S-Video

From Free net encyclopedia

Separate video, abbreviated S-Video and also known as Y/C (or erroneously, S-VHS and "super video") is an analog video signal that carries the video data as two separate signals (brightness and colour), unlike composite video which carries the entire set of signals in one package. S-Video works in 480i or 576i resolution.

1 Overview
2 Connector
3 Usage
4 See also
5 References

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Overview

The luminance (Y; brightness) signal and modulated chrominance (C; colour) subcarrier information are carried on separate synchronized signal/ground pairs.

In composite video, the luminance signal is low-pass filtered to prevent crosstalk between high-frequency luminance information and the color subcarrier. S-Video separates the two, and detrimental low-pass filtering is unnecessary. This increases bandwidth for the luminance information, and also subdues the color crosstalk problem. The infamous dot crawl is eliminated. This means that S-Video leaves more information from the original video intact, thus having a much-improved image reproduction compared to composite video.

Due to the separation of the video into brightness and colour components, S-Video is sometimes considered a type of component video signal, although it is also the most inferior of them, quality-wise, being far surpassed by the more complex component video schemes (like RGB). What differentiates S-Video from these higher component video schemes is that S-Video carries the colour information as one signal. This means that the colours have to be encoded in some way, and as such NTSC, PAL and SECAM signals are all decidedly different through S-Video. Thus, for full compatibility the used devices not only have to be S-Video compatible but also compatible in terms of colour encoding.

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Connector

Image:SVideoConnector.jpg Today, S-Video signals are generally connected using 4-pin mini-DIN connectors using a 75 ohm termination impedance. Apart from the impedance requirement, these connectors are identical to ones used for the now-obsolete Apple Desktop Bus; ADB cables can be used for S-Video transfer if no other cable is available, but picture quality may not be as good, and there is a risk of damage to unshielded cathode ray tubes via incompatible resistances.

The pins in the connector are bent rather easily, but this can be prevented using care when properly inserting the connector to the receptacle. If a pin is bent, the result could be the loss of colour, or other corruption (or loss) of the signal. A bent pin can be forced back into shape, but this carries the risk of further damage, or even the pin breaking off.

Before the mini-DIN plug became standard, S-Video signals were often carried through different types of plugs. For example, the Commodore 64 home computer of the 1980s, one of the first widely available devices to feature S-Video output, used an 8-pin standard size DIN plug on the computer end and a pair of RCA plugs on the monitor end. The S-Video connector is the most common video-out connector on laptop computers, however many devices with S-Video outputs also have composite outputs.

Today, S-Video can be transferred through SCART connections as well. However, it was not part of the original SCART standard, and not every SCART-compatible device supports it for this reason. Also, S-Video and RGB are mutually exclusive through SCART, due to the S-Video implementation using the pins allocated for RGB. Most SCART-equipped televisions or VCRs (and almost all of the older ones) do not actually support S-Video, resulting in a black-and-white picture if attempted to use, as only the luminance signal portion is used. Black-and-white picture in itself can also be a sign of incompatible colour encoding, for example NTSC material viewed through a PAL-only device.

A hack exists to possibly attain color on devices that do not support S-Video through SCART. This is done via joining the pins 15 and 20 in the SCART connector (either directly or using a 470pF capacitor), and may not yield optimal results.

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Usage

S-Video is commonly used in USA, Canada, and Japan, found there on consumer TVs, DVD players, video tape recorders and game consoles. Almost all TV-out connectors on graphics cards are S-Video, even in Europe, where the standard failed to make a significant impact due to the preference of the higher-quality RGB signal.

Some particularly cheap S-Video cables are notorious for degrading the signals considerably, when transmitted across more than 5 meters.

Because it is very simple to convert S-Video to composite signal (just the logical merging of the two through a filter is required) or vice versa, many electronics retailers offer converter adaptors for signal conversion. Obviously, no conversion will improve image quality, but will allow connecting to otherwise-incompatible devices.

S-Video does not carry audio. Hence, a separate audio connection is required.

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