Modding VGA 31kHz CRT monitor to EGA 15kHz. It's been done?

[Paranoid_Andy]

When I searched this once before a while back I didn't get any results. This time however I found something interesting. It seems somebody has made this work, I think anyway. Some of this stuff is over my head for sure but I thought in case anybody here is interested and isn't aware of this that I would share.

Here is the video that appears to be demonstrating it:
https://www.youtube.com/watch?v=5ta5wtzdpXo

Here is the forum discussing the project. (I used google translate. It's in Russian):
https://translate.google.com/translate? ... -kgts.html

Does this mean you could possibly hook up a retro game console directly(or almost directly) to a PC CRT with this mod?

If this is useful, cool! If not, feel free to delete my post then

[FinalBaton]

David Murray (8-Bit Guy on Youtube) showed something similar recently and got his PC to output VGA graphics over 15kHz, but it was analog RGBS. It worked but only on a portion of games and programs. a bunch of them didn't behave nicely.

If you're going for EGA then you'll need a circuit to transcode from RGBHV to EGA pinout of course.

[headlesshobbs]

Usually an ATI card is needed as programs like 15khz could make it work, but I'm not so sure of things now. Nvidia on the other hand will probably allow 480i at best.

I've at least got one of my Dreamcast vga boxes set to output 240p, but that really needs a source that will accept it and I had to stick with the ossc on that.

[Fudoh]

just to clarify: the topic is about modding VGA monitors to lower the minimal scan rate from 31 to 15khz. It's not about modding a source or converting a signal.

[headlesshobbs]

oh whoops!

I'm used to seeing TV's with RGB mods. I didn't know that EGA on a 31khz was a thing.

[Josh128]

Really interested in this. Seems on the surface like it should be technically possible. 25KHz as well. I wonder what prevents it from working outright with no mods?

[headlesshobbs]

I think the reason why computer crt's dropped 15khz at a certain point was due to a timing limit or something. This sounds really touchy since forcing a display beyond it's limits can actually destroy a tube from what I've heard.

That's also why I see people concerned when they do their mame setups and have a worry about switching 15khz, 24khz, and 31khz with some modes being clearly avoided.

Also thought some of the info in this discussion was interesting. https://www.vogons.org/viewtopic.php?t=50069

[NJRoadfan]

Also thought some of the info in this discussion was interesting. https://www.vogons.org/viewtopic.php?t=50069

As per my old post, the reason was cost. VGA monitors were originally simple fixed frequency displays and cheap to make. NEC's "Multisync" became a genericized trademark since they were the leaders in the market. The original Multisync and Multisync II could scan from 15khz up to about 40 or so khz (supports roughly 800x560, 800x600 works with the top and bottom cut off), along with full TTL RGB support (both 4bit and 6bit CGA/EGA varients along with MDA).

Thing is, this was expensive to build in a market that was always about cost. IBM's dual-sync 5154 EGA monitor was never a hot seller and everyone kept buying the "cheap" 15khz 5153 CGA display. The multisync monitors were sold as top-of-the-line products with a price to match and were never big players in the market. Their primary marketing angle was that they would not be "obsolete" when new computers came out.

[Josh128]

Kind of the opposite of the topic, but I recently got a Groovymame / CRTEmuTools setup going on my 27" Trinitron FD Wega TV and let me tell you it works like a charm. Beautiful composite sync 15KHz signal coming from the H-Sync and RGB pins. When I saw and heard some of the late 80's and early 90's classics on this setup I almost had to wipe away a tear.

It just so happened I had one of the most ideal VGA cards for this type of thing given to me, the AMD/ATI HD 5450. Only downside is that CGA games really need to be played in interlace mode to get all the detail (and that works fine, to be sure), but yeah, fantastic. The resolution auto-switching of GroovyMAME + the ability to tweak and lock any one of the 15KHz modes as you see fit is just fantastic.

