[mythtv-users] Solved: 4:3 content always fills my widescreen TV resulting in an incorrect aspect ratio

[Christopher David Petersen]

Helpful keywords: aspect ratio Fill 4:3 15:9 EDID DDC dpi Xserver SiS
Xorg.conf driver Asus Pundit Olevia Syntax LT26HVE MythDora UseEDID
IgnoreEDID

If 4:3 content is always filling your TV / monitor regardless of the
screen's true aspect ratio, then this information might help you.

I have a Olevia LT26HVE LCD screen whose native resolution is 1280x768
(a 15:9 aspect ratio).

After a clean install of MythDora 4.0 on a Asus Pundit-S (the original
Pundit with the SiS chipset), all my 4:3 content would always fill my
whole 15:9 screen. As a result, standard definition TV would always
look stretched horizontally.

After many false starts and dead-ends, I arrived at the following theory:

- My TV was reporting to the video card that it had a 4:3 aspect
ratio. Apparently, monitors and TVs can report such information to
video cards via several methods including DDC and EDID. But apparently
many TVs and monitors report incorrect information.

- Even though I had specified my TV's true dimensions in the
/etc/X11/xorg.conf file (and these dimensions indicated a 15:9 aspect
ratio), the SiS driver was believing the bogus EDID information. Some
nVidia drivers have an option that can be set in xorg.conf to force
the driver to ignore EDID information (Option "UseEDID" "false"). The
SiS driver that comes with MythDora (which I believe is the xorg
driver) may have an option to ignore EDID information, but I could not
find it.

- I theorize that MythTV determines the monitors aspect ratio by
asking Xserver for the screen dimensions. I do not know this for sure,
but MythTV needs the aspect ratio of the screen from somewhere.

- MythTV thought I had a 4:3 screen, so it put 4:3 content into the
whole screen. Therefore, 4:3 content always filled the screen. The
aspect ratios "Off", "Fill", "4:3" all look the same. Just like one
would expect on a 4:3 screen. This is correct behavior on MythTVs
part.

The first confirmation of my theory came from the output generated by
a command called "xdpyinfo".

When I ran xdpyinfo from a command prompt, it reported that my screen
was 1280x768 pixels (good). And that my screen was 40x30mm (very bad).
First of all, 40x30mm is like the size of a iPod screen. And worst of
all, its a 4:3 aspect ratio. It also seems like the sort of value that
might get returned by bad EDID data.

So, if I could get the SiS driver to ignore the EDID information, the
SiS driver would respect the DisplaySize information in my xorg.conf
file, and MythTV would realize I had a 15:9 screen.

The simple solution would be to set an option in the xorg.conf file
that would tell the SiS driver to ignore EDID information. There most
likely is an option, but I couldn't find it. (Anybody know the option
for xorg SiS driver ignore EDID?)

The drastic solution would be to hack the xorg SiS driver source and
force it to return that EDID information wasn't available. Although
do-able, more work than I wanted.

The really drastic solution would be to break pin 6 and 7 off on my
DVI cable. These are the pins that transmit the EDID information. This
may not work. It's quite possible that video cards would not even know
that a monitor is there at all without these pins. It's also possible
your monitor or computer could let off the "magic smoke" as the
hardware one or bath fried (unlikely, but fire is always a possibility
when modifying the hardware).


Many helpful people indicated that if one forces a -dpi setting to X
server, then X server will ignore DisplaySize settings from the
xorg.conf file. Also the DPI setting will cause drivers to ignore the
EDID information.

So, where does one put the magic -dpi command line argument.

After many false starts, a helpful reply on this list from Alex
Halovanic revealed that it's easy with Gnome.

For a default MythDora installation (Gnome):

1) From the MythDora customized MythTV frontend, select MythDora
Tools->Launch Xterm. Run a xdpyinfo command to see your screen
dimensions. If the ratio is 4:3 (and your screen isn't), then this
solution might help.

2) From Gnome's main menu, select System->Administration->Login Screen
(you will need your root password for this)

3) On the dialog box entitled "Login Window Preferences", there's a
tab called Security.

4) On the Security tab, press the button titled "Configure X Server"

5) On the dialog box titled "X Server Login Window Preferences",
there's a text box titled "Command:". It will already contain text
like "/usr/bin/Xorg -br -audit 0"

6) Add a new option at the end "-dpi 57" so that the text reads
something like "/usr/bin/Xorg -br -audit 0 -dpi 57" (without the
quotes). Note that 57 is my monitors true dots per inch. I have a 26"
(diagonal) monitor with a 15:9 aspect ratio (remember geometry? It's
back...). Of course, the easiest way to determine your DPI is to
measure your screen with a ruler.

7) Reboot your machine.

8) Open a Xterm window and type "xdpyinfo". See what it says from
screen dimensions in millimeters. Is the ratio better?


Now my 4:3 content has black bars on the side (like it should) and
everything runs great!

Thanks to everyone who helped.

-- Christopher David Petersen (not the Chris Petersen of nuvexport
fame, just an ordinary programmer)