The Dreamcast outputs both 480i and 480p video. Rather than using a regular scart cable and a VGA box I wanted to combine the two into a single scart cable. To do this it's necessary to combine the TTL logic level H and V sync siggnals generated by the Dreamcast and produce a 75ohm impedance SCART compatible signal.
The Dreamcast has two RGB select lines pin 7 (rgb mode select) and pin 6 (VGA mode select). Pin 7 should be tied to ground as it is grounded for either type of rgb. I used a DPDT switch to switch pin 6 between floating and ground and the sync from csync to the combined 480p sync signal.
This is a simplified version of the sync circuit at this site.
I'm only running the sync signals through one XOR gate, and I replaced the transistors with 2n3904s. I had them on hand and they seem to work just fine.
I tried twisting the sync wires together after the 150 ohm resistors and connecting them directly. This produces a clear but slightly shaky signal. touching the wire made the image rock solid. It would appear you could just run a resistor (in this case me) to ground to drop the sycn signal to an acceptable level. While this works, it seemed best for the Dreamcast and my RGB to YUV converter to put in a buffer circuit.
I had a dreamcast scart cable, and a cheap and broken vga box. I used the vga box plug as the scart cable was missing the hsync and vsync lines. The vga box plug had the sync line 150 ohm resistors built into it. The scart cable included the 220uF RGB line caps. The rest of the circuit was built on a bit of perfboard.