Specifications

May/June 2000 ExtroNews 11.2 23
MISCONCEPTIONS
CAT 5 is low cost—Yes, you did see this comment
under the "Advantages" section. However, to transmit
CAT 5 video a transmitter-receiver pair is needed. These
can run up to approximately one thousand dollars per
pair. Compare the cost of a CAT 5 system versus a system
using a P/2 DA2 and an Extron 100 foot VGA cable. As
you can see, a CAT 5 system is not economical in all cases.
CAT 5 is easy to terminate—CAT 5 is easy to
terminate, but doing it right is another matter. Care
needs to be taken when terminating CAT 5 so that it
won't affect the quality of the signal. When
terminating CAT 5 it is important to not untwist a
twisted pair more than 1/2" at the termination point.
Untwisting the pairs just 1/2" will increase near end
crosstalk (NEXT) by 1.5 dB. Untwisting the pairs 2" will
cause NEXT to increase by 3.8 dB. Untwisting the pairs
6" will cause NEXT to increase by 11.6 dB.
Also, when terminating, the cable sheath should not
be removed more than needed. Again, this will effect
the NEXT specification, reducing performance.
Removing just 1" of cable sheath will cause NEXT to
increase 1.2 dB. Removing 12" of cable sheath will
cause NEXT to increase 2.0 dB.
I saw it at the show and it looked good—Keep in
mind that any manufacturer can make an image look
good at a tradeshow. When you have a known image
resolution, known cable quality, and known cable
length, the video signal can be equalized to completely
compensate for the system. Extron does this at every
INFOCOMM Shoot-Out to ensure equal signals are
presented to each projector. Since all the signal
distribution parameters are known and fixed, we have
been able to develop custom amplifiers that provide flat
signal response over the long coax cable runs.
CAT 5-based video is digital—This is incorrect. CAT 5
based video is actually a differential analog signal.
Small wire = low voltage—Where as coax is actually
low voltage 0.7V p-p (see figure below), CAT 5 actually
uses two amplifiers, resulting in a voltage differential of
1.4V p-p.
All phone wire is CAT 5—As mentioned earlier there
is a difference between twisted pair phone wire and
CAT 5. CAT 5 is made in accordance with more
stringent specifications. Using phone wire to run video
will likely result in a poor image.
*Near end crosstalk (NEXT) is a measure of how much
electrical signal is coupled from one pair of wires to the
next. It is measured at the near end (signal source side)
where the transmitted signal is the strongest.
0.7V p-p
1.4V p-p
Analog Amplifier for Coax Cable
Analog Amplifier for CAT 5
TECH CORNER