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

R. G. Newbury newbury at mandamus.org
Tue Sep 26 16:39:21 UTC 2006

Michael T. Dean wrote:
> 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 ).

I tried playing with over/underscan while running mythfrontend in a 
window, output on my NTSC TV. The window size can be set on a per-pixel 
basis, but underscan reduces the size of the picture *in that window*, 
ie set underscan to -10 both vertically and horizontally and you get a 
black border around the picture, *within the window*. The window, as 
usual can be moved around on the desktop, which itself is subject to 
losing its edges due to the TV's overscan.

How does one control the desktop size, so that I can run mythfrontend 
without using a window?

              R. Geoffrey Newbury			

        Helping with the HTTP issue
<a href="http://www.w3.org/Protocols/">HTTP</a>

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