Hall Research URA-XT - Manual (Page 13)

PC (VGA) & HD (YPbPr) AV over Twisted-Pair Receivers
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3.4 Why Skew Adjustment?
UTP cables have 4 twisted pairs inside. The Hall Research UVA/URA
video transmission on UTP uses 3 individual pairs for each color
(Red, Green, & Blue).
Figure 6 - Skew mechanism
As shown in the figure above, a characteristic of the Category 5/5e/6
cables is that the pairs of wires are twisted at different rates.
Therefore, for a given length of Cat6 cable the total length of any
particular pair could be longer than other pairs in the same cable.
Since the signals travel along the length of each pair at a fixed speed,
the arrival times of signals will be skewed in a long cable (those that
have to travel farther arrive later and the corresponding color shifts to
the right).
This is seen on the monitor as separation, or lack of convergence in
colors. For example, a vertical white line on the screen may seem to
have a red tinge on the left edge and blue tinge on the right edge.
This effect gets worse at high resolutions, high refresh rates, long
cables (in excess of 200 feet), and depends on the cable construction
itself.
If you are using special UTP cables that are specifically designed for
video transmission (such as Hall Research Zero-Skew™), then there
should be no shift in color alignment regardless of the cable length. In
many applications, standard Cat6 cables may be utilized, but this will
necessitate a receiver that can move each color component to the left
and right in order to realign them. Models URA-SKU and URA-XT
both are capable of doing this. Hall Research also offers a stand-
alone skew corrector unit (Model SKU-RGB).