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Thread: CGA colors as displayed on monochrome TTL monitors

  1. #1

    Default CGA colors as displayed on monochrome TTL monitors

    hey all,

    I'm not a collector of old systems (...yet?...), and it seems like the answer to my question can't easily be found online, so here I am on this fine forum.

    Basically I'd like to know what the 16 CGA colors look like when translated into 16 monochrome shades.

    The setup I'm talking about is a CGA/MDA dual-mode card, driving a TTL monochrome monitor, as was common back in the XT clone days (that was the combination on the first PC I ever used). Most emulators of old PCs don't tackle this kind of thing, so I've starting working on a DOSBox patch myself.. the catch is, I don't know anything about the exact correspondence of each CGA (RGBI) color to the resulting shade of green / grey / amber.

    So this is kind of a mini research/educational project for me. All I've been able to find is this:

    - An image showing 16 colors on a monochrome green monitor
    - A video (maybe the creator posts here? ) of an ATI Small Wonder card displaying CGA graphics on an IBM 5151 - check out 2:31 to 2:47 for the color bars test.

    Interestingly it looks like the colors progressively increase in brightness, within each half of the palette... but this doesn't make much sense compared to the relative brightness of each CGA color. e.g., you'd expect color #4 (red) to appear darker than color #3 (cyan).
    On the monitor in the first photo, color #6 (brown) is darkened, just like on IBM's color CGA monitors - but all other colors just follow the increasing brightness trend. I'd especially like to know if there were any implementations that worked differently.

    I imagine there could've been at least two ways to do the CGA-to-mono conversion... it could either be done by the graphics card (and then sent to any TTL monochrome monitor), or on the monitor itself (if it's dual-mode and accepts color CGA signals). And the exact method could vary between different cards and monitors... so it gets still more confusing.

    Here's how you can help:

    1. If you have a working setup with CGA graphics on a mono TTL monitor - and a few spare minutes:
      Could you post a nice, clear photo of it, showing the 16 color bars? (such as one of IBM's test utilities, or the "color testplate" from Trixter's CGA compatibility tester)
    2. Any technical info on how the color-to-mono conversion is done? (whether in principle, or with a specific card or monitor)
    3. Bonus question: the IBM 5155 PC Portable also had CGA graphics, but its monochrome monitor was composite (rather than TTL). Still, I'd also like to know what the 16 shades looked like on the 5155, since it would show how IBM meant it to look.
      So if you've got a working 5155, could you post a 16 color bar photo? That would be cool too.


    Thank you for your time.

  2. #2

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    My model 25 has a monochrome monitor wired to MCGA, but the input internally is analog, would you still want that to run the color tests?

    EDIT: http://img.photobucket.com/albums/v3...7212011223.jpg
    Last edited by evildragon; July 20th, 2011 at 09:52 PM.
    IBM PS/2 Model 25, NEC V30 8MHz, 640KB RAM, ATI VGA Wonder XL, 2GB SSD, Ethernet, DR DOS 6/GeOS, Xircom PE3 Ethernet

  3. #3

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    Quote Originally Posted by evildragon View Post
    My model 25 has a monochrome monitor wired to MCGA, but the input internally is analog, would you still want that to run the color tests?

    EDIT: http://img.photobucket.com/albums/v3...7212011223.jpg
    Hey, very good picture, and could give me something to compare with - thanks
    but yeah, analog MCGA/VGA works very differently, so it doesn't really give me the answers...

    Anyone with mono-CGA / MGA(?) card and monitor, who could do the same thing?
    (or even with an IBM 5155?)

    cheers!

  4. #4

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    I've got an ATI Small Wonder card like the one in the video, and with the switches set for CGA emulation on a TTL mono display this is what I get:


    (click for bigger)

    The monitor is 18KHz mono only rather than dual-mode, so any conversion must be being done by the card rather than the monitor.

    The card manual doesn't give any details, it just says "colours converted to shades". There's some slight flicker on some of the bars, so maybe it's doing it by toggling them on/off between frames, or by using moire patterns?

