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TP Receivers • Installation and Operation
Installation and Operation, cont’d
2-18
Skew exists between pairs when the physical length of one
wire pair is different from another. As the transmission cable
length increases, the amount of skew increases. Skew affects
the displayed image when the differential length between wire
pairs exceeds 2 feet, causing the timing of red, green, and blue
video signals to appear out of alignment (horizontal registration
errors). A white vertical line on a black eld can appear as
individual red, green, and blue lines that are close together;
the signal transmitted on the shortest wire pair leads the other
colors and appears to the left on the display.
UTP cable test equipment measures and reports wire pair
length. The report on the various pair lengths can be used in
equalizing pair skew. The nominal velocity of propagation
(NVP — the speed at which the signal travels on the
transmission line, measured as a percentage of the speed of
light) of TP cable is very close to that of conventional coaxial
cable. The similarity in NVP means that an additional length
of coax equal to the length of pair skew, placed on the receiver
output, equalizes the effects of pair skew (gure 2-11).
If UTP cable test measurement cannot be done, pair skew can
still be equalized by viewing a test pattern with a critical eye.
Examine the test pattern for loss of horizontal registration and,
through a process of trial and error, equalize any pair skew with
coax extensions on the red, green, and/or blue outputs.
TP T 15HD A
TP R BNC A
TP T 15HD A
H
-S
H
IF
T
B
U
FF
E
R
E
D
C
O
M
P
U
TE
R
IN
P
U
T
A
U
D
IO
ID PIN 4
ID PIN 11
L
O
C
A
L M
O
N
IT
O
R
PC Computer
CAT 5 UTP Cable
Audio
LCD Projector
... THEN insert a four foot
extension cable to equalize
TP skew for red video.
IF cable measurement
indicates that the pair
with wires 1 and 2 is
four feet shorter than
the other pairs...
RGB INPUT
RGB OUTPUT
R G B H/HV V
A
AUDIO
L
R
B
SOG
C SYNC
POWER
1
5
V
.5
A
D
C
L
R
Pair RGB video
1, 2 Red
4, 5 Green
7, 8 Blue
Figure 2-11 — Pair skew equalization