Guide - Modding your Dreamcast VGA Box for 15/31KHz output

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dmauro
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by dmauro »

If I were to build my own LM1881 circuit, could someone please tell me if I've got this right:

The composite sync I need would come out of pin 1.
The composite video would go through a 0.1μF capacitor and then into pin 2.
pin 4 would be grounded.
pin 6 would be grounded with a 0.1μF capacitor and a 680kΩ resistor in parallel.
pin 8 would be connected to +5v.

Seems doable if that is all correct.

And if I wanted to get composite sync from hsync and vsync, how would I do that?

edit: I found this, which might actually be what I'm after:
http://www.tkk.fi/Misc/Electronics/circuits/vga2rgbs.html wrote:Analogue composite sync signal converter

First circuit is for connecting VGA card to video projector or a monitor which accept VGA card frequencies and has RGB + Composite sync input (1 Vpp signal level, 75 ohm input impedance).

Image
This picture is available in GIF and Postscript versions.

Note: The circuit diagram does not show the wire giving power to IC1. The +5V power is connected to pin 14 and ground is connected to pin 7.

This circuit has been succesfully used with Electrohome Projection Systems ECP 4100 video projector in many VGA and SuperVGA modes. The circuit have been also used succesfully with one old Barco video projector using my VGA to TV drivers to get the VGA card to generate suitable signal frequencies which that old video projector can handle.

Component list

C1 22 uF 10V electrolytic
C2 22 uF 10V electrolytic
R1 2.2 kohm
R2 2.2 kohm
R3 1.8 kohm
R4 1.8 kohm
R5 2.7 kohm
R6..R8 47 ohm
U1 74LS86 or 74HCT86
T1 BC 547
T2 BC 547
This way I can get the composite sync from the VGA signal, rather than having to put an LM1881 chip inside the VGA box. This would be good because I'm going to be connecting a PC as well.

another edit:
This page suggests that I can simply tie the hsync and vsync lines together to get csync on the Dreamcast with a VGA box.
Its possible to hack a Dreamcast VGA Box to output at 15khz ... If you are using a Jamma connector (which only has a connection for composite sync), then twist the Hor and Vert sync lines together and connect it to the composite sync on the Jamma connector.
I'll try this and report back how that works, and if that method works, perhaps it will work with an Nvidia card on the PC as well.
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notsonic
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by notsonic »

all i can say is that the cyberlead is a jvs cab so i just hooked the vga cable to the vga box. cut the wire so it would be in 15khz, and it worked fine.

the video runs through a board in the cabinet which i think is just for video out (composite and svid) i dont know for sure if it does any sync separation or amplification.
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by capcomguru »

notsonic wrote:the video runs through a board in the cabinet which i think is just for video out (composite and svid) i dont know for sure if it does any sync separation or amplification.
It more than likely does do sync separation. I know for a fact that Nanao MS9-29x monitor can not sync on composite, which is what the dreamcast attempts to sync on. They need H + V sync.
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by dmauro »

I tried connected the vsync to the hsync and it did not work. :(

Also, I can not get SFIII 3S to boot up in 15Khz without getting a black screen. I thought maybe it was because of the Toodles disc, but I tried with just a standard NTSC-U copy of the game and had the same problem. (This is on the tri-sync monitor).
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by dmauro »

I figured out what was going on with Third Strike:
A patch is just a small part of code that is replaced by another in order to change some features in a program. In your case, you need a RGB fix because some DC games have some kind of protections against european DC so that if you play with a RGB cable, you'll get a black screen or your game will reboot.
My Dreamcast is PAL so I need to apply this patch for RGB to work on it. Weird. :/

Would anyone be willing to make this circuit for me, and what would it cost me:
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by dmauro »

I think perhaps the reason tying hsync and vsync together didn't work is because maybe it's using a positive sync and the monitor needs a negative sync. Does anyone know if this is true of the Nanao MS8? I'll have to check if that's the case and then use a 7404 to invert the polarity of the sync signals. Hopefully then I can tie them together.
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by dmauro »

Okay, this is very interesting. I got my PC set up for low res finally with an Nvidia 7800GT and made sure it was working on the tri-sync. Then I hooked it up to my low res Astro going through a VGA cable into an RGB amp into Jamma. The signal came through fine o_0 Perhaps the RGB amp is combining the syncs for me into composite, okay. So then I tried the same thing, but routing it through a VGA splitter so I could display it on both monitors and suddenly the sync is messed up. It's still displaying just fine on the tri-sync after going through the splitter, but the low res monitor gets all messed up when I try to use the splitter. :? Any ideas what this is all about?
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by dmauro »

