[mythtv-users] Re: Custom Modeline

[Cory Papenfuss]

 	It's certainly not the fault of the modelines that the TV is 
overscanned.  Rather, it's that historically TV pictures have always been 
overscanned and the switch to HDTV apparently hasn't fixed that (?).  If you 
raster the specified visual resolution with the standard timings, the picture 
will be overscanned.  Since you (and I for that matter) are essentially driving 
the TV directly from the VGA port, it's *REQUIRED* to output an ATSC standard 
video signal raster with standard timings.  If you're TV is tolerant of 
something other than that, great... but make sure it is by the book or you can 
blow it up.

 	With all that said, remember that although the spec might specify 
visual size (e.g. 1920x1080) in addition to the raster size/speed, it's the 
raster that must be correct to avoid TV damage.  Think of the raster as the 
black canvas that's painted at a certain rate consisting of a certain number of 
lines.  For illustrative purposes, I scoured the 'net and found a 1080i 
modeline that does appear to have a 33.75kHz horizonal frequency (that appears 
to the be correct standard rate for 1080i... 45kHz for 720p BTW):

Modeline "1080i" 74.250 1920 2008 2048 2200 1080 1084 1094 1125 interlace

 	You could use the same modeline, but have a letterboxed 1024x768 
*visual* (upper left hand corner) by modifying it as follows:
Modeline "test" 74.250 1024 2008 2048 2200 768 1084 1094 1125 interlace

 	Or try to move it towards the center by 200 pixels H/V:
Modeline "test" 74.250 1024 1808 1848 2200 768 884 894 1125 interlace

 	In all those cases, the oscillators in the TV won't change at all... 
just when and for how long the video guns turn pixels on and off.

 	So if you are trying to vertically "underscan" the mode and can live 
with running a "standard" HDTV raster with a "nonstandard" HDTV 
visual resolution,

Modeline "test2" 74.250 1920 2008 2048 2200 980 1034 1044 1125 interlace

will keep the TV happy, and you should have a 1920x980 resolution, with 50 
additional black pixels on both the top and the bottom.  Although it's a funky 
resolution, playing video on a linux box usually uses XV for scaling anyway, so 
it usually scales to fit accordingly.  If you're trying to actually play all 
1080 lines of a bone-fide 1080i orginal source material without losing any 
lines off the screen, you'll need more than a modeline.... like a TV fancy 
enough to adjust the screen position, or unfancy enough to take a screwdriver 
to it and adjust the raster.

Cheers
-Cory


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On Thu, 15 Jul 2004, Chris Vargas wrote:

> Cory: your initial reply to John caught my eye,
> because I'm in John's same boat. At first I thought
> I'd be able to correct it given your great explanation
> of modelines, but now I'm not so sure.
>
> I have a 9A60 transcoder (thanks yet again, Jarod!),
> and am trying to adjust the vertical dimension as
> well. My 480p modeline plays back anamorphic DVDs
> correctly, but my 540p and 1080i modelines (derived
> via Powerstrip) are all stretched too far vertically,
> so that an anamorphic test pattern displays circles as
> ovals.
>
> Wouldn't I need to add vertical lines to get this
> compression? I tried using one of your examples to add
> (say) 40 pixels vertically, but if it's not going to
> work and blow up my TV, maybe not. Then again, it's 3
> years old and doesn't do 720p, so maybe I should blow
> it up...
>