Calculating resistor values - Digital-To-Analogue

I came across this a while ago...

... and need to implement something similar but I can't figure out how to calculate the output voltages/current etc. I know Ohms law but am not so hot at applying it. So yeah, this is a pretty basic electronics question. :)

According to http://www.javiervalcarce.eu/wiki/TV_Video_Signal_Generator_with_Arduino where I stole the image from, the outputs are as follows:

P7 = 0, P6 = 0, Vo = 0v P7 = 0, P6 = 1, Vo = 0.3v P7 = 1, P6 = 0, Vo = 0.6v P7 = 1, P6 = 1, Vo = 1v

But could someone spare a few minutes to show me how those output voltages were calculated? You'd really be helping me out.

Cheers.

Does this: http://en.wikipedia.org/wiki/Voltage_divider help?

(75 / 1075) * 5 =0.34V

Ah... I think I get it now.

The key bit I was missing was the way the resisters change from being in a serial or parallel relationship with the 75r impedance depending on whether their pin is high, or grounded.

Cheers mate.