|MadSci Network: Computer Science|
Edward: 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 Rebecca, 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 down). 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 firstname.lastname@example.org. I like to help...if my computer lets me! ;) Your MadSci, -Matt
Try the links in the MadSci Library for more information on Computer Science.