Video Attenuation on Arcade Boards (Neo Geo MVS in particular)


#1

So not to long ago there was this huff on the internet about proper sync levels on the RGB cables on various Game Consoles.
And there these two competing standards for Csync on consoles, TTL Sync and 75Ω Sync (75Ω is named after the impedance on Consumer video displays).
Hence what spawned this post. The post here is copied from my Face Book post.

So I been looking into proper video attenuation for a Neo Geo MVS. I am posting my findings and hoping on some feedback.

I would post pics, but everything covered in heat shrink tubing and I do not feel like undoing everything now. And I didn’t have the foresight to photograph before hand.
[Edit] I No longer have the attenuation circuits in the Scart head.

[EDIT 9/26/2017]
So I neglected to mention I have a MVS board MVH-MV1T. Apparently different boards have different output levels.

The most common thing to do is just top add some 1k adjustable Pots to the RGB lines. This works mostly fine for the bulk of Superguns for overall/generic compatibility. But I want to get the precise attenuation Neo Geo MVS.
I found I got the proper RGB Video Balance when I had a 75Ω Resistor and a 220 µF capacitors to the RGB Lines. This gives the NEO GEO MVS video the right levels of brightness, and the Caps provide some AC current filtering so there no return voltage on the input RGB lines.

As for the Sync, I believe the sync comes in at 5 volts peek TTL Sync. To get the sync closer NTSC specs, I followed the same attenuation circuit that is used properly wired Sega Genesis/Megadrive and Sega Master System Scart Cables. 470Ω going into a 10µF cap.

This does nothing for the picture quality per say but rather it avoids future burn-out consumer video equipment by bringing the sync signal down to the 75Ω impedance that consumer video equipment and TVs use.

Sources I used for my findings includes

Voultar’s Poor SCART Cable Design w/ Analysis and Rework Tips
https://youtu.be/khl6x0S_bV0

René’s dbGrafx Booster – Not Every SCART Cable is Created Equal


#2

Instead of putting the attenuation circuits in the Scart head, I put the components on a small pref board and installed it inside my Neo Geo Deticated Super Gun


#3

So I don’t have proper figures or sources yet as much of what I read is anecdotal, but I been reading around and it appears to be different versions/revisions of the MVS board require slightly different.

I am thinking a quick and dirty solution to most people’s RGB level issues can be solved with a 500 ohm or 1000 ohm trim pots and some 220 µF capacitors.
The pots can be adjusted to the desired brightness and worst case scenario you turn the pots too low you get a dark image (or a black screen if you turn all 3 lines down all the way on each pot).

I asked people about the issue with proper sync, and apparently the question got passed off to other people so I am kinda waiting on them for feedback.


#4

Just wanted to let you know I’ve been fiddling with NeoGeo sync as well.

This started as an idea to fix “S rolling” edges on my 32inch consumer trinitron.

I threw the CSync line on the scope and didn’t find anything too out of the ordinary. I did not connect an external trigger to trigger on the V/H refresh.

System has a strange refresh rate too…I’ll update here if I find anything worth mentioning.


#5

Yeah the Neo Geo does not follow the NTSC specs when it comes to refresh rate.
The NTSC Refresh rate is actually 59.94Hz (29.97Hz for interface) and not a perfect 60hz, this has to do with the color subcarrier.

Video that explains it better than I can.

Spoiler

https://youtu.be/3GJUM6pCpew

I understood it as the MVS puts out estimate 59.1856060~ Hz Refresh Rate. Some HDMI devices can tolerate this, others has there parameters programed too tightly and not accept the input.
The SNES is another gaming system that didn’t follow the NTSC specs when to comes to Refresh, it outputs like a 60.08 Hz refresh.

As for your 32 Inch Trinitron? I am not sure why a CRT is being thrown off. Do you have a 'newer" Model Trinitron? Some Sony PVMs also have a issue with sync from game systems. Specifically I forgot what model has major issue with the Sega Master system and Sync on RGB video.
You could look into a dedicated Sync processor, I forgot the model number from extron, but it something to look into.


#6

Yeah, I totally understand.

The one I am having issue with is a Sony 32FV310. I don’t get this issue at all on my D24E1WU or 20M4U.

