An S-Video to RCA adapter is nothing more than a single capacitor joining the signal wires, and both grounds tied together. Look here:http://www.epanorama.net/circuits/svideo2cvideo.html

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This works for converting an S-Video out port into a composite out port. Usually that's all the people use anyway. (The same thing works to convert a composite in port into an S-video in port.)

But if you want to convert a composite out port into S-Video out port, (or an S-video in port into a composite in port), this will not work. The color/chroma signal needs to be separated from the brightness/luma signal somehow.