Converting RGB composite output to VGA

Question

We've got an old Arcade machine at work that we're busy converting to being powered by a PC running an emulator. The controls have now been sorted, and audio should be a breeze.

Remaining is hooking its existing screen up to the VGA port on the PC we're going to insert into the cabinet.

Just to give some background, here is the way the Arcade machine looks like. It doesn't seem like a "standard" Arcade machine as it doesn't have any kind of branding at all. We don't know the history of the machine, but it does have a cathode tube ray setup inside.

This is how it looks like from behind the screen:

Now, we identified the output coming from the controller board above, and it has four outputs: RGBES.

Now, I know it cannot be extremely hard to convert RGB to VGA, but I'd like some pointers here. I also don't know what the "E" and the "S" stand for, and I wouldn't be surprised if they are also important.

If we could leverage existing cables available such as typical RGB to VGA adapters that would be swell, but I suspect it's not going to be that easy.

Answer 1

As Cheibriados has pointed out , E stands for Earth, and S stands for Sync.

Most likely, a video game used simple 1-bit encoding for the RGB levels, and if so that will make your life simpler. You'll need to get the use of a scope to look at the outputs, making sure to reference the scope to earth. With luck, you'll see essentially binary levels, although the actual levels may be somewhat high. If so, you can simply put 3 comparators on the RGB lines, and buffer the outputs so that they drive 75 ohms from 0 to 0.7 volts. This will drive VGA properly. If the color lines show a wide range of values, you'll need to get some op amps with relatively high speed (~5 MHz or better at operating gain - not gain-bandwidth) and buffer the signals to a range of 0 to 0.7 volts into 75 ohms. There may or may not be a sign inversion, and you may need to adjust offsets.

A single sync line establishes the use of composite sync, which means you'll need to provide a sync separator. South Africa uses PAL, so you'll need a unit which works in this format (I'm US, so I have no pertinent experience). The separator will produce separate horizontal and vertical syncs which are sent to the monitor. Signal levels are TTL, not 0 - 0.7 volts. I expect that any modern monitor will have no trouble handling PAL format syncs, but it's barely possible that you'll have to do some modification of the both pulse width and position wrt the video data.