1296 Chapter 34
34.5.1.5 Cable and Connectors for Digital Audio
Devices
Cable and connectors for digital audio devices are
derived from computer cabling and are often exactly the
same. For example, many digital audio devices utilize
Ethernet-style CAT5 cabling and connectors or USB
cable and connectors. As a rule, these com-
puter-derived connectors and cabling are not rugged
enough for portable sound system usage. For this rea-
son, some audio cabling companies have introduced
specialty connectors designed for portable usage such as
the one shown in Fig. 34-58.
The AES/EBU digital audio standard utilizes
conventional XLR connectors. The digital audio output
coaxial connector found on home-theater receivers is a
high-quality RCA-type connector. As with all computer
connections, digital audio connections should use
high-quality cable and connectors of the right imped-
ance and length. Consult the device manufacturer for
recommended cable specifications.
Fiber optic cable and connectors may be used in
large installed audio systems because of their ability to
carry multiple channels of digital information (audio,
video, control, and other) on a single cable and because
of their relative immunity from hum and noise pickup.
Fiber optic suppliers often provide seminars to teach
designers and installers how to specify, design, and
install fiber optic cabling systems.
See Chapter 15 for a detailed discussion of fiber
optic cabling systems. See Chapter 39 for a detailed
discussion of digital audio networking and the associ-
ated connections.
34.5.1.6 Understanding Balanced and Unbalanced
Lines
Every audio signal requires at least two wires. In an
unbalanced line, the shield (outer conductor) is also one
of the audio signal wires. Thus, an unbalanced line, Fig.
34-59A, needs only the shield and one additional wire
(a total of two wires). In a balanced line, the shield does
not carry audio signals. Thus, a balanced line, Fig.
34-59B, requires the shield and two additional wires to
carry the audio signal (for a total of three wires). Also
see Chapter 37 for a more detailed discussion of bal-
anced and unbalanced circuits.The primary advantage of a balanced line is that it is
much less likely to pick up external electronic noises
(hum, buzzing, static, radio stations, etc.) than an unbal-
anced line.34.5.1.7 Impedance and Level WatchingWhile some passive devices require impedance match-
ing, most active audio devices do not require matched
impedances. What they do require is impedance com-
patibility. In addition, all audio devices require signal
level compatibility. Thus, impedance and level watch-
ing means establishing and maintaining required imped-
ance and level compatibility (as will be discussed).34.5.1.7.1 Terms: Source, Input, Output, LoadIn Fig. 34-60, the source is the microphone, the input is
the input to the mixer/amplifier, the output is the output
from the mixer/amplifier, and the load is the loud-
speaker, but these four terms are relative. For example,
the input to the mixer-amplifier can be called a load
from the viewpoint of the microphone. And, the
mixer-amplifier output can be called a source from the
viewpoint of the loudspeaker.
Thus, the input impedance of the mixer/amplifier can
be called the load impedance for the microphone, andFigure 34-58. An ethernet-style connector designed for
portable audio systems. Courtesy Neutrik USA, Inc.
Figure 34-59. Balanced and unbalanced lines. Courtesy
Fender Musical Instruments.ShieldShield+ ++ +(^)
A. An unbalanced line connecting
two audio devices.
B. A balanced line connecting
two audio devices.