[mythtv-users] overscan problems with nvidia 8400 GS

Matthew Harrison lists at mwharrison.co.uk
Mon Jul 13 20:45:06 UTC 2009


----- Original Message ----- 
From: "Matthew Harrison" <lists at mwharrison.co.uk>
To: "Discussion about mythtv" <mythtv-users at mythtv.org>
Sent: Monday, July 13, 2009 10:31 AM
Subject: Re: [mythtv-users] overscan problems with nvidia 8400 GS


{...}

I wonder how consistent modern sets are. I've not dug out the relevant
standards to check, but Wikipedia has this interesting nugget on the
following page.

http://en.wikipedia.org/wiki/Overscan

"With the accuracy attainable with digital type displays (e.g. plasma
<http://en.wikipedia.org/wiki/Plasma>, LCD
<http://en.wikipedia.org/wiki/LCD> etc.), the area presented to the
viewer is precisely defined. For 1080i or 1080p material only the
central 1776x1000 pixels are presented to the viewer. For 720p material
it is only the central 1152x648 pixels."

I hope that doesn't mean that I have to scale HD video up in order to
correctly display it on my 1920x1080 LCD.
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Tortise wrote:
> Relevant Wikipedia references are
>
> http://en.wikipedia.org/wiki/720p
>
> "720p has a widescreen aspect ratio of 16:9, a vertical resolution of 720 pixels and a horizontal resolution of 1280 pixels, or 
> 1280x720, for a total of 921,600 pixels." and
>
> http://en.wikipedia.org/wiki/1080i
>
> "The term usually assumes a widescreen aspect ratio of 16:9, implying a horizontal resolution of 1920 pixels and a frame resolution 
> of 1920  1080 or about 2.07 million pixels."
>
>   
Restating the transmitted signal size doesn't change the recommended 
number of pixels that a set is meant to display. Unfortunatly, I no 
longer have access to the SMPTE standards and haven't been able to find 
any other sources to confirm the info in wikipedia, but assuming it's 
correct, it would be entirely correct for consumer displays to have a 
resolution of 1776x1000 or 1152x648. Giving you more pixels would either 
require scaling (nasty) or would risk displaying junk (ugly).

I suppose a more usefull question is: Have broadcasters stopped letting 
junk into the edges of their transmissions to allow us to safely reduce 
overscan?
> I've not introduced the additional complexity of frame rates, that is also potentially an issue, however getting the promised pixels 
> is indisputably a black and white issue.  Either you are getting them or you are not.
>   
If the set is sold as a 1920x1080 pixel monitor and has less pixels, 
that's clearly wrong. If it's sold as a TV, surely is should respect the 
recommended overscan?
> Mathew it depends on what your HD native resolution is, if it is 576i or 720p then it will have to be scaled, that not unreasonable 
> in itself and works well - especially when the overscanning is turned off - if the capability is there.
>   
Yes, if you are scaling anyway, it shouldn't be a problem.
> I've calculated a 5% edge deficit overscanned equates to 19% of the image missing and not seen!  Would you accept 19% less pay - if 
> you knew about it?
>
>   
The recommended overscan does seem to remove just under 17% of the 
pixels, but it's removing pixels you were never ment to see.

If my tax and national insurance only came to 19%, I would be very happy.


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