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EGA with OSSC is a big YES

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First post, by wbahnassi

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Hi all,

So I got this late EGA card (Gemini EGA Ultra GC 201) for my TurboXT, but I don't own any EGA nor CGA monitors, nor did I intend to fork a fortune on one.

I decided to build an EGA-to-VGA cable using resistors to bring down TTL to analog VGA voltage range. Of course my LCD monitor couldn't fathom the horizontal scan rate, so it just refused to display anything.

However, I routed the signal to my OSSC's VGA input, and BAM! Picture shows up! With some tweaking on the OSSC, I managed to get REALLY nice results... matching my memories of how my EGA monitor from late 80's looked like.

The OSSC's ability to add scanlines brings a key aspect of EGA that can't be achieved if I fed my custom cable to a VGA CRT (assuming it can handle the frequency ranges).

One behavior I couldn't explain well though, is that even though DOS text mode looks excellent, games that switch to graphics mode are showing with some small green tint. I programmed the OSSC to offset green under EGA graphics mode.. so now all works great.. but I'm not sure why graphics mode (most old games) have this behavior to start with. I'm doubting my cable, but I used the same type of resistor for each of the color channels (1K Ohm for low bit and 500 Ohm for high bit). Not sure why this green shift is occuring.

Anyways, now I leave you with cable photos in this post, and some screen captures in the next post.

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Turbo XT 12MHz, 8-bit VGA, Dual 360K drives
Intel 386 DX-33, Speedstar 24X, SB 1.5, 1x CD
Intel 486 DX2-66, CL5428 VLB, SBPro 2, 2x CD
Intel Pentium 90, Matrox Millenium 2, SB16, 4x CD
HP Z400, Xeon 3.46GHz, YMF-744, Voodoo3, RTX2080Ti

Reply 1 of 6, by wbahnassi

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Some screen shots of the setup in action

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Turbo XT 12MHz, 8-bit VGA, Dual 360K drives
Intel 386 DX-33, Speedstar 24X, SB 1.5, 1x CD
Intel 486 DX2-66, CL5428 VLB, SBPro 2, 2x CD
Intel Pentium 90, Matrox Millenium 2, SB16, 4x CD
HP Z400, Xeon 3.46GHz, YMF-744, Voodoo3, RTX2080Ti

Reply 2 of 6, by wbahnassi

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Replying to self here. The reason of the green tint is because such a circuit doesn't handle CGA's brown properly. A proper circuit would require active components, thus a power supply. This cable won't be able to do it by itself.

HOWEVER, I don't see why the OSSC can't handle the CGA brown color logic itself. Unfortunately I don't know how to customize the OSSC's core logic, otherwise I would've coded this feature. Any OSSC experts around who can comment on the feasibility of this idea?

Turbo XT 12MHz, 8-bit VGA, Dual 360K drives
Intel 386 DX-33, Speedstar 24X, SB 1.5, 1x CD
Intel 486 DX2-66, CL5428 VLB, SBPro 2, 2x CD
Intel Pentium 90, Matrox Millenium 2, SB16, 4x CD
HP Z400, Xeon 3.46GHz, YMF-744, Voodoo3, RTX2080Ti

Reply 3 of 6, by wbahnassi

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Feature requested:
https://videogameperfection.com/forums/topic/ … fix/#post-62847

Matt seems excited for it, but I'm disappointed that it wouldn't fit in the classical OSSC. I don't have an OSSC Pro... but hey, you never know 😅

Turbo XT 12MHz, 8-bit VGA, Dual 360K drives
Intel 386 DX-33, Speedstar 24X, SB 1.5, 1x CD
Intel 486 DX2-66, CL5428 VLB, SBPro 2, 2x CD
Intel Pentium 90, Matrox Millenium 2, SB16, 4x CD
HP Z400, Xeon 3.46GHz, YMF-744, Voodoo3, RTX2080Ti

Reply 4 of 6, by willinliv

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Looking to do similar. Is the circuit you used for the cable similar to the one described at this post where you used 1k / 470k resistors on the msb and LSB EGA end to bring together for the single analogue RGB pins on the VGA side? Thanks for posting

Reply 5 of 6, by wbahnassi

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Yes, that's what I did.

If OSSC adds support for the brown-fix via a "compare-replace color" option, this passive cable would be just enough.

Until then though, it's possible to use Necroware's active converter circuit and connect it to the OSSC to have it work with correct CGA colors on any modern monitor. But really, this kind of pixel processing is OSSC's domain, so I'm really hoping it gets added.

The passive cable is also capable of supporting MDA/Hercules very nicely. Just needs slight changes to the wiring, then OSSC picks up the rest. It can even add any kind of tint you like to simulate green or amber phosphor monitors. Here is my test of such setup.

Turbo XT 12MHz, 8-bit VGA, Dual 360K drives
Intel 386 DX-33, Speedstar 24X, SB 1.5, 1x CD
Intel 486 DX2-66, CL5428 VLB, SBPro 2, 2x CD
Intel Pentium 90, Matrox Millenium 2, SB16, 4x CD
HP Z400, Xeon 3.46GHz, YMF-744, Voodoo3, RTX2080Ti

Reply 6 of 6, by butjer1010

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Looks awesome, but i bought MCE2VGA (now it is MCE2HDMI) from serdashop, and it looks also fantastic. I didn't have this "greenish" color problem. Now i don't have it anymore, 'cause i managed to find EGA monitor..... aaaaah, that brought me to early nineties..... Wooow, when i remember first color monitor i saw at my friend 386 DX40....... I had 386SX 33, with MD display, but on VGA connector.... I was so jealous on EGA 😀
Now You forced me to go to attic to turn on Monkey Island on EGA....

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