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Title: Cooperation between screen and graphis device

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Cooperation between screen and graphis device

General:

If you commence a discussion on graphics devices and on controlling monitors you will be confronted with quite a lot of different frequencies. They all have their special task for synchronizing the screen. Here we want to present you a short overview on how they work and the cooperation between the different monitor signals and your fraphics device. Lets start with the monitor itself. There are mainly two signals for creating a screen:

The highest values of these frequencies as well as your settings are the frequencies needed for creating a modeline. More in this may be found in: (maddin_videomodes.html)

Unfortunately there is more that needs to be done. There is another important parameter called pixel frequency (or dot clock). This is a changeable value that is created by the graphics device up to a maximal value (maximum dot clock). It sets the interval of how the graphics device creates pixels. Typical values range from 25MHz to 135MHz. It creates n Ticks per second; this is a common phrase. You may find clock or pixel frequency as a synonym.

Lots of questions point into one direction: How does the colour depth depend on the pixel frequency? The DAC (digital analogue coverter) transforms any digital screen information it gets to analogue current. This frequency is called the ramdac. The higher the colour depth the more information is needed for creating this pixel. Transformation needs some time so the pixel frequency is decreased. The faster the DAC the less the pixel frequency has to be decreased.

It is also important to know that the dot clock increases with increasing resolutions. This should be clear as more pixels need to be visualized. If the maximum pixel frequency has been reached no higher resolutions may be set. The same applies to the horizontal and vertical synchronization frequencies of the monitor.

On other operating systems it may occur that the maximal allowed values are superseded by 25%. This of course exploits your device to its limits. As the complete hardware failure will not be reached as the device is changed or modified before one doesn't notice this. The XFree86 X servers strictly follow the values given by the manufacturer. This makes sure that the card lasts longer and doesn't crash. Well, you might notice that you are not able to set exactly the same resolution in X as you are accustomed to from other operating systems. Anyway, there are only little exceptions.

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Keywords: APPLIKATION, SOFTWARE, XSERVER

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Categories: X Applications , X Window System

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Mainpage o Searchform o History o Versions o Categories o Contents
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SDB-ms_video, Copyright SuSE GmbH, Nuremberg, Germany - Version: 04. Dec 1997
SuSE GmbH - Last generated: 07. Oct 1999 17:48:25 by maddin with sdb_gen 1.00.0