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

raphael rpooser at gmail.com
Tue Sep 26 22:34:13 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).
Just to second that and give some personal experience and expound a little.
For my Nvidia card downstairs this works beautifully, with TV out.  
There is a utility that comes with the Nvidia drivers, something like 
nvsettings.  Run that and you get a menu allowing you to adjust the 
amount of overscan easily when using TVout.
Or, you can use xvidtune, if you start with a modeline that is readable 
already.  Using xvidtune you can adjust the over/underscan to your 
heart's content.
You will probably have to go the xvidtune route if you use an ATI card.  
My frontend machine with a 9600 uses a custom modeline for an HDTV with 
1280x768 native resolution (using hdmi input, not vga, so resolution 
choices were limited to be able to display an image in X at all).  All 
my shows were horribly underscanned, just as you described, until I set 
a custom modeline with help from xvidtune that fit my preferences 
perfectly.  Further, you don't have to worry about overscanning your 
whole X image - it's ok to do that, because you can then set a custom 
resolution for the mythtv frontend after that.  Just set a custom 
resolution and offset for your frontend instead of using "fullscreen" to 
fit it all in your screen to your preference.  You shouldn't have to 
mess with it too much, as the program seems to be built with the idea of 
fitting onto an overscanned screen in mind anyway.  You then select 
"fullscreen" for the playback of recordings and you'll have your image 
overscanned to the desired amount you set with your custom modeline.

Also, you may want to google the term "modeline" or "X modeline", etc.  
To see just what parts of the modeline
 move the image around on your screen, what parts account for overscan, 
and what parts acount for total resolution.  You can manually adjust 
that in your xorg.conf.  Most people I heard of who wanted a perfect 
image on their HDTVs had to go through the steps of editing their own 
modelines through trial and error, unless their HDTV was the same model 
of someone with the same setup.  In addition to using xvidtune I also 
had to manually modify my modeline.

More information about the mythtv-users mailing list