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

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

Postby dmauro » December 17th, 2008, 6:13 pm

There are not a lot of reasons to do this since most games look much better in 31KHz and almost every game can be forced to output at this frequency, but there are a couple of reasons. In my case, I have two arcade monitors and only one of them is capable of 31KHz. If I want to play Dreamcast games on the low-res monitor, the best option is to have a VGA box that also outputs at 15KHz. Note: Noami games at 15KHz will be at 480i (yuck), but Bangai-O runs in 240p and I believe SFIII Third Strike can be forced in 240p as well.

The VGA box I used for this is one of the easiest to get your hands on.

Image

It can be purchased at Racketboy or can be had for a few bucks cheaper on eBay.

I used this guide to help me figure out what exactly I needed to do, but it's really very straight-forward: if pin 6 and 7 are both grounded you will get 31KHz, if only pin 7 is grounded, you will get 15KHz. Dreamcast A/V pinout here. Since pins 6 and 7 only serve this purpose, to prevent 6 from being grounded, you can simply cut it. since we want to switch between 15 and 31KHz, we are going to add an on/off switch to complete or break the pin 6 connection.

If you're using the same VGA box I used, this is nice and easy. Pin 6 should be the dark blue wire. Pull the hot glue off of the PCB where the wires meet the board being careful not to damage anything. Use your desolder braid and soldering iron on the back of the board to remove the solder from the dark blue wire's connection and pull it out of the hole. Now you need to use a multimeter to verify that this is pin 6. Use an alligator clip to connect one end to the dark blue wire, and use the other end to touch pin 6 on the part of the cable that makes contact with the Dreamcast's A/V out (remember that because this mirrors the A/V pinout, you're counting up from RIGHT to LEFT). If it turns out that your wire coloring is different, you might have to solder this back in and try a different one. You can do your best to deduce which one might be pin 6, but because pin 6 is grounded as long as it is soldered into the board, you won't know for sure until you desolder and remove the wire.

Once you have identified the correct wire as pin 6 and removed it from the board, connect it to one of the connections on your on/off switch. Now you'll need another stranded wire, ideally smaller than 22 gauge, but this is what I used by clipping a few of strands off so that it would fit into the hole on the PCB which we removed the pin 6 wire from. You need to solder one end of this wire into the pin 6 hole and the other end onto the other connection of on/off switch.

You should now have something that looks as below:

Image

When the button is on, pin 6 is grounded and you'll have a 31KHz signal, and when the button is off, pin 6 is not grounded and you will get a 15KHz signal. To test this out, set it to 15KHz and see if you can boot up Bangai-O. Bangai-O will not be able to boot in 31KHz, so if it it does successfully boot, your 15KHz setting is working. Now boot a Naomi game in 31KHz and make sure you have a beautiful, crisp 480p image to ensure your 31KHz setting is working.

Assuming there are no problems, you can drill a hole in the plastic enclosure. There is plenty of room right next to the TV/PC switch on the VGA box, and the blue wire should be able to just reach this far. Hold the two parts of the enclosure together without the PCB inside and drill a hole just to the left of the switch. The hole should be about half on the bottom side of the enclosure, and half on the top. Now simply reassemble the VGA box with the on/off switch fitting snugly in the hole you just made.

Image

If you have a different VGA box you're trying to do this too, things can get more complicated. I originally had a different VGA box that I accidentally destroyed while trying to mod. It turned out that pin 6 was connected to pin 7 way up near the pin header, so there was no pin 6 wire in the VGA box to cut. So other VGA boxes might not be so simple to work with, but the general idea is the same: install an on/off switch that completes/breaks the connection between pin 6 and the ground (or any grounded wire, including pin 7).

Enjoy Bangai-O in 240p!

Image

Edit: To get this signal to display on most low-res monitors, you're going to have to toy with the sync signals to get a negative composite sync. I'm currently working on that now and will update this post with the details once I've got it working.

Important Note:
If you're doing this mod, there's a good chance you're looking to play SFIII Third Strike in 240p, but the American and Japanese versions have some sort of video protection that won't allow the game to run in RGB mode on a PAL console. You will simply boot into a black screen without applying this patch to the disc image:
1) look for
09 00 09 00 09 00 09 00 09 00 30 88 07 89 06 A0 09
replace it with
07 89 32 88 07 89 33 88 07 89 30 88 07 89 06 A0 09

2) look for
00 E0 00 E0 04 7F 26 4F 0B 00
replace it with
02 E0 00 E0 04 7F 26 4F 0B 00

3) look for
30 E0 02 2E 00 E0 26 4F 0B 00 F6 6E
replace it with
4C 84 02 2E 00 E0 26 4F 0B 00 F6 6E

4) look for
07 D3 31 60 19 40 03 C9 02 88 00 8B 03 E0 0B 00 09 00
replace it with
22 4F 06 B0 09 00 26 4F 03 E4 19 40 09 24 0B 00 43 60
Last edited by dmauro on January 15th, 2009, 4:16 pm, edited 1 time in total.

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

Postby jonny5 » December 17th, 2008, 6:43 pm

are you running that through a JPAC or similar product or is that a tri-sync monitor?
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Re: Guide - Modding your Dreamcast VGA Box for 15/31KHz output

Postby dmauro » December 17th, 2008, 6:49 pm

That 15KHz image is on a tri-sync monitor, and to run it on my low-res monitor I have a video amp that uses VGA in and plugs into a jamma fingerboard.

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

Postby capcomguru » December 17th, 2008, 7:41 pm

Just an FYI- you can run the 15khz VGA through a Capcom I/O to utalize the RGB boost and Sync Separation. Nanao MS8 and MS9 series can not sync on a dreamcast 15khz signal as-is, they need to have H + V sync separation to lock on, opposed to composite sync that the dreamcast outputs. The other option is a LM sync separator IC.

I have the same exact VGA box, thanks for the guide, I'm probably going to do the mod on my own (even though I'll probably never run it 15khz hehe). It might be cool to integrate the sync separation circuit AND rgb booster as well as the 15/31khz switch.

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

Postby thegreathopper » December 17th, 2008, 7:48 pm

What a great tutorial, well done that man.

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

Postby dmauro » December 18th, 2008, 1:50 am

Hmmm, yeah I think I'm going to have to build a sync separator into my video amp. I thought the Dreamcast's sync would work with the MS8. :(

edit: because I'm not able to test yet, will my PC work without a sync separator?

I've tested the Dreamcast on the MS8 and you're right in that it doesn't work, but I'm curious as to why. The Dreamcast A/V pinout shows that it has both H + V sync and composite sync.

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

Postby capcomguru » December 22nd, 2008, 6:27 pm

dmauro wrote:Hmmm, yeah I think I'm going to have to build a sync separator into my video amp. I thought the Dreamcast's sync would work with the MS8. :(

edit: because I'm not able to test yet, will my PC work without a sync separator?

I've tested the Dreamcast on the MS8 and you're right in that it doesn't work, but I'm curious as to why. The Dreamcast A/V pinout shows that it has both H + V sync and composite sync.


Yes, a PC monitor that can accecpt 31khz will work just fine... I ran my Dreamcast on a CRT monitor for years before I had cabinets.

The reason the Nanao does not like the sync is because the Dreamcast outputs Composite video IN PLACE of composite sync (there is a difference). To strip out usable sync, you'll need to build a LM1881 circuit (super easy to build):

Image
http://www.mameworld.net/pc2jamma/images/circuit1.gif

Drop me a PM if you need a Sync separator chippie. I have a spare DIP package LM1881 that I'm not using. The circuit is super-easy to build... you feed it composite video, voltage and ground and it outputs composite sync.

EDIT It appears I can't link directly to the schematic :( Here's the page that has the schematic embedded:
http://www.mameworld.net/pc2jamma/arc_dc4.html

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

Postby dmauro » December 22nd, 2008, 7:25 pm

Actually, what I meant was, will my PC display on my MS8 monitor without needing a sync separator. Will just a video amp do the trick? I haven't been able to test it out yet because I haven't set my PC up for 15KHz display just yet.

Also, because I might be running the 360 on the MS8 eventually (in 480i unfortunately), does anyone know if a 360's VGA output puts out the proper sync signals to work with an MS8?

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

Postby capcomguru » December 22nd, 2008, 7:40 pm

In my experience, you MUST have a sync separator when running a PC in 15khz on a Nanao MS8/ MS9 monitor. A couple years ago, I had a mame setup that used Arcade VGA running on my MS8 monitor, and I needed to pass the video through my capcom I/O (which has an integrated sync separation circuit). Unfortunately since then, I upgraded and sold the MS8 (to a MS2932 31khz), and ditched the ArcadeVGA mame setup. Xbox 360, I'm not too sure about the video mode. If I had to guess, it passes composite sync as the entire composite video signal... not stripped.

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

Postby Jason » December 22nd, 2008, 8:23 pm

nice guide, so let me get this right this will allow you to play 15k games through VGA like Last Blade 2?

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

Postby dmauro » December 22nd, 2008, 8:48 pm

capcomguru wrote:In my experience, you MUST have a sync separator when running a PC in 15khz on a Nanao MS8/ MS9 monitor.

That's sad news :(

Now I'm starting to wonder if I should just get another tri-sync chassis. :/

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

Postby capcomguru » December 23rd, 2008, 5:25 am

Cmon now, don't despair :D

What I would do if I were you was construct a VGA pass thru "box" sporting the sync separator. VGA in and VGA out (even though it'll be 15khz signal through the vga connector). This way, it can be modular and used on all applications. IMHO, no tri-sync chassis upgrade will ever look as good as your nanao. Just my 2cents though...

Offer's still good if you want one of the LM1881 chippies to toy around with.

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

Postby dmauro » December 23rd, 2008, 6:39 am

Yeah, I'm probably going to take you up on that offer. I could easily build a LM1881N chip into my existing video amp I suppose (what would I need to make the circuit? some resistors and a bread board?). The only reason I start to consider getting another tri-sync is for vertical 31KHz like Under Defeat, and for running 360 games in 480p instead of 480i. :/ Probably I'll go ahead with the sync separator route for now and see how that goes. Can I paypal you a few bucks for you to ship me that chip?

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

Postby capcomguru » December 23rd, 2008, 4:26 pm

Yeah just drop me a PM with your details, and we'll go from there. All I ask is you cover shipping.

I hear ya on the tri-sync vert need. My blast city (which has a tri-sync monitor) is currently set to 'tate... 15khz for my PCB's and Mame, 31khz for Under Defeat and Shikigami 2. I also have a New Astro City (15/24khz nanao) on hori for low rez fighters, and an Astro 2 (upgraded to a Nanao 31khz monitor) also on hori for Naomi/ 246/ JVS games. If you have the cabs to do the upgrade with, I highly recommend it. If you only have one machine, I'd probably stick with just the 15khz nanao. Just my opinion, take it or leave it :mrgreen:

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

Postby CPS2 » January 2nd, 2009, 10:18 pm


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

Postby channelmaniac » January 3rd, 2009, 1:29 am

Just buy the sync separator from JROK. It's a tiny SMT loated board and for the price (<$20 USD) it's not worth building yourself.

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

Postby dmauro » January 5th, 2009, 6:25 pm

CPS2 wrote:http://cgi.ebay.co.uk/VGA-CABLE-Connect ... otohosting

Anyone every hacked one of these??

I haven't, but simply because it's so small, they've probably grounded the 6 and 7 wires right in the pin header rather than in the little box part.

channelmaniac wrote:Just buy the sync separator from JROK. It's a tiny SMT loated board and for the price (<$20 USD) it's not worth building yourself.

I didn't see one on jrok.com. Do you have a link?

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

Postby notsonic » January 7th, 2009, 6:53 am

for what its worth i had no problems using my dreamcast on my ms9 in my cyberlead using a modded vga box. (this was obviously before i swapped the chassis to a tri sync.) the picture was amazing to be honest.

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

Postby dmauro » January 7th, 2009, 5:06 pm

That's interesting. Did you have to use a video amp? I wonder if the video amp I was using could have caused problems.

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

Postby dmauro » January 8th, 2009, 4:34 pm

How are you getting the video from the VGA box to the monitor? Is it possible that my VGA box is dropping the composite sync signal, or does the Dreamcast not actually put out a composite sync in VGA mode (in which case I guess your monitor gets the sync from the composite video)?

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