XFree86 Video Timings HOWTO
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# 7. Interpreting the Basic Specifications

This section explains what the specifications above mean, and some other things you'll need to know. First, some definitions. Next to each in parentheses is the variable name we'll use for it when doing calculations

horizontal sync frequency (HSF)

  Horizontal scans per second (see above).

vertical sync frequency (VSF)

  Vertical scans per second (see above). Mainly important as the upper limit on your refresh rate.

dot clock (DCF)

  More formally, `driving clock frequency'; The frequency of the crystal or VCO on your adaptor --- the maximum dots-per-second it can emit.

video bandwidth (VB)

  The highest frequency you can feed into your monitor's video input and still expect to see anything discernible. If your adaptor produces an alternating on/off pattern (as in an interlaced mode), its lowest frequency is half the DCF, so in theory bandwidth starts making sense at DCF/2. For tolerately crisp display of fine details in the video image, however, you don't want it much below your highest DCF, and preferably higher.

frame length (HFL, VFL)

  Horizontal frame length (HFL) is the number of dot-clock ticks needed for your monitor's electron gun to scan one horizontal line, including the inactive left and right borders. Vertical frame length (VFL) is the number of scan lines in the entire image, including the inactive top and bottom borders.

screen refresh rate (RR)

  The number of times per second your screen is repainted (this is also called "frame rate"). Higher frequencies are better, as they reduce flicker. 60Hz is good, VESA-standard 72Hz is better. Compute it as

      	RR = DCF / (HFL * VFL)

  Note that the product in the denominator is not the same as the monitor's visible resolution, but typically somewhat larger. We'll get to the details of this below.

The rates for which interlaced modes are usually specified (like 87Hz interlaced) are actually the half-frame rates: an entire screen seems to have about that flicker frequency for typical displays, but every single line is refreshed only half as often.

For calculation purposes we reckon an interlaced display at its full-frame (refresh) rate, i.e. 43.5Hz. The quality of an interlaced mode is better than that of a non-interlaced mode with the same full-frame rate, but definitely worse than the non-interlaced one corresponding to the half-frame rate.

## 7.1. About Bandwidth

Monitor makers like to advertise high bandwidth because it constrains the sharpness of intensity and color changes on the screen. A high bandwidth means smaller visible details.

Your monitor uses electronic signals to present an image to your eyes. Such signals always come in in wave form once they are converted into analog form from digitized form. They can be considered as combinations of many simpler wave forms each one of which has a fixed frequency, many of them are in the Mhz range, eg, 20Mhz, 40Mhz, or even 70Mhz. Your monitor video bandwidth is, effectively, the highest-frequency analog signal it can handle without distortion.

For our purposes, video bandwidth is mainly important as an approximate cutoff point for the highest dot clock you can use.

## 7.2. 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:

         _______________________
        |                       |     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_____________|

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:

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

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.

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