I used my MiSTer with a VGA-Board on a VGA-LCD (Samsung Syncmaster 931) quite a while now. Since a few days i have an hdmi-vga-dac (based on AG6200 chip).
Over VGA-Board my monitor catches the 15 kHz video signal, but over the hdmi-vga-dac and direct video mode i have to force scandoubler otherwise i get a black screen.
Is it normal to force scandoubler, when using direct video?
Doesn't the 931 have a DVI input? If so I would strongly recommend using a cheap HDMI->DVI adapter, it will give you a pristine signal and the Mister will do a better job scaling and applying filters than the monitor will. Turn off direct_video and set the video_mode to match the monitor's native resolution. (video_mode=1280,1024,60,cvt) and go from there.
I'm surprised this monitor can handle 15khz at all over VGA, it doesn't seem rated for it. But know that direct_video uses some weird resolutions (like 2048x240) to try and force 15khz into HDMI specs, so some converters may not work with it. The minimum standard for HDMI is generally 480p after all.
Shame on me, i messed up the numbers. I am sorry! The Monitor is a Syncmaster 943N. It has only a VGA-Port.
But if direct_video uses resolutions for 15 kHz to go over HDMI specs where as the VGA-board generates the analog video signal in a true 15 kHz hsync way, it gives me a reason for a black screen over HDMI-VGA-DAC at least.
Well, a CRT generally can't tell the difference. The digital resolution going out of the Mister's HDMI might be 2048 wide, but once its analog a CRT just sees a rate of change for the analog signal and follows it as long as it can keep up with the sync side. An LCD monitor looking at analog has to guess a resolution and sample it so maybe that's more complicated, but I'm more likely to think the HDMI converter itself doesn't know what to do with 2048x240p and gives up, so nothing even comes out of its analog out. Its a bit of a crapshoot which converters work and don't for direct video these days.
Anyway if you don't want to use native resolution (video_mode=1280,1024,60,cvt), you might as well try the direct_video scandoubled. Its not like your LCD monitor is drawing scanlines or something. (Or is it?)
Yes, hard to say what the reason for the black screen, HDMI-VGA-DAC or the LCD's „smart“-VGA-electronic. Okey, i could lend a DSO..., but not sure if it is worth the effort... Finally, with force_scandoubler option, the image is quite good.
The monitor has no hardware caused scanlines by the way.