[mythtv-users] Underscan in playback and recording, driver issue?

Michael T. Dean mtdean at thirdcontact.com
Tue Sep 26 03:35:43 UTC 2006


On 09/25/06 19:39, David McKenzie wrote:

>  On 9/25/06, Michael T. Dean <mtdean at thirdcontact.com> wrote:
>
> >
> > On 09/24/06 22:54, Carl Lewis wrote:
> >
> >> David McKenzie wrote:
> >>
> >>
> >>> The problem is that recordings and live TV using the PVR-150
> >>> doesn't fill the screen completely
> >>>
> > ...
> >>> Has anyone seen this before or can offer a resolution? Right
> >>> now it is the only thing from letting me switch over to Myth
> >>> completely.
> >>>
> >> David, I've been seeing the same thing in Australia, with ABC in
> >> particular. I run a 100" screen (projector :-) with no overscan.
> >> The affected channels/programs have about a 2" vertical
> >> stripe/black-bar down one or both sides. I've only noticed it
> >> since setting up a border around the screen :-/ so I've no idea
> >> when it started. Same as you I've looked at the recordings
> >> themselves and they contain the vertical space.
> >>
> > Because the recording device was designed for use with a system
> > that /requires/ overscan. I.e. NTSC and PAL were designed such
> > that display devices would overscan the image to cut off
> > black/green/rainbow bars, static, jagged edges, and any other
> > "ugly" attributes of the image allowing the user to see only pretty
> > and without requiring significantly advanced technology (remember
> > also we're talking about technologies that are over 50 and almost
> > 40 years old, respectively).
> >
> > That means, it's not a driver issues, but an encoding system issue
> > (and we're talking the encoding that comes before the PVR does its
> > encoding--garbage in/garbage out and all). So, if you don't want
> > the edges, overscan the image to get rid of them--make your display
> > device work as the designers of NTSC/PAL intended.
>
>  I'd like to do this but I'm not sure how I can, my LCD display is a
>  tv and only runs at certain resolutions, native 1366x768. I don't
>  know how I can force it to overscan in X11.

You use either MythTV to overscan the video during playback or use the 
video drivers (i.e. the NVIDIA drivers) to overscan the entire desktop 
(which gets the video, the menus in MythTV, the desktop outside of 
MythTV, and every other application you run).

Note that your LCD (if it's a TV) automatically overscans--all TV's do.  
(If it's a monitor, it may or may not depending on the design of the 
monitor (and will probably only overscan for certain inputs).)  If 
you're seeing the edges of your desktop, you're already overscanning 
with the video drivers--just using a negative overscan (meaning an 
underscan).  An underscan simply scales the image to use fewer of the 
1366x768 pixels, and an overscan simply scales the image to use more 
than 1366x768 pixels, and in both cases the resulting scaled image is 
centered on the available (1366x768) pixels.  The signal sent to the TV 
is 1366x768 pixels, but contains black pixels on the edge (for 
underscanned) or crops the image (for overscanned).

If you have a TV that's already overscanning, you just need to use less 
underscanning on the Myth box.

In Myth, there are the settings:

Vertical[Horizontal] over/underscan percentage
Adjust this if the image does not fill your screen vertically[horizontally].

and

Scan displacement (Y[X])

Adjust this to move the image vertically[horizontally].

and, you can also use the crop filter on playback ( 
http://mythtv.org/docs/mythtv-HOWTO-10.html#ss10.4 ).

Mike



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