[mythtv-users] Troubleshooting a VGA-SCART converter

Alex Butcher mythlist at assursys.co.uk
Sun Oct 2 18:55:28 UTC 2011

On Mon, 3 Oct 2011, 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 successfully used the nexusuk adapter with nVidia MX440, 7600GT and
7600GS cards...

> 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.

...however, when I bought a new GT240, I found that X would accept the
modelines, but the card wouldn't output a signal that my TV would display. 
My (relatively untested) theory is that any card with an analogue
composite/S-Video output can output modelines with <15MHz dot clocks, but
that cards with only HDMI TV outputs can't. I suspect you're out of luck
with your GT520.

> 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'm pretty skeptical that'll work. But it might, I guess.

Have a look at
<http://www.mythtv.org/wiki/User:Cowbutt#.2Fetc.2FX11.2Fxorg.conf> which is
my working xorg.conf for a 7600GS and a PAL TV.

> 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).

Hmm, that's hopeful, then - I got nothing at all out of my GT240 through the

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

This should work with your VGA/SCART and give you the best quality over
SCART. See my theory above as to which cards will work. I got a pair of
7600GS PCIe cards on eBay for less than £20, so you don't need to spend much
if you're only doing SD.

> d. give up and buy a new TV
> e. something else?


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