[mythtv-users] Troubleshooting a VGA-SCART converter
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.
> 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
> 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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