Definición y significado de S-Video

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S-Video

A standard 4-pin S-Video cable connector, with each signal pin paired with its own ground pin.
Type	Analog video connector
General specifications
Hot pluggable	Yes
External	Yes
Video signal	NTSC, PAL, or SECAM video
Pins	4, 7, or 9
Connector	Mini-DIN connector
Pin out
Looking at the female connector
Pin 1	GND	Ground (Y)
Pin 2	GND	Ground (C)
Pin 3	Y	Intensity (Luminance)
Pin 4	C	Colour (Chrominance)
The shells should be connected together by an overall screen/shield. However, the shield is often absent in low-end cables, which can result in picture degradation.

Separate Video,^[1] more commonly known as S-Video and Y/C, is an analog video transmission scheme, in which video information is encoded on two channels: luma (luminance, intensity, "Y") and chroma (colour, "C"). This separation is in contrast with lower-quality composite video, in which all video information is encoded on one channel, and higher-quality component video, in which video information is encoded on three channels. S-Video carries standard definition video (typically at 480i or 576i resolution), but does not carry audio on the same cable. It's often referred to by JVC (who introduced the DIN-connector pictured) as both an S-VHS connector^[2] and as Super Video.^[3]

The four-pin mini-DIN connector (shown at right) is the most common of several S-Video connector types. 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.

  Overview

The luminance (Y; grey-scale) signal and chrominance (C; colour) information are carried on separate, synchronised signal and ground pairs.

The luminance signal carries horizontal and vertical synch pulses in the same way as a composite video signal, but S-Video luminance information can have a higher bandwidth than in composite video, which must devote some of its bandwidth to the chrominance information (beginning at roughly 3 Megahertz, depending on the encoding standard used). Both luminance and chrominance information in composite video therefore have to be low-pass filtered else crosstalk between high-frequency luminance information and the colour subcarrier will lead to unwanted video artifacts patterning when viewed. As S-Video maintains the two as separate signals, but still encodes two colour-difference signals into one chroma subcarrier, such detrimental low-pass filtering for luminance is unnecessary, although the chrominance signal still has limited bandwidth, and the colour crosstalk problem is subdued. The infamous dot crawl is eliminated. This means that S-Video leaves more information from the original video intact and offers an improved image reproduction compared with composite video.

  Other strengths and weaknesses

S-Video, with its two signals for video, is a compromise in terms of quality and convenience between composite video with one, and three-wire (or more) component video schemes. Using two video lines will, for example, use two inputs of video processing Integrated Circuits where two composite video inputs could have been accommodated, e.g. in the TVP5154A.^[4]

Compared with component video schemes where separate Red, Green and Blue (or luminance and two colour-difference signals) are given their own cables, S-Video is:

• poorer quality, because the colour information encoding (with a subcarrier frequency of perhaps 3.57 to 4.43 Megahertz, depending on standard) limits the maximum theoretical chrominance bandwidth possible, although the signal source may have its own limitations.
• Carrying the colour information as one signal means that the colour has 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 connected devices not only have to be S-Video compatible, but also compatible in colour encoding. In addition, S-Video suffers from reduced colour resolution. NTSC S-Video colour resolution is typically 120 lines horizontal (approximately 160 pixels edge-to-edge)^{[citation needed]}, versus 250 lines horizontal for the Rec. 601-encoded signal of a DVD, or 30 lines horizontal for standard VCRs.

Compared with Digital video systems, S-Video:

• requires less processing to feed analog televisions but more complex processing (and hence quality loss) to interact with digital systems, including computer storage and processing, as well as most modern televisions.
• Depth of colour depends in S-Video, and other analog video systems, on the Signal-to-noise ratio, but in digital systems it depends on the number of bits allocated (e.g. 2 bits to each colour-difference signal in 8-bit ITU-R BT.656 may mean S-Video has more realistic colours, even if the resolution and noise performance is worse than the digital system).

When used for connecting a video source to a video display that supports both 4:3 and 16:9 display formats, the PAL television standard provides for signalling pulses that will automatically switch the display from one format to the other. The S-Video connection transparently supports this operation. The S-Video connection also has general provision for widescreen signalling through a DC offset applied to the chrominance signal; however, this is a more recent development that is not widely supported.

  Connector and Cable

An S-Video signal is generally connected using a cable with four-pin mini-DIN connectors. Apart from the impedance requirement, these cables are equivalent to regular mini-DIN cables (like Apple's ADB). Apple-type cables can be used for S-Video transfer if no other cable is available, but picture quality may not be as good. Due to the wide use of S-Video connections for DVD players, S-Video cables are fairly inexpensive compared to component or digital connector cables.

The mini-DIN pins, being weak, sometimes bend. This can result in the loss of colour or other corruption (or loss) in the signal. A bent pin can be forced back into shape, but this carries the risk of the pin breaking off.

The cable should be made up with two twisted pairs (one for the luminance (pins 3/1) and one for the chrominance (pins 4/2)), with an overall screen(shield)connecting the shells. Preferably, the pitch of the twists should be different, as in a Cat 5 LAN cable. However, Cat 5 cable has solid core wires and, generally, no overall screen, so it is not suitable for this application. Alternatively, two separately screened cables with an overall screen may be used. This will significantly reduce crosstalk between each signal pair. Use of ordinary or unscreened cables causes impedance mismatches, which will degrade the picture.