I just emailed Andy to confirm, but my suspicion is that the VGA cable he included with the RGB amp combines the hsync and vsync into the hsync pin. (edit: Andy says: "The amp simply connects the sync signals together, although they can be separated." so that's explained, but still no idea why the VGA splitter caused problems)

So I need to check the polarity of the sync signals in the Dreamcast VGA box to see if I need to invert one or both. I just connect one end to the sync wire and the other to the ground and check for a small voltage reading right?
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by dmauro »

Just confirmed that both sync signals are positive. :/

edit: I was able to find a 74LS04 chip in eBay for $2.50 shipped :D

edit2: I have the chip and I'm going to try installing it tonight. I'll update the OP if this works.
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by dmauro »

The Dreamcast av port does not seem to be supplying +5v. This seems odd to me since pin 5 is dedicated to that purpose, but I hooked up a meter, and it certainly wasn't powering the inverter. I'm going to have to wire this up to the jamma fingerboard to draw the power I guess. This is going to be a mess of wires now. I wonder if there is some way to force my Dreamcast to put out the +5v...?

edit: The Dreamcast seems to ouput some voltages when the VGA box is unplugged, but when I read all the pins with it plugged in, the only things putting out any voltage are the h and vsyncs which look to be about +3v I think.
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by dmauro »

I drew +5v from the Jamma edge and connected that to the chip, but it's not inverting the sync signals. I am actually get a very weak positive signal coming out rather than a negative signal at the same voltage. Any ideas? The chip says HD74LS04P, but as far as I can tell this should be wired up just as I have it (same as in the schematic above).
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by Andypc »

Thanks for the great guide :D I just modded my VGA box at it is working great on my tri-sync Sanwa PFX.
15Khz
15khz.jpg
31Khz
31khz.jpg
I do have a couple of questions?
How can I ensure I am running SF3 SFIII Third Strike in 240P rather than 480i. I am using a modded Japanese Dreamcast and an original Japanese SF3 SFIII Third Strike?

Also I have tested a few games like Puzzle Bobble 4 that will not normally work with the VGA cable and they run fine in 15KHz, but I am having a problem with the American version of NFL Blitz 2000. In 31Khz I get the message that the VGA cable is not supported, but in 15Khz it starts to load and then resets the console. Any Idea's? Do other versions of this game work?
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by shaktazuki »

Well, this turned out to be quite a pleasant introduction to basic electronics and soldering for me. :awe:

And something of a disappointment. SF3S is not in 240p on my American DC - it's 480i, as is everything else. :cry:
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by dmauro »

You have to hold a button down when you boot up 3S to get 240p (just once and then save something in the settings and it will be remembered). I can't remember which button it is, but a quick google search should turn it up.
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Post by shaktazuki »

http://dreamcast.onlineconsoles.com/php ... 5eb58fbb0d
240p mode for scanlines (flawless) - Hold the Start button + L button until the Capcom logo appears on the screen.

240p mode for scanlines + anti-flicker/sprite filter - Hold the Start button + Y button until the Capcom logo appears on the screen.

480i screen, no scanlines + anti-flicker/sprite filter - Normal Bootup
It's better. Not arcade perfect, but a big improvement!
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz outp

Post by lacanian »

Thanks for the great guide, I have just modded my VGA box.

I have a Atomiswave SD cabinet. When I play some games they seem to shake, even afer I modify all the holds etc. I have also tried changing the impedance on my monitor. This happened with games like Marvel vs Capcom 2 DreamSNES and some others
Is there any reason why some games are like this and others are not? Do some games support RGB while others do not?

My last test was on Jo Jo's Adventure which I know doesn't work on the VGA box. When I put it in 15hz mode it displayed perfectly without flicker.

Thanks.
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz outp

Post by Red5 »

Thanks for the ace guide.
Just mode'd my vga box, to allow Dreamcast Hori goodness on my NAC. through my Capcom I/O adapter, i must say its very nice :D
now have Hori and tate DC's, set up on my New Astro City and Net City.
:D :D
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz outp

Post by lacanian »

Can you please tell me what Hori and TATE DC's are?
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz outp

Post by fagin »

He means he has:

1) Horizontally (HORI) screened cab (normal TV aspect ratio)
2) Vertically (TATE short for rotated) screened cab for shooters that enable rotation in the menu's (same as your TV turned on it's end)
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz outp

Post by fagin »

Anyone know of any information around to complete this mod with the official VGA lead?
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