the axia guide to choosing category cable
TRANSCRIPT
The Axia Guide to Choosing Category CableBy Stephen H. Lampen, Multimedia Technology Manager, Belden Electronics Division, Belden CDT
Abstract
If you’ve gotten to this paper, you’ve probably
read the promotional literature for the Axia
line of products, and maybe the “Introduction
to Livewire”. If you have, then you know that
this is a new type of audio control product,
one that is an amalgam between computer
networking and audio processing. It is a
100Base-T network running audio as the data
stream, with all the advantages of cost, simplicity,
power and elegance of a computer network.
But it also means that you will be installing
a network based on network cables, usually
called Category cables. If you’re an old broad-
cast engineer, like me, this move to a networked
architecture, and these strange un-
shielded cables, may be a leap of
faith. But let me assure you, many
other engineers have taken this
leap and not only survived but
prospered.
There are lots of other people
(those IT network types) on the
other side. And the Telos/Axia
folks are more than eager to help you through
the learning process. Because that’s really all
you have to do, learn about this “new way” of
running audio. There are tens of thousands of
very reliable data networks running Ethernet®
around the world, and have been for many years.
Hey, it’s not lonely at all. In fact, it’s kind of
crowded! Now you probably didn’t become a
broadcast engineer or installer to end up a data
dude (or gal), but, sorry, you are now offi cially
a network installer.
And one of the things you may have to learn
about is cable, specifi cally “premise/data”
cable that comes in a number of fl avors, called
Categories. Many people call these “Cat”
cables, short for Category. We’ll discuss Cat
5e, and Cat 6 (such as Figure 1), a bonded-pair
Category 5e, Belden 1700A. We’ll examine
how Category cables are different, and how
they are the same, how to choose good cable
from bad cable, and how to get the most bang
for the buck.
A (Very) Short History Of Computer Cables
When computers were fi rst invented, they were
running at lightning-fast speeds like 1 megabit
per second. (That was fast for the 1950s.) No
twisted-pair cable could carry a signal like that
so computers ran on coaxial cable. Computer
designers looked longingly at twisted pairs for
one reason: noise rejection. Twisted-
pairs can be run as “balanced lines”
that dramatically reduce electromag-
netic noise picked up by a cable.
Where does this noise come from?
Everywhere! Motors, generators, fl uo-
rescent light ballasts, lighting dim-
mers, even computers, like the one
you’re probably looking at right now,
are all sources of electromagnetic noise. Two-
way radios, medical machinery, and, of course,
radio and TV broadcast transmitters are won-
derful sources. And then we have broadband
sources, like the sun, an excellent source of
electromagnetic noise, which is why things get
electrically “quiet” at night and you can hear
that AM radio station a thousand miles away.
How Twisted Pairs Work
To understand how twisted pairs work, we have
to understand “balanced lines”. To understand
balanced lines, we start with Figure 2.
Figure 1
Figure 2 shows a battery and a
light bulb. We get the electricity
to fl ow through the light bulb by
attaching two conducting paths,
usually wires. I’ve put two
arrows to show how the electric-
ity will fl ow, out of the negative terminal of the
battery, through the light bulb, and back to the
positive terminal.
This often confuses readers.
Why does the electricity move
in opposite directions? Because
it is a “circle” of conducting
pathway. It’s like a race track. If
you’re in a race and you look across to the other
side of the track, those cars are going in the
opposite direction.
It doesn’t matter how close These
wires are together, like Figure
3, or even if we twist them to-
gether like Figure 4. Nothing
has changed. It’s still a circle of
electricity, which is why we give it the Latin
name for circle: circuit.
Now that we have a twisted pair, as in Figure 4,
we have to cover the wires with
a non-conductor, like plastic, so
that they can’t touch each other.
If they did, electricity takes the
path of least resistance where
they touch, instead of the light
bulb that has a lot of resistance. Our circuit
won’t be as long as it’s supposed to be. It would
be a short circuit!
If we replaced the battery and
light bulb with something else
that produces electrical signals
and something at the other end
that uses them, amazingly little
has changed, such as Figure 5.
In this example we have a micro-
phone as the source of electricity.
