Interpreting the Basic Specifications

# 4 Interpreting the Basic Specifications

Contents of this section

This section explains what the specifications above mean, and some other things you'll need to know. First, some definitions. Next to each in parens 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'; sometimes loosely called `bandwidth'. The frequency of the crystal or VCO on your adaptor --- the maximum dots-per-second it can emit.

video bandwidth (VB)

  The highest frequency at which your monitor's video signal can change. This constrains the highest dot clock you can use and the overall sharpness of fine details in the video image.

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. 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.

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

Sync Frequencies snd 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 frame length
>      |->->->->->->->->->->-> |     is the time in dot clocks
>      |                      )|     required for the
>      |<-----<-----<-----<--- |     electron beam to trace
>      |                       |     a pattern like this
>      |                       |
>      |                       |
>      |                       |
>      |_______________________|

>      _______________________
>      |        ^              |     The vertical frame length
>      |       ^ |             |     is the time in dot clocks
>      |       | v             |     required for the
>      |       ^ |             |     electron beam to 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.

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