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Thread: [Megadrive/Genesis model 1&2] Dual Crystal PAL/NTSC color fix restoration.

  1. #16
    Video Game Modder Outrunner segasonicfan's Avatar
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    Quote Originally Posted by t0nito View Post
    Is your console crystal modded or original? That is a weird problem as the image is shaking horizontally, dot crawl usually "travels" up and down, not sideways.

    EDIT: Actually I just connected my Megadrive in composite mode (I usually use RGB) and the dot crawl is actually scrolling sideways when in NTSC mode. Your problem may be related, also some TVs deal with chroma better than others.
    This is a Game Gear, so it's modded with a CXA1645 and its own crystal. This is a problem I've encountered for a long time with all my mods- any time I use a TTL 4-pin 3.579 crystal I get this horrible dot crawl/rainbow banding stuff. The image itself isn't shaking horizontally - its rainbow artifacts on all the edges on things that is moving left to right or up and down. I just wish I could make it stationary.

    attached is the kind of crystal I have been using.

    Thanks for the help!!

    -Segasonicfan
    Attached Images Attached Images

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    Those are crystals with internal oscillators, I'm not sure how you adjust them, maybe someone here can help on how to wire a pot for adjusting, but your problem does in fact seem related to chroma timing. I just opened my Megadrive because NTSC was looking horrible, there were huge stripes near the edges and the whole picture was wavy/wobbly looking quite similar to your video, I found that adjusting the crawl to "move" the fastest is the way to make them less visible and more compatible with other equipment. All is well now.

    Though I do use RGB with a chroma killer switch to eliminate the banding completely as here in Europe nearly every colour TV has RGB capability, but I like to have the dual mode composite just in case.
    Last edited by t0nito; 04-09-2015 at 06:05 PM.

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    Video Game Modder Outrunner segasonicfan's Avatar
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    Quote Originally Posted by t0nito View Post
    Those are crystals with internal oscillators, I'm not sure how you adjust them, maybe someone here can help on how to wire a pot for adjusting, but your problem does in fact seem related to chroma timing. I just opened my Megadrive because NTSC was looking horrible, there were huge stripes near the edges and the whole picture was wavy/wobbly looking quite similar to your video, I found that adjusting the crawl to "move" the fastest is the way to make them less visible and more compatible with other equipment. All is well now.

    Though I do use RGB with a chroma killer switch to eliminate the banding completely as here in Europe nearly every colour TV has RGB capability, but I like to have the dual mode composite just in case.
    thanks for the informative reply would you mind posting what u did to adjust the dot crawl on your megadrive? id love to try doing that with my circuit. i also have the PLL oscilatiors for 3.579mhz on hand, so i can built the 7404 circuit with the cap and resistor network. i just want to make sure i know what to build / try first

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    If you look at the previous posts there is a link to my mod.

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    Quote Originally Posted by segasonicfan View Post
    I would LOVE a video showing what the tweaking looks like for fixing the banding. If I could fix that issue I would be in heaven
    Hey, I didn't do a video on HOW to adjust the chroma but I've done a video that shows what to look out for, I'll be uploading it in a few hours.

    EDIT: Video uploaded:



    As you can see at the beginning of the video the dot crawl on PAL 60Hz is nice and stable and on PAL 50Hz you can't even see them, on 0:17 you can see that the dots are drifting slowly on NTSC, I believe that my variable capacitor on my NTSC crystal isn't as good as the PAL one I might replace it some day. Ideally the dots should be as stable as the PAL ones. When you adjust the pots the dots will change their speed and pattern.

    Actually there are two stable patterns, a checker board pattern and a striped pattern when turning the pot you'll find that the striped pattern begins to move faster and faster until it starts to to slowdown again but into a checkered pattern, if you keep turning it will move again until a striped pattern shows up again then back to checked and so on. For my equipment I found that the checkered patterns are the ones that look better and increases compatibility between TVs.

    EDIT 2: I just noted that I contradicted myself when I said that the dots moving the fastest is the way to go, well it still remains true, it so happens that when I posted that I was adjusting on my CRT, and on the CRT the croma dots move so fast that you almost can't see them, the LCD on the other hand due to the way is processes the image it shows the dots as a checkered pattern so when adjust do as follows:

    - CRT: dots moving the fastest until you can't pratically see them
    - LCD: Aim for a stable checkered pattern


    Sorry for the confusion
    Hope this helps and I hope you learned about dot crawl.
    Last edited by t0nito; 04-13-2015 at 04:11 PM.

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    Quote Originally Posted by t0nito View Post
    Hey, I didn't do a video on HOW to adjust the chroma but I've done a video that shows what to look out for, I'll be uploading it in a few hours.

    EDIT: Video uploaded:



    As you can see at the beginning of the video the dot crawl on PAL 60Hz is nice and stable and on PAL 50Hz you can't even see them, on 0:17 you can see that the dots are drifting slowly on NTSC, I believe that my variable capacitor on my NTSC crystal isn't as good as the PAL one I might replace it some day. Ideally the dots should be as stable as the PAL ones. When you adjust the pots the dots will change their speed and pattern.

    Actually there are two stable patterns, a checker board pattern and a striped pattern when turning the pot you'll find that the striped pattern begins to move faster and faster until it starts to to slowdown again but into a checkered pattern, if you keep turning it will move again until a striped pattern shows up again then back to checked and so on. For my equipment I found that the checkered patterns are the ones that look better and increases compatibility between TVs.

    EDIT 2: I just noted that I contradicted myself when I said that the dots moving the fastest is the way to go, well it still remains true, it so happens that when I posted that I was adjusting on my CRT, and on the CRT the croma dots move so fast that you almost can't see them, the LCD on the other hand due to the way is processes the image it shows the dots as a checkered pattern so when adjust do as follows:

    - CRT: dots moving the fastest until you can't pratically see them
    - LCD: Aim for a stable checkered pattern


    Sorry for the confusion
    Hope this helps and I hope you learned about dot crawl.
    can you explain the circuit you used to controll the dot crawl?
    i have a single crystal on my genesis but i just want to control the dot crawl as mine genesis only has the crystal installed in the circuit

    pretty much like this
    SeJIMGs.jpg


    Edit:

    just done this tutorial that real phoenix sent me and dot crawl is completely controlable
    actually i've tweaked it so that it moves so fast that you pratically can't see it, this results on the best picture possible

    http://www.mmmonkey.co.uk/mega-drive...ylfYxu9Zxuqt-o
    Last edited by victorstk; 10-07-2019 at 01:06 AM.

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