A microphone converts acoustical
energy into electrical energy. At the
other end we have a speaker that turns electri-
cal energy into acoustical energy, i.e. sound.
(Of course, we would have to have a preampli-
fi er and power amplifi er inside the speaker, for
all you nit-pickers! That’s why I put
knobs on the speaker!)
If we put the microphone in a piano
and hit “Middle A” (440 vibrations
per second) then the diaphragm in the micro-
phone would move in and out 440 times per
second, the two arrows on our twisted pair
would reverse direction 440 timers per second,
and the speaker cone would move
in and out 440 times per second.
We would hear that note: 440 Hertz.
If this were a perfect microphone,
perfect cable, and perfect speaker,
we would hear the note as if our ear were in the
piano where the microphone is.
Twisted pairs, when connected as a balanced
line, reduce noise. And the secret
is a device at each end of the pair
called a “transformer”. Figure 6
shows a twisted pair with a trans-
former at each end. Transformers
are just coils of insulated wire, but
they can pass signals between them.
You’ll notice noise (big yellow arrow) coming
from outside. The insulation on the wire can’t
stop it, so when the noise hits each
wire it creates (“induces”) a signal
on the wire (those little yellow ar-
rows). Since there are two wires,
there are two noise signals. Those
Figure 2
Figure 6
Figure 5
Figure 3
Figure 4
- 2 -
noise signals travel to the end of the cable
where they meet each other inside the trans-
former and cancel each other out.
Fancy Words
The noise signals are moving in the same
(“common”) direction, so they are called
“common-mode noise”. If we measure them,
we’re interested in how well the transformers
cancel out the noise, compared to the noise
that might get through and not get cancelled
out. So this measurement is a ratio of the stuff
that gets through to the rejected noise signal,
called “common mode rejection ratio” or just
CMRR, for short.
You might recall that the signal we want to
travel down the
cable is traveling
in opposite direc-
tions (as in Figure
6). Since those ar-
rows are moving in
different directions, we call this a “differential”
signal. And the key is that, if we could measure
the signal on the two wires and mathematically
add them together, the total should always
equal zero, since they’re opposite signals.
If the differential signal doesn’t equal zero, it
means that the twisted pair is not balanced and
any noise on the cable would not be completely
cancelled out and that noise would be added to
the desired signal.
Once noise is added, it is very diffi cult to get
rid of. It’s much easier to get rid of it before it
gets included with the signal.
An Ideal Twisted Pair
Table 1 shows basic factors that go into making,
and measuring, a twisted pair. We have a say-
ing in cable manufacturing: “Physicals equal
Electricals”. That means, anything you do that
physically changes the cable will also change
its electrical performance.
For instance, if the two wires in our twisted
pair are not the same length, then the noise
signals will not arrive at the same time. They
will be “out of phase” and the cancellation will
not be perfect. Also one wire being longer, will
have more resistance, so the two noise signals
will not have the same intensity and therefore
not cancel out.
The same applies to size (wire gage, AWG).
Even if they are slightly different in size, it can
have a big effect on noise rejection. We know
how long the cable is,
and we know what the
resistance per foot (or
per meter) should be,
so we can easily cal-
culate what the value
should be. If it’s different than that theoretical should be. If it’s different than that theoretical should
value, then we have an unbalance, one wire is
more or less resistance than the other, and will
allow some percentage of noise to be passed to
the next device.
The two wires also need to be close togeth-
er. Notice in Figure 6 how the two wires are
spread apart. Now you might understand that
this is a very poor twisted pair. And the reason
is that the noise signal hits one wire and, a
tiny fraction of a second later, hits the second
wire. This bit of time means that the two noise
signals will not arrive at the same time at the
transformer. They will be out of phase and not
completely cancel out.
The distance between the two wires also affects
the “impedance” of the cable. The impedance
Table 1
- 3 -
is a combination of capacitance, inductance,
and resistance, everything that tries to “impede”
signal fl ow. At low frequencies, below 1 MHz,
the cable isn’t long enough for the impedance
to make a difference so the impedance at those
frequencies is often ignored.
Above 1 MHz, impedance becomes more and
more important. At 100 MHz (like Cat 5) it is
very important. Cables that are not the correct
impedance (100 ohms for Category cables) will
refl ect part of the data signal back to the source.
