RGB Video Output from the Game Gear (!)

Digital image from an emulator.<-->Image taken from my RGB video monitor!
(See more game screenshots HERE!)

I cannot claim to have invented the method of displaying the Game Gear's image on a TV screen. Credit for that must go to Victor Kemp (and others) who did the analysis and drew up a schematic for a simple decoder of the GG's RGB output. I did, however, have my GG pulled apart and I isolated the composite signal(s) before I came across his page. My results using my old (B&W) decoder are on this old page.

THEN! A guy named Xavier came along in late 2004 and started experimenting with extracting true colour RGB output from the GG, and a few months later, we had detailed designs for doing exactly this. I started building the RGB decoder for the 2 ASIC Game Gear (using Xavier's "ggtv25nm" design), and finally after a long time of troubleshooting, it works almost perfectly. From the decoder board, I can connect an analogue RGB monitor and view the GG's image on its much larger tube.
A guy in Japan named Sankichi also was helpful. The diagram he made on his page is a little different from Xavier's, and matches how I hooked mine up.

Finally, a guy named "viletim?" has made up his own fully-integrated decoder board, so be sure to check it out!

Some spare Game Gears, in case I ever needed replacement parts.

Step One... getting inside...

First off, an old-model Game Gear (no "licensed by" screen, no BIOS, 2 ASICs inside) is necessary for Xavier's RGB schematic.

The red wire goes to IC2 pin 10, blue to IC2 pin 30, yellow is sync, then the 2 brown and 2 grey wires are the 4 data lines. (Whoops! I've connected the data lines on the wrong ends of the resistors!)

So, after I correctly connected the data lines (see last pic) and covered up some bare metal with red tape, we can see which data lines go to which resistor.

Step Two... building the decoder...

The Decoder Board

I connected all the wires inside the GG to a 9-pin connector. To this, I can plug and unplug a shielded cable with ease. The longer the cable, however, the higher chance for interference in the image on-screen.

All of this digital data goes from the Game Gear to the decoder board to the right. -->

On the left, the signals come in; and on the right, the R,G,B,Sync, and Ground exit to the monitor. Never mind the mess of wires on the NAND ICs in the lower-half. I used them to connect and disconnect to try different logic configurations. I used 2 ICs just to make wiring by hand easier... I have lots of space, anyway. The big thing in the centre is a 500 Ohm variable resistor, used for adjusting blue separation.

Step Three... calibrating...

Something had gone wrong, because when I connected everything correctly (according to Xavier's specs), the expected data from the IC pins, and the resultant colour information, seemed mismatched. I had to swap the Red, Green, and Blue outputs before the colours even began to look correct. Hmm... why?

Here is what I used for testing the colours... Tototek's menu program hacked to display some primary colours + shades + a few greys. You can compare this pic with those to the right.

IC2 pin 10 should be some kind of pixel clock. It seems to be triggering in all instances of Red or Green. There are many lines in the areas of only red or green, but that's expected.
IC2 pin 30 appears to be triggering for all instances of Blue.
 Here is the output from the "Red_valid" pin of the decoder. "Red_valid" is made from a ((NOT pin 30) NAND (NOT pin 10, with an RC filter for delay)). The image is nice and solid, but it appears to be active when blue is triggered, doesn't it?
"Green_valid" is made from (NOT(NOT(pin 30)). It appears to be valid for red, not green!
"Blue_valid" (NOT(pin 30)). It looks like it should be active for green. :(

Anyway, after swapping the lines, and adjusting the blue separation, everything looks pretty good. I used a couple of my own demos for testing:

Emulator shots. Pixel-perfect, but an emulator is never quite like the real thing...
There are a few lines in the picture, but they are much less pronounced on a monitor tube than in the video capture above.

So, let's check out all the shots in the PHOTO GALLERY, then!

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