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CGA <-> VGA converters: (how) do they work?

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ropersonline

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During some curious googling, I've run across this ad for a cheap CGA <-> VGA converter:

http://www.ebay.com/itm/121805953708

I actually have both CGA and VGA monitors, but I am curious. Do converters like the above one work? And if so, how? I'm especially curious about the claimed bidirectionality, about the claimed possibility to connect a VGA card to a CGA monitor (whose built-in resolution is lower). How would that work? Would it work on the basis that you pinkie promise to only run software that uses 200-line modes with the VGA card, and otherwise: puffs of smoke?

Could I order these and connect my CGA card to a VGA monitor or connect a VGA output to my CGA monitor? Is it safe to try this, even with software that may run in non-CGA-ish modes?
What about the special handling of the colour brown with RGBI CGA?

Does anyone know anything about this?
 
When those eBay sellers say "CGA" what they really mean is analog 15 kHz RGB video, commonly used by arcade games. It is not the same as the actual CGA used by computers, which is digital (TTL) 15 kHz RGBI video.

That cheap adapter has no active components and does nothing to convert the signal. All it does is adapt it to a VGA-style connector, and maybe use the trimpots to reduce the video signal levels a little bit. You'd still need a monitor or TV set capable of displaying 15 kHz video.
 
vwestlife said:
When those eBay sellers say "CGA" what they really mean is analog 15 kHz RGB video, commonly used by arcade games. It is not the same as the actual CGA used by computers, which is digital (TTL) 15 kHz RGBI video.

That cheap adapter has no active components and does nothing to convert the signal. All it does is adapt it to a VGA-style connector, and maybe use the trimpots to reduce the video signal levels a little bit. You'd still need a monitor or TV set capable of displaying 15 kHz video.
Click to expand...

Thank you! :)

So basically, this means that the CGA card --> VGA monitor is only possible for certain monitors, and the VGA card --> CGA monitor claim is flat out false?
(Or is there still some limited mode in which a VGA card --> CGA monitor setup could sort of work?)
 
You may be interested in this thread:
http://www.vintage-computer.com/vcforum/showthread.php?40965-Commodore-128-Video-DAC-Board-Available

Where it appears to me that with the CGA converter you've linked to on ebay PLUS the adapter described in this link, would give you the CGA->VGA you're looking for. Someone on the PCjr forum has used this to get their PCjr to display on a flat panel VGA monitor:
http://www.brutman.com/forums/viewtopic.php?f=1&t=484

This is something I'd like too. All the pieces are there, we just have to put them together.
 
hargle said:
Where it appears to me that with the CGA converter you've linked to on ebay PLUS the adapter described in this link, would give you the CGA->VGA you're looking for.
Click to expand...

Actually it won't do anything with that adapter in the eBay link in this thread; that DAC board is for use with one of these cheap and cheerful video scaler boards. And they do indeed work with CGA cards; the only thing they *might* not do quite correctly is that hack that real IBM CGA monitors have in them to turn "dark yellow" into a brown color. It's actually sort of possible to do the same thing with just a set of resistors. (IE, see how they've tied the Intensity line to the three color lines; you can ignore the parts relating to the sync signals.) I know someone that banged out a simple board using a schematic like that for his C128 to work with one of those scaler boards and it allegedly works okay. (I actually have a copy of it but I don't at the moment have anything with RGBI out to try it on; I use the scaler board with an Apple IIgs, an example of a computer with CGA scan rate *analog* output.)

