[mythtv-users] LCD and traditional CRT TVs, overscan and UI size and widget placement

Brian J. Murrell brian at interlinx.bc.ca
Wed Nov 11 18:52:05 UTC 2009

On Thu, 2009-11-12 at 07:31 +1300, Tortise wrote: 
> It seemed to me this wow effect is really the "edge artefact" that overscan was made to hide.

Exactly, and overscan will hide it if you actually overscan the image
you give to the TV.

The myth UI is not overscanning though once you adjust it to get all of
the widgets (i.e. in setup screens) inside the viewable area because
rather than just moving widgets on the overscanned canvas, they are just
making the canvas smaller, to fit inside the viewable area and not into
the overscan area.

To put it more clearly, my X screen is 640x480 and that stretches out
into the overscan area once I use /usr/bin/nvidia-settings to enable the
overscan.  When I view a television station I see the same visible
portion of the picture as I would watching real TV.

However, when I go into the setup menus the various widgets (checkboxes
down the left for example) are off the visible area because by default
the myth UI starts at 0,0 and stretches the canvas out to 640,480, with
the edges hidden in the overscan area.  So I go to the screen adjustment
to bring it all back into the visible area again but that simply reduces
the size of the canvas (and sets an offset so it's centered) to try make
the edges of the UI meet the spot where visible area turns into
overscan.  In my case, it's reduced the UI to 571x443 offeset to display
at 31,22 which is roughly (I say roughly because of the in-exactness of
the visible area borders) where visible area turns into overscan.

But as expected with analog CRTs, this "meeting" is not exact
everywhere.  Which is why this resizing of the canvas is the wrong
approach.  IMHO.

> I have a 32" "semi HD" CRT TV which has the "wow" shown but on the right hand side, I was going to get the dealer to come and adjust 
> the set as I had assumed that was a minor malignment of the tube that they might be able to correct.

No.  AFAIK, this is normal and to be expected with the analog nature of
a CRT.

> Interestingly the inner bow moves depending on content movement

Indeed.  Again, an artifact of the analog-ness of the CRT.

> and it is a little disconcerting however I can seem to come to 
> ignore it.

Sure.  And I have ignored mine for years now, but this problem is also
now having the ripple effect of not working on a gnome desktop properly
when the UI window is not the same size as the gnome screen, but when it
is the same size, and overlaying the task bars properly, the edges are
off into the overscan again.


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