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PostPosted: Sat Aug 22, 2015 7:52 am 
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edit: I tested the sync combiner RGB cable on another Dreamcast and it works perfectly. But I still have no green signal on the first Dreamcast, and I can't determine why. Jump to current post here: viewtopic.php?p=153976#p153976

Hey all,

As you might know, the Dreamcast can output 240p/480i over RGB SCART and 480p over VGA. So people on the net have posted sync combiners that allow you to use the VGA signal over SCART, and force 480p over RGB with composite sync. I decided to try the following:

http://dcemulation.org/?title=Dreamcast ... ster_Cable

I'm using the XRGB mini Framemeister. When I tested it, the image is stable on the screen. So that suggests to me that the new composite sync is working as it should. The problem I have is that the red level is WAY too intense.

I snapped a couple of crappy cell phone pics. My cell phone camera picked up on the red a lot, it doesn't actually look this red in person, but it's enough to get the idea...

Image

White areas look pink. For example, the start up screen that is mostly white, I have a very pink screen. When I turn the red level down, it makes it clear that red is too intense, but that can't be used to fix the problem...

Image

(Again, my crappy cell phone camera picked up on the red too much, but this still give you an idea)

So I know that 480p over SCART is not standard, and as I understand it, it's pushing the limits. But numerous sources on the net claim that this works (here's another source). Can sync effect color intensity? Could the sync combiner circuit that I'm using be causing the problem?

I double and triple checked everything. I confirmed that the R, G and B lines a wired up to the correct pins on both ends of the cables. I checked for shorts to ground on the RGB lines. I checked to make sure the polarity of all the caps are correct, and the diodes are facing the right way.

Pins 6 and 7 on the Dreamcast's multi AV port are used to select modes. Ground one for RGB 480i mode, ground both for VGA 480p mode. Some sources say to ground pin 6 and switch pin 7 between ground and floating, other sources say vise versa, so I don't think it matters. I grounded pin 6 and switch pin 7. I confirmed that when my switch is in one position that a (VGA compatible) game loads in 480p, and when the switch is in the other position the game loads in 480i. Doesn't make a difference though, I still have a very red screen either way.

Unfortunately, the cable that I used didn't have enough wires to include the Dreamcast's composite sync signal. So I have no way of bypassing the sync combiner circuit to see if that's the problem.

Thanks for any input.


Last edited by Ziggy587 on Mon Aug 24, 2015 8:01 pm, edited 1 time in total.

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PostPosted: Sat Aug 22, 2015 8:06 am 
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It looks like your TV might be interpreting the signal as component video (YPbPr).

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PostPosted: Sat Aug 22, 2015 8:25 am 
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thefox wrote:
It looks like your TV might be interpreting the signal as component video (YPbPr).


Sorry, I should have been more clear. This is being inputted into the XRGB Framemeister, specifically the RGB input. Every other console I use with the RGB input on the Framemeister works perfectly.


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PostPosted: Sat Aug 22, 2015 8:29 am 
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Given the overwhelming magenta tint and the very not-green color of the Y button, I'd say you lost the green signal somehow.


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PostPosted: Sat Aug 22, 2015 8:39 am 
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Joe wrote:
Given the overwhelming magenta tint and the very not-green color of the Y button, I'd say you lost the green signal somehow.


The thing is, when I first saw it I thought the same thing, that either the blue or green signal was missing or shorted. For red, green and blue, individually, I checked for continuity between the Dreamcast AV connector and the positive leg of the capacitor. I also checked for continuity between the negative leg of the capacitor and the SCART connector. I confirmed that the RGB lines were all connected from end to end, to the correct pins on both ends of the cables, and not shorted to ground. Although, this seems like it's the most likely answer. I'll check it over again when I get home just to be sure.

edit: Someone on another forum suggestion that the AV connector might not be making good contact with the AV port on the console. I forgot to consider this. If it's not making good contact, that would explain why I have continuity from end to end of the cable, but the green signal is missing.

When I get home I'll test for this. I'll plug the cable into the Dreamcast and test continuity from the solder points on the motherboard and the other end of the cable. That should be a definitive test. Hopefully that's it.

Thanks for the help so far! I'll report back my findings.


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PostPosted: Sun Aug 23, 2015 4:58 pm 
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Are you combining sync into the green channel? It looks like you are just missing green entirely. The photo exposure could be making it unclear, though.


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PostPosted: Sun Aug 23, 2015 6:09 pm 
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Indeed, green is missing entirely as far as I can tell. Good call, Joe, I didn't notice that the Y button on the control was actually black.

Turns out there was never anything wrong with the sync combiner or cables! :lol:

I tried the sync combiner cable with a second Dreamcast and it works PERFECTLY. So, there's something wrong with the first Dreamcast. I tried cleaning the AV pins but that didn't help. I'm gonna have to open the console up and see if I can find out what's wrong with it.

Thanks, everyone, for your help!


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PostPosted: Mon Aug 24, 2015 7:59 pm 
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I can't get that green signal to work...

The sync combiner RGB box thing works perfectly on another Dreamcast, so that rules that out.

Composite video works, and looks fine. I have no other Dreamcast AV cables to test with.

RGB is missing the green signal. From the pics I posted above, red is present (obviously) and you can see blue too. Joe pointed out the Y button on the controller is actually black. So it's clear that green is simply missing.

I cleaned the AV output pins. Also, with the RGB cable plugged into the console, I checked continuity between the cable and the solder points on the motherboard. So I'm 100% sure it's not a bad connection.

Having ruled out the above, I checked continuity from the green pin on the AV output all the way back to the video encoder. There were no gaps.

The only thing I can see is, there's a screw mark on the motherboard. It was made from one of the screws that mount the GD-ROM drive to the shield on top of the motherboard. And it just so happens that the screw mark is RIGHT by the via for the green signal before it hits the AV output.

So, even though I had continuity everywhere, that screw mark was the only thing I could find wrong. I soldered a jumper wire between two points that jump the screw mark but don't omit any components. I tested and I still have the same (mostly red) screen.

So I'm really stumped here. Did the video encoder just decide to stop outputting the green signal?


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PostPosted: Tue Aug 25, 2015 8:40 am 
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If you have a multimeter, you can use it to check the voltage at different points in the green output circuit. You can compare with red and blue to see if the values you're getting are reasonable.


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