Video timing details revealed

Video timing picture

Video timing picture

Green dots are the visual side of sceen.
Black dots  are the black border or the screen and the pixels where the cathode ray is outside the picture.
White dots are the sync pulses. The beginning of thse pulses position the picture on the screen.

Video terms to describe letters in picture

A is the amount of dots. It is usually 640, 800, 1024 ,1280 but it can be any other number as well. Used to use 1048x832 pixels in my monitor before i found that i can get that work with windows too.

B is the startplace of horisontal sync-pulse. If will remain in constant plase, so if you increase it the picture will move to left and if you decrease it the picture will mode to right.

C is the end of the sync pulse. It contains no relevant information, just put something between B and C that is something like b+20 or b+100, it won't really make much difference.

D if the end of the line. It affects to 2 things. First the ratio a/d affects the wideness of the visual picture. Secondly Horisontal frequency "J" (khz) is counted from the following formula:

   1000*I                   1
J= --------       time= ----------
     D                  frequency  

(this is needed nowhere here but  knowing it makes things easier to understand)
This means that if you decrease dot clock "I" then the horisontal frequncy will decrease and vise versa. If you increase the line lengtht "D" then the horisontal frequency will decrease and vise versa.

Xfree86 timing settings

The modline is:

"name" I  A B  C D     E  F G H
the name if often resolution, but it doesnt have to be. And changing the name, cannot change the resolution.

Matrox Windows driver settings

MGA.MON file has the following settings

[*User-Defined_mode,(640X480)]
PIXEL_CLK   =   needed dot clock 
H_DISP   =   horizonal display resolution (640,800,1024 etc.)
H_FPORCH   =   B - A from picture
H_SYNC   =   C - B from picture
H_BPORCH   =   D - C from picture
H_SYNC_POL   =   1
V_DISP   =   vertical display resolution (480,600,768 etc.)
V_FPORCH   =   F - E from picture
V_SYNC   =   G - F from picture
V_BPORCH   =   H - G from picture
V_SYNC_POL   =   1
INTERLACE_ENABLE  =  0  (normally non-interlaced)

Online calculator

Try my online video timing calculator. It needs JavaScript to work.


Tomi Engdahl <Tomi.Engdahl@iki.fi>