CGA to RGB convertor


A simple single board adapter to convert CGA style digital RGBI into analogue RGB.

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SKU: cga2rgb Category: Tags: , , ,


This board is a single device solution for converting digital RGB to analogue RGB more suitable for modern displays. It does NOT do frequency conversion (scandoubling). A 15 pin VGA style output connector is used, but unless your VGA monitor is capable of the frequency present on the input (usually 15khz for CGA) it will not work be able to display the output. Please verify your monitor frequency range or use a scandoubler if required.

Using a single GAL this device does RGBI+HV into RRGGBB+s. Thats 4bit into 6bit colour and h/v sync into proper composite sync. The 6bit output (2bits per colour) feeds a resistor DAC to provide the most perfect colour output I think it’s possible to get from CGA output.

As the conversion is done digitally there is no loss in quality or washing out of colours often seen in cheap passive adapters using resistors, this also makes it possible to include the “brown fix” that is done inside most CGA monitors to turn one of the yellow shades into brown widening the palette available.

As well as the RGB output there is a pin header on the board for connecting to an S-Video source as well as mono audio and the 5v supply. When used with the Commodore 128 this allows feeding 5v from the 128 to the adapter board as well as hooking up the VIC-IIe’s S-Video output and audio in an easier way using a single Din cable to the AV socket on the back of the machine. The S-Video from the VIC goes to a real S-Video socket and the audio goes to a 3.5mm audio jack.

Having a VGA style 15pin connector for RGB output makes hooking up to multi-sync monitors or RGB to VGA/HDMI adapters easy. The Gonbes style arcade RGB to VGA adapters work and can be connected easily with a normal VGA cable between this board and the Gonbes. It is also possible to make a SCART to 15pin Dtype cable for use on Euro TVs that support RGB on SCART input.

5v is passed through to pin 9 on the 15 pin connector output. Please check that your cable/monitor does not short pin 9 to ground before connecting. This voltage is present so that auto switching SCART cables can be made that force the TV/Monitor into RGB mode.

There is a pad near the power led and pin header to insert a resistor on the chroma line but I have not had need of that yet and this is shorted on the PCB. To use it and add your own resistor you will need to cut the track shorting the 2 pads before adding the resistor, I have heard 330ohm recommended however as mentioned, I have not needed it.

An onboard power LED confirms the presence of 5v to the board. Please double check the power connections as reverse polarity or incorrect voltage will cause damage to the board.

The 6pin header is as follows,
1 Ground
2 5v+
3 Audio
4 (not connected)
5 Luma (connected to S-Video socket)
6 Chroma (connected to S-Video socket)

Additional information

Weight 60 g
Dimensions 50 x 50 x 20 mm