Building a Video-DAC

[ArcadeTV]

I want to rebuild the DAC inside an Amstrad GX4000 videogame. The conversion in this system happens inside an IC which adds a noticable amout of blurriness to the picture so I'd like to replace the IC with an R2R-ladder. It's a 4-Bit-DAC, the video signals for Red, Green and Blue have 4 signal-lines labeled r0 to r3, g0 to g3 etc.

Here's what I see on the scope while the system is displaying a pure white screen on the input of the DAC-IC:


R0 to R3 all look the same.
The Vpp is 4.92V, I need to build the R2R-ladder with a maximum output-level of 0.7V, that would be the correct input level for the THS7314 that I'm going to use for amping the signals.

I've never build a DAC before, I understand its concept and the function of a voltage-devider but I need some help please to calculate the values for R and R2.
Also in an R2R there should be an opamp I guess, I'm pretty lost because I wouldn't know which one would be suitable for my application.

Any help with this is very much appreciated, thanks guys.

[zerorisers]

Vout=R2/R1+R2*Vin according to Wikipedia on voltage simple voltage dividers http://en.m.wikipedia.org/wiki/Voltage_divider

[radix]

Maybe you could use this as an inspiration:https://github.com/fenlogic/vga666, http://raspi.tv/2014/gert-vga-666-review-and-video.

[katzohki]

Give this a look too:

http://www.circuitstoday.com/digital-to-analog-converters-da

[ArcadeTV]

Thanks guys, that is enough to get going.
Much appreciated!!!