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Refresh Rates and Interlacing
The RAMDAC is the device in the video card that is responsible for reading the contents of the video memory, converting the digital values in memory into analog video signals, and sending them over the video cable to the monitor. The RAMDAC's ability to translate and transfer this information directly controls the refresh rate for the video mode it is operating in. The refresh rate is the number of times per second that the RAMDAC is able to send a signal to the monitor and the monitor is able to repaint the screen.
Refresh rate is measured in Hertz (Hz), a unit of frequency. Support for a given refresh rate requires two things: a video card capable of producing the video images that many times per second, and a monitor capable of handling and displaying that many signals per second. The refresh rates are somewhat standardized; common values are 56, 60, 65, 70, 72, 75, 80, 85, 90, 95, 100, 110 and 120 Hz. This is done to increase the chance of compatibility between video cards and monitors.
Note: Do not confuse the
refresh rate with the term "frame rate", often used for games. The frame rate of
a program refers to how many times per second the graphics engine can calculate a new
image and put it into the video memory. The refresh rate is how often the contents of
video memory are sent to the monitor. Frame rate is much more a function of the type of
software being used and how well it works with the acceleration capabilities of the video
card. It has nothing at all to do with the monitor.
The refresh rate is important because it directly impacts the viewability of the screen image. Refresh rates that are too low cause annoying flicker that can be distracting to the viewer and can cause fatigue and eye strain. The refresh rate necessary to avoid this varies with the individual, because it is based on the eye's ability to notice the repainting of the image many times per second. My experience has generally been as follows:
Note that this also depends on the size of the monitor. Flicker is easier to see on a larger monitor than on a small one for two reasons: first, you're just looking at that much more screen; second, you are seeing much more screen using your peripheral vision, which is much more sensitive to noticing flicker.
Also note that going to very high refresh rates generally has no positive impact; no person I have ever met could even tell the difference between a video system running at 120 Hz and one running at 100 Hz. In fact, going to too-high a resolution can be counter-productive. The reason is that the higher the refresh rate, the faster the electron guns have to switch between colors for adjacent pixels. At very high refresh rates there can theoretically be a reduction in the contrast in the displayed image. I therefore recommend running only at as high a refresh rate as you need to eliminate flicker. 85 Hz is generally more than enough for virtually everyone.
The refresh rate is related directly to the resolution of the image--higher resolution images generally have lower refresh rates. Higher refresh rates require the RAMDAC to generate the video images more times per second. The ability of the RAMDAC to do this depends on several variables:
Refresh rates are normally specified for non-interlaced operation, since that is what modern video systems typically use. Some older monitors can only display some of the higher resolutions when using interlacing. Interlacing allows the refresh rate to be double what it normally would be, by displaying alternating lines on each refresh. In essence, half the screen is redrawn at a time. Interlaced operation is normally done at 87 Hz (really 43.5 Hz because of the interlacing) and hence produces flicker that is noticeable by most people.
The table below shows the relationship between screen resolution, refresh rate and the amount of data the RAMDAC must process. The numbers in the table are in MHz, representing how many millions of pixels per second the RAMDAC must output to support a given resolution at a given refresh rate. Many video cards rate their RAMDAC in MHz and you can use this table to see if the card is likely to support the resolution and refresh rate you need. This table includes a 1.32 conversion factor to take into account retrace times (the time that the electron guns are in non-visible areas of the monitor):
Note: Don't forget that at
higher resolutions and color depths, video memory bandwidth
becomes a limiting factor. The speed of the RAMDAC doesn't matter if the video memory
can't provide the necessary data fast enough.