TVs and monitors on live TV, what's with the scrolling bar?

Any time a computer, and sometimes TV, is in the background of a live TV show (most notably news programs), there is a scrolling bar of slightly darker color than the rest of the screen slowly making it’s way up and down the screen. Seems to be a refreshing thing of some sort. Does this have anything to do with degaussing or interlacing?

Sorry if this has been covered before, I just wasn’t sure what to search for.

The TV or monitor in the background is not precisely in sync with the camera recording the image. The scrolling bar is indeed being caused by the screen being refreshed slightly out of synch. The problem is much worse for CRT computer monitors, which can have wildly varying refresh rates.

The scrolling bar you see is the result of aliasing. Ever see a movie or TV program where the spokes on a wheel appear to be rotating in reverse while the vehicle is moving forward? Same idea. The difference between the frame rate of video capture (in live broadcast, there is no video, but still may be A/D conversion of the signal which produces a similar phenomenon) and the refresh rate of the CRT (monitor or TV) in the background produces the pseudo-effect.

I thought aliasing was the jagged edges you get from the low resolution of a picture.

“Aliasing” is a general term for artifacts caused by inadequate sampling interval. If you sample a diagram at only 640x480 points you get artificial (false) features like jagged edges. Similarly, if you sample a real-life scene at only 24 frames per second, you get some strange artifacts such as wheels that appear to rotate backwards. If you record sound at a low sampling rate you get a whole different set of artifacts.

You can also think of it as beating created by two signals of slightly different frequency.

You can get a good beat effect with sound as well. Try it.

In the U.S. films are shot at 24 frames per second and television monitors run at about 30 fps. What this means (as friedo said) is that there is part of the television frame that is “off” at the time the camera shutter is open, so that little “bar” is underexposed. It scrolls because the portion of underexposed screen varies depending on when the half-screen refreshes and when the shutter opens. My Eclair NPR has an adjustable shutter that can be set at 145°, which is very close to the ideal 144°. I haven’t experimented with it, but it’s supposed to be adequate to get rid of most of the scrolling.

There is also a device called a “millispeed controller” that allows the filmmaker to adjust the speed of his camera so that it matches the television frame rate (I’d have to look up the exact speed – 29.xxxx fps). friedo also points out that the problem is worse with monitors. A millispeed controller can be adjusted to deal with those as well.

Some professional video cameras have a synch button so the shooter can get rid of the scrolling. In a live broadcast (such as a news story) it’s probably not worth it to synch up the camera if you’re more interested in getting a shot of a person.

The timer / synchronization features of cameras like that presumably would not help if you had more than one monitor in the frame with differing refresh rates.