 |
 |
|
|
|
|||
|
|||
 |
|||
Ask the Bridge
Why is it that when a computer screen is shown in a film or on television, the screen has flickering lines?
“This effect occurs because there are two different methods — interlaced scanning and progressive scanning — used for creating television, movie, and computer images,” says Azer Bestavros, a CAS professor and chairman of the computer science department. “Interlaced scanning is a nifty little analog compression trick that made television a viable reality using 1930s technology. Unfortunately this leaves us with a bit of a problem when dealing with newer and better displays that use progressive scanning, such as computers and digital TVs.
“Interlaced video is an artifact from the early days of TV technology, when transmission rates were too low to enable all the pixels on the screen to be sent at a high enough rate for a playout that does not flicker — for example, at a rate of 60 images per second, which would make our eyes see a movie quite comfortably. The ingenious solution at the time was to play a trick on the human eye. Rather than draw 30 whole images per second, which would have resulted in flickering, an interlaced system draws 60 half-images per second. This is often referred to as 30 frames per second, since a whole frame consists of both the odd and even lines, or ‘fields.' On an interlaced system, odd lines are drawn, then the even lines are filled in, and so on.
“With progressive scanning, each image is drawn in its entirety from top to bottom on each pass. Computer screens and modern digital TVs use progressive scanning (one can actually control this ‘refresh' rate; the higher, the better). In fact, on new DVD players, it is possible to select whether to use a progressive or an interlaced scan.
“This is all well until we start mixing the two, such as videotaping a TV screen, especially when the refresh rates are not in synch — for example, videotaping at the rate of 30 frames per second a computer screen that is refreshed at the rate of 75 frames per second.
“With interlaced scanning, 1/60th of a second passes between the odd and even lines. During that time, the image can change, resulting in two images — one drawn by the odd lines and one by the even lines — that are ‘fused' together. The fadeout rate of a TV screen, combined with the ‘persistence of vision' of the human eye, makes television screens look stable and clear. However, when you play interlaced video on a computer monitor and there is a difference between the odd and even fields, you see what is often referred to as ‘combing,' or the ‘venetian blind' effect.”
“Ask the Bridge” welcomes readers' questions. E-mail bridge@bu.edu or write to “Ask the Bridge,” 10 Lenox St., Brookline, MA 02446.
9
March 2004
Boston University
Office of University Relations