S-video cables are used for the transmission of analogue video signals, split into luminance and chrominance (Y/C), which gives substantially better sharpness and contrast than a conventional composite video signal.
For transmission of S-Video signals, as for all other analogue video signals, coaxial S-video cables with an impedance of 75 ohms are used for optimum quality. If the cable does not have the correct impedance, you will often get shadows in the image. S-Video cables do not transmit audio signals, usually a separate cable is routed for this. S-video signals can operate at a resolution of up to 480i (NTSC) or 576i (PAL).