VOGONS


First post, by Gustavo

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Hi Everyone!

We all know how the CGA palette has been criticized.
Recently, I acquired a 15-Khz-capable VGA monitor, and have been experimenting with various ideas to connect a CGA card to it.

Eventually, this is the circuit I have come to:

circuitopaleta.png

It has some interesting features:
- three new palettes (white, blue, salmon/rosy; white, orange, blue and light yellow, orange, blue);
- pure white is found in Palette 1 (white, cyan, magenta) and Palette 3 (alternative white, cyan, red);
- colors are somewhat close to the original palette;
- uses only passive components (resistors).

Last edited by Gustavo on 2024-01-24, 18:30. Edited 1 time in total.

Reply 1 of 11, by Gustavo

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Reply 2 of 11, by BitWrangler

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Nice, I have two TVs with VGA inputs, will have to see if they still synch 15khz thru the VGA or not.

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 3 of 11, by Gustavo

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15 kHz is like a lottery. Some very cheap TFT monitors do accept 15 kHz.

This is the one I got:
MONITOR TGT ST17G, 17 POL., TN, HD, 3MS, 75HZ, HDMI, TGT-ST17G-BL01

tgt-st17g-bl01.jpg

Reply 4 of 11, by majinga

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Gustavo wrote on 2024-01-24, 14:53:

...
Eventually, this is the circuit I have come to:
...

That's interesting. I don't know much on the subject.
Why do you choose this particular resistor arrangement?

What's the deal on adapting the CGA signal to VGA?

Reply 5 of 11, by Gustavo

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A CGA card outputs red, green, blue, and intensity signals in TTL voltage. That means +5 volts when each of these signals is active and 0 volt when it is not. There are 16 possible combinations, and so CGA can only display up to 16 colors. A VGA monitor accepts red, green, and blue signals in "analog" form, from 0 volt to 0,7 volt, when each signal is at maximum intensity. So a VGA monitor can generate an "infinitude" of hues.

The resistors are needed to lower the signals voltages (some say that it is not the correct way to do it, because of impedance and such).
Also, the way resistors are linked makes the colors less harsh. Instead of pure red, or pure green, we get more 'pastel' tones. Most circuits use a direct approach to RGB values, so you get regular CGA colors. Please check out the following picture:

circuitoexpl.png

Reply 6 of 11, by majinga

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I see. Thank you.

I still have a question on the resistors arrangement.
I mean:

The RED is the only one with two resistor in parallel so is like to have one 340 ohm resistor to the RED signal.

The GREEN is connected in the same way to the GREEN and BLUE, so is present in the same way on both.

The BLUE seems the most logical, where you have one resistor to the BLUE, 3 to the GREEN, 4 and 1/2 to the RED.
Even if probably on the RED, the signal became to faint, so it is basically 0.

Why this strange arrangement?

Reply 7 of 11, by mkarcher

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The arrangement looks only strange if you look at it the way you do. Assume (even that isn't perfectly true) that the CGA card as the sender keeps the voltage at the RED, GREEN and BLUE constant at 0V and "5V" (will be more like 4V on TTL logic). The signal from the BLUE output will go to the BLUE input, and you are right that there will be some effect from the BLUE output visible at the GREEN output. The effect will not spread past the GREEN output, though, because the CGA card will actively keep the GREEN output at a constant level, whether it gets "low" or "high" feedback from the BLUE output.

An easier way to look at the circuit is thus to just look at the VGA monitor inputs, and see how they are driven: The "blue" input is driven to the average of the GREEN and BLUE outputs. The GREEN input is driven to the average of the GREEN and RED outputs, and the RED input is driven to by the RED output only. The will create false colors, but this is intended by the OP, because the genuine CGA colors don't look that great, and a lot of people are tired of the genuine CGA colors, so this adapter gives CGA graphics a yet unknown (and thus fresh) look.

This circuit is not connecting the "intensity" output of the CGA card, which causes the distinction between "bright" and "normal" colors to disappear. This is not that much an issue in CGA graphics modes, as the 4-color graphics modes support only one kind of "normal" or "bright" colors (except for the "background" color), so you won't see mixtures between the normal and bright in graphics modes. In text modes the loss of 8 colors might be more notable - but in text mode you might not like the false colors anyway. You could modify this adapter to take care of the intensity bit by using 1K resistors instead and adding a 1K resistor from the "INTENSITY" output to each of of the "RED", "GREEN" and "BLUE" VGA monitor intput.

Reply 8 of 11, by Gustavo

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I have tried many other arrangements, but one thing that I needed was pure white to be present in the main palette (white, cyan, and magenta originally).

Notice that when all three signals are high at the CGA card, the VGA connector gets the same value on R, G, and B, giving a white pixel.

Reply 9 of 11, by VileR

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Cute - that palette reminds me of the Apple ][, specifically those games that used HGR mode with the high bit always set (e.g. https://www.mobygames.com/game/23779/the-goon … enshots/apple2/): you get black, orange, sky-blue, and white. 😀

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Reply 10 of 11, by dr.zeissler

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Gustavo wrote on 2024-01-24, 20:20:
15 kHz is like a lottery. Some very cheap TFT monitors do accept 15 kHz. […]
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15 kHz is like a lottery. Some very cheap TFT monitors do accept 15 kHz.

This is the one I got:
MONITOR TGT ST17G, 17 POL., TN, HD, 3MS, 75HZ, HDMI, TGT-ST17G-BL01

tgt-st17g-bl01.jpg

Nec 51vm / 71vm / 1550m / 1970nxp

Retro-Gamer 😀 ...on different machines