[mythtv-users] Custom Modeline

John Patrick Poet john at BlueSkyTours.com
Sun Jul 11 19:37:24 EDT 2004


Cory Papenfuss wrote:

>     I looked at it, but I fail to see how the 'videogen' program is 
> any more intuitive than calculating the modeline yourself.  They are 
> easy to figure out:
>
> Modeline  mymode FF H1 H2 H3 H4 V1 V2 V3 V4 FLAGS
>
> FF: dotclock frequency (MHz)
> Hx: horizonal description (number of FF dots to count)
> Vx: vertical description (number of horiz. lines to count)
> FLAGS: special stuff maybe necessary for drivers (sync-on-green, sync 
> polarity,
>     interlacing, etc)
>
> Lines are drawn by counting pixels of the dotclock, but also needs to 
> describe what happens off the edge of the screen.
> H1    is picture width in pixels (e.g. 720)
> H2-H1 is the black space on the right side of the screen
> H3-H2 is the length of the sync pulse to tell the monitor to reset to the
>       beginning and start drawing another line.
> H4-H3 is the black space on the left side of the screen
>
>
> HHHHHHHHHHH___----___HHHHHHHHHHH___----___HHHHHHHHHHH___----___HHHHH...
> 0         H1 H2  H3 H4         H1 H2  H3 H4
>
> The vertical is exactly the same, except it uses complete lines to 
> count, rather than individual dots.  Here's the line I use for my 
> custom NTSC hardware board (720x480 at 59.94Hz interlaced)
>
> Modeline coryntsci 14.318 720 760 824 910 480 484 492 525 interlace
>
> 14.318 MHz dotclock
> H:
> 720    picture pixels
> 760-720 (40/14.318e6)=2.78us black on right side
> 824-760 (64/14.318e6)=4.47us sync pulse width
> 910-824 (86/14.318e6)=6.01us black on left side
> Total horiz line is 910/14.318e6 = 63.556us => 15.73 kHz (sound 
> familiar?)
>
> V:
> 480    picture lines
> 484-480 (4/15.73e3)=254us black on bottom
> 492-484 (8/15.73e3)=508us vertical sync pulse width
> 525-492 (33/14.73e3)=2.1ms black on top
> Total vert frame is 525/15.73e3 = 33.36ms => 29.97 Hz (sound familiar?)
>
>
>     It might seem complicated, but it's really pretty simple once you 
> understand what's it's doing.  Just a matter of counting dots.  The 
> tough part (sometimes) is finding the spec for the video signal (sync 
> pulse widths, etc). If you wanted to move the picture to the left a 
> bit without changing anything else, add 5 to H2 and H3.
>
> Hope that helps some folks
>
> -Cory
>
Cory, can you explain how to modify a modeline to decrease VERTICAL 
overscan?  I am able get the horizontal just the way I want from your 
excellent explanation, but I seem to have real problems with the 
vertical.  I have even tried increasing the dotclock, but still do not 
seem to have the ability to adjust the veritical overscan the way I need 
to.  Maybe I just can't do it with my TV?

Thanks,

John



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