This is called “return loss” and is a good way
to compare cables. It is especially interesting
to note if this is “typical” return loss (can be
higher or lower), or “maximum” return loss
(no worse than). Just having a maximum return
loss number is a good indication of a superior
Category cable.
Now you might begin to under-
stand what the defi nition of a
perfect balanced line is. It is a
pair of wires, with all passive
components attached to them, where each wire
is the same impedance in respect to ground.
In other words, two wires that are electrically
identical.
Perfect Pairs
Of course, there’s no perfect anything, but
manufacturers have ways of getting closer to
perfection. One of these ways is to make your
own bare wires. Many cable manufacturers buy
their bare wire from a “wire mill”. If they buy
a 24 AWG (gage) wire, how will they know
it’s perfect? (They don’t.) And the wire mill
probably doesn’t know what this wire will be
used for. It could be for door bell wire, where
almost anything would work just fi ne. Varia-
tions in size may already be in the wire when
it is bought.
But if a manufacturer has its own wire mill, like
Belden, then the wire for Category cable will
have much greater precision than the same gage
wire used in other applications. And that preci-
sion is crucial to good performance.
One way of keeping a twisted pair close to each
other, is just that: twist them together. That
may sound easy but it is not. If the tension on
one wire is slightly more (or slightly less) than
the other wire, the two wires will be different
lengths, one wire will be closer to straight with
the other wire wound around it, bad news for
data cable. This pair will suffer from resistance
unbalance and timing problems, as listed in
Table 1.
And twisted pairs in a fi nished cable, when bent
and fl exed while installed, cause the
pair to “open up” changing the im-
pedance, increasing the return loss,
changing capacitance, and allowing
noise to get in.
Bonded Pairs
One solution for this problem is bonded pairs.
This technique sticks the two wires together as
they are twisted. This dramatically improves
impedance stability, reduces capacitance un-
balance, and reduces return loss. The only bad
thing is that you have to split the wires apart
when you install them, which adds a few sec-
onds to the installation of each connector.
That minor amount of time is more than off-
set by the fact that bonded-pair cables retain
their performance specs after they are in-
stalled. And this means fewer call-backs to fi x
bad cables. Figure 7 shows bonded-pair Belden
1872A MediaTwist Category 6.
Figure 7
- 4 -
Base What?
Axia uses 100Base-
T and 100Base-T
signals. 100Base-T
means 100 Mega-
bits of data per sec-
ond (“100 Mbps”)
based on twisted
pairs (“T”). So
you can probably guess that 100Base-T means
1000 Megabits (1 Gigabit) of data (Gbps)
on twisted pairs. Most Axia devices run on
100Base-T, although much of the hardware can
also handle 100Base-T.
Standards
Category cable standards are set by a joint
committee of the Telecommunication Industry
Association (TIA) and the Electronic Industry
Association (EIA). The committee is called
TIA/EIA 568. Their current set of standards is
called TIA/EIA 568-B.2
Table 2 and 3 show the standards for Category
5e and Category 6. Some of the Belden prod-
ucts that are made
to this standard
are in the last col-
umn.The 568-B.2
standard contains
sp e c i f ica t ions
for Category 3
(now used as
telephone cable),
Category 5e, and
Category 6. They
have dropped
Category 4 and
Category 5 from
the standard, so
it’s very hard to
buy that type of cable. They’re working on the
next standard, 10GBase-T, 10 gigabits on four-
pair UTP.S
Be aware that Table 2 and Table 3 are the “stan-
dards”, the minimum (or maximum) that all
Cat 5e or Cat 6 must meet. The Belden cables
listed at the end easily meet, and very often
exceed, these requirements.
What They Mean
The specifi cations in Tables 2 and 3 are written
in the acronym language of the data world.
Here’s what those acronyms actually mean,
along with some other terms used to describe
data cables.
Table 2
Table 3
- 5 -
NEXT means “near-end crosstalk”. At the
near end (source end), the transmitted signal is
the strongest. Transmitting pairs can interfere
with other signals on other pairs. This is called
“crosstalk”. The 568B.2 standard specifi es the
minimum crosstalk value at various frequen-
cies.
PSNEXT is “power sum near-end crosstalk”
that looks at the effect of all adjacent pairs to
the one under test rather than just pair-to-pair.
Such testing is then the “worst case” where
all pairs are energized, such as in 100Base-T.
Results of each combination are averaged
together.
ATTENUATION is signal loss and is common
to all signal carrying systems. Attenuation is
measured in decibels (dB). Decibels are loga-
rithmic. Data signals -40 dB down (one-ten
thousandth of the original intensity) are fully
and easily recovered. This is not surprising to
most audio/video engineers considering that
analog microphone signals are often –60 dB.
FEXT is “far-end crosstalk”. The far end of the
cable is where the signals are weakest, where
attenuation has already reduced the signal level
so that crosstalk can have a signifi cant effect
ELFEXT is “equal level crosstalk”. If you’re
interested in just the crosstalk numbers, then
you would subtract the attenuation from FEXT.
What is left is crosstalk, all at the same level
(“equal level”)
ACR is “attenuation-to-crosstalk ratio”. ACR
subtracts the crosstalk from the attenuation,
to indicate the overall performance of a cable.
Positive ACR, especially at high frequencies,
can be an indicator of superior cable perfor-
mance. ACR is very similar to “signal-to-noise
ratio” in the analog audio/video world. There-
fore, ACR can be valuable where multiple data
signals travel down a four-pair cable, such as
100Base-T networking.
PSACR is “power-sum attenuation-to-crosstalk
ratio” where all pairs are energized around the
measured pair and the ACR results averaged.
This shows the “signal-to-noise” ratio with
everything running, a very good test.
RETURN LOSS shows the variations in im-
pedance within a cable. Impedance variations
cause the signal to refl ect back to the source,
so return loss is the ratio between direct signal
and refl ected signal. It is measured in decibels
(dB). With a larger negative number, more
signal reaches its destination, and less of the
signal is refl ected back to the source. (-30 dB
return loss is better than -20 dB return loss.)
Return loss is especially effective in showing
fl aws in cable construction and installation,
such as excessive bending or stretching, which
affect the impedance of the cable. In link and
channel tests, return loss can show the effect
of poor, or badly installed, connectors, patch
panels, and other passive hardware.
DELAY SKEW is timing differences on a
multipair cable. For all cables listed above, the
maximum allowed by TIA/EIA 568B.2 is 45
nsec/100m (nanoseconds per 100 meters, 328
feet). Delay skew is especially interesting to
designers who are using more than one pair to
simultaneously deliver data. Formats such as
Gigabit Ethernet® (100Base-T) require that the
data be split between the four pairs. In such
systems, it is essential that the signals arrive at
the other end of the cable at the same time.
Timing variations for a complete system should
not exceed 50 nsec (nanoseconds) between any
of the four paths. When looking at cable alone,
the maximum delay is 45 nsec/100m. This is
- 6 -
one of the reasons Belden’s MediaTwist® (delay
skew 25nsec/100m maximum, 12nsec/100m
typical) is popular in applications where this
is critical.
Be aware that there are “no skew” cables.
These have vanishingly low skew, such as 2.2
nsec/100m for Belden 7987R (7987P). But
all these cables, and all “no–skew, zero-skew”
designs from other manufacturers, accomplish
this by having all four pairs with identical
twists. While these cables might be good for
non-data applications, such as RGB and VGA,
they are not Category anything. They won’t
even pass Cat 3 (telephone cable), and should
not be considered for anything requiring true
Cat 5e or Cat 6 performance.
On the other hand, there are category cables
(Belden 7988R and P, 7989R and P) that are
true Cat 5e and Cat 6, where the cable design
concentrates on ultra-low delay skew with
values of 9nsec/100m (7988) and 10nsec/100m
(7989). This allows an installer to use the same
cable for data network applications (such as
Axia) and also use it for RGB or VGA display.
PAIR TWISTING (“Lay Length”). Tight pair
twisting can greatly reduce crosstalk but also
has a number of negative effects on the cable.
There is more copper used per unit length so
the price goes up. And more copper means the
signal will take longer to travel down that pair
(compared to other pairs) so attenuation and
delay skew are worse. It doesn’t matter how
high your ACR is, or how low your crosstalk is,
if you don’t have enough signal strength to be
recovered at the receiving end! What you truly
want is a cable that improves both crosstalk and
attenuation to improve ACR, to have positive
ACR at a higher frequency, without affecting,
or possibly even reducing, delay skew.
IMPEDANCE indicates the ability of a data
cable to transfer a signal from one box to an-
other. The impedance of the systems, and boxes
it is attached to, specifi es the impedance of the
cable. The TIA/EIA standard for Category 5e
and 6 is 100Ω ± 15Ω (ohms). Some cables meet
this spec. Others require the use of a smooth-
ing formula called “Zo-fi t”. This allows manu-
facturers to ignore rapid changes in impedance.
Belden bonded-pair data cables are tighter than
±15Ω without the Zwithout the Zwithout o-fi t function.
BANDWIDTH is the range of frequencies
available to be used for signal carrying. It is
the “width of the tunnel”. However, knowing
the width of the tunnel tells you nothing of how
the traffi c will move through it. This is because
data can be compressed to take up less band-compressed to take up less band-compressed
width.
For instance, a 100 Mbps data signal can fi t
in a 100 MHz bandwidth. Or the data can be
arranged and coded to fi t in a 50 MHz band-
width, a 30 MHz bandwidth, or even smaller. In
fact, the 31.25 MHz numbers commonly seen
in cable specifi cations are for a compressed
155 Mbps (“ATM”) protocol.
Since the coding scheme is not apparent,
only the bandwidth in Megahertz (MHz) can
be used to compare potential data handling
capacity. You will know the size of the tunnel.
Knowing how many cars will fi t depends on
how you arrange them. If you want to com-
pare the signal-carrying capacity of two cables,
compare bandwidths.
Isolation
Another technique to improve performance
in found in all Category 6 cables. These
work at much higher frequencies (250 MHz)
than Cat 5e. And they are intended to carry
- 7 -
1000 Mbps (megabits-per-
second), also called 1 Gigabit
per second (Gbps).
The crosstalk requirements of
Cat 6 are 10 dB harder than
Cat 5e, so most manufacturers have solved
this problem by putting a divider in the cable,
as shown in Figure 8, Belden 7851A “600e”
Category 6.
Most dividers are simply an “X” that divides
the four pairs into separate quadrants. The
dividers in the Belden 7851A, and
other cables in that family, are just
a little different. It’s more a back-to-
back “Y” like Figure 9.
In this way, the pairs that are most likely to
“talk” to each other are kept as far apart as pos-
sible. Another way of keeping the pairs apart
to meet Category 6 crosstalk is used in Belden
1872A MediaTwist (Figure 10). MediaTwist
spreads the pairs apart, giving each pair a little
channel inside the jacket. This is why the cable
is “crescent-moon” shaped and
not round.
It should be noted that this
cable is now over ten years old,
ancient for a data cable, and
still exceeds Category 6 specifi cations. In
some specs, such as “bend radius” and “pull
strength”, MediaTwist is still unequalled in the
industry.
Like all products, there can be a Chevy, a Jag,
or a Ferrari. Figure 11 shows
a “Chevy” Category 6, Belden
7881A. Note the divider between
the pairs is just a tiny plastic
wire. The pairs in this construc-
tion are non-bonded. Changes
like these make such a cable smaller,
lighter, cheaper, and easier to install.
You’re just trading performance for
all those nice things.
Fire Ratings
Fire ratings are defi ned in the National Elec-
trical Code (NEC). This is a voluntary code,
so each city, county, or state may or may not
follow this code. It is also interpreted in differ-
ent ways by Fire Marshals, building inspectors,
permit boards and other bodies “having juris-
diction”. If your area does not subscribe
to the NEC, then you must obtain a copy
of their own rules, or at least have some-
one who can advise you on the l ocal
requirements.
Within the NEC, there are different fi re ratings,
tests that are performed on cables to deter-
mine their reaction to a fi re. Most often these
involve fl ame spread and smoke production.
Most category cables come in two different
fi re ratings, riser (CMR) and plenum (CMP).
There are lower grades (CM, CL2
for example) or higher (LC, lim-
ited combustible) that are available.
The choice of cable and fi re rating
is between your architect or system
designer and the appropriate legal
body having jurisdiction.
Riser rating (CMR) allows cables to be placed
vertically between fl oors without use of a metal
conduit. Plenum ratings (CMP) allow cables to be
used in drop ceilings or raised fl oors that are con-
nected to an air conditioning system.
Before you buy the cable for your
installation, be sure you have
determined which fi re rating is
appropriate. An inspector can easily
Figure 8
Figure 9
Figure 10
Figure 11
- 8 -
require that an
entire wiring job
be removed if
the wrong rating
is used.
Diff erent Cables, Diff erent Choices
There are many different kinds of Category 5e
or Category 6. Like any manufactured product,
these can minimally meet the standard, or they
may exceed the standard. Designers and end-
users are urged to obtain the test data for all
cables that might be considered and to compare
them.
Belden makes four kinds of Category 5e and
four kinds of Category 6, and each of those
four types is available in plenum and riser fi re
ratings. Just within Belden, this gives you 16
choices of cable, a bewildering selection.
The “e” in Category 5e means “enhanced”. It’s
an enhanced Category 5. What is enhanced is
the set of parameters and tests that this cable
must pass. These new tests allow this cable
to run 100Base-T. In that application, all four
pairs are running and the signal is divided into
four parts. Not only that, but the signals run in
both direction
simultaneously
(“duplex”), just
like a telephone
where you can
speak and listen
on the same two
wires.
Table 4 shows
a list of these
cables and how they differ generally. Guaran-
teed performance specs for any cable should be
available from any manufacturer. For Belden,
these can be found in the Belden catalog, or
even more detailed specifi cations at www.
belden.com .
While Cat 5e meets the minimum requirements
for 100Base-T, it became apparent that a much
better cable design was needed for really good
100Base-T performance. This was Category 6.
Belden Cat 6 products are listed in Table 5.
Note that some Cat 5e and some Cat 6 cables
have ultra-low skew. Those low-skew numbers
allow you to use these cables for RGB and
VGA (and other analog and digital applications
that benefi t from precision multi-pair delivery)
as well as for 100Base-T or 100Base-T data
applications.
Table 4
Table 5
- 9 -
Color Me Fast
Most UTP data cable is available in a number of
colors. While there is no “color standard”, you
might want to consider using different colors.
For instance, all the Axia stuff might be one
color, and your regular in-house networking
another color, just so you can tell them apart.
If you have a low-latency network (Layer 2 vs.
Layer 3), you might want to color code that
differently too. One broadcaster had every
data installer use a different color so he could
tell which installer put in which cable. Pretty
clever.
Connectors And Connections
One of the most critical parts to a data network is
the connections. And I do mean “ connections”,
not “connectors”, because there are two ways
to make connections between cables.
The fi rst is with a punch block, commonly
called a 110-block. This is a 100-ohm, low
capacitance, high quality means of connecting
cables. The punch points are gas tight, so con-
nections last a long time. This is the highest
performance way of connecting category data
cables together.
However, punch blocks are permanent. It is
diffi cult to remove and reconnect cables. To
disconnect and re-connect, you really need
a connector. The second type of connection
is a connector. The connector of choice for
Category cables is called an RJ-45. This con-
nector is very simple and fast to install.
Most data installations put jacks at the end of
the installed cable, such as a plate on a wall.
Then patch cables are bought pre-made to
connect from the wall to the equipment. Just
be aware that this point is probably the most
critical for good network performance. More
network failures happen here than all other
places combined.
To start with, the jack must be the equal of the
cable. If you put a 5e jack on Category 6 cable,
you will get 5e performance. Be sure and put a
Cat 6 jack on Cat 6 cable. If you can get data
from the connector manufacturer, you should
be able to choose the best. Belden now makes
connectors for Category cables (Belden IBDN)
that are very high quality and highly tested.
There are other excellent brands around as
well.
If you buy pre-made patch cable be aware that
stranded conductor patch cables are inherently
worse than the solid-conductor backbone
cables. Therefore, the fewer patch cables, the
shorter they are, and the higher the quality of
their assembly, the better your network will
run.
Be sure that your patch cables are the same
Category (5e or 6) as your network. If they
come with test data, or a warranty, so much the
better.
Analog Applications For Data Cables
About ten years ago, with the advent of Belden
MediaTwist, it became apparent that Category
data cables were getting so good that they
might be suitable for some non-data applica-
tions, such as analog or digital audio.
Most data cables are never tested below 1 MHz
(in some cases 772 kHz). This is way above
analog audio, so the actual performance of
audio was not known. Figure 12 was the fi rst
test to look at the analog audio performance of
premise data cable. Figure 12 shows the cross-
talk performance of all four pairs averaged
- 10 -
together, so you see the
“worst case”. The cable
chosen was Belden
1752A, Category 5e
stranded patch cable.
Patch cable is possibly
the worst cable made
for data. But look at the
results in Figure 12.
The worst case cross-
talk is -95 dB at
around 40 KHz. In
the standard audio
frequency band (to 20
kHz) the average of all
pairs is typically -100
dB. Compares this to a
CD that, when it goes
“quiet” drops to maybe -90 dB, and you have to
wonder why we put shields on cables. In truth
foil shields are RF (high frequency) shields.
They do virtually nothing at audio frequencies,
and seriously nothing below 1,000 Hz. Only
the twisting of the pair (and the CMRR of a
balanced line).
Some observant view-
ers noticed that Figure
12 was FEXT (far-end
crosstalk) where the
signal is the weakest.
Perhaps crosstalk is
terrible at the other
end (NEXT, “near-end
crosstalk”) where the
signals are strongest,
as in Figure 13.
You can easily compare
Figure 12 and 13 and
see that the numbers
are even better in Fig-
ure 13. And what do we see? Typically, -100 dB
of crosstalk averaged from all four pairs. Worst
case NEXT is 45 kHz, way beyond human
hearing, where the crosstalk is slightly better hearing, where the crosstalk is slightly better
than -95 dB.
So we tested 1872A MediaTwist (now Cat 6). So we tested 1872A MediaTwist (now Cat 6).
Figure 12
Figure 13Figure 13
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Unfortunately, I have no charts or graphs to
show you because they couldn’t measure it.
The crosstalk was below the noise fl oor of the
$60,000 Agilent network analyzer (-110 dB).
For digital audio, it’s even easier. Digital
signals are naturally noise resistant. The
sampling frequency used in Axia (48 kHz) is
very common. Digital audio channels on video
machine are 48 kHz sampling. But that is not the
frequency running on the cable. To determine
that, according to the AES addendum, we must
multiply that by 128. So the actual bandwidth
of a two-channel digital audio bit stream at 48
kHz sampling is 6.144 MHz. We use 6 MHz as
a simple frequency to test our data cable.
What is the crosstalk of Cat 5e at 6 MHz? More
than -50 dB for Cat 5e, and -60 dB for Cat 6.
The amazing thing is that digital signals are
inherently noise-resistant. (It’s very easy to tell
a square wave from noise.) You only need a few
dB to tell one from the other. Cat 5e and 6 give
you thousands of times more crosstalk protec-
tion than you actually need.
These category cables work great for analog
and digital audio, as long as they are run as
a balanced line, and you can also run them
as 100Base-T or 100Base-T. You can even use
them to wire up a telephone!
So What Do I Choose?
Now you are well-armed to choose a “ category”
cable. You understand many of the consider-
ations in design and manufacturing these
cables. Besides these, you have many other fac-
tors to infl uence your decision. Here is a list:
Availability. (If you can’t buy it, it doesn’t
matter how good it is.)
Consistency. (Is the roll from last year
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identical to next year?
Quality.
Price.
Ease of installation.
Performance.
Performance after installation.
Company track record/history.
Your familiarity with manufacturer and
other products.
Recommendations from others.
If you take this list and apply it to any particu-
lar cable from a particular manufacturer, give
a + for each point that cable meets, it should
be very easy to judge which cable is the best
for your installation. You want a cable with as
many +’s as possible. A + on price alone could
easily be the hardest to install with the worst
performance. You are now loaded with enough
questions to impress any manufacturer.
Good luck with your Axia install!
©2005 Axia Audio . If you’d like to re-purpose portions of this text, please contact [email protected] for prior permission. Don’t worry, we’re nice guys.
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