[mythtv-users] Re: Custom Modeline

[John Patrick Poet]

Cory Papenfuss wrote:

>     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


Thanks Cory.  Your unadaltered "1080i" modeline results in a visual 
which looks very much like the one Doug came up with.

The vertical overscan really is not that much, when viewed as a 
percentage.  It is just that the KDE task bar gets so small at that res, 
that it almost completely disapears off the bottom of the screen.

Going with a non-standard HDTV vertial res would probably not be that 
big of a deal.  After all, my local stations which broadcast in "1080i", 
are actually sending out 1088 lines.  I assume that XV is having to do a 
tiny amount of scaling, anyway.

I guess that is another reason why it is cool to have different display 
resolutions for the GUI and the Video.

My current TV will not do 720p, only 1080i.  I hope to get a new TV by 
the end of the year, which will do both.  Then I will use a 720p 
modeline for my 720 stations, and a 1080i modeline for my 1080i 
stations.  The only time I currently use a "video" display resolution 
other than 1080i, is for non-HD channels.  For my non-HD channels I came 
up with a 720x480 modeline which is just about perfect for my TV.

John