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 Inquiry@AxiaAudio.com for prior permission. Don’t worry, we’re nice guys.

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