VGA -> CGA... Is it possible?

[Pickelhaube808]

I have an IBM 5153 that I would like to do more with and it is much more portable than the other CRTs I own. While I have converted CGA to VGA before, I haven't really ever seen anything that goes the other way around, except for one sketchy looking adapter that claims to go from VGA to CGA, yet has two HD15 ports instead of an HD15 and a DB9 for CGA out.

Are there even adapters out there that can accept a 30KHz VGA signal and convert it down to a 15KHz CGA output for use on old CGA monitors? Even if they are not commercially available, I wouldn't be against buying parts and assembling one myself (if possible).

 

[Plasma]

The 5153 only has 240 active scanlines and 16 colors. You could try to squeeze more out with interlacing and scaling, but in general it's going to look like crap. Which is probably why such a converter does not exist...

 

[Eudimorphodon]

except for one sketchy looking adapter that claims to go from VGA to CGA, yet has two HD15 ports instead of an HD15 and a DB9 for CGA out.

What you probably found was a device to scale VGA down to 15khz analog RGB. The reason these exist is to convert arcade game cabinets to run off of PC hardware, and for some reason the people selling them insist on conflating 15kHz analog RGB with “CGA”, which is *not* compatible.

In theory you could possibly use one of these devices plus some kind of cobbled together high speed analog to digital converter to display some kind of picture on a 5153, but the conversion to the 16 color RGBI palette would make for some pretty terrible results.

 

[Plasma]

I searched around a bit and found this project. The input is a video stream (not VGA) but the output is CGA. It looks fine when displaying 320x200 at 16 colors, as expected. But when displaying 320x200 at 256 colors, you can see how bad the flicker is with interlacing to get additional colors. Trying to display 640x480 would look even worse.

 

[Timo W.]

Are there even adapters out there that can accept a 30KHz VGA signal and convert it down to a 15KHz CGA output for use on old CGA monitors?

That's not the issue, any VGA-to-SCART converter does that. But CGA is digital/TTL, while VGA is analog. CGA has 16 fixed colors. There is no way to map VGA colors to CGA, since you have no options on the CGA side. It's not about mapping 256 colors to 16. VGA can display 256 colors at once, but these colors are not fixed - you have 16.7 million to choose from. So the end result will always look like crap.

 

[dhau]

You can find some cheap ISA Oak VGA cards (Oak-037, Oak-057 etc.) with both VGA and TTL 9-pin outputs. The card detects the monitor you connect. If you connect VGA, you get VGA, if you connect CGA to 9-pin port, you get low-rez modes of EGA (200 lines): 320x200 16 colors and 640x200. I have Oak-057 and it works fine in 8-bit XT ISA slot.

There are also ATI and Trident cards with the same VGA/TTL outputs.

 

[g4ugm]

Really what you are asking is equivalent to "it possible to look at a UHD tv picture on a 1950's black and white TV" , so possible but probably not useful.
You can go CGA to VGA because VGA has more lines, more colors, an a higher frame rate. Trying to go the other way isn't really usefull...

 

[Eudimorphodon]

You can find some cheap ISA Oak VGA cards (Oak-037, Oak-057 etc.) with both VGA and TTL 9-pin outputs. The card detects the monitor you connect. If you connect VGA, you get VGA, if you connect CGA to 9-pin port, you get low-rez modes of EGA (200 lines): 320x200 16 colors and 640x200.

FWIW, I had an 037 in a 286 back in the day, and my recollection was:

A. There was no autodetect; you had to set some dip switches for which monitor you had, and:

B: If you set the switches for a CGA monitor you were stuck with strict CGA emulation, it wouldn’t let you do the low-res EGA modes. This was incredibly annoying.

Maybe this was fixed on the 057?

