Converting RGBHV to RGBs for the GBS-Control

The GBS-Control (GBS-C) is an amazing piece of affordable hardware. For around 30 bucks worth of parts, I have a good scaler which does the job for me. It also has excellent down-scaling to 240p which I was hoping to take advantage to hook up a more modern image to a 15khz CRT.

The GBS-Control has a quirk due to it being based on off the self hardware. It only outputs RGBHV, not RGBs. You can enable an experimental component output mode, but for my purposes I need that RGB out.

Whats the difference between RGBHV and RGBs? How the sync signal is encoded. The VGA standard says that sync is carried across 2 wires, one for horizontal sync and the other for vertical sync. Even when “VGA” is outputting low resolutions like 480i or 240p, it separates H & V sync. This is what the GBS-Control does.

Most 15khz CRTs which accept RGB, accept RGBs. In this case the H & V sync signals are combined into one sync – C sync – which you might recognize the name. This is also called “composite sync” because it’s a “composite” of H & V, but it’s still only a sync signal. This is different to “sync on composite”, which is a composite video signal which has R, G, B and S all combined into one wire. You might know this as a regular yellow video cable plug.

In this case, I am trying to get RGBs out of the GBS-Control. I can then use VGA to BNC cables (with a BNC plugs for R,G,B and S) and connect this to my 15khz BVM.

What I wanted to do was design a dongle with a VGA connector on each side which you plug into the output of the GBS-Control. The input will be the RGBHV from the GBS-C and the dongle will have a similar D-Sub VGA port on the other side, but the output will RGBs.

Lets look at how we wired the input and output pins of these two d-sub connectors.

Picture source

When looking at your pin-out, be careful to number the pins directly. I always look at pin 6 and remember its the one which sticks out one side.

Pins 1, 2 & 3 are the red, green and blue wires. These will be passed straight through to the equivalent pins on the output connector. No change is needed to this signal. When creating this dongle, just wire pin 1 of the input connector to pin 1 of the output connector.

You need to do the same for pins 9 and 10. Pin 9 in the VGA spec has an optional 5v to help drive the output device. I have wired up pin 9 to pass the 5v because I will need it later for the connecting the Antonio Villena S-Video and composite adapter to this. More on that in another article. For now, I recommend wiring pin 9 directly across.

I also wired up pin 10, which is sync ground. Again, just a pass through wire is needed.

The tricky part here is combining H & V sync. You can not just twist the H&V together, we need to build a simple combining circuit which will input

H sync
V sync

and then output C Sync, along with a ground connector.

To create this circuit, I followed this guide on RetroRGB. Ste from HD Retrovision has designed this small circuit which safely combined H & V using only a dollar worth of parts.

If you have never done something like this before, the diagram will look confusing.

It all centers around a 2N3904 transistor, labelled here “Q1”. Just search your local electronics parts store for one of these. This picture explains how you identify the legs

Picture source

  • V sync (pin 14 of the input d-sub) needs to be connected to a 100 ohm resistor (labelled “R1”) and then to pin 3 of the transistor
  • H sync (pin 13 of the input d-sub) needs to be connected to a 10K ohm resistor (labelled “R2”) and then to the middle pin of the transistor

The output of the transistor (it’s pin 1) gets wired into two outputs:

  • Connect a 470 ohm resistor (labelled “R3”) and then connect this to pin 13 of the output d-sub, this is your c sync
  • Connect a 1K ohm resistor (labelled “R4”) and connect that to ground. I chose pin 5 of the input d-sub but other ground pins should also work

Just connect both outputs to the transistor pin 1. I took the 470ohm and 1K resistors, twisted them together and then soldered this onto pin 1.

Then wire this into your dongle and you are good to go. I don’t have a 3D printer but I found my dongle is fairly rigid, the combining circuit gives it some strength. For your own home purposes, it’s totally fine to use as is.

When I was making mine, I added small bits of shrink tubing to the connection points to ensure electrical isolation. I didn’t heat the heat-shrink, it short of just stays on by itself and I can move it back if I need to rewire my pins.

Now I can putout 480i or 240p from my GBS-C, connect my sync combining dongle and then add BNC breakout cable and it works great on my BVM.

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