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I'm trying to convert arcade video signals (JAMMA) to VGA specifications.

From what I've gathered, arcade video signals aren't standardized. They can range from 0-2 V or 0-5 V, etc. However, VGA requires 0 - 0.7 V p-p (75 Ω double terminated).

Are there any ICs that can detect RGB voltage and then convert it to VGA level?

Can anybody give me any advice?

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  • \$\begingroup\$ Nothing on ebay? \$\endgroup\$
    – user76844
    Jul 26, 2019 at 18:30
  • \$\begingroup\$ RGB to VGA was a very big thing, back when RGB and VGA were big things. Have you googled? If it's just a level conversion thing, you may be able to get by with attenuators. \$\endgroup\$
    – TimWescott
    Jul 26, 2019 at 18:32
  • \$\begingroup\$ It appears to me that JAMMA does have video standards. Have you really searched for the JVS? Judging by the appearance of similar products on the usual Amazing shopping sites, the conversion will take more than just an IC, \$\endgroup\$ Jul 26, 2019 at 18:43
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    \$\begingroup\$ Not sure of source impedance. If you can see the signals are 2V and you make a Thevenin Equivalent at 1.4V @ 75 Ohms then when terminated , it will be 0.7V THis might only be approx a 150 Ohm series R to VGA for each RGB. ( 150+75)/75 * 0.7 =2.1 V \$\endgroup\$ Jul 26, 2019 at 19:06
  • \$\begingroup\$ If you could actually examine the signals on a scope and determine their levels and that the timing and synch schemes and interlacing are compatible then you could presumably use a resistive voltage divider. But it's quite likely that the incompatibilities go beyond this. It's possible that the sampling stage in a VGA-in LCD monitor might have more flexibility. \$\endgroup\$ Jul 26, 2019 at 19:08

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You don't have to worry about this too much; all but the oldest VGA monitors typically have very robust input conditioning on the RGB inputs to clamp input voltages that are out of range. They also AC-couple the input so the DC offset doesn't matter.

Therefore it is sufficient to just use a voltage divider circuit; a potentiometer will do. On my JAMMA to VGA adapter I use three 500 Ω potentiometers. You won't be able to damage anything if the potentiometer isn't set correctly: the display will just be too dark or too bright.

To calibrate them, enter the game's diagnostic mode and view the color bar screen. Almost all games can display this. Then adjust the potentiometers for red, green, and blue until the color bars are not saturated and each gradient is visually distinguishable. Now you are good to go.

I've tested this circuit with many arcade boards from different manufacturers and it works fine. As you mentioned, there's a lot of variation across games so it can be worth it to use potentiometers that are easy to turn (the kind with big knurled knobs for example), since you'll need to twiddle them on a game-by-game basis. It won't always be a "set it and forget it" solution.

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  • \$\begingroup\$ I can't imagine this to work well with cables of any significant lengths due to impedance mismatches, but maybe on board-to-board connections. \$\endgroup\$
    – pipe
    Dec 22, 2022 at 0:52
  • \$\begingroup\$ Agreed, most wiring within an arcade cabinet is 3 feet long and it works fine there. For long runs you'd definitely want something more sophisticated. \$\endgroup\$ Dec 22, 2022 at 17:39

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