[What is S-Video]
S-Video (AKA Y/C, Super Video, Separate Video) is an upgrade from composite signal.
It separates the luminance signal (Y/luma) and the chrominance signal (C/chroma).
It uses a 4 pin mini-din connector also known as ushiden.
Many consoles support it natively, many have fairly easy mods for it.
In north america it's fairly easy to find a CRT TV with S-video input and for some devices like projectors it's the best connection for a console.
It's video only so you need to carry audio with separate cables.
It's a huge quality upgrade over RF or composite ; going from S-video to RGB/component yields a much smaller quality jump.
240p consoles benefit the most, DC and beyond should use RGB, component or VGA if possible.
Some games were made with composite in mind, this is especially true with many US Genesis games because its 320 horizontal resolution interferes with composite encoding. In these games you will see typical vertical dithering pattern with S-video instead of expected blending. Despite this, many times the clarity gain is worth it because the game is colorful enough (both Earthworm Jim are good exemples). Some others like Vectorman 1 get plain ugly though.
Many low quality console cables are floating around, OEM cables are often rare and expensive.
For some reason, starting with Gamecube Nintendo doesn't want S-video on their PAL consoles. This is a problem mainly for australian users, as here S-video TVs are plentiful but SCART ones are rare.
[Consoles with native support]
Super Nintendo / Super Famicom
JP/NA Wii U
[Consoles with mods that enable S-video]
Megadrive / Genesis (most of them)
NES / Famicom
[Getting the best out of S-video]
While S-video is a great upgrade, you can still run into quality troubles.
It's best to add an S-video port to use a regular S-video cable even if your console natively supports it because high quality ones are cheap. Search the pinout on the AV port, and solder wires to the s-video socket, preferably with shielded wires.
If you want to use a S-video cable made for your console, you risk running in the dreaded checkered pattern.
Contrary to what info is mostly found on the internet, the pattern is not caused by composite signal being routed on Y/C pins, most cables are actually wired correctly ; it's caused by luma and chroma wires being too close to each other and/or wires too thin.
Some say that a cable with no yellow composite plug is less likely to have the pattern, but this is false as well. Look how thin and close those wires are.
Outside of using OEM cables which are generaly high quality, you can try opening the connector and replacing the S-video wire with a thicker, higher quality wire ; even if they're not shielded, the simple fact that each have thicker insulation and that each has a ground wire can be enough to stop interference. It's then up to you to make it look as good as possible.
If you want to try your luck and buy one, try to find a cable with thick wires, or at least with the S-video wire being thicker than the audio wires.
Comparison between 2 cables :
look at the light orange lines above mountains. Colors are uniform and clean.
Each line seem to have alterning dots, and you can also see it on darker orange. In motion it's worse as the dots are moving.
See links for original 4000x3000 shots in the archive.
PAL Gamecube, Wii, Wii U don't support S-Video.
N64 stock can't output more than S-video.
PAL N64 requires additional components on the Y/C lines, can be put in either the cable plug or in the console itself.
PAL Super Nintendo requires additional components but only on the C line. Note that exact capacitor value does not matter much, I used 100nF and it worked fine.
Super Nintendo/Famicom can exhibit a vertical line, it might get more noticiable because of quality jump. It's fixable with 2 220µF caps, one one the regulator, they other on the video encoder power pins. Be sure to check proper pinout.
Supposedly, OEM Nintendo AV and S-video cables are only downward compatible, meaning you can use a Gamecube cable on a Super Nintendo / Famicom, but not the other way around.
NES/Famicom requires a NESRGB board which is a fairly extensive modification. Once done, wiring is straightforward. If you mod a HVC-101 AKA AV Famicom, you can wire it to the multi-AV out and use a SNES/SFC/N64/GC cable, even one made for PAL consoles.
PAL Megadrive modded to 60hz won't output color because chroma subcarrier frequency is derived from master clock divided by 15 for NTSC or 12 for PAL. On a PAL Megadrive, 53.2034/15=3.546893 mhz which is too far from NTSC 3.579545 mhz. Solution is to either also replace the master clock with a 53.693175 oscillator (but you won't get color for 50hz).
Or build a small circuit that will always provide a correct frequency to the encoder no matter what mode is chosen. This might also be true for Master System.
Megadrive / Genesis, depending on encoder used, the circuit is not the same and some cannot be S-Video modded.
The vast majority of model 1 and some model 2 use Sony CXA1145, which require a transistor and a resistor on the luma line, and a capacitor and resistor on the chroma line. See links for a web page that explains it.
Some late model 1 and many model 2 use Fujitsu MB3514 which has the same pinout for S-video. Wiring is simpler, each line only needs a 75 resistor and a 220µF cap. However you can't use color mod mentionned above, you have to use a 4 pin oscillator and some resistors / caps to feed a proper sub carrier.
Late model 2 and model 3 use Sony CXA1645. Same wiring as MB3514 but different pinout so be careful.
Some model 2 use a Samsung KA2195, which doesn't support S-Video at all. Only found in NTSC consoles.
Some rare model 2 use a ROHM BH7236. Judging by datasheet, S-video pinout and wiring is the same as MB3514, including an optional color mod circuit.
French RGB Master System 1/2 have no video encoder and thus cannot be modded.
PAL color encoding is slightly better than NTSC, even with S-video ; reds tend to bleed less. If your display supports it, you might want to set the encoder in the console permanently to PAL. This is especially true with PAL Megadrive as switching to 60hz also changes encoding. Super Nintendos are already set permanently.
PAL encoding + 480i/60hz + 4.43mhz subcarrier = PAL60, not a standard but usually largely supported on euro TVs.
PAL encoding + 480i/60hz + 3.57mhz subcarrier = PAL-M, used in Brazil. Subcarrier isn't exactly NTSC but very close.
NTSC encoding + 480i/60hz + 4.43mhz subcarrier = NTSC443, also not a standard but usually supported well enough.
Exemples of checkered pattern (go for pics, not for explanation)
S-video mod for consoles using Sony CXA1145
Gallery, some pics found, some taken myself