is that the one that comes out the back of the onboard graphics ??
More aboutwhat converter onboard graphics
Usually the onboard has VGA and sometimes HDMI or DVI. If it's blue it's probably VGA.
Also I'd have to guess you're plugging into an older TV, because any modern TV or monitor uses VGA, DVI, Display Port, or HDMI. They do not have S-video. S-video is actually pretty bad, I think it's like 720p max or something.
It is possible to get VGA to S-video/RCA converter.
Does your monitor or tv only have s-video?
The onboard video is the output directly connected to your motherboard. As for a converter you need one that converts whatever that output is to whatever input you are using... obviously.