As to going the other way, displaying VGA on a CGA monitor. it should in principle be possible with some ugly software and hardware hacks. It's possible to program a VGA card to output CGA (NTSC) compatible interlaced video. Years ago I played with the monochrome composite compatible hardware setup, which again uses just a few resistors. (In addition to the Linux-related stuff there used to be a little DOS program you could run that would set up compatible scan rates, although it could be defeated by any program that tweaked the VGA registers directly.) Of course, for CGA you'll want color, and for that you'd have to design some sort of 1 bit Analog-to-Digital converter. I think we had a thread about this idea at some point not that long ago, I don't think it went anywhere. The simplest version would dither everything down to just 8 colors, which would probably make horrible hash out of most VGA displays. Doing better would require some sort of lookup table and other digital wizardy.
 
hargle said:
Someone on the PCjr forum has used this to get their PCjr to display on a flat panel VGA monitor:
http://www.brutman.com/forums/viewtopic.php?f=1&t=484

This is something I'd like too. All the pieces are there, we just have to put them together.
Click to expand...

Wow, even the brown looks correct.
Is this because the PCjr takes care of that onboard, or does your adapter/converter actually emulate the CGA brown adjustment monitor feature (like in a proper RGBI CGA monitor, where the monitor is supposed to tweak color 6)?
 
Eudimorphodon said:
Actually it won't do anything with that adapter in the eBay link in this thread; that DAC board is for use with one of these cheap and cheerful video scaler boards. And they do indeed work with CGA cards; the only thing they *might* not do quite correctly is that hack that real IBM CGA monitors have in them to turn "dark yellow" into a brown color. It's actually sort of possible to do the same thing with just a set of resistors. (IE, see how they've tied the Intensity line to the three color lines; you can ignore the parts relating to the sync signals.) I know someone that banged out a simple board using a schematic like that for his C128 to work with one of those scaler boards and it allegedly works okay. (I actually have a copy of it but I don't at the moment have anything with RGBI out to try it on; I use the scaler board with an Apple IIgs, an example of a computer with CGA scan rate *analog* output.)

Actually, that converter won't work standalone to convert to CGA. It is designed for analog CGA, whereas PC's use TTL CGA. The C128 converter works in tandem with a board like you listed to convert to VGA. I have such a setup and it works nicely. I have not paid attention the color problem you mention, so I can't say if that is correct or not...but is does convert, which is awesome, especially when using an old Tandy such that you can keep the original Tandy CGA graphics.

Wesley
Click to expand...
 
wesleyfurr said:
Actually, that converter won't work standalone to convert to CGA. It is designed for analog CGA, whereas PC's use TTL CGA.
Click to expand...

I think you mean 'RGB' instead of 'CGA'. CGA uses RGBI, as in: RGB and an extra intensity bit. So you have to translate the high and low intensity RGB signals to analog ones, where yellow/brown is the tricky part (this is hardwired in a TTL CGA monitor. A C128 monitor is expected to do the same... I use a Commodore 1084S monitor myself, as a CGA display, and it has proper brown, so it would do the same on the C128, which it was meant for).
 
wesleyfurr said:
Actually, that converter won't work standalone to convert to CGA. It is designed for analog CGA, whereas PC's use TTL CGA. The C128 converter works in tandem with a board like you listed to convert to VGA.
Click to expand...

But that's what I said. ;) (The first part was clarifying that the simple pinout converter dingus the OP linked to wouldn't do the job followed by "The DAC board is for use *with* (one of those cheap scaler boards)". It's the DAC board that does the RGBI to RGB conversion but you need the scaler to take care of the scan frequency issues.

Based on the evidence of the PCjr screenshots in the link above it does indeed look like that DAC board with the IC's handles the brown correction properly. I wish I had something with RGBI output lying around so I could try the simple resistor-based converter I have kicking around and see what a non-corrected conversion look like.

One "interesting" experiment you could do with that DAC board would be to see if it could work to let you display RGBI on one of the rare NTSC-frequency RGB Analog monitors there are kicking around. That would include, I guess, the monitor Apple sold for the IIgs and that monitor Tandy had for the CoCo3. (I know the magazines usually recommended that people buy a different monitor for use with the CoCo3 for the very reason the... CM-8 *only* supported analog RGB while there were several competing monitors at about the same price that could do RGB, RGBI, and Composite, and the composite input in particular was useful for backwards compatibility with CoCo games using artifact colors.)
 
