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Thread: CGA to SCART adaptor

  1. #1

    Default CGA to SCART adaptor

    This is something I made the other day, and I thought people here might find it handy. It's a little adaptor that lets you connect a CGA card to the SCART socket on a TV.

    Many CGA cards already have a composite output, but the quality is often dubious at best, and they all use NTSC colour, which may not be supported by your TV if you live outside the US.


    (click for big)

    It uses a few resistors to convert the TTL RGBI signals down to 0.7V RGB, and a circuit based on this VGA-SCART adaptor, which uses a 74HC86 (quad XOR) to generate a composite sync signal from the CGA's seperate HSYNC and VSYNC.

    As well as connecting to the CGA port it also needs +5V and +12V, which aren't available on the CGA connector, so it needs a molex connector for plugging into the PSU. (I left this off the diagram for clarity).

    NOTE:
    - Although SCART itself has pins for RGB input it may not be supported on all TVs - mostly only very old, or very cheap/crappy ones though.
    - Some TVs with multiple SCARTs may only support RGB on one of them.
    - The display is still 60Hz, so your TV must support that too (all TVs in the US do, and most non-US TVs made in the last 15 years or so should as well)
    Last edited by FishFinger; October 15th, 2009 at 03:30 AM.

  2. #2

    Default

    Great!

    However, note that color 6 should be brown, not dark yellow as it will appear with this adapter.
    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 ).

  3. #3

    Default

    Yeah, I couldn't think of any way around that, not without making things three times as complicated anyway.

  4. #4

    Default

    I took a few pictures to show the results. These are on a cheap 14" TV.

    Colour Test:


    80-column text:


    80-column text (close-up):

  5. #5

    Default

    CGA on 800x600 ? Wow!!

    Just kidding

    Anyway, great work! Thanks!
    “Thus, we see that one of the obvious origins of human disagreement lies in the use of noises for words.”


  6. #6
    Join Date
    Jul 2008
    Location
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    Default

    Stupid question perhaps, but would a setup like this support the 160x200/16 color NTSC composite hack that is supported in some games, such as older Sierra AGI adventures? Probably not, right?

    http://en.wikipedia.org/wiki/Color_G...o_an_advantage

  7. #7

    Default

    I don't think so.

    According to the wikipedia page 160x200 is really 320x200, but it exploits the smearing effect you get with NTSC composite video to give the illusion of 160x200 with a more varied palette. The adaptor connects to the 9-pin RGB socket, so any effects that rely on composite/ntsc trickery won't work.

    You can always just connect the composite output directly to the TV with an RCA cable though.

  8. #8
    Join Date
    Jul 2008
    Location
    Amsterdam, the Netherlands
    Posts
    325

    Default

    Yeah, I should've figured that.

    It's actually kind of ironic how you have to use a lower quality video interface in order to get better graphics.

  9. #9

    Default

    I'll be the first to admit that my electronics knowledge is pretty rusty, so maybe I'm looking at this wrong. I'm not following how your resistor array works for the RGB signal. It look to me like your RGB signals will be about 2V when intensity is low, and about 5V when intensity is high. What am I missing here?

  10. #10

    Default

    Looks good, but unfortunately SCART is unknown here in North America, so we just have to live with composite. I've found that 80-column text mode is actually readable on a color composite monitor or TV, as long as you keep the CGA in black & white mode. As soon as you turn on the color burst, the image gets unreadably smeared and rainbow-fringed. But 40-column text mode and color graphics modes work perfectly, and like "digger" mentions, many games were designed to take advantage of composite video artifacts in order to produce extra colors on the screen.

    The same applies to other vintage computers as well. The Apple II high-resolution graphics mode is based entirely on composite color artifacting, and many Atari 800/XL/XE games use it in order to produce high-res color as well; if you play these games in an emulator on your PC, they will show up as black & white unless the emulator is able to simulate the artifacting effect.

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