[mythtv-users] Custom Modeline

[Cory Papenfuss]

>> Modeline  mymode FF H1 H2 H3 H4 V1 V2 V3 V4 FLAGS
>> 
> 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

 	If you're doing what I am (direct VGA->NTSC), you're stuck with the 
NTSC specification of 525 scanlines (approx 480 visible).  All you could do in 
that case would be to reduce the number of visible pixels to less than 480. If 
you're like most people using a tvout card, you can program up a modeline that 
pads the vertical areas with extra black lines.  For example, consider 
800x600 at 75.  V1 will be 600 visible pixels, with 640 total.

ModeLine "800x600" 53.76 800 840 1040 1120 600 602 614 640

 	The easiest way to underscan would be to reduce the visible pixels on 
this mode but leave the total alone, such as:
                                           -100 -50 -50
ModeLine "800x500" 53.76 800 840 1040 1120 500 552 564 640

 	That would make a strange visible resolution (800x500), but would get 
the job done.  You could make it vertically underscan and keep the 800x600 by 
adding 50 pixels on top and bottom by using:
                                                +50 +50 +100
ModeLine "800x600" 53.76 800 840 1040 1120 600 652 664 740

 	That will also have the effect of lowering the vertical frequency by a 
ratio of 640/740.  If you've still got horizonal bandwidth available (this 
original modeline uses 53,760,000/1120=48kHz), you can increase the dotclock by 
the same ratio to get the same vertical refresh:

ModeLine "800x600" 62.16 800 840 1040 1120 600 652 664 740

 	For a total of 55.5kHz horiz, 75Hz vert... usually well within the 
specs of modern vid cards/monitors.

 	Hope that helps... as you can see, horizonal twidlings are done first, 
and their outcome affects the vertical twidlings.  Most TVOUT cards are 
dependent on some funky deinterlacing/scaling unit to allow 
frequency/size/scanline mismatches in the modeline to be converted into an NTSC 
(or PAL) compatible signal.  Unless you really know how its chip works, you 
never quite know what you will actually get out the svid/composite port in the 
end.

-Cory