GTIAdigitizer (for Atari 8bit) - c0pperdragon/LumaCode GitHub Wiki

The GTIAdigitizer is a small board to be used in Atari 8-bit machines to create a LumaCode video signal by sniffing the relevant signals inside the machine and re-creating the intended video output.

Mod kit

The kit consists of the board that sits under the GTIA chip, and a cable with alligator clips to attach to the existing RCA jack.

Installation

After removing the GTIA chip, insert the mod board in its socket and put the GTIA on top.

Use the cable to attach it to the RCA jack of the obsolete RF output port. You will need to first cut the internal connection from the RF modulator to the jack. When directly using the clips for a solderless install, you may have to leave the lid of the RF can open. When you have the tools for soldering, you should directly solder the wires to the jack.

Upscaling the lumacode signal

The new output signal is not directly usable with a TV or monitor. You need a compatible upscaler to create HDMI. One possibility is the RGBtoHDMI with analog input.

Details on color encoding

The Atari 8-bit computers can generate at total 256 colors: 16 hues, each of which can be shown in one of 16 luminosities. This would require a total of 8 bits of information needed for every pixel. But because of the way the colors are generated by the Atari internally, every two consecutive pixels share the same hue. So only a total of 12 bit is needed for every pair of pixels. These 12 bit will be encoded in the LumaCode signal with 6 samples (each transfering 2 bits). Grouping always two samples together (high bits first) to form a 4-bit number, three such numbers encode the data for a pixel pair in the following order: HUE, LUM1, LUM2

Installation in the special 800XL SECAM (Rose).

This machine is a very rare french variant of the 800XL which uses the equally rare FGTIA chip. Unfortunately the pinout is very different from the regular GTIA/CTIA, so installation is more involved. You will need to solder the GTIAdigitizer on top of the FGTIA, but in a pretty unusual way. Also take care to identify the FGTIA correctly. It is labeled U20 and sits nearer to the center of the main board than usual.

1. Cut away the legs 1 - 7 of the GTIAdigitizer (counting always starts at the notch and goes counter-clockwise).
2. Place the GTIAdigitzer on top of the FGTIA so that it is rotated 180 degrees and its pin 30 sits on top of pin 1 of the FGTIA. That means, it will hang over the left side of the FGTIA by 9 pins. This will bring its pins 30 - 40 in alignment with pins 1 - 11 of the FGTIA. With the exception of pin 31, which is not connected, these pins now already get the correct signals.
3. On the other side, pins 8 - 11 of the GTIAdigitizer should be on top of FGTIA pins 37 - 40. These pins are not connected in the GTIAdigitizer and are mainly used as mechanical stabilization and to bring signals up for later use.
4. Solder the aforementioned pins to the FGTIA. Note that pins 1 - 7 of the GTIAdigitizer have been cut away and will not be soldered.
5. On the top side of the GTIAdigtizer, forward some signals that are brought up from below through an internally unconnected pin. Solder wires between the top holes of these pins:

Signal	GTIAdigitizer	GTIAdigitizer
GND	11	3
HALT	31	26
FO0	10	29

6. The remaining signals need to be wired directly from the FGTIA below to the GTIAdigitizer above. Take care to not mix up the pin numbering, as the orientation of the two components is different now. Use this wiring table:

Signal	FGTIA pin	GTIAdigitzer pin
VCC	36	27
AN0	27	18
AN1	28	19
AN2	29	20
D0	17	7
D1	16	6
D2	15	5
D3	14	4
A0	13	2
A1	12	1