Handbook for Sound Engineers

430 Chapter 14

UTP for analog and digital connections. Where the
source is not a balanced line, a device must change from
balanced (UTP) to unbalanced (coax, for instance).
Such a device matches Balanced-to-Unbalanced and is
therefore called a balun. There is more on baluns in
Section 14.9.3.6.7, Baluns.

14.18 AES/EBU Digital Audio Cable

Digital audio technology has been around for many
years, even decades, but until recently it has not been
used much for audio. This has now changed and digital
audio is overtaking analog audio. For this reason it is
important that the cable used for digital signals meet the
digital requirements. To set a standard, the Audio Engi-
neering Society (AES) and the European Broadcast
Union (EBU) have set standards for digital audio cable.
The most common sampling rates and equivalent band-
width are shows in Table 14-28.

It is important that the line impedance be maintained
to eliminate reflections that degrade the signal beyond
recovery. Standard analog cable can be used for runs
under 50 ft (15 m) but beyond that, reliability decreases.
The impedance and capacitance of analog cable is 40 to
70 :"and 20 to 50 pF/ft. The impedance and capaci-
tance for digital cable is 110: and 13 pF/ft with a
velocity of propagation of 78%. Proper impedance
match and low capacitance are required so the square
wave signal is not distorted, reflected, or attenuated.

Broadcast cable is most often #24 (7 × 32) tinned
copper wire with short overall twist lengths, low-loss
foam insulation, and 100% aluminum polyester foil
shield for permanent installations. Braided shields are
also available for portable use. If required, #22 to #26
wire can be obtained. Digital audio cable also comes in
multiple pairs with each pair individually shielded, and
often jacketed, allowing each pair and its shield to be
completely isolated from the others. One pair is capable
of carrying two channels of digital audio. Cables are
terminated with either XLR connectors or are punched
down or soldered in patch panels.

14.18.1 AES/EBU Digital Coaxial Cable

Digital audio requires a much wider bandwidth than ana-
log. As the sampling rate doubles, the bandwidth also
doubles, as shown in Table 14-28.

Digital audio can be transmitted farther distances
over coax than over twisted pairs. The coax should have
a 75: impedance, a solid copper center conductor, and

have at least 90% shield coverage. When transmitting

audio over an unbalanced coax line, the use of baluns

may be required to change from balanced to unbalanced

and back unless the device contains AES/EBU unbal-

anced coax inputs and outputs. The baluns change the

impedance from 110: balanced to 75: unbalanced

and back.

14.19 Triboelectric Noise

Noise comes in a variety of types such as EMI (electro-

magnetic interference) and RFI (radio frequency inter-

ference). There are also other kinds of noise problems

that concern cables. These are mechanically generated or

mechanically induced noise, commonly called triboelec-

tric noise.

Triboelectric noise is generated by mechanical

motion of a cable causing the wires inside the shield to

rub against each other. Triboelectric noise is actually

small electrical discharges created when conductors

position changes relative to each other. This movement

sets up tiny capacitive changes that eventually pop.

Highly amplified audio can pick this up.

Fillers, nonconductive elements placed around the

conductors, help keep the conductor spacing constant

while semiconductive materials, such as carbon-impreg-

nated cloth or carbon-plastic layers, help dissipate

charge buildup. Triboelectric noise is measured through

low noise test equipment using three low noise stan-

dards: NBS, ISA-S, and MIL-C-17.

Mechanically induced noise is a critical and frequent

concern in the use of high-impedance cables such as

guitar cords and unbalanced microphone cables that are

constantly moving. The properties of special conductive

tapes and insulations are often employed to help prevent

mechanically induced noise. Cable without fillers can

often produce triboelectric noise. This is why

premise/data category cables are not suitable for

flexing, moving audio applications. There are emerging

flexible tactical data cables, especially those using

bonded pairs, that might be considered for these

applications.

Table 14-28. Sampling Rate versus Bandwidth

Sampling Rate

kHz

Bandwidth

MHz

Sampling Rate

kHz

Bandwidth

MHz

32.0 4.096 48.0 6.144

38.0 4.864 96.0 12.228

44.1 5.6448 192.0 24.576