CGA to VGA scaling with GBS-8220 board

Connecting an old PC machine with a CGA video output to a modern VGA monitor requires changing the horizontal display frequency from 15kHz to 30kHz. Professional video scalers are very expensive devices, but fortunately there are popular Chinese alternatives, which cost only around 20$. The problem with them is that they rarely work out of the box.

I will describe here how I forced GBS-8220 video scaler to display my Schneider Euro PC CGA screen on a Sony VGA monitor.

CGA output and board inputs

A CGA video output consists of separate RGB signals, horizontal and vertical frequencies and ground (here facing the connector in a computer).

On the other hand, there are three input connectors on GBS-8220:

• 15-pin VGA (marked RGBHV signal input)
  It conforms to the standard VGA pinout with GND, R, G, B, HS, and VS pins used.
  
• 8-pin white connector (marked 8-pin RGBS input)
  Pins are: GND, GND, VS, HS, S, B, G, R.
  The S pin is connected to the HS pin – they are the same.
• 5-individual-pin connector (marked CGA/EGA signal input)
  Pins are: R, G, B, GND, S.
  The S pin is the same as HS pin on other connectors.

The pins on these three connectors are exactly the same – the are all wired together on the board. Some people experiencing sync issues attempt to try different connectors. If they notice a difference in the stability of the synchronization, this is most probably due to changes in the capacitance of the sync wires, that reduces the crosstalk between the signals. Nevertheless, it should not dramatically change the result. My suggestion is to use the VGA connector wherever possible, because the VGA cable attached is usually shielded.

Similarly, the two power connectors available on the board are exactly the same – it should not really matter which one you want to use.

Using RGBHV mode

GBS-8220 has a special RGBHV mode, in which (in the absence of any decent user manual) as I deduct from the name, it should interpret the separate horizontal and vertical syncs.

However, connecting the CGA signals directly to my GBS-8220 does nothing – the board can’t detect any signal. Search in the Internet reveals many people have this issue and it is suspected that it depends on the firmware version you have installed on the board. Unfortunately, there are no firmware updates available.

This makes RGBHV mode useless for me.

Using RGBS mode

In RGBS mode, GBS-8220 board expects a Composite Sync (CS) signal on its S or HS input. VS input seems to be not used in this mode at all. This is written nowhere in the instruction.  Marking the 5-pin connector as “EGA/CGA input” is also pretty misleading, because there is no composite sync on EGA/CGA connectors.

Combining HS and VS signals into Composite Sync (CS)

CS = not (HS xor VS)

This operation can be achieved with two chips:

They can be powered from the GBS-8220 board, using one of the two power connectors.

This is my bench for the experiment:

How the horizontal and vertical sync signals look on the Euro PC:

The results of HS xor VS using 7486 chip:

And finally the Composite Sync, which is a signal from the previous picture negated on 7404 chip:

Reducing the crosstalk

Producing the CS signal from HS and VS and connecting it to the S input of GBS-8220 in RGBS mode may be not enough though. It occurs, the HS and VS inputs may be disturbed by the interference from RGB signals. I am not sure, if this is due to the Euro PC, GBS-8220 or the cables between them – probably a combination of all. When the interference is strong, the scaler will not be able to properly sync to the video signal.

I did the following things to reduce the crosstalk effects:

1. Used VGA input on GBS-8220 and attached half of a VGA shielded cable to it
2. On the other (computer) end used a half of RS-232 shielded cable
3. Soldered R, G, B, GND wires and shields from both cables together close to the computer connector (RS-232 cable was of a poorer quality than VGA cable).
4. The HS and VS wires from both cables moved outside of the cable in order to connect it to the CS composition circuit
   • VS, HS from the computer side as inputs
   • HS from GBS-8220 side as output
   • VS from GBS-8220 side connected to the ground
5. In the CS composition circuit, added a 150 pF capacitor between VS and GND and 220 pF capacitor between CS and GND – the values need to be found experimentally.

This reduced the visible crosstalk effects to a level that allowed the GBS-8220 to finally display the picture.

Bingo!