Handbook for Sound Engineers

Transmission Techniques: Wire and Cable 417

are the bandwidth specified on the cable and the perfor-
mance of the cable (attenuation, return loss, crosstalk,
etc.) at those higher frequencies.

Passive baluns can also change the source imped-
ance in audio devices. This dramatically extends the
effective distance of such signals from only a few feet to
many hundreds of feet. Consult the balun manufacturer
for the actual output impedance of their designs.

Some baluns can include active amplification, equal-
izations, or skew (delivery timing) compensation. While
more expensive, these active baluns can dramatically
increase the effective distance of even marginal cable.

14.9.3.6.8 Adaptors

Users and installers should be aware there are adaptors,
often that fit in wall plates, where keystone data jacks
are intended to be snapped in place. These adaptors
often connect consumer audio and video (RCA connec-
tors) to 110 blocks or other twisted pair connection
points. However, there is no unbalanced-to-balanced
device in these, so the noise rejection inherent in twisted
pairs when run as a balanced line is not provided. These
adaptors simply unbalance the twisted pair and offer dra-
matically short effective distances. Further, baluns can
change the source impedance and extend distance.
Adaptors with no transformers or similar components
cannot extend distance and often reduce the effective
distance. These devices should be avoided unless they
contain an actual balun.

14.9.3.6.9 Power Over Ethernet (PoE)

PoE supplies power to various Ethernet services as VoIP
(Voice over Internet Protocol) telephones, wireless LAN
access points, Blu-tooth access points, and Web cam-
eras. Many audio and video applications will soon use
this elegant powering system. IEEE 802.3af-2003 is the
IEEE standard for PoE. IEEE 802.3af specifies a maxi-
mum power level of 15.4 W at the power sourcing
equipment (PSE) and a maximum of 12.95 W of power
over two pairs to a powered device (PD) at the end of a
100 m (330 ft) cable.

The PSE can provide power by one of two
configurations:

1. Alternative A, sometimes called phantom power-
   ing, supplies the power over pairs 2 and 3.


2. Alternative B supplies power over pairs 1 and 4, as
   shown in Fig. 14-11.

The voltage supplied is nominally 48 Vdc with a

minimum of 44 Vdc, a maximum of 57 Vdc, and the

maximum current per pair is 350 mAdc, or 175 mAdc

per conductor. For a single solid 24 AWG wire,

common to many category cable designs, of 100 m

length (328 ft) this would be a resistance of 8.4:. Each

conductor would dissipate 0.257 W or 1.028 W per

cable (0.257 W × 4 conductors). This causes a tempera-

ture rise in the cable and conduit which must be taken

into consideration when installing PoE.

14.9.3.6.10 Power Over Ethernet Plus (PoE Plus)

PoE Plus is defined in IEEE 802.3at and is capable of

delivering up to 30 W. Work is being done to approach

60 W or even greater. This requires the voltage supply to

be 50 to 57 Vdc. Assuming a requirement of 42 W of

power at the endpoint at 50 Vdc, the total current would

be 0.84 A, or 0.21 A per pair, or 0.105 A (105 mA) per

conductor, or a voltage drop of only 0.88 V in one

24 AWG wire.

Figure 14-11. Two methods of supplying power via the

Ethernet. Courtesy of Panduit Int’l Corp.

RX

TX DC/DCconverter

TR

RX

Spare pair

Signal pair

Signal pair

Spare pair

(^45)
1
2
3
6
(^78)

• −
  48 Vdc 4
  5 1 2 3 6 7 8
  RX
  TX DC/DCconverter
  TR
  RX
  Spare pair
  Signal pair
  Signal pair
  Spare pair
  4 5 1 2 3 6
  (^78)


• 
  

  −
  48 Vdc
  (^45)
  1
  2
  3
  6
  (^78)
  Power sourcing
  equipment (PSE)
  Power sourcing
  equipment (PSE)
  Powered device (PD)
  Powered device (PD)
  B. Power supplied by spare pairs
  A. Power supplied via data pairs