4BITdigitizer
A small mod board to create a lumacode signal from 4-bit digital color information available in some retro computers.
Some retro computers of the late 1970s and early 1980s expose their video color signal internally in digital form. For color depths of up to 4 bits this can faily easily be serialized into a lumacode signal using this board. For this to work, the pixel clock as well as the sync signal must also be available.
Some computers that meet these criteria are:
- Philips gamepack G7000
- Magnavox Odyssey II
- Fairchild Channel F
The board contains the necessary logic to turn 4 parallel digitial bits into a sequence of lumacode voltage levels. The sync signal is directly taken from the host machine and also mixed into the output.
The row of holes on the one side need to be connected to power (GND, 5V) and all necessary input signals. SYN is the composite synch signal. If the machine only provides seperate HSYNC and VSYNC, connect HSYNC here and VSYNC to VSY (which should be left unconnected otherwise). Color data goes to D0 - D3. The pixel clock needs to be provided at CLK.
All inputs can also be inverted by closing the corresponding solder jumper. For the G7000, for example, this needs to be done for the sync signal.
The resulting lumacode signal and its accompanying ground return can be taken from the LUM and GND holes.
There are various board layouts for this console. In my machine (a PAL variant), the installation to the underside of the main board looks like the picture provided.
I used some double-sided tape to stick the mod board to a convenient location and wired up GND and 5V to some nearby solder points with silver wire. All the input signals are connected to the corresponding points on the underside of the PCB. Refer to the picture for where to take what. Also close the solder jumper labeled "INV SYN".
If you have a different board layout you may have to do some research on what is what. In any case, the main graphics chip is the only one with 28 pins, so should be easy to find. To check if you have the correct IC and orientation, see that pin 14 is GND, pin 28 is +5V and pin 1 is CLK (which will show on a multimeter as a voltage somewhere in between). Dont forget that the pin ordering is clockwise when looking from the underside.
The second relevant IC (for some extra signal encoding) has always 16 pins, as far as I know, so this should narrow down the possibilities. It may or may not be located near the first IC, depending on your specific board. For PAL machines, you need to get two signals from there. For NTSC only one as the other is already available at the main IC. To make sure you find the correct IC and its orientation even with a different mainboard layout, you can check signal continuity from the video chip to this 16-pin IC on the following pins: 18,19,20 to 10,13,12 .
The lumacode signal and its GND can be taken from the LUM and GND pads and brought out of the case using some RCA connector (I propose a black one).
| Machine variant | Signal | IC | pin |
|---|---|---|---|
| both | CLK | main video chip | 1 |
| NTSC | SYN | main video chip | 5 |
| PAL | SYN | encoder chip | 8 |
| both | D3 (VIDEO) | encoder chip | 11 |
| both | D2 (R) | main video chip | 18 |
| both | D1 (G) | main video chip | 19 |
| both | D0 (B) | main video chip | 20 |