Q U E S T I O N:

    Have you ever noticed how computer monitors flicker and have a scrolling black bar running through the monitor's picture when you see them on TV? Why is that?


A N S W E R:

    The scrolling, flickering monitor is a phenomenon that most of us have observed and wondered about at one time or another.

Persistence of Vision

    The main thing at work here is the fact that human eyes perceive images on a television screen rather differently than a video camera does. An image on a TV screen is created by an electron beam that scans the inside of the picture tube, lighting up phosphor pixels arrayed in lines across the screen. In the U.S., that electron beam makes 30 scans every second. But an individual phosphor dot doesn’t actually stay lit for the full 1/30th of a second that it takes for the electron beam to strike, finish a complete scan, and then strike again. By the time the beam returns to a row, the phosphor pixels have faded to black. However, your eye doesn’t register that split second of darkness. The way human vision works, those dots appear to stay lit the entire 1/30th of a second. This visual phenomenon is called “persistence.” Our brain then takes the series of still pictures created by the glowing phosphor dots and reassembles them into a steady image that recreates what we see in the normal world with a great deal of accuracy.

The Camera’s Eye

    A video camera doesn’t have visual persistence. Essentially, a camera records a scene by taking a series of still images, which are preserved on videotape as frames. In each of those individual frames, the lines of phosphors that have faded show up as a black stripe. That accounts for that line you see. The reason it scrolls is that your television and the video camera that recorded the scene had different scanning frequencies. Since they aren’t synchronized, the video camera sees the line in a different place each time it records an image. When viewed on television, the speed at which the bar travels up or down depends on the difference in the scanning rate between the two systems. Occasionally the difference will yield a full-fledged flicker. For television news programs and other shows where computer monitors appear frequently, monitors are set so that the refresh rate — the number of times per second that the electron beam scans the inside of the tube — is faster than the speed at which the phosphors completely fade. Then the camera and the monitor are synced up so that the vertical blanking interval — which is the black bar that appears briefly when the electron beam is turned off as it returns from the bottom of the screen back to the top to start a new scanning pass — falls in between frames recorded by the camera.