate.  Otherwise, you'd see the image flicker.

Now the TV camera (and recorder), take pictures in much the same
way, by scanning a "vidicon" tube (or "CCD" integrated circuit chip)
at roughly the same rate.  If the scanning rate of the TV set (or
monitor) is not equal to the camera recording the scene, you'll see a
dark bar on the screen.  Usually, this bar isn't stationary, and will
"roll" from top to bottom (or vice versa) depending on the different
scanning rates of the two systems.

When the bar isn't rolling, the two systems (monitor and camera) just
happen to be in "sync" with each other, and this is known as "beating
together".  When the the systems aren't in sync, the beat frequency
(higher or lower than the normal scanning frequency) will determine
how fast (and in which direction) the bar on the screen rolls (up or

Hope this helps, and have a great day!

Keith Little"

Keith pretty much says it all. Should you have more questions, need more 
details, or, disagree with my (quoted) answer, please feel free to e-mail 
me at I like to help...if my computer lets me! ;)

Your MadSci,

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