VGA CRT Direct Video Intermittent Black Screen 28.5MHz?

[etherlore]

I am running direct video to a VGA CRT (ViewSonic P775) through an AG6200 dongle. I have:

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direct_video=1
forced_scandoubler=1

Everything else is default. I get a picture, but a black screen occurs every few seconds, looks like loss of sync but I could be wrong. The monitor says I am getting a 640x480 59.9hz 31.5khz signal. MiSTer says its output is 640x480 60hz, 28.5Mhz. The monitor works fine at 640x480 59.9hz 31.5khz from other sources, like my 486 with a cirrus logic board. For instance SC2000 and Tie Fighter both output 640x480 59.9hz 31.5khz according to the monitor and does not have issues (same cables etc)

I have an LCD VGA monitor that does not exhibit the problem, which is an LG Flatron L1750S, using the same VGA cable and MiSTer output.

Any ideas for what could be going on are appreciated. What could be different enough from the MiSTer signal compared to the 486 signal that we loose sync? Is the 60hz 28.5Mhz output the MiSTer tells me it's outputting normal? Happens on all cores I have tried, including Menu.

[FoxbatStargazer]

An actual 640 wide is going to be rare when using scandoubler. Remember scandoubler only doubles the vertical resolution, not the horizontal. Things in the ballpark of 256x448 are going to be super common from console cores (although this reports as 31.1khz by Mister), not something you'd see from PCs very often and therefore some monitors might not be compliant? Be sure to watch mister's video information when it shows up (You can lengthen its appearance in the settings,.ini), the top line is the analog output and the bottom is that of the scaler. Not sure if vga_scaler works with direct_video or only the analog I/O board, but you might try using it to force particular resolutions/video modes from the mister to try and troubleshoot.

[etherlore]

Be sure to watch mister's video information when it shows up

It looks like the MiSTer always says the output is 640x480 60hz 28.5mhz when I run with direct_video and scan doubler, in all cores I have tried, including menu. This is true regardless of vga scaler or display mode settings. Disabling the scan doubler gets me no signal on either monitor, saying out of range, 15.7 khz.

What’s odd to me is that the MiSTer says horizontal refresh is 28.5Mhz, while the monitors say 31.5khz. But maybe that’s normal?

[FoxbatStargazer]

I'm using the Analog I/O board to go from VGA into a VGA CRT. I don't have a hdmi->vga dongle to test with.

I am seeing similar behavior when I turn on scandoubler, direct video, AND VGA Scaler. The resulting screen is usually a sync mess, with it appearing discolored and split in half. The stats seem to report 640x480, 60hz and 25khz. However the OSD appears perfectly intact in center... So I'm guessing that turning on VGA Scaler is making that same "direct video" HDMI output head over my I/O board VGA out. Because the OSD looks fine, I'm not sure if this is actually a sync problem between Mister and monitor or rather how the cores output is being synced with an internal framebuffer.

The one core that actually looks not horrible is AO486, centered a bit odd and DOS text looks bad but at least it syncs properly. 320x200 DOS games look scaled right although there is extra black space at bottom, and it seems to force 60hz output instead of 70hz. You might try the core just to see if it is at least usable.

Its possible that direct_video has not been set up to work with the scandoubler too well, or only for AO486 right now. I am certainly getting a lot more resolutions out of the Analog I/O port with direct_video turned off. If you don't have or want to get an Analog I/O board you may have to wait for someone to further adjust how direct_video interacts with the scandoubler.

[FoxbatStargazer]

OK I used a Portta powered HDMI to VGA converter. This adapter is supposed to convert any normal HDMI signal to VGA. This let me bypass using vga_scaler. Using this adapter the exact same cores display perfectly on my Compaq S910 VGA CRT with direct_video=1 and scandoubler=1, where I was having trouble using vga_scaler=1 from the I/O board. So contra my speculation earlier, it is possible to have a good experience with the scandoubler and direct video, just that maybe you have to stay away from vga_scaler.

