Handbook for Sound Engineers

416 Chapter 14

• The desired impedance for all digital audio on
  coaxial cable is 75:.

With 1000Base-T systems, the pairs simultaneously
transmit and receive. As the transmitter sends data, it is
also listening for data being sent from the opposite end
of the same pair. Any reflected signal from the sending
end that reflects back to the sending end mixes with the
sending signal from the far end, reducing intelligibility.
With 10Base-T or 100Base-T data networks, one pair
transmits while another receives, so reflections (RL,
return loss) are not a major consideration and were not
required to be measured. Now, with pairs simultane-
ously transmitting and receiving, called duplex mode,
RL is a critical measurement for data applications.

Delay Skew. Since every pair (and every cable) takes a
specific amount of time to deliver a signal from one end
to the other, there is a delay. Where four pairs must
deliver data to be recombined, the delay in each pair
should, ideally, be the same. However, to reduce cross-
talk, the individual pairs in any Category cable have dif-
ferent twist rates. This reduces the pair-to-pair crosstalk
but affects the delivery time of the separate parts. This is
called delay skew.
While delay skew affects the recombining of data, in
1000Base-T systems, for instance, the same delay skew
creates a problem when these UTP data cables are used
to transmit component video or similar signals, since
the three colors do not arrive at the receiving end at the
same time, creating a thin bright line on the edge of
dark images. Some active baluns have skew correction
built in, see Section 14.12.5.

ANEXT. ANEXT, or alien crosstalk, is coupling of sig-
nals between cables. This type of crosstalk cannot be
cancelled by DSP at the switch level. Alien crosstalk can
be reduced by overall shielding of the pairs, or by insert-
ing a nonconducting element inside that cable to push
away the cables around it.

14.9.3.6.6 Terminating Connectors

All structured cabling use the same connector, an RJ-45.
In LANs (local area networks) there are two possible
pin-outs, 568A and 568B. The difference is pair 2 and
pair 3 are reversed. Both work equally well as long as
they are not intermixed. The termination is shown in Fig.
14-10.
In the past decade, the B wiring scheme has become
the most common. However, if you are adding to or
extending and existing network, you must determine

which wiring scheme was used and continue with that

scheme. A mixed network is among the most common

causes of network failure.

It is very important that the pairs be kept twisted as

close to the connector as possible. For 100Base-T

(100 MHz, 100 Mbps) applications, a maximum of

½ inch should be untwisted to reduce crosstalk and

noise pickup. In fact, with Cat 6 (250 MHz) or Cat 6a

(500 MHz) it is safe to say that any untwisting of the

pairs will affect performance. Therefore there are many

connectors, patch panels, and punch-down blocks that

minimize the untwisting of the pairs.

14.9.3.6.7 Baluns

Baluns (Balanced-Unbalanced) networks are a method

of connecting devices of different impedance and differ-

ent formats. Baluns have been commonly used to con-

vert unbalanced coax, to balanced twinlead for television

antennas, or to match coaxial data formats (coaxial

Ethernet) to balanced line systems (10Base-T,

100Base-T etc.). Other balun designs can allow unbal-

anced sources, such as video or consumer audio, for

instance, to be carried on balanced lines, such as UTP

Cat 5e, 6, etc.

Since there are four pairs in a common data cable,

this can carry four channels. Since category cables are

rarely tested below 1 MHz, the audio performance was

originally suspect. Crosstalk at audio frequencies in

UTP has been measured and is consistently better than

90 dB even on marginal Cat 5. On Cat 6, the crosstalk

at audio frequencies is below the noise floor of most

network analyzers.

Baluns are commonly available to handle such

signals as analog and digital audio, composite video,

S-video, RGB or other component video (VGA,

Y/R-Y/B-Y,Y/Cr/Cb), broadband RF/CATV, and even

DVI and HDMI. The limitations to such applications

Figure 14-10. Termination layout for EIA/TIA 568-B.2 cable.

1 2 3 4 5 6 7 8

T568A T568B

1 2 3 4 5 6 7 8

(^31)
2
4 2
3
1 4