Before the mini-DIN plug became standard, S-Video signals were often carried through different types of connectors. For example, the Commodore 64 home computer of the 1980s, one of the first widely available devices to feature an S-Video output, used an eight-pin connector similar to the DIN connector on the computer end and a pair of phono plugs on the monitor end. (Also available via third-party vendors was an eight-pin DIN-to-4-pin mini-DIN to connect the Commodore directly to a television.) The S-Video connector is the most common video-out connector on older laptop computers; however, many devices with S-Video outputs also have composite outputs.

The Atari 800 home computer featured S-Video outputs in 1979 (three years before the Commodore 64), via a five-pin DIN plug.

Both S-Video and audio (mono or stereo) signals can be transferred through SCART connections as well. However, it is not part of the original SCART standard, so many SCART-compatible devices do not support it for this reason. Also, S-Video and RGB are mutually exclusive through SCART, due to S-Video using some of the pins allocated for RGB. Most SCART-equipped televisions and VCRs (and almost all of the older ones) do not support S-Video, resulting in a monochrome picture if such a connection is attempted, as only the luminance signal portion is usable. A monochrome picture could also be a sign of incompatible colour encoding: for example, NTSC material viewed through a PAL-only device.

Another incompatibility (due to S-Video not being part of the original SCART standard) is when connecting a SCART output device such as a cable TV box to a TV with a mini-DIN S-Video input. In many cases when this connection is made, the result will be a predominantly black-and-white picture, with most of the colour (chrominance signal) washed out. An example of this is when connecting the SCART output of a FOXTEL Digital Box (Australia) to a mini-DIN S-Video input of a TV. An impedance mismatch between the SCART and mini-DIN interfaces causes the signal levels to be reduced at the TV end, resulting in a poor picture. This problem can be overcome by terminating the chrominance line of the SCART plug with a 75-ohm resistor, correcting the mismatch. Many high-end sets do support this connection, however (without the termination), due to their inputs having a larger dynamic range.

At least some Fujitsu laptops (S-7020, S-7110) use so-called mini S-Video connector ^[5] for TV-Out. This mini S-Video connector is actually a 3.5mm TRS connector where tip and ring carry Y/C, both using the sleeve as ground. Such a mini S-Video is rare, and it is very difficult to obtain a proprietary cable for it.

  Non-4-pin variants

  7-pin mini-DIN

  A 7-pin mini-DIN socket.

  7 pin plug diagram

This is a non standard 7-pin mini-DIN connectors (this variant is called "7P"). These are used on computer equipment (PCs). A 7-pin socket accepts and is pin compatible with standard 4-pin S-Video plug.^[6] The three extra sockets may be used to supply composite (CVBS) or an RGB or YPbPr video signal, or an I²C interface. The pin out usage varies between manufacturers.^[6] In some implementations, the remaining pin needs to be grounded to enable the composite output (or disable the S-Video output on some of those implementations).

  9-pin Video In/Video Out

These are used on graphics systems that feature the ability to input video as well as output it.^[7] Again, there is no standardisation between manufacturers as to which pin does what (also given that there 2 known variants of the connector in use).^[8] 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 this latter case, it becomes all too easy to misalign the plug when inserting it with consequent damage to the small pins.

  Usage

In many European Union countries, S-Video is less common because of the dominance of SCART, which allows RGB quality and is usually fitted to every TV. It is not usual to find S-Video outputs on equipment such as DVD players, although the player may output S-Video over SCART, but the TV may not be compatible with S-Video wired this way, and so would just show a monochrome image.^[9] In this case it is sometimes possible to modify the SCART adapter cable to make it work. Games consoles usually do not output S-Video, due to the dominance of SCART with its better RGB quality. However, in the US and other non-SCART countries, S-Video is provided but no RGB. The Nintendo 64 was an exception - NTSC models could output S-Video, but only with modification could they output RGB. PAL Nintendo 64 models could output S-Video but not RGB despite, that being the easiest way to connect if done via SCART. There is, however, a German company that is able to modify PAL N64's to output true RGB ^[10] although this service is rather expensive compared to the way you can make an NTSC model output RGB.

  Converting S-Video signals

• Simple mixing (addition of voltages) after appropriate low-pass filtering is required to convert S-Video to composite video. Conversion to or from any other standard requires more sophisticated circuitry.
• Encoding the R, G, and B analog components of a video signal into S-Video luminance and chrominance, or composite video, requires some analog processing, such as that provided by the AD723 integrated circuit (chip).^[11]
• Converting S-Video to or from digital video signals, depending on the digital standard involved, requires more processing - such as the CS4954^[12] NTSC/PAL Digital Video Encoder when converting from 27 MHz 8-bit YUV, 8-bit YCbCr, or ITU R.BT656.

  Gallery

  See also

• Audio and video connector
• RF connector
• Composite monitor
• List of video connectors
• Video In Video Out (VIVO)

  References

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.