I will note that when I plugged the same direct_video HDMI into a flatpanel 4K TV, using the Atari ST core in low resolution mode, the TV reported 2784x515, while the mister claims the analog output is supposed to be 424x484. I believe when Mister reports 640x480 for the HDMI output in direct_video it is lying, and you are actually getting that "superresolution" with an insane horizontal value. Bob from RetroRGB claimed that these super-resolutions were how they worked around HDMi limitations to push out 240p/15khz signals. Anyway this weird resolution could be causing problems somewhere along the chain, especially when it is basically doubled to around 480p, such as your dongle or the monitor itself. If forcing 640x480 video modes works fine for example (i.e. direct video off) then the wide resolution may be suspect. You could try creating some wide video modes, with thousands of pixels of horizontal resolution, to try and narrow down exactly what point your sync goes bad. Maybe change out the adapter if you can too. Hopefully its not your monitor that is at fault.

For the record the AO486 core tells my 4K TV it's making a more normal 694x517 resolution. That might be why it isn't screwing up the vga_scaler and it might also work with your current setup.

[etherlore]

I do not understand what is going on with the direct_video output, in addition to the 240p cores doing a 26.5mhz output with forced_scandoubler=1 instead of just doubling to 31.5 khz, some native 480p cores exhibit strange behavior.

For instance MultiComp, according to video_info natively outputs 640x400 31.26Khz 59.4Hz, gets the following ouput with direct_video=1:

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Native Core Output:         640x400 31.26Khz 59.4Hz
DV=1, forced_scandoubler=0: 640x240 12.59Mhz 59.4Hz
DV=1, forced_scandoubler=1: 640x480 24.18Mhz 59.4Hz

Why would direct_video=1 and forced_scandoubler=0 not output the native 640x400 31.26Khz 59.4Hz from the core, and instead half the vertical resolution to 240? What's this 12.59Mhz and 24.18Mhz? Maybe VGA is not compatible with the 640x400 signal?

It's more than likely I just don't have good enough of an understanding for these video signals, but this all looks odd to me.

[FoxbatStargazer]

I don't know about Multicomp but cores can have multiple modes and they only get one stat for "native." The Atari ST is listed as a 31 khz core purely because of its 400p/70hz monochrome mode, however its color modes are 240p/60hz/15 khz and you need scandoubler on to use on a VGA monitor. Then there's cores like ao486 that don't seem to have a real genuine analog output of the original resolution, and direct_video from that will be 240p unless you use scandoubler, even though that's not how the OG hardware works.

Just realized that the "MHz" on the Scaler line is likely the pixel clock rather than horizontal refresh. This applies even when using real HDMI instead of direct video. Note that a range of pixel clocks are possible for the same resolution depending on the timings for your video mode. 25 Mhz happens to be the reported pixel clock when you force the built-in 640x480 video mode (mode 6). Note also that I think this has nothing to do with the resolution actually being output by direct_video!

If you don't believe me about the super-wide resolutions on direct_video (depending on the core), read Bob's recent article on the topic:

https://www.retrorgb.com/mister-240p-over-hdmi.html

Direct Video is a way of sending “super resolution” 240p: A trick that sends a horizontal signal wide enough for most HDMI chips to understand, while sending a vertical signal that a standard 15KHz CRT can read (something like 1174×253). The CRT doesn’t know the difference, the same way it doesn’t know the difference between horizontal resolutions of a console like the PlayStation; As long as the vertical signal is correct, the entire horizontal signal is drawn.

[etherlore]

I don't know about Multicomp but cores can have multiple modes and they only get one stat for "native."

Just realized that the "MHz" on the Scaler line is likely the pixel clock rather than horizontal refresh.

I think you are right, that is good information. It turns out the ViewSonic flickers using the dongle without direct_video as well. I ordered a different one, will see if this changes anything.

[etherlore]

Got the new dongle, a UGREEN one: https://www.amazon.com/gp/product/B01GFMW91E/ No longer get the black screens. The brand of the dongle that produced the intermittent black screens was GTMEDIA, which supposedly uses the AG6200 chipset. Either way seems like all is good for me now.

[anderse77]

Important info: You cannot use the scaler with direct_video=1. Direct_video=0 must be set. This is not written anywhere in documentation.