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CGA convertion.

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per

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Is it possible to build a CGA to VGA converter by using some volt regulators and transistors?

I guess the output from the CGA card is 5v, and I know the input to the VGA monitor is about 1v... Is this right?

Is there any differences in the pattern of the sync signals (or volt diffrerences)?

I do have an old VGA monitor that doesn't care about the v sync/h sync... I guess I can run 16KHz*60Hz on it.

Can somebody tell me if it is possible to convert from CGA to VGA this way?
 
Google is your friend.

CGA is digital TTL; VGA is analog RGB. It is normally not easy to convert these.

There are adapters that have been made, primarily from arcade suppliers, that do this; this is because a lot of old arcade machines use CGA/EGA signals. Use Google a bit and you'll find them, although one drawback is that color index #6 will most likely be yellow and not brown (because CGA monitors have special circuitry in them to specifically darken color index #6).
 
yeah, you're basically out of luck unless find some sort of rare converter hardware. imo, the best/cheapest way to do this kind of thing would be to find a CGA board that has an RCA jack on it for TV-out then use some little 15 or 17 inch TV if you have one sitting around.

trying to hook up a CGA/EGA card's TTL output to an analog VGA monitor is roughly equivalent to trying to connect your DVD player's SPDIF output coax to a regular analog audio line-in on the back of your stereo receiver.
 
Back in the day, "scan doublers" were popular to let Amiga users display their CGA-like 640x200, 15.7 kHz video on a VGA monitor. However, the Amiga's output was analog RGB, the same as VGA uses, so that's what allowed the scan doubler to work -- it just doubled the scan rate from 15.7 kHz to 31.5 kHz, which is what VGA monitors are designed to accept.

On PCs, however, MDA/Hercules, CGA, and EGA are all digital RGB, which will not work with a VGA monitor (analog RGB), even with a scan doubler. :(
 
Uh, is it really as much rocket science as you pretend it to be?!?

I've built my own cable to convert the TTL signal from an Acorn BBC to display on the analog RGB input on a Commodore 1084, and am in the process of making another cable to get SCART. Sure, in those cases the sync rates are the same, only the signal strength differs. But as Per pointed out, his monitor can sync to almost any frequency, so it would "only" be a matter of bringing down the voltage levels.

The crude and perhaps not optimal solution seems to be using voltage dividers made up of resistors in the 200-500 Ohm range. You may have an issue if CGA outputs only one type of sync and VGA needs two; I haven't looked up that. There are more sophisticated solutions using resistor-capacitor (R-C) combinations as well.

Search for "bbc micro scart" and you will get some hits that might give you some ideas.
 
The Commodore 1084 had the TV scanrate that CGA outputs, so no doubler was needed.

The OP wants to use a VGA monitor, and will likely need it as they usually only sync down as low as 30KHz (or was it 31KHz?)
 
I'm with mr Carlsson on this one, but you might need a line driver/transistors to buffer the ttl if you don't want it very very dim, as the vga monitor has 75ohm input impedance that'll look like a short to ground as far as the ttl outputs are concerned!
 
Doesn't all TVs and monitors - RGB, CGA, VGA or otherwise - have 75 ohm impedance that the voltage dividers take advantage of?

Again, I repeat what Per wrote:
I do have an old VGA monitor that doesn't care about the v sync/h sync... I guess I can run 16KHz*60Hz on it.
Click to expand...

Perhaps he is mistaken, that there are no such multisync VGA monitors but assuming he knows what he is talking about, no scan doubler or other more advanced electronics would have to be involved. It is true that if you consider this discussion for general purpose, those 99% VGA monitors who are not able to display a signal at CGA sync rates, it won't work easily.
 
I'm posting some info about the monitor:

It is manufacted by Chun Yun Electronics (Nowdays, they're called Albatron) in september 1992. On the front it is labbeled "Chun 35.5K L.R.". The screen size/position/brightness/contrast is changeable by scrolling some wheels on the underside of the frame. Swiching it on/off is done with a toggle swich nex to the screen control weels.

