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S-video just uses 2x the number of scan lines, so all the adapters do is run 2 of the s-video pins straight through to composite and end the other 2. >>
That's not true at all. The guy at Best Buy must have told you that
S-video keeps the luminance (brightness) and chrominance (color) signals separate. This allows the video signal to bypass one combine/split step in the transmission from the video source to the TV. This allows for a sharper, higher resolution picture because there is less processing happening to the signal. The comb filter in your TV is used to separate the signals when composite video is used.
2 of the s-video wires carry the luminace signal (1 wire is signal, the other is ground). If you connect only those you will get a black and white picture. The other 2 wires carry the color information (again, 1 wire is signal, the other is ground). If you connect just those you will not get any kind of picture. A s-video to composite adapter combines the signals into two wires, signal and ground. This has to be done in a certain way, you can't just tie all the s-video conductors together or just not connect a few and expect to get a composite signal.
Just thought you'd like to know.
<<edited for additional clarity>>