  5. #5

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    Here is the IBM 5155, IBM CGA card on the built-in display.

    http://home.earthlink.net/~russellbaker/cgatest.jpg

  6. #6

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    Quote Originally Posted by FishFinger View Post
    The card manual doesn't give any details, it just says "colours converted to shades". There's some slight flicker on some of the bars, so maybe it's doing it by toggling them on/off between frames, or by using moire patterns?
    Yep, the card uses extreme flickering to create shades. On the long persistence phosphor used in the IBM 5151 and various other monitors, you won't even see the slight flickering.

    As for the display, yes, it's not 100% emulated (50Hz vs 60Hz vsync, cursor differences, slightly different font, etc..). I assume it's based on the technology of ATI's fitst card, which purpose was mainly to display CGA graphics to MDA monitors.
    Current systems owned by me:
    Vintage:IBM PC/XT submodel 087 ( 1983 ), [Kon]tiki-100 rev. C (1983), Compaq Portable I ( 1984 ), IBM PC/XT submodel 078 ( 1985 ), IBM PC/XT286 ( ~1986 ), 3x Nintendo Entertainement Systems ( 1987 ).
    Obsolete:Commodore A500 ( ~1990 ), IBM PS/2 model 70/386 type 8570-161 ( 1991 ), Atari Lynx II ( ~1992 ), Generic Intel 486SX PC ( ~1993 ), AT/T Globalyst Pentium w/FDIV bug MB ( 1994 ), Compaq 486DX4 laptop ( ~1995 ).

  7. #7

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    Quote Originally Posted by RJBJR View Post
    Here is the IBM 5155, IBM CGA card on the built-in display.

    http://home.earthlink.net/~russellbaker/cgatest.jpg
    Thanks man. Interesting - looks like a monochrome composite screen really doesn't deal well with color modes... not only does it generate those vertical lines in place of color, but most of the color combinations make the text completely unreadable Guess it's the color-burst signal not being processed and noising things up?


    Quote Originally Posted by FishFinger View Post
    The monitor is 18KHz mono only rather than dual-mode, so any conversion must be being done by the card rather than the monitor.

    The card manual doesn't give any details, it just says "colours converted to shades". There's some slight flicker on some of the bars, so maybe it's doing it by toggling them on/off between frames, or by using moire patterns?
    Guess it has to use some combinations of "video on/off" and "intensity on/off", but yes - sounds like it might be doing some quick toggling of the electron gun on top of that, to produce even more shades. On a white-phosphor monitor that could indeed create some flicker... green screen monitors had much longer phosphor persistence, so the flickering must have been far less noticeable on those.

    Anyway, many thanks for the photo! Looks similar to what I've found so far, so we can at least assume that the ATI generates this "increasing brightness" pattern of shades whenever it's set to drive a 18KHz-only screen, such as yours or the IBM 5151 in the video.

    I guess my next question is: Can anyone produce a similar photo, but from a different setup?

    (a dual mode monochrome screen, receiving CGA-like signal in 15.75 KHz / 200-line mode... this could show what happens when the monitor itself does the conversion and assigns a shade to each color.)

  8. #8

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    If you are interested I can get you photos of a Compaq Portable II (which is a dual mode MDA/CGA card and dual mode green phosphor CRT combination) and of a Portable III (which is a fixed resolution amber colored 640x400 Plasma display that emulates CGA)
    My Vintage computer/blog site
    Searching for a keyboard for a WYSEpc WY-1100.

  9. #9

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    Hi RWallmow,

    Quote Originally Posted by RWallmow View Post
    If you are interested I can get you photos of a Compaq Portable II (which is a dual mode MDA/CGA card and dual mode green phosphor CRT combination) and of a Portable III (which is a fixed resolution amber colored 640x400 Plasma display that emulates CGA)
    that would be nice indeed - especially the Portable II in CGA mode. Not sure how the III's CGA emulation works, but perhaps it could also be relevant to what I'm looking for.

    cheers!

  10. #10

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    - Color bar test on Compaq Portable II
    - Contrast test on Compaq Portable II
    My Vintage computer/blog site
    Searching for a keyboard for a WYSEpc WY-1100.

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