I have added some screenshots:
1.jpg 1024*768 (32 bit color)
2.jpg 800*600 (32 bit color)
3.jpg 1152*864 (32 bit color)

(Note that the higher resolutions that where not avalible when the screen was made does not return in an error message; the monitor is actually trying to display it. that's why I think it doesn't care about the sync frequency)

*Edit*
The monitor is about 13 inch diagonal.
 
Last edited:
Great Hierophant said:
You could get an early NEC Multisync, they support both CGA/EGA and VGA sync rates. Whether they also support both types of connectors is not known to me.
Click to expand...

The NEC MultiSync 2a or 3d both support CGA/EGA/VGA but you need
an adapter to adapt the 9->15 pin connectors. Probably would need to
build this adapter yourself.
 
per said:
(Note that the higher resolutions that where not avalible when the screen was made does not return in an error message; the monitor is actually trying to display it. that's why I think it doesn't care about the sync frequency)
Click to expand...
That's because the adjustment controls are knobs. Old monitors will try to display any kind of signal you can throw at them, up to and including burning out the monitor's circuitry if it's way beyond what it can handle!

BTW, in my experience, CGA looks best on a real CGA monitor. My Mitsubishi Diamond Scan can display CGA, just like an older NEC Multisync, but you can see a gap between each scan line, which makes the image look crude and gritty. On my Tandy CM-11, a real CGA(-only) monitor, there are no gaps between the scan lines, making the image look much better.
 
vwestlife said:
BTW, in my experience, CGA looks best on a real CGA monitor. My Mitsubishi Diamond Scan can display CGA, just like an older NEC Multisync, but you can see a gap between each scan line, which makes the image look crude and gritty. On my Tandy CM-11, a real CGA(-only) monitor, there are no gaps between the scan lines, making the image look much better.
Click to expand...

Well, I'm only thinking of this as an option if I get an American card (with NTSC composite output)... And I really don't know anybody around in my area that has a real CGA monitor...
 
For 200 line modes, it should be relatively easy to convert. For each line, double it. Voltage conversion can be done by a lookup table. For 350 line modes, a larger conversion table (64 compared with 16) would be needed. The circuitry would have to add longer blanking intervals (50 dummy lines) on either end of the active picture. (I assume this is what VGA does when emulating 350-line modes.)
 
Great Hierophant said:
For 200 line modes, it should be relatively easy to convert. For each line, double it. Voltage conversion can be done by a lookup table. For 350 line modes, a larger conversion table (64 compared with 16) would be needed. The circuitry would have to add longer blanking intervals (50 dummy lines) on either end of the active picture. (I assume this is what VGA does when emulating 350-line modes.)
Click to expand...
Real VGA doesn't "emulate" 350-line mode. It actually outputs 350 scan lines to the monitor. VGA does scan-double 200-line graphics modes (CGA, low-res EGA, MCGA) up to 400 scan lines. Thus, a standard VGA monitor has to handle three different amounts of scan lines: 350, 400, and 480. All three VGA modes have a horizontal frequency of 31.5 kHz, versus 21.8 kHz for EGA color, 18.4 kHz for MDA/Hercules and EGA monochrome, and 15.7 kHz for CGA.

There were also some non-IBM intermediate standards, like 26.4 kHz for Tandy 2000 color graphics and 22.25 kHz for classic Macintosh monochrome video (usually not seen on an external monitor).
 
vwestlife said:
BTW, in my experience, CGA looks best on a real CGA monitor. My Mitsubishi Diamond Scan can display CGA, just like an older NEC Multisync, but you can see a gap between each scan line, which makes the image look crude and gritty. On my Tandy CM-11, a real CGA(-only) monitor, there are no gaps between the scan lines, making the image look much better.
Click to expand...

CGA looks best on an IBM PGC/PGD combo (Professional Graphics).
This adapter has an emulated CGA mode that looks great. The adapter
actually has its own microprocessor (8088 ) so its pretty fast too :)
Too bad they didnt also emulate EGA on this card :-(

More info on the card can be found here :

http://en.wikipedia.org/wiki/Professional_Graphics_Controller
 
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