The Model 1 Genesis has an RF plug and an 8-pin plug for Composite video, RGB and Mono audio with Stereo audio available from the front headphone jack, the Model 2 on the other hand has a RF plug and a 9-pin plug for Composite/RGB and Stereo sound.
Here's the plug pin-outs:
To squeeze out S-Video out of a Genesis, modifications
are required because S-Video uses Chroma and Luma, not Red/Green/Blue or Composite. As such, you have to extract those signals from the graphics chip itself as they are not provided on the standard plug.
RF - typical radio frequency plug for old televisions, video and audio jumbled together
Composite - yellow plug for video, colours composited together, separate plugs for audio
Component - three separate plugs for YPbPr
(Luma, Difference between Luma and Blue, Difference between Luma and Red) with additional plugs for audio
S-Video - one four-pin plug with Chroma, Luma, C-Ground and Y-Ground
RGB - 21-pin SCART connector, colours are separate