Would using screened audio cable such as **broken link removed** work for sending S-Video or composite video over 10-15m? Either using the two cores to carry the chrominance and luminance signals of s-video or just using one core to carry the composite signal. What kind of signal degredation would I get?
75Ohm refers to characteristic impedance, not a resistance Ohms/Km measurement. Also SVideo cables tend to have two seperate shields, one for each signal.
15m's is a long distance for a video signal. Seeing as how there is a fair number of variables, you're really not going to know unless you try it, and trying it will be a lot faster than trying to calculate it.