[mythtv-users] Troubleshooting a VGA-SCART converter

Lawrence Rust lvr at softsystem.co.uk
Sun Oct 2 19:26:01 UTC 2011

On Mon, 2011-10-03 at 00:50 +1030, Andrew Cranwell wrote:
> Trying to not spend money on a new TV, I thought I'd try VGA-SCART.
> Per http://www.mythtv.org/wiki/RGB_Scart and 
> http://www.nexusuk.org/projects/vga2scart/ I've been trying to connect a 
> new system (so far running from a livecd) with an NVIDIA GT520 card to 
> an old CRT via the sync-converter circuit described, without much success.
> I suspect the first problem is that the scan frequencies are simply too 
> high - X doesn't seem to accept the 15.6kHz modelines (from 
> http://www.mythtv.org/wiki/Modeline_Database) but will use the 31kHz 
> non-interlaced modes - hopefully this will actually work as the TV can 
> supposedly take 100Hz progressive input.

I have found that some graphics card won't accept modelines in xorg.conf
but can be configured using xrandr.  E.g:

# Set TV display mode 16:9
mode="1024x576 at 25i"
xrandr --newmode "$mode" 19.7 1024 1032 1124 1259  576 587 592 625 interlace -hsync -vsync
xrandr --addmode $out "$mode"
xrandr --output $out --mode "$mode"

Using xrandr is a lot quicker than restarting X.  Just type xrandr on
its own to list the available 'out' names.

> RGB is selectable as an A/V mode (though I did try the secondary select 
> circuit at 
> http://www.nexusuk.org/projects/vga2scart/voltage_from_vsync/view), and 
> I've at least got RGB wired through correctly, but sync is eluding me 
> (the image is blocky and madly scrolling everywhere).

Classic signs of inverted or too fast sync.

> Has anyone got some suggestions on what I should be looking for to test 
> this? I'm reading through the following for advice:

If at all possible I would recommend a 'scope.  Just a cheapo 10MHz
handheld DSO is sufficient.

> http://web.archive.org/web/20070402195239/http://www.tkk.fi/Misc/Electronics/circuits/vga2tv/vgatv_troubleshoot.html

> I'm also trying to get hold of a CRO to test what's going on, but I'd 
> like some advice on what to do

Once you can 'see' the output signals it becomes a lot easier.  You
could try an analog multimeter in DC mode to check whether you have
inverted sync.  If the CRO has x, y and rear z inputs than you could use
it to display a basic image.

>  - should I
> a. persist with/modify the circuit (e.g. to sync-on-green?)
> b. buy a VGA-composite converter
> c. buy a suitable NVIDIA card (e.g. 8600GT) & use either VGA/SCART or 
> S-Video
> d. give up and buy a new TV
> e. something else?

Most graphics cards can be configured to output TV compatible timings.
It just takes time playing with xrandr and a pair of eyes - i.e. a good

Don't give up without a fight.  You can save a fortune and learn a lot.

NB - most TV's require the RGB Commutation on SCART pin 16 to accept RGB
in and it also helps to have the AV Commutation pin 8 wired so that the
correct input is selected on powering the TV.


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