Combating electrical noise inside an arcade machine .. What can I do?

leonki

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Combating electrical noise inside an arcade machine .. What can I do?

Background:

Got a New Astro Cab that has been running MAME (via ArcadeVGA/JPAC). I recently acquired a Naomi 2 setup for more modern games. I decided to get both of them installed at the same time, and use 2 switchers to select what system should run (DB25 2-way switch for controls, and DB15 2-way switch for video)

I purchased harness and finger board from JAMMA boards, and wired it all up. For the most part it works GREAT! I can now select what system I want to control/display, and I managed to actually get a full MAME setup + Naomi 2 setup (2 power supplies, and all!) inside a NAC!

To accomplish this I made the following adapters:

- 2 * JAMMA->DB25/DB15 (between Naomi and switchers + between JPAC and switchers)
- 1 JAMMA edge->DB25/DB15 (between NAC harness and switchers)

I got only 1 small problem, and I'm not sure what I can do, and figured maybe someone can point me in the right direction:

When I have: JPAC->NAC JAMMA harness, the video is nice and clean.

When I have :

JPAC->JAMMA Harness->DB15 -> high quality shielded 6' VGA cable -> DB15 -> JAMMA fingerboard-> NAC JAMMA harness

The video is nice, but in Windows (in 480i) I can almost see a faint shadow around the mouse cursor which suggests noise being pickup somewhere. When playing games, one doesn't see/know it's there .. but it kills me that I'm picking up noise somewhere, and not sure where!

I know the JPAC amplifies the signal, so it should be nice and clean. I made the wire harness/fingerboard wires as short as possible (~2") before they enter the DB15 on both . Inside the DB15, each has R,G,B,composite sync and GND is connected to GND + GND for R,G,B and grounded to actual plug.

Any other suggestions? Has anyone tried something similar?
 
The video is nice, but in Windows (in 480i) I can almost see a faint shadow around the mouse cursor which suggests noise being pickup somewhere.
Not traditional noise -- I'd call what you're describing "ghosting". That's when there's an impedance mismatch in wiring that causes ringing or reflections in the signal. Here's an example I found on the web.

An ideal pulse (imagine your video signal is drawing a certain pixel):
backmatching.jpg


What happens when you have a severe impedance mismatch (see the poor edges mean the pixel looks not fully-on and the next one looks not fully-off):
no-backmatching.jpg


I didn't follow your entire description but it sounds like you have a long series of connections that are electrically mismatched -- unshielded harness, into shielded wiring, through some fingerboard and a bunch of other connections. I don't have too much advice on how to fix it but something in your complex setup is probably what's causing it.
 
Thank you. That actually makes a lot of sense!

So ghosting is caused by impedance mismatch rather than noise pickup. If that's the case, it could be that the impedance in the VGA cable is different than the unshielded standard JAMMA wires.

Do you think replacing the VGA cable with CAT5/6 wire make a difference? I was also doing a small test. Using a VGA gender changer to connect the 2 VGA ports together (this will eliminate the VGA cable) and be able to tell me if the noise is in the cable of the harness.
 
Yeah it could be the wiring. It could also be the electronics. It wasn't clear from your description if you're mixing and matching PC, JAMMA boards, monitors, etc. I thought that JAMMA boards output a high-impedance RGB while PCs output a low-impedance VGA. Check your system end-to-end for other sorts of mismatch.

CAT 5/6 is twisted pair wire; the twists help to reject noise on differential signals. Since rgb/vga is not differential it's not what I would recommend. Usually you'd use straight wiring possibly with shielding.
 
Update:

I purchased a gender changer and used it to connect the 2 harnesses directly. Shockingly, the signal is perfectly clean! I think I know what's going on here (correct me if I'm wrong - hope this helps someone in the future)

Signal coming out of pcbs or jpac is at arcade specs (2.2 Vp-p, 2.2k ohm input impedance) VGA on the other hand has input impedance of 75 ohm! So a VGA cable can't be used as a connector between standard CGA monitor and pcb/jpac! It will cause ghosting.

Sounds right?
 
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