[mythtv-users] overscan problems with nvidia 8400 GS

[Brian Wood]

On Tuesday 07 July 2009 12:27:39 David Brodbeck wrote:
> 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.

All true, convergence especially is hard to get right at the edges, and your 
linearity starts to become a problem as well. The main problem was 
picture "shrinkage" due to aging components in the TV sets.

The main cause of picture width variation is non-standard horizontal blanking, 
since a lot of sets key (bad word here) on the blanking pulse, though they 
shouldn't. Sort of like keying black level to the tip of sync instead of the 
baseline, which is harder to do and thus more expensive.

Closed captioning in NTSC is on line 21, which *should* not be visible, but 
often is.

> 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.

I don't think it's so much "spec" as it is "tradition", and economics.

Then there are the "Matrix" switches on high-quality CRT monitors, which cause 
the monitor to screw up the reds the way most consumer sets do, so the 
director gets a better idea what the home viewer is actually seeing.


-- 
Brian Wood
beww at beww.org