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Thread: 32x S-video mod - shows ringing when a switch is added to enable/disable composite

  1. #1
    WCPO Agent CrossBow's Avatar
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    Default 32x S-video mod - shows ringing when a switch is added to enable/disable composite

    So I'm sure most of us here know of the simple 3904 transistor mod to get s-video from the 32x. Just requires a 3904 with the base attached to the luma output pin directly, remove R45 to disable the composite circuit further down the board, Add in +5 to the 3904 collector and attach a 75Ω resistor off the emitter back to the Luma out on your s-video jack. Chroma is wired directly off a point before the rest of the 32x straight to the s-video chroma out pin and tied them to ground somewhere.

    Well, you have to disable the composite side of things or else you get a very smeary looking s-video output as you can see here:

    https://drive.google.com/file/d/10ov...ew?usp=sharing

    If you disable the composite from the luma side of things, then you get a much more clear s-video picture. Quite clear in fact:

    https://drive.google.com/file/d/14BQ...ew?usp=sharing

    However, if you disable the composite side of things, then you find out that you can NO longer use HDretrovision cables from a 32x because HDR apparently decided to use composite sync in their cable circuitry instead of C-Sync. And this causes a problem for the client whom this 32x belongs to. Because my client wants to be able to use HDRetrovision cables and s-video at the same time. That means you have to leave the composite in place for it to all work. Now as a compromise, I was trying to wire in a switch to essentially disable the connection to the rest of the composite off the luma output pin. This kinda works and I will explain. It doesn't matter what kind of switch I use, as soon as I attach a wire off the luma pin that is more than 1inch in length, I then get bad ringing on the s-video output that can really only be seen when you have graphics against a lighter contrasting background. Notice the high score screen on Afterburner for the 32x here in s-video with the composite side disabled via the switch so that only the 3904 is getting the luma signal:

    https://drive.google.com/file/d/1P8S...ew?usp=sharing

    In case it is hard to see, I zoomed in on it in this pic here where you really see how the text trails off towards the right:

    https://drive.google.com/file/d/1Ifg...ew?usp=sharing

    Now, I have confirmed that standard RGB works and looks great with the composite disabled but that uses C-sync in that case and my client only has the ability to use component to their displays as they apparently don't want to invest into RGB stuff to go that route. So yeah, RGB via C-sync works without the composite of course and that gives you the ability to have clear s-video and RGB. But I can't find a simple way of having composite and s-video from the default encoder and circuitry within the 32x.

    Anyone have any ideas? Or is this something where it would require having to wire in a separate 1145 or something off a spare Genesis on a separate perf board to extra s-video from? I've tried to add diodes off the switch to further isolate the signals from back feeding and nothing gets rid of the ringing. Adding in caps just softens the image more and you still have the ringing like effect. I even attached the base directly to the Luma pin, and then soldered my wire to enable/disable the signal to the composite off the pin and that still results in the ringing on s-video output. Any amount of wire seems to introduce the ringing to the s-video and acts like an antenna that is being picked up on the base side of the transistor. If I hadn't already added the s-video output to this 32x months ago, I would have simply told my client this isn't possible currently and left it at that. But again this 32x was sent back to me to try and find a solution that wouldn't cost a ton from the client.
    Last edited by CrossBow; 02-19-2021 at 10:25 AM.

  2. #2
    WCPO Agent CrossBow's Avatar
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    Okay, so I've found two solutions to this. One better than the other in most cases.

    Solution 1:- Just solder a jumper wire from the composite input from the Genesis on the 32x AV in to the composite out on the 32x AV out. Now of course this will only give you the composite image from the Genesis doing this, but the sync in the composite is all you are looking for so it works fine in this case. However, if you are using a model 1 genesis, then it also brings back something the 32x cleans up somewhat. And that is the jail bars because they are present on that composite output from the Genesis and carry over. So that takes us to the next solution.

    Solution 2?:- is to solder in a 470Ω resistor off the c-sync from the 32s AV in off the genesis along with a 220f cap and attach that to the composite out on the 32x AV out. That allows us to just use the sync from the Genesis in a manner the HDRetrovision cables will accept and limits the jail bars. They are still there of course but not as pronounced as they were when using the composite straight off the genesis.

    So, if you are in the same situation where you need s-video from a 32x and HDR cables working, you might look into one of these solutions to get your going.

  3. #3
    Video Game Modder Outrunner segasonicfan's Avatar
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    Quote Originally Posted by CrossBow View Post
    Okay, so I've found two solutions to this. One better than the other in most cases.

    Solution 1:- Just solder a jumper wire from the composite input from the Genesis on the 32x AV in to the composite out on the 32x AV out. Now of course this will only give you the composite image from the Genesis doing this, but the sync in the composite is all you are looking for so it works fine in this case. However, if you are using a model 1 genesis, then it also brings back something the 32x cleans up somewhat. And that is the jail bars because they are present on that composite output from the Genesis and carry over. So that takes us to the next solution.

    Solution 2?:- is to solder in a 470Ω resistor off the c-sync from the 32s AV in off the genesis along with a 220f cap and attach that to the composite out on the 32x AV out. That allows us to just use the sync from the Genesis in a manner the HDRetrovision cables will accept and limits the jail bars. They are still there of course but not as pronounced as they were when using the composite straight off the genesis.

    So, if you are in the same situation where you need s-video from a 32x and HDR cables working, you might look into one of these solutions to get your going.
    Very cool, thanks for documenting this! Someone could also do the extra mile and build a buffer circuit to double the gain of the CVBS. I've never understood why any adapters use composite video to strip sync, when a C sync is readily available..

    You also might want to check out my YouTube channel if you're interested in this upgrade. I go into some exhaustive detail with schematics on how the 32x s video signal works.
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    WCPO Agent CrossBow's Avatar
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    Quote Originally Posted by segasonicfan View Post
    Very cool, thanks for documenting this! Someone could also do the extra mile and build a buffer circuit to double the gain of the CVBS. I've never understood why any adapters use composite video to strip sync, when a C sync is readily available..

    You also might want to check out my YouTube channel if you're interested in this upgrade. I go into some exhaustive detail with schematics on how the 32x s video signal works.
    I thought it might have been your vid I used to do this for my client?

    When I did this on my personal 32x years ago, I didn't care about any other outputs at all and just wanted the s-video output. So I removed the original 9-pin AV out on mine and replaced it with a panel PCB mount s-video output jack that would fit in the same holes and cut the other traces that weren't used. Since I had RCAs coming off the 1134 in the Genesis already on the back of my Genesis, the audio out from the 32x wasn't needed since that backfeeds through the Genesis amp circuits anyway.

    This was the first time I had been requested to do the 32x s-vid mod for a client. I knew at the time they were planning to use the s-video and HDR cables but I stupidly thought that HDR was using c-sync from the Genesis so I didn't think anything of it when I disabled the jumper resistor off the luma output pin that goes to the composite circuit on the 32x. I figured since my RGB Scart cables were still working from the AV out without the composite, that the client was golden. It wasn't until over 6 months later they contacted me to tell me that their HDR cables weren't working and so I got to looking into it. That is when I found out that HDR is actually using the composite video for sync on both their Genesis, and SNES cables. Doesn't make sense to me why they are doing this unless it was for some sort of increased compatibility between variant systems?

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