If you have ever tried to point your video camera at your TV set or your computer monitor to record the image you see, you know that it does not work -- instead of the stable image that your eyes see, there is either incredible flicker or a black rolling bar. This short video file shows you what happens when you try it. The video shows two different frequencies for the monitor: 70 Hz and then 60 Hz.
The flicker is caused by two things:
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- A difference in the scanning frequency between the TV and the camera
- A difference in the way the phosphor dots are perceived between the human eye and the camera's image sensor
If you have read the HowStuffWorks article How Television Works, then you know how a standard CRT display works. A single electron beam scans horizontal lines of pixels across the screen, lighting up each pixel when the beam hits it. The pixels are made of individual phosphor dots that glow when the beam hits them. To our eyes, the dots glow for about 1/30th of a second, so we see a steady image. For a video camera, however, the dots do not appear to glow nearly as long -- the camera is much less sensitive to persistence than our eyes.
When you watch the video, the second shot has a monitor refreshing every 60th of a second and a camera taking a frame every 60th of a second. The wide black bar that you see shows you a collection of pixels that have faded by the time the camera tries to image them. The bar rolls because the camera and monitor are not exactly synchronized. TV stations use special monitors and cameras that are able to sync with one another, so the camera's scan follows the monitor's scan.
Some camcorders use 30 frames per second (fps) and do not have the problem if you can get the monitor to refresh at a rate of 60 fps progressive. They may also work on TVs (see How HDTV Works for a description of progressive vs. interlaced scanning). Some camcorders use a 1/100th of a second shutter speed (causing rolling black bars) unless you turn the "steady shot" mode off to get it back to 1/60th or 1/30th of a second shutter speed.
To record a computer monitor image, you need to experiment with different refresh rates in the monitor settings or in the host computer software. There are display cards for personal computers whose video drivers allow you to pick the refresh rate. This rate setting may take some experimentation to get it to match your camera setting. If your camera uses 24 fps, use a multiple of 24 -- use a refresh rate of 48 Hz or 72 Hz to get rid of the annoying scrolling bars. Use 640 x 480 mode on the monitor to help lessen the problem. Television sets with the usual cathode ray picture tube do not have an adjustable screen refresh rate. They will either work with your camera or they won't.
For a person trying to take a screen shot using a video camera, the easiest solution is to shoot off an LCD panel on a portable computer. Flat-screen LCDs do not have this problem.
These links will help you learn more:
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