Next thing Im going to try is scan-doubling CGA / medium res games on a VGA monitor. 30 fps games could get away with a 90fps mode on limited range monitors while 40+ KHz capable monitors should be able to something like 512x448120p quite handily!

Exciting stuff!

[FinalBaton]

I wonder if it's dangerous for PC monitors? To ''unlock'' 15kHz on them? in regards to what headlesshobbs is saying

[Josh128]

The only way I think it could be dangerous is if the beam intensity would not be attenuated enough to compensate for the slower scan. Could perhaps cause some early phosphor burn in. Im no CRT engineer, but on a standard multisync monitor logic would dictate that the electron beam intensity needs to be increased as you move to faster scan rates, so Im sure the opposite must be true as well.

[Bassa-Bassa]

Next thing Im going to try is scan-doubling CGA / medium res games on a VGA monitor. 30 fps games could get away with a 90fps mode on limited range monitors while 40+ KHz capable monitors should be able to something like 512x448120p quite handily!

Why 448, though? Wouldn't you need to get something between 360-400 for a better fit?

[Josh128]

I was just thinking that was about the upper range that any 25KHz game ever ran. Probably the most common would be something like 512 x 384 or 496 x 384.

[jd213]

Guess there still isn't any way to make a standard PC CRT do 15khz. I already have a PVM but really like how my Mitsubishi Diamondtron looks with retro games, and would love to be able to use it with 15khz signals as well.

Any case that it might be possible with some kind of mod someday? Is it more of a technical limitation, or just one that hasn't been investigated thoroughly enough?

[Josh128]

Best bet is to get an OSSC and use to line double + scanlines (not perfect, but by far the best solution currently available, or wait for the OSSC Pro and see if Marqs and the other devs can implement the 240p/120Hz mode that Marqs said was possible. If they can do that, that would be the best technical option. I have tested 240p/120 on a PC CRT using CRU and Retroarch, and its beautiful.

[jd213]

I know about those already, I was just wondering if it's even theoretically possible for a PC CRT to be modded to display an actual 15khz signal.

If it's just an issue of cost or man-hours, it might still be worth it considering the current costs of PVMs.

[headlesshobbs]

I keep thinking when they went from those IBM multi sync displays, they could have just included a built in feature that upped 15khz to 31khz internally as we continued seeing many displays with functional processing in their hardware (i.e. YUV component).

I didn't believe that modding television sets for RGB were possible until it was done, so how far off would it be for someone to mod signals at a higher frequency?

[maxtherabbit]

Not only would you have to redesign the chassis, you would probably have to rewind the yoke as well.

[BazookaBen]

Not to mention that the stock of PC monitors is much lower than CRT TV's. PC monitors were lighter and thus easier to throw away, so people threw them away.

And even if you could display 240p 60hz on a normal PC CRT, they would look basically the same as 480p 60hz with added scanlines, which looks nothing like a PVM.

[headlesshobbs]

I'm fairly certain that all crt displays have a focus dial that connects from the main board. You can turn that to get thicker 240p lines, but it's not really a suggestion if there's a shock hazard involved and you have to still be able to view the screen in order to see what you're doing.

Just keep in mind when you adjust for one resolution, other resolutions higher or lower will not be as sharp unless you have a specific range in mind.

[LukeEvansSimon]

Not only would you have to redesign the chassis, you would probably have to rewind the yoke as well.

Looking at the schematics for the Weiya arcade CRT chassis, which is tri-sync and supports both 15khz and 31khz, this mod is possible without rewinding the yoke. Instead, there is a separate branch of the horizontal deflection circuit for 15khz and for 31khz. The circuit automatically switches between the subcircuit depending on the frequency.

Modding a 15khz CRT to do 31khz would require a different yoke. The 31khz yokes have lower resistance and inductance than the 15khz yokes. So using a 15khz yoke at 31khz would probably generate too much heat.

[Josh128]

Luke, where did you find the Wei-ya schematic? Can you please share?

Thanks