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Eudimorphodon said:
That would include, I guess, the monitor Apple sold for the IIgs and that monitor Tandy had for the CoCo3.
Click to expand...

Also the Atari SC1224 monitor for the ST series, and I believe several Amiga monitors only supported analog RGB, although they were quite rare amongst the myriad Commodore/Amiga-branded monitors that also supported digital RGBI, luma/chroma, and/or composite video.
 
Eudimorphodon said:
But that's what I said. ;) (The first part was clarifying that the simple pinout converter dingus the OP linked to wouldn't do the job followed by "The DAC board is for use *with* (one of those cheap scaler boards)". It's the DAC board that does the RGBI to RGB conversion but you need the scaler to take care of the scan frequency issues.

Based on the evidence of the PCjr screenshots in the link above it does indeed look like that DAC board with the IC's handles the brown correction properly. I wish I had something with RGBI output lying around so I could try the simple resistor-based converter I have kicking around and see what a non-corrected conversion look like.

One "interesting" experiment you could do with that DAC board would be to see if it could work to let you display RGBI on one of the rare NTSC-frequency RGB Analog monitors there are kicking around. That would include, I guess, the monitor Apple sold for the IIgs and that monitor Tandy had for the CoCo3. (I know the magazines usually recommended that people buy a different monitor for use with the CoCo3 for the very reason the... CM-8 *only* supported analog RGB while there were several competing monitors at about the same price that could do RGB, RGBI, and Composite, and the composite input in particular was useful for backwards compatibility with CoCo games using artifact colors.)
Click to expand...

Doh...sorry...apparently I wasn't reading closely enough! :)

I have read that the GBS-8220 will work for the IIgs, though I gather the image quality isn't the greatest. I've not tried it yet, though I should, as I have the 8220 I've used with the DAC. Very intersting about the CoCo3...I have one here, I'll have to look into that as well. Apparently I didn't look closely enough, as I thought it just had RF or composite video out...

Wesley
 
wesleyfurr said:
I have read that the GBS-8220 will work for the IIgs, though I gather the image quality isn't the greatest.
Click to expand...

I'm not sure what model my scaler is (probably the same or similar) but it works okay with the IIgs. Game programs looks fine, anyway. The GS-OS desktop kind of looks like *ss, but it looks even worse on composite so... is what it is, I guess. (It might look better on a CRT VGA monitor, but with an LCD that checkerboard background is essentially getting non-linearly scaled twice.)

Very intersting about the CoCo3...I have one here, I'll have to look into that as well. Apparently I didn't look closely enough, as I thought it just had RF or composite video out...
Click to expand...

The RGB comes out that mysterious 10 pin connector in the middle of the bottom of the unit. ;) I haven't tried mine on the scaler yet either, mostly because all I have is a pair of bare 128k CoCo3s; no drives, no software, nuttin, so I haven't played with them much.
 
Eudimorphodon said:
I'm not sure what model my scaler is (probably the same or similar) but it works okay with the IIgs. Game programs looks fine, anyway. The GS-OS desktop kind of looks like *ss, but it looks even worse on composite so... is what it is, I guess. (It might look better on a CRT VGA monitor, but with an LCD that checkerboard background is essentially getting non-linearly scaled twice.)

The RGB comes out that mysterious 10 pin connector in the middle of the bottom of the unit. ;) I haven't tried mine on the scaler yet either, mostly because all I have is a pair of bare 128k CoCo3s; no drives, no software, nuttin, so I haven't played with them much.
Click to expand...

Sounds about right from what I've read and seen...not great quality in GS/OS. I'll have to look at that connector, thanks for the tip. I've actually got two CoCo3's...one new looking in the box. Have only played with a few cartridge games that came with one of them, though I do actually have a floppy drive...haven't messed with it yet. Too many fun vintage computers, not enough hours in the days. :)

Wesley
 
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