[Novalug] xorg.conf -- gtf command and mode lines

Jon LaBadie novalugml@jgcomp.com
Fri Oct 16 19:33:34 EDT 2009


On Fri, Oct 16, 2009 at 08:59:32AM -0400, Nino Pereira wrote:
> On defining scan frequencies:
> 

Nino mentioned he used a program, "gtf" in /usr/bin to generate
his X timings.  Although he did not use them, gtf can also
create a mode line appropriate for the resolution.  For example:

$ gtf 1920 1200 60 -x 

  # 1920x1200 @ 60.00 Hz (GTF) hsync: 74.52 kHz; pclk: 193.16 MHz
    Modeline "1920x1200_60.00"  193.16  1920 2048 2256 2592  \
                                        1200 1201 1204 1242  \
					-HSync +Vsync

I thought I knew a tiny bit about X mode lines, having figured
one out from a manufacturer's spec's a long time ago.  If I
recall correctly, the 4 numbers "1920 2048 2256 2592" define
a horizontal sweep timing as totalling 2592 pixels, 1920 of
which are in the viewable area, ending at 2048 pixels, and
horizontal sync can start at 2256 pixels.

The thing I don't get is how can gtf know that mode line will
be suitable for any specific monitor.  I.e. that it will run
at a 193.16MHz pixel clock rate, that the leading overscan
will be 128 pixels before the viewable area, that each screen
update will be 1242 scan lines, etc.?  Seems to me that it is
just one of many possible mode lines depending on the properties
of the monitor electronics.  Or am I just too old ;)

Jon
-- 
Jon H. LaBadie                  jon@jgcomp.com
 JG Computing
 12027 Creekbend Drive		(703) 787-0884
 Reston, VA  20194		(703) 787-0922 (fax)



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