Advanced Deskew Technology For Sharper Video in Extenders and Switches


Baseband Member

Analog video presents a high quality image when the computer is directly connected to the monitor by short cables. In this case, image quality is limited only by the quality of the computer's video card and of the monitor itself. Unless they are of very poor quality, short video cables do not tend to degrade the image.

However, in today's data centers, labs, stores, and with digital signage, video cables may be quite long. Conventional VGA video cables are not a good choice here because they are bulky, expensive, and difficult to run at great distances (e.g., they cannot be pulled through conduit).

To overcome these practical video cable length limits, KVM switches –as well as both KVM extenders and VGA-only extenders – are used that send video through Category 5 or Category 6 cables, which we'll collectively abbreviate as “Cat x” here. One of the potential drawbacks of using Cat x cables is that their very design tends to induce video distortion, as we explain below.

ATEN has developed patented technology to correct a particular cable-induced distortion described as skew (or time base error). This is not a new idea, but most deskew implementations rely upon a time-consuming, two-way process of sending test signals down a cable, then waiting for a response from the equipment at the other end in order to make an automatic adjustment. Some systems even require the user to make tedious manual adjustments. ATEN's deskew cuts video distortion automatically with one-way signal detection and is therefore very fast.
Why Cat x cabling requires video compensation
Cable construction

Cat x cables were originally designed for data and voice communications, not analog video or keyboard/mouse signals. Each pair of wires in the cable was intended to carry one signal, and that signal is reversed in polarity in each of the two conductors. A balanced receiving circuit flips the polarity of the two wires, thereby instantly doubling the signal strength and simultaneously canceling noise that has entered the cable. To reduce leakage (crosstalk) of signals between the four twisted pairs, each wire pair is twisted at a different pitch or twist rate; the more twists per unit length of cable, the longer the ACTUAL wires in a given pair. The difference in length of the four pair of conductors in the same 305 meter (1000 foot) can be up to 15 m (50 ft)!
Skew – A degradation daused by cable construction and the laws of physics

The progressively longer conductors in each pair result in progressively more overall wire resistance (R). Capacitance (C) in the wires also increases with wire length. Basic electronic principles explain that the greater the resistance-capacitance coefficient (R x C), the longer the time it takes a signal to arrive at the other end of the wires. That translates to an increasingly shifted location for the display of the Red, Green, or Blue pixels. The result is a smeared, unsharp, color-fringed image which becomes visibly worse with longer cables, higher resolutions, and higher refresh rates. While such skew distortion is nothing new, it is a more serious concern in today's long-cable capable switch systems and extenders.
How is skew corrected?
The Basic Parameters

You may wonder, “How much skew is there?” Or, “How much difference in length is there between the individual wires in long cables?” The maximum length difference between pairs falls typically at 2 to 2.5 meters per 100 meters of cable (6.6 to 8.2 feet per 328 feet of cable). However, some cables have as much as 4 to 5 meters wire pair length difference per 100 meters, (13 to 16.5 feet per 328 feet). KVM switch (and VGA video extender) applications can involve up to three times this cable length, so the worst-case cable is about 305 meters (1000 feet), in which the length difference between the shortest and longest wire pairs (if untwisted) may be about 15 meters (49 feet). The difference in signal arrival time between the shortest (lowest pitch) wire pair and the longest (highest pitch) wire pair will vary from about 30 to 120 nanoseconds (billionths of a second), enough to noticeably shift the displayed red, green and/or blue pixels on the monitor, and inevitably the shift is different for each pixel color due to the different twist pitch of each pair of wires.

There is no physical means to speed up the late-arriving signals (from the longer wire pairs), so instead skew compensators delay the faster arriving video signals. This is the essence of skew correction (deskew) – adding proportionally more time delay (phase shift) to the signals in the shorter wire pairs so they “match up” with the arriving signal in the longest wire pair.
What is really needed for effective Deskew?

The method used to apply deskew must be largely automatic and rapid so that users' don't have to put their time and energy into setup. It has to be effective in correcting the video so a sharp, clear, undistorted image appears at any specified cable length. The correction has to sense which are the longer pairs (this is not a standard), and how much each pairs differs in signal arrival time at the monitor. Moreover, the deskew method has to provide independent correction at each user console (monitor) on a multi-user KVM switch or an extender/synchronizer with multiple consoles. The deskew system should work seamlessly with a wide variety of possible cable types that may be encountered.

In 2006, ATEN developed a new, very fast, and effective deskew process for which they have been granted multiple patents. Legacy deskew systems send test signals the length of the cable (encountering the first cable-induced delay); remote circuits then measure the arriving signal and generate a correction signal, and a second cable-induced delay occurs as the correction signal travels back down the same cable to complete the auto-adjustment cycle. With ATEN's one-way process, test signals are applied to all the wire pairs at the signal source end, these are detected at the far end of the cable, their relative phase (delay) is established, and variable delay elements are automatically and precisely adjusted at that point – no wait for a return trip down the cable.
Deskew enhances ROI (Return On Investment)

The deployment of KVM extenders, switches and synchronizers that rely upon Cat x cables, enhanced by automatic deskew technology embedded in the hardware, can save a lot of money. A single Cat 5e or Cat 6 cable now can be used where previously an expensive fiber optic device (and its expensive-to-install fiber cable) was required. With deskew technology, it may even be possible to use existing Cat x infrastructure and thus avoid the highest part of installation costs. Because deskew improves video clarity, operators can work more efficiently with less fatigue and fewer human errors; think of the time saved by avoiding incorrectly deleted files or directories because now the filename and cursor are sharp and clear! This translates into more productivity per worker.
ATEN Models using Advanced Deskew

* CE350 PS/2 KVM Extender
* CE370 USB KVM Extender
* ATEN KM0532/KM0932 5/9 Console x 32 Port Matrix KVM Switch with KA7240 Console Module
* VE510 Video Synchronizer

About ATEN

ATEN International Co., Ltd. is specialized in connectivity solutions in information technology. Established in 1979, ATEN is today considered the leading manufacturer of KVM Switches worldwide. This prominent position was reached by continuously high investment in research and development, resulting in numerous patents and exemplified by the sophisticated ASIC, developed in the ATEN labs and manufactured by ATEN. The product range today covers hundreds of connectivity products, providing complete KVM solutions from entry level to the enterprise market. Among the customers of ATEN are large companies with global operations, midrange and small businesses, as well as ambitious private users. For further information, please visit