XFree86 Video Timings HOWTO : Interpreting the Basic Specifications : Sync Frequencies and the Refresh Rate:
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## 5.2. Sync Frequencies and the Refresh Rate:

Each horizontal scan line on the display is just the visible portion of a frame-length scan. At any instant there is actually only one dot active on the screen, but with a fast enough refresh rate your eye's persistence of vision enables you to "see" the whole image.

Here are some pictures to help:

<hr>

         _______________________
        |                       |     The horizontal sync frequency
        |->->->->->->->->->->-> |     is the number of times per
        |                      )|     second that the monitor's
        |<-----<-----<-----<--- |     electron beam can trace
        |                       |     a pattern like this
        |                       |
        |                       |
        |                       |
        |_______________________|
         _______________________
        |        ^              |     The vertical sync frequency
        |       ^ |             |     is the number of times per
        |       | v             |     second that the monitor's
        |       ^ |             |     electron beam can trace
        |       | |             |     a pattern like this
        |       ^ |             |
        |       | v             |
        |       ^ |             |
        |_______|_v_____________|

<hr>

Remember that the actual raster scan is a very tight zigzag pattern; that is, the beam moves left-right and at the same time up-down.

Now we can see how the dot clock and frame size relates to refresh rate. By definition, one hertz (hz) is one cycle per second. So, if your horizontal frame length is HFL and your vertical frame length is VFL, then to cover the entire screen takes (HFL * VFL) ticks. Since your card emits DCF ticks per second by definition, then obviously your monitor's electron gun(s) can sweep the screen from left to right and back and from bottom to top and back DCF / (HFL * VFL) times/sec. This is your screen's refresh rate, because it's how many times your screen can be updated (thus refreshed) per second!

You need to understand this concept to design a configuration which trades off resolution against flicker in whatever way suits your needs.

For those of you who handle visuals better than text, here is one:

<hr>

            RR                                      VB
             |   min HSF                     max HSF |
             |    |             R1        R2  |      |
    max VSF -+----|------------/----------/---|------+----- max VSF
             |    |:::::::::::/::::::::::/:::::\     |
             |    \::::::::::/::::::::::/:::::::\    |
             |     |::::::::/::::::::::/:::::::::|   |
             |     |:::::::/::::::::::/::::::::::\   |
             |     \::::::/::::::::::/::::::::::::\  |
             |      \::::/::::::::::/::::::::::::::| |
             |       |::/::::::::::/:::::::::::::::| |
             |        \/::::::::::/:::::::::::::::::\|
             |        /\:::::::::/:::::::::::::::::::|
             |       /  \:::::::/::::::::::::::::::::|\
             |      /    |:::::/:::::::::::::::::::::| |
             |     /     \::::/::::::::::::::::::::::| \
    min VSF -+----/-------\--/-----------------------|--\--- min VSF
             |   /         \/                        |   \
             +--/----------/\------------------------+----\- DCF
               R1        R2  \                       |     \
                              min HSF                |    max HSF
                                                     VB

<hr>

This is a generic monitor mode diagram. The x axis of the diagram shows the clock rate (DCF), the y axis represents the refresh rate (RR). The filled region of the diagram describes the monitor's capabilities: every point within this region is a possible video mode.

The lines labeled `R1' and `R2' represent a fixed resolutions (such as 640x480); they are meant to illustrate how one resolution can be realized by many different combinations of dot clock and refresh rate. The R2 line would represent a higher resolution than R1.

The top and bottom boundaries of the permitted region are simply horizontal lines representing the limiting values for the vertical sync frequency. The video bandwidth is an upper limit to the clock rate and hence is represented by a vertical line bounding the capability region on the right.

Under Plotting Monitor Capabilities) you'll find a program that will help you plot a diagram like this (but much nicer, with X graphics) for your individual monitor. That section also discusses the interesting part; the derivation of the boundaries resulting from the limits on the horizontal sync frequency.

XFree86 Video Timings HOWTO : Interpreting the Basic Specifications : Sync Frequencies and the Refresh Rate:
Previous: About Bandwidth:
Next: Tradeoffs in Configuring your System