[mythtv-users] Custom Modeline

[John Patrick Poet]

On Thu, 15 Jul 2004, Cory Papenfuss wrote:

>
> >> 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

Okay, I think I get it.  I am actually after a good 1920x1080i modeline.
Doug came up with one which is "pretty good", but has too much vertical
overscan on my TV.  At that spec, I may be pushing the horizontal bandwidth,
but I think I will try adding vertical lines.

Thanks,

John