Handbook for Sound Engineers

Grounding and Interfacing 1203

Unlike balanced interconnections, such noise pickup is

not nullified by the receiver.

32.5.5 Bandwidth and RF Interference

RF interference isn’t hard to find—it’s actually very
difficult to avoid, especially in urban areas. It can be
radiated through the air and/or be conducted through
any cables connected to equipment. Common sources of
radiated RF include AM, shortwave, FM, and TV
broadcasts; ham, CB, remote control, wireless phone,
cellular phone, and a myriad of commercial two-way
radio and radar transmitters; and medical and industrial
RF devices. Devices that create electrical sparks,
including welders, brush-type motors, relays, and
switches can be potent wideband radiators. Less
obvious sources include arcing or corona discharge
from power line insulators (common in seashore areas
or under humid conditions) or malfunctioning fluores-
cent, HID, or neon lights. Of course, lightning, the ulti-
mate spark, is a well-known radiator of momentary
interference to virtually anything electronic.
Interference can also be conducted via any wire
coming into the building. Because power and telephone
lines also behave as huge outdoor antennas, they are
often teeming with AM radio signals and other interfer-
ence. But the most troublesome sources are often inside
the building and the energy delivered through ac power
wiring. The offending source may be in the same room
as your system or, worse yet, it may actually be a part of
your system! The most common offenders are inexpen-
sive light dimmers, fluorescent lights, CRT displays,
digital signal processors, or any device using a
switching power supply.
Although cable shielding is a first line of defense
against RF interference, its effectiveness depends criti-
cally on the shield connection at each piece of equip-
ment. Because substantial inductance is added to this
connection by traditional XLR connectors and
grounding pigtails, the shield becomes useless at high
radio frequencies. Common-mode RF interference
simply appears on all the input leads.^34 Because the
wire limitations discussed in Section 32.2.4 apply to

grounding systems, contrary to widespread belief,

grounding is not an effective way to deal with RF inter-

ference. To quote Neil Muncy:

Costly technical grounding schemes involving

various and often bizarre combinations of

massive copper conductors, earth electrodes, and

other arcane hardware are installed. When these

schemes fail to provide expected results, their

proponents are usually at a loss to explain why.^35

The wider you open the window, the more dirt flies

in. One simple, but often overlooked, method of mini-

mizing noise in a system is to limit the system band-

width to that required by the signal.^36 In an ideal world,

every signal-processing device in a system would

contain a filter at each input and output connector to

appropriately limit bandwidth and prevent out-of-band

energy from ever reaching active circuitry. This RF

energy becomes an audio noise problem because the RF

is demodulated or detected by active circuitry in various

ways, acting like a radio receiver that adds its output to

the audio signal. Symptoms can range from actual

reception of radio signals or a 59.94 Hz buzz from TV

signals or various tones from cell phone signals to much

subtler distortions, often described as a veiled or grainy

audio quality.^37 The filters necessary to prevent these

problems vary widely in effectiveness and, in some

equipment, may not be present at all. Sadly, the perfor-

mance of most commercial equipment will degrade

when such interference is coupled to its input.^38

32.6 Solving Real-World System Problems

How much noise and interference are acceptable

depends on what the system is and how it will be used.

Obviously, sound systems in a recording studio need to

be much more immune to noise and interference than

paging systems for construction sites.

32.6.1 Noise Perspective

The decibel is widely used to express audio-related

measurements. For power ratios,

(32-9)

For voltage or current ratios, because power is propor-

tional to the square of voltage or current:

Figure 32-39. Magnetic coupling between shield and
center conductor is 100%.

Lