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CGA to SCART adaptor

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    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 15, 2009, 02:30 AM.

    #2
    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 ).

    Comment


      #3
      Yeah, I couldn't think of any way around that, not without making things three times as complicated anyway.

      Comment


        #4
        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):

        Comment


          #5
          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.”

          Comment


            #6
            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

            Comment


              #7
              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.

              Comment


                #8
                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.

                Comment


                  #9
                  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?

                  Comment


                    #10
                    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.

                    Comment


                      #11
                      Originally posted by tomasont View Post
                      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?
                      The TV inputs are 75 ohms internally, so the resistors effectively form a potential divider with the CGA output on one leg, and 75 ohms to ground on the other:

                      You end up with approx 0.7v for high intensity, and 0.5v for low.


                      Originally posted by vwestlife View Post
                      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.
                      There are limitations here too. Pretty much any TV made in the last 15 years will handle 60Hz refresh just fine, but 99% of them won't do NTSC colour, so if you use the CGA composite output you get black and white whether colour is enabled or not. That's actually what made me build this adaptor in the first place - the composite image was good enough, but there was no way for me to get colour from it.

                      Comment


                        #12
                        Originally posted by FishFinger View Post
                        There are limitations here too. Pretty much any TV made in the last 15 years will handle 60Hz refresh just fine, but 99% of them won't do NTSC colour, so if you use the CGA composite output you get black and white whether colour is enabled or not. That's actually what made me build this adaptor in the first place - the composite image was good enough, but there was no way for me to get colour from it.
                        That's because PAL TVs are looking for the color subcarrier at 4.433618 MHz, while the NTSC color subcarrier is at 3.579545 MHz -- generated in the PC by taking the 14.31818 MHz clock crystal and dividing it by 4. If you replaced it with a 17.73447 MHz crystal (if there is such a thing!) then the CGA's composite output would generate so-called "NTSC 4.43", which many modern European TVs will reproduce in color. However, this would also overclock your PC from 4.77 MHz up to 5.91 MHz! (The CPU clock speed is derived from the clock crystal's frequency divided by 3.)

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                          #13
                          Would this actually work with an EGA card, too?
                          And DID somebody find a way to produce dark brown?

                          Also, I'm thinking about making a cable with an additional plug for the composite mode, as my TV should support NTSC - then I could just switch easily between the modes...

                          Comment


                            #14
                            Originally posted by NsMn View Post
                            Would this actually work with an EGA card, too?
                            And DID somebody find a way to produce dark brown?

                            Also, I'm thinking about making a cable with an additional plug for the composite mode, as my TV should support NTSC - then I could just switch easily between the modes...
                            As you see about the age of this thread, it's several months old so I don't think any advancements in the design has been implemented.

                            The color of brown can be implemented, but that will require some transistors/curquits to limit green (to appromaxely 1/2 the color strength in terms of fully on) only if intensity/blue is low and red/green is high. As told earlier, that makes the design qutie a lot more complicated.

                            EGA uses seperate intensity lines for each color, and it should be easy to implement. Just take the CGA version, and instead of connecting one intensity signal to all of the three color lines, connect one seperate signal to each of the three color lines instead. but certanly, you can connect the adapter as it is with one intensity input to an EGA card (you'll only get 16 colors then, though).
                            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 ).

                            Comment


                              #15
                              Originally posted by NsMn View Post
                              Would this actually work with an EGA card, too?
                              And DID somebody find a way to produce dark brown?
                              It should work with EGA with only a small alteration. Instead of connecting the three 1.5K resistors together at pin 6 connect them to each of the secondary RGB pins (2, 6, and 7 iirc)

                              Is the yellow/brown issue a problem with EGA? I thought EGA solved that by changing the palette rather than hardware trickery in the monitor.


                              Originally posted by NsMn View Post
                              Also, I'm thinking about making a cable with an additional plug for the composite mode, as my TV should support NTSC - then I could just switch easily between the modes...
                              I'd double check. Most TVs claim to support NTSC, but it seems they really just mean 60Hz, very few of them seem to actually support NTSC colour.

                              Adding a composite input to the adaptor is a little tricky, as you need to change the voltages on some of the pins. Also some TVs automatically switch to RGB mode if they detect an RGB signal, so you'd need to switch those on/off too. The easiest way would be to either use a separate composite input on the TV if it has one, or a cheap SCART switch with a SCART-composite adaptor if it doesn't.

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