 

[dhau]

FWIW, I had an 037 in a 286 back in the day, and my recollection was:

A. There was no autodetect; you had to set some dip switches for which monitor you had, and:

B: If you set the switches for a CGA monitor you were stuck with strict CGA emulation, it wouldn’t let you do the low-res EGA modes. This was incredibly annoying.

Maybe this was fixed on the 057?

Hi, sorry, I didn't know about 037 experience. With 057, I got auto-detection of connected monitor and with CGA monitor, it worked like EGA in 200-line only mode (8x8 text, 320x200 16 color, 640x200 4 color). I didn't mess with any jumpers.

 

[vwestlife]

Irrespective of resolutions and scan rates, the main problem with squeezing higher resolution out of a CGA monitor is the large dot pitch of the CRT. In the early '90s there were some VGA monitors with CGA-grade picture tubes, and the quality of the image was very poor:

 

[dhau]

Hi, sorry, I didn't know about 037 experience. With 057, I got auto-detection of connected monitor and with CGA monitor, it worked like EGA in 200-line only mode (8x8 text, 320x200 16 color, 640x200 4 color). I didn't mess with any jumpers.

I don't have one of those terrible budget VGA monitors from ~1991 with dot pitch of 0.41, but I do have a gloriously terrible Tandy CM5 CGA monitor with dot pitch of 0.51. You can clearly see each individual hole in the shadow mask.

 

[vwestlife]

I don't have one of those terrible budget VGA monitors from ~1991 with dot pitch of 0.41, but I do have a gloriously terrible Tandy CM5 CGA monitor with dot pitch of 0.51. You can clearly see each individual hole in the shadow mask.

It's actually worse than that. Depending on the OEM, the CM-5's dot pitch is either 0.63 or 0.64 mm.

 

[Chuck(G)]

On the other hand, monochrome VGA monitors have superb resolution--but no colors.

 

[NeXT]

Okay I see what you are doing here.
You want to take a high-resolution and high color depth analog VGA signal and downconvert it down to something resembling a digital CGA signal for your monitor, correct?

Doesn't exist...yet. Only recently have I seen converters from HDMI down to modulated RF for ancient TV sets. The fact is at this time due to how uncommon CGA monitors have become most people are going the other direction. I'm sure like the HDMI RF modulator such a day might come where someone will develop such an adapter but for now the only product I'm aware of on the market that took a newer video standard and converted it back to a CGA output was the ATI EGA Wonder which in a select mode will let you display EGA on a CGA monitor.

 

[skate323k137]

I've got nothing for downscaling to true CGA. But the Extron Emotia series as far as I know will downscale HD video, with the caveats mentioned above regarding displays not withstanding.

 

[Eudimorphodon]

Taking a devil’s advocate approach here, well, I guess if you don’t mind hacking the 5153 it probably wouldn’t be too difficult to bypass the TTL RGBI decoders and convert it into an analog RGB monitor. It wouldn’t surprise me if at some point in the later 1980’s some computer magazine already published plans for such a modification, in order to allow an old CGA monitor to be repurposed for use with something like a Commodore Amiga.

Rip out the TTL circuitry and one of those arcade scan converters will work... as well as they can, but I’m pretty sure those converters are themselves limited to specific video modes and are intended to be used with software like AdvanceMAME.

In short, well, I know CRT VGA monitors are getting rare-ish, but it still makes way more sense to look for a decent one of those than wrecking an even rarer CGA monitor just to end up with at best a barely better picture than you could get from an old TV with an S-Video port.

 

[vwestlife]

If you want a monitor that can display VGA graphics but matches the family look of the original PC, XT, and AT, it used to be a popular modification to take a surplus IBM 5175 Professional Graphics Display, which was designed for the enormously expensive Professional Graphics Controller (often incorrectly referred to as "PGA") and convert it to work with VGA:

Displaying VGA on a stock IBM 5175 PGA monitor? \ VOGONS

www.vogons.org

In 1993, Computer Reset was selling VGA-modified 5175 monitors for $199. But good luck finding one today...