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Refresh Rates


Now that you know what phosphors do, we can get back to the refresh rate. Because phosphors only glow for a very short duration after being struck by energy, they need to be constantly bombarded with it (refreshed). A CRT is refreshed in what is called a "Raster" pattern. This means the electron beam starts at the top left, scans to the right, drops down one line and repeats. There are two numbers associated with this: horizontal and vertical scan rates. The numbers are how many times the screen is updated per second. You'll typically find the horizontal scan rate to be between 31KHz and 90KHz, and is generally ignored as the vertical scan rate is considered to be much more important because it is so much slower.

As the vertical refresh rate drops, you'll notice an annoying flicker beginning around 60Hz (unless you are a little more sensitive like myself where 70Hz is the absolute minimum.) Generally speaking, the lower your resolution, the higher your vertical refresh rate. This is because it doesn't have to rasterize as many vertical pixels. Take for example a screen resolution of 640x480. There are 480 vertical pixels, and you could scan through this relatively quickly as compared to say a resolution of 1600x1200.

If you notice flicker, you might try to up your refresh rate, but remember this is dependent upon your video card and monitor specifications. Consult your manuals first, as setting an incorrectly matched refresh rate could possibly damage your monitor. Another thing to check is to make sure that your monitor is not running in interlaced mode. Interlacing is a means of refreshing the screen every other line and relying on the human eye to combine two adjacent lines. Interlaced mode also rasterizes the screen, but it does so in two passes. The first pass draws all of the odd lines, and the second, all the even. This produces a much more apparent flicker.

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