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How To Calculate Skew Limits

Ideally, all of a video signal's components (R, G, B, and sync) should arrive at a display at exactly the same time. When they don't ? as a result of passing through multi-pair cable ? the condition 9/22/2005 6:49 AM Eastern

How To Calculate Skew Limits

Ideally, all of a video signal's components (R, G, B, and sync) should arrive at a display at exactly the same time. When they don't ? as a result of passing through multi-pair cable ? the condition is termed ?skew.? While a certain amount of skew usually isn't detrimental, the effect becomes more noticeable as the resolution level of the video source or the unshielded twisted pair (UTP) distance is increased.

Ideally, all of a video signal's components (R, G, B, and sync) should arrive at a display at exactly the same time. When they don't — as a result of passing through multi-pair cable — the condition is termed “skew.” While a certain amount of skew usually isn't detrimental, the effect becomes more noticeable as the resolution level of the video source or the unshielded twisted pair (UTP) distance is increased.

Skew isn't a problem when twisted pair cable is used for composite video signals because the luminance and chrominance signals are sent together over a single pair of wires. Skew also isn't a problem for any NTSC resolution-level signals — even when transported on alternate pairs —because the scan rates are considerably slower than for VGA-type signals. This is also true for S-video, component (Y, U, V video), and distances of less than 2,000 feet of UTP.

For high-resolution video, the observable effects of skew are primarily dominated by three factors:

• Cable characteristics (skew per unit length)
• Cable length
• Video signal resolution and scan rate
• Skew compensation

There are two approaches to compensate for detrimental skew effects:

• Use a “skew-free” cable designed for transmitting computer video.
• Use a skew compensation device. These devices may be integrated or standalone, with varying ranges and resolutions. They work by introducing timing delays to one or two colors, allowing R, G, and B to concurrently arrive at the display.
• Source: Magenta Research

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