[mythtv-users] Stupid overscan question
Cory Papenfuss
papenfuss at juneau.me.vt.edu
Wed Nov 9 16:07:49 EST 2005
> 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
--
*************************************************************************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
*************************************************************************
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