Specifications

May/June 2000 ExtroNews 11.2 21
TECH CORNER
Y
ou probably know by now that “CAT 5” refers to twisted pair
wire, and not a rock group or new breed of feline. You prob-
ably also know that CAT 5 is being used to carry video. This leads
many in the AV industry to the wonder about the specifics of
CAT 5. What exactly is CAT 5? How reliable is it for carrying video?
What are the advantages and disadvantages of CAT 5? Let’s start
with answering the first question, “What exactly is CAT 5?”
CAT 5 originated in the computer
industry as a means of carrying data. In
the late 1970s IBM experimented with
using special constructions of twisted pair
wire to carry computer network
communications. The wire was much bigger
than the type used today, but it still showed there
were advantages in using wire to carry computer
data. Later it was found that smaller unshielded
twisted pair (UTP) wire could carry high speed digital
data reliably. Various grades of UTP wire were created
and categorized. The higher the category number, the
more stringent the design and manufacturing
requirements, and the higher the possible data rate.
The categories are referred to as “category 1”,
“category 2”, and so on, but they are typically
abbreviated as “CAT 1”, “CAT 2”, etc. Here is a list of
the categories and capabilities as defined by the
computer industry:
CAT 1: This category had no basic performance
characteristics specified, and is basically defined as
telephone voice-grade copper cable.
CAT 2: Capable of a frequency bandwidth of up to
1 megahertz (MHz). It was used on early slow-speed
(1-2 Mbps) LAN technologies like Arcnet.
CAT 3: CAT 3 was designated as the minimum quality
twisted-pair cable that would handle 10 Mbps Ethernet
and 4/16 Mbps active Token Ring without errors at the
desktop.
The In’s and Out’s of CAT 5
Advantages, Disadvantages & Misconceptions Part 1
by Roger McCarten, Product Development Manager
CAT 4: Rated up to 16 Mbps.
CAT 5: Typically consists of four pairs of unshielded
twisted copper wires covered by an outer jacket. It has
a bandwidth speed of 100Mbps. Special construction
methods are used to ensure category 5 performance
and limit crosstalk in these multiple cables. CAT 5 is
primarily used for Ethernet networks.
Currently, only category 3 and category 5 wire are
commonly used. CAT 5 looks much like the unshielded
twisted pair wire that is used in the telephone industry.
However, as just mentioned, it is made to a more
stringent specification. Substituting standard twisted
pair “telephone” wire for CAT 5 would decrease system
performance considerably.
VIDEO TRANSMISSION ON CATEGORY 5
It is important to keep in mind that CAT 5 was
developed by the computer industry for transmitting
digital data over computer networks. It was only more
recently that CAT 5 began to be used to carry analog
video signals. Using CAT 5 for video is not simply a
matter of soldering BNC or VGA connectors to the wire
and then connecting equipment together. A special
type of signal (differential analog) is used to make the
best use of CAT 5 wire, in spite of its limitations.
To transmit video over CAT 5, a special
transmitter/interface is needed to convert the video to
a differential analog signal that is a better match for
CAT 5 wire. At the receiving end a receiver is needed to
convert the differential analog signal back to a video
signal.
There are many misconceptions about using CAT 5
for video. To the question “Is CAT 5 wire a good choice
for running video?”, the best answer is “CAT 5 may be
a good choice if...” CAT 5 has definite advantages, in
certain situations. It also has definite disadvantages, in
certain situations. The following page is a list of
advantages, disadvantages, and misconceptions.
continued on next page