[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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