S-Video (also known as separate video and Y/C) is a signaling standard for standard definition video, typically 480i or 576i. By separating the black-and-white and coloring signals, it achieves better image quality than composite video, but has lower color resolution than component video.
Standard analog television signals go through several processing steps on their way to being broadcast, each of which discards information and lowers the quality of the resulting images.
A signal with three components is no easier to broadcast than the original three-signal RGB, so additional processing is required. The first step is to combine the Pb and Pr to form the C signal, for chrominance. The phase and amplitude of the signal represent the two original signals. This signal is then bandwidth-limited to comply with requirements for broadcasting. The resulting Y and C signals are mixed together to produce composite video. To play back composite video, the Y and C signals must be separated, and this is difficult to do without adding artifacts.
Each of these steps is subject to deliberate or unavoidable loss of quality. To retain that quality in the final image, it is desirable to eliminate as many of the encoding/decoding steps as possible. S-Video is an approach to this problem. It eliminates the final mixing of C with Y and subsequent separation at playback time.
In composite video, the signals co-exist on different frequencies. To achieve this, the luminance signal must be low-pass filtered, dulling the image. As S-Video maintains the two as separate signals, such detrimental low-pass filtering for luminance is unnecessary, although the chrominance signal still has limited bandwidth relative to component video.
In many European countries, S-Video was less common because of the dominance of SCART connectors, which are present on most existing televisions. It is possible for a player to output S-Video over SCART, but televisions' SCART connectors are not necessarily wired to accept it, and if not the display would show only a monochrome image. In this case it is sometimes possible to modify the SCART adapter cable to make it work.
The four-pin mini-DIN connector is the most common of several S-Video connector types. The same mini-DIN connector is used in the Apple Desktop Bus for Macintosh computers and the two cable types can be interchanged. Other connector variants include seven-pin locking "dub" connectors used on many professional S-VHS machines, and dual "Y" and "C" BNC connectors, often used for S-Video patch panels. Early Y/C video monitors often used phono (RCA connector) that were switchable between Y/C and composite video input. Though the connectors are different, the Y/C signals for all types are compatible.
9-pin connectors are used in graphics systems that feature the ability to input video as well as output it. Again, there is no standardization between manufacturers as to which pin does what, and there are two known variants of the connector in use. As can be seen from the diagram above, although the S-Video signals are available on the corresponding pins, neither variant of the connector will accept an unmodified 4-pin S-Video plug, though they can be made to fit by removing the key from the plug. In the latter case, it becomes all too easy to misalign the plug when inserting it with consequent damage to the small pins.