Funny you mention the SNES because that gives me a rock solid image without any waves. JROK sync “Cleaner” does nothing to fix the signal on the 32inch. I’ll look into a Extron but that would be such a crazy signal flow for my setup.

The 32inch TV also does not have any of the “sync processing” service menu options that the foreign versions have that do PAL/SECAM etc.

I do have a Shinybow SCART to YUV box that is collecting dust. Maybe it will handle the sync pulses better than the JROK but I think it is just the TV at the end of the day.

I haven’t had time to scope around more on the MVS but we will see!


#7

Oh you got one of the later CRT Trinitron, one of the WEGA Tvs, I am assuming Component input, 1 front and 2 rear S-video inputs, and 2 rear composite inputs?
I would try the Shinybow, but there no guarantee it would fix the issue. One Youtube review went to say he can’t get NES and SNES to work on the Shinybow.

I don’t know how it helps, but here is a interesting thread on one guys 240p setup.

I also try http://www.neo-geo.com/forums/ and see if anyone there had the same issues.


#8

Another person had issues with the Neo Geo Frame rate, where he was modding the arcade board for HDMI

His solution was to slightly overclock the MVS CPU itself by1% bumping the frame rate closer to NTSC standards.
As much of the frame rate of these old game consoles were sometimes a byproduct of the CPU.

I don’t remember if there was a Bios mod for this or not.
[Edit] - it’s not the standard Unibios, I just checked with my MVS board that uses a v3.2 Unibios and the frame rate shows up on my OSSC as "59.18"hz


#9

Hi Darksakul,

Thanks for putting this together, it was very useful info to me.
I used these values in a MVS 2slot.
I left pots on because it was the same ammount of work, on the two slot I found 150OHM to be the sweet spot.

I am now working on making a board for this with SMD components as I am going to consolize 2x1 slots and a friend needs a couple as well. It is my first attempt at designing a board, not sure how it will come out.


#10

So, Voultar has clearly stated that MVS boards should not be connected to 75ohm equipment…
https://www.youtube.com/watch?v=qFwL-1cxgEY&t=5989s


#11

Got a time mark where he said that, 2h our videos is alot of video to search for a single quote.

And I am sure he said it should be connected directly to 75 ohm equipment.
Its a similar issue with the C Sync on the Sega Genesis.


#12

I had included the timestamp in the link I posted (watch?v=qFwL-1cxgEY&t=5989s) not sure why it is not working for you.
If you need the timestamp, it is 1:39:49.

This is all over my head but from my investigation looking at the boards, on AES/CD equipment there’s a Sony video encoder (CXA1145P) whereas on the MVS all I could find behind the RGB lines was these Hex inverters (HD74LS05P)
So I think what Voultar and Ste are saying is that you need some kind of IC to properly drive a 75ohm compatible signal for consumer electronics like SCART TVs and Upscalers.
Voultar is working on some kind of AES/MVS RGB board though.


#13

Yeah I remember RGB Bob saying something to me about that

the forum software here prevents the timestamp in the link, the video starts from the beginning


#14

Sorry for jumping in mid conversation. Does this mean I should not be connected other arcade boards (CPS3/NAOMI) to an ossc/framemiester directly?


#15

Don’t Know. We focusing on just the Neo Geo at the moment

If you have the know how and a Oscilloscope you can get the levels for your self.


#16

Haha I was the one that sent in the question about hooking up the MVS to the framemeister. I got video working with the framemeister using the resistors and capacitors mentioned above, but I have no idea if this is good for the framemeister. Also there is some snow (interference?) on the picture but i’m not sure what is causing that.


#17

it’s probably one of these cases where it works but is most likely bad for your framemeister.


#18

Its bad as it slowly wears down your equipment where months or years down the road they fail

Short term, there no issues.


#19

So follow up,

I have discovered this

http://etim.net.au/shop/shop.php?crn=211&rn=556&action=show_detail

It’s a AV-Driver board by Tim Worthington, it can be set to amplify video signals for game consoles or Attenuate those video signals to lower levels for arcade boards.

Here is the instructions/ work sheet for the Attenuation side of the board
http://etim.net.au/av-driver/arcade/


#20

I suggest you look at this: