Normally, Mythbuntu does a pretty good job of detecting the display connected to it and configuring it automagically. However, if your video display does not play well with Mythbuntu (i.e. it cannot be probed and set up automagically), you may have to configure it yourself.
This section describes what to do about this situation but first, a note about how to get out of a bind, if for some reason your display does not work properly and you cannot recover from this state because the picture is too garbled to run the usual configuration tools.
Using SSH on another system, connect to the system that is in trouble and go to the /etc/X11 directory. There, you should find files named xorg.conf*. You should rename xorg.conf to something like xorg.conf.brokey and then symlink xorg.conf to the xorg.conf.failsafe file:
cd /etc/X11
sudo mv xorg.conf xorg.conf.brokey
sudo ln -s xorg.conf.failsafe xorg.conf
This should get you up and running, at which point you can begin monkeying around with the display configuration to arrive at one that works for your display.
Also, before you start playing around with any of the display settings, be aware that some of the NVidia drivers don't handle modelines properly so you may want to check into that first, before you waste a whole lot of time (like we did) trying to set something that's going to have no effect.
If the display you are trying to work with is a TV, you may find some useful notes about setting it up at this URL:
http://www.linuxis.us/linux/media/howto/linux-htpc/
video_card_configuration.html
Basically, you are trying to figure out one or more modelines that can be used to set the various screen resolutions supported by your display. There should be a chart in the back of your display's manual that lists all of the supported modes (beware that some of the charts we've seen have the vertical and horizontal frequencies exchanged - a good clue is that the vertical frequency is usually the one that matches the number in the VESA name). First off, you should try to pick one of the standard modes from the list below. Also, it is best to try and pick the highest vertical refresh rate (in Hz) supported by your display:
# 640x400 @ 85Hz (VGA VESA 85) hsync: 37.9kHz
ModeLine "640x400" 31.5 640 672 736 832 400 401 404 445 \
-hsync +vsync
# 640x480 @ 60Hz (Industry standard) hsync: 31.5kHz
ModeLine "640x480" 25.2 640 656 752 800 480 490 492 525 \
-hsync -vsync
# 640x480 @ 72Hz (VGA VESA 72) hsync: 37.9kHz
ModeLine "640x480" 31.5 640 664 704 832 480 489 491 520 \
-hsync -vsync
# 640x480 @ 75Hz (VGA VESA 75) hsync: 37.5kHz
ModeLine "640x480" 31.5 640 656 720 840 480 481 484 500 \
-hsync -vsync
# 640x480 @ 85Hz (VGA VESA 85) hsync: 43.3kHz
ModeLine "640x480" 36.0 640 696 752 832 480 481 484 509 \
-hsync -vsync
# 720x400 @ 85Hz (VESA 85) hsync: 37.9kHz
ModeLine "720x400" 35.5 720 756 828 936 400 401 404 446 \
-hsync +vsync
# 800x600 @ 56Hz (SVGA VESA 56) hsync: 35.2kHz
ModeLine "800x600" 36.0 800 824 896 1024 600 601 603 625 \
+hsync +vsync
# 800x600 @ 60Hz (SVGA VESA 60) hsync: 37.9kHz
ModeLine "800x600" 40.0 800 840 968 1056 600 601 605 628 \
+hsync +vsync
# 800x600 @ 72Hz (SVGA VESA 72) hsync: 48.1kHz
ModeLine "800x600" 50.0 800 856 976 1040 600 637 643 666 \
+hsync +vsync
# 800x600 @ 75Hz (SVGA VESA 75) hsync: 46.9kHz
ModeLine "800x600" 49.5 800 816 896 1056 600 601 604 625 \
+hsync +vsync
# 800x600 @ 85Hz (SVGA VESA 85) hsync: 53.7kHz
ModeLine "800x600" 56.3 800 832 896 1048 600 601 604 631 \
+hsync +vsync
# 1024x768 @ 43Hz (industry standard) hsync: 35.5kHz
ModeLine "1024x768" 44.9 1024 1032 1208 1264 768 768 776 817 \
+hsync +vsync Interlace
# 1024x768 @ 60Hz (XGA VESA 60) hsync: 48.4kHz
ModeLine "1024x768" 65.0 1024 1048 1184 1344 768 771 777 806 \
-hsync -vsync
# 1024x768 @ 70Hz (VESA 70, HP1070) hsync: 56.5kHz
ModeLine "1024x768" 75.0 1024 1048 1184 1328 768 771 777 806 \
-hsync -vsync
# 1024x768 @ 75Hz (XGA VESA 75) hsync: 60.0kHz
ModeLine "1024x768" 78.8 1024 1040 1136 1312 768 769 772 800 \
+hsync +vsync
# 1024x768 @ 85Hz (XGA VESA 85) hsync: 68.7kHz
ModeLine "1024x768" 94.5 1024 1072 1168 1376 768 769 772 808 \
+hsync +vsync
# 1152x864 @ 75Hz (SXGA VESA 75) hsync: 67.5kHz
ModeLine "1152x864" 108.0 1152 1216 1344 1600 864 865 868 900 \
+hsync +vsync
# 1280x960 @ 60Hz (SXGA VESA 60) hsync: 60.0kHz
ModeLine "1280x960" 108.0 1280 1376 1488 1800 960 961 964 1000 \
+hsync +vsync
# 1280x960 @ 85Hz (SXGA VESA 85) hsync: 85.9kHz
ModeLine "1280x960" 148.5 1280 1344 1504 1728 960 961 964 1011 \
+hsync +vsync
If you didn't find your display's resolution in this list, there are more standard modes listed at:
http://m.domaindlx.com/LinuxHelp/resources/modelines.htm
Another way to obtain a modeline for your display is to use videogen. If it is not installed on your Mythbuntu system, you can obtain it with:
sudo apt-get install videogen
To use it, you select the maximum dot or pixel clock frequency from your display's specifications, along with the maximum vertical and horizontal refresh rates. Feed the desired screen resolution plus these values into videogen:
videogen -m640x480 -mdc=108 -mhf=87 -mvf=72
This will generate a modeline with the highest refresh rate that can be supported by your monitor for the resolution selected. In the above example, we determined that the maximum dot clock was 108 (from perusing all of the resolutions listed for the monitor) and the maximum horizontal frequency was 87 KHz, while the maximum vertical frequency was 72Hz.
This would yield a modeline for a dot clock of 30MHz, a horizontal refresh rate of 36.4 kHz and a vertical refresh rate of 72Hz.
Yet another alternative is to use the modeline calculator at:
http://xtiming.sourceforge.net/cgi-bin/xtiming.pl
Instead of determining the maximum refresh rate for your display, this will let you put in the refresh rate and calculate the modeline from that. If you are trying to match your display's refresh rate with that of a video broadcast so as not to see jitter from interpolated frames, you might find this method more to your liking.
Now, make a copy of your working xorg.conf file in /etc/X11 and begin hacking the Monitor section. Set the device identification fields to something useful:
Identifier "Syntax LT27HV LCD Display"
VendorName "Syntax"
ModelName "Olevia LT27HV"
Next, you should probably set the minimum and maximum horizontal and vertical sync frequencies from the chart of your display's capabilities:
HorizSync 56-87
VertRefresh 24-72
These values are not used to calculate any actual display settings but the values in the modelines are checked against these limits to ensure that you do not exceed your display's capabilities (and possibly destroy it).
Then, add the modelines to this section:
# 640x480 @ 85Hz (VGA VESA 85) hsync: 43.3kHz
ModeLine "640x480-85" 36.0 640 696 752 832 480 481 484 509 -hsync -vsync
# 800x600 @ 85Hz (SVGA VESA 85) hsync: 53.7kHz
ModeLine "800x600-85" 56.3 800 832 896 1048 600 601 604 631 +hsync +vsync
# 1024x768 @ 85Hz (XGA VESA 85) hsync: 68.7kHz
ModeLine "1024x768-85" 94.5 1024 1072 1168 1376 768 769 772 808 \
+hsync +vsync
# 1280x720 @ 60Hz (1280x720-60) vsync: 60.39Hz, hsync: 45.72kHz, clk: 74.25
# This is the only true native mode for the LT27HV display.
Modeline "1280x720-60" 74.25 1280 1312 1592 1624 720 735 742 757 \
+hsync +vsync
For the Syntax Olevia models, you may want to add the following, as well:
# Power saving mode
Option "DPMS"
# We know everything about this display
Option "UseEDID" "false"
# We know what the dot pitch is
Option "DPI" "96×96"
# Make sure the video card always picks the CRT. Or, if you have the
# display plugged in on the DVI connection, use "DFP" instead of "CRT-0".
Option "UseDisplayDevice" "CRT-0"
Next, hack the Screen section to use the display that you just defined by setting the Display SubSection to point to it:
# The color depth
Depth 24
# All of the supported modes that we defined in modelines. The first is
# the default mode chosen at startup.
Modes "1280x720-60" "1024x768-85" "800x600-85" "640x480-85"
Now, you can either copy this file over the top of xorg.conf or symlink it:
sudo rm -f xorg.conf
sudo ln -s xorg.conf.failsafe xorg.conf
If you have any more questions about the configuration of display devices under Xorg, you can look at the man page:
man xorg.conf