[mythtv-users] overscan problems with nvidia 8400 GS

Brian Wood beww at beww.org
Mon Jul 6 14:29:17 UTC 2009


On Monday 06 July 2009 07:52:25 Johnny wrote:

>
> It is true that overscan is caused by the TV and not the video card,
> however, the problem of displaying stuff from a computer on a TV is
> shared by the TV and the video card. In the case of SD TVs, they were
> never intended to be hooked up to a computer and so it is unreasonable
> to hope for them to fix this issue. So in my opinion, if Nvidia
> provides a TV out option they do have some responsiblity to help me
> display stuff correctly on that TV (especially since overscan is
> standard in many many TVs).

Another history lesson:

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.

Back in those days, people would actualy call a repair man to the home if the 
set needed service, and jokes were common about how much the TV repairmen 
earned. Manufacturers valued their record of reliability, and neither the 
manufacturer or the user liked the repairman having to be called. Having the 
set overscan a bit delayed the inevitable service call due to normal aging of 
components, and contributed to a better "reliability" record.

Today most TV sets do not use vacuum tubes, and in general all of the reasons 
for the makers to set up sets to overscan no longer exist, but inertia is a 
powerful force in manufacturing.

Most commercial or broadcast TV monitors have an "underscan" switch, which 
causes the set to underscan by a slight amount, in order to be sure that the 
entire picture can be seen, important in a TV control room for example. This 
would not be necessary but for the fact that most consumer sets or cheap 
monitors did overscan by a bit.

The best solution was to use regulated power supplies, but that was very 
expensive with voltages in the hundreds and currents in the hundreds of 
milliamps, so this was not done except in the most expensive commercial 
units. It was simpler and cheaper to just overscan the sets, and nobody 
seemed to mind. It's why a TV picture is divided by a SMPTE standard 
into "safe action", "safe title" and the "rest", so nothing important would 
happen in the area missed by many TV sets. Typical manufacturer's solution: 
screw the customer in the interest of making more money.

</history lesson>

-- 
Brian Wood
beww at beww.org


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