[mythtv-users] overscan problems with nvidia 8400 GS
David Brodbeck
gull at gull.us
Tue Jul 7 18:27:39 UTC 2009
Brian Wood wrote:
> Why is overscan standard?
>
> In the "Old Days" (defined as a block of time that ended the day you were
> born) virtually all TV sets came from the factory overscanning to an extent.
> The reason is because as the TV set aged, many things tended to change with
> time: capacitors would get leaky, tubes would lose emission and other bad
> things would happen. Unfortunately just about all these changes tended to
> reduce voltages in the set, and result in under scanning.
>
> The manufacturers figured (probably correctly) that consumers were more likely
> to notice and be unhappy about a blank area around the picture than they were
> to complain about overscan, which most consumers would not even notice. So
> the makers set up the sets to intentionally overscan.
This is true, but there are other reasons, too. As anyone who has
played with a CRT computer monitor knows, the edges of the picture on a
CRT are very difficult to control accurately. Wavy edges or edges that
bow in or out are common and difficult to correct, and overscanning
hides these ugly defects behind the CRT bezel. The vertical blanking
interval at the top of an NTSC picture also contains closed captioning
data, which shows up as distracting flashing lines if it isn't
overscanned off the edges of the screen. Finally, the actual width of
an NTSC picture varies a bit from source to source, and overscan hides
that, too.
In theory none of this should apply to a digital signal on a flat panel
monitor, but overscan seems to have persisted in the HDTV world. I
suspect this is because there are CRT HDTV sets out there and the spec
has to accommodate them.
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