[mythtv-users] Stupid overscan question

[Cory Papenfuss]

> So are you saying that I should use something else
> instead of 640x480?  If so then what would you
> suggest?
>
 	What I'm saying is that if you're using a "tvout" card, what your 
computer renders is only loosely related to what shows up on the TV.  The 
signal to the TV *MUST* be NTSC-compliant.  That means *exactly* 525 lines 
drawn every 1/29.97th of a second, where each line takes exactly 
1/15,734th of a second.  The 640x480 pixels the computer is rendering is 
likely being spatially interpolated (in both horiz and vert dimensions) by 
the tvout chip on the video card so 1 pixel rendered != 1 pixel displayed.

   The number of visible horizontal pixels can be just about anything, so 
long as it take 1/15734th of a second to draw a whole line's worth of 
them.  One can pad the modeline horizontally to make 640 or 720 or 
whatever "visible" pixels visible.

   The number of vertical lines is *fixed* at 525/2 per field... If 
you draw 480 of them (the generally accepted number of "visible" lines for 
NTSC), the TV will draw some of them off the top and/or bottom screen so 
you cannot see them.  If you use a "640x465" resolution, the video player 
will scale the presumably 480-line source into 465-line output.... now the 
video's been abused.

 	Bottom line, overscanned is the way TVs have been mal-adjusted 
forever.  The *correct* way to not have overscanning is to adjust the TV 
so it doesn't do it.

-Cory


> I know that my TV is capable of displaying a picture
> which fills the screen without the top being slightly
> narrower than the bottom since the normal antenna
> input does so, as does the xBox boot screen.  I've
> noticed that some xBox games have this resolution
> problem while some do not.  I'm sure that there's some
> setting in xorg.conf that will fix this but I am not
> familiar enough with TV out to figure out what.
>
> Please let me know if you have any ideas,
>
 	Sounds like poor geometry control on the TV if it's not a 
rectangular picture.  Again... overscanning hides such misadjustments.

-Cory

-- 

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* Cory Papenfuss                                                        *
* Electrical Engineering candidate Ph.D. graduate student               *
* Virginia Polytechnic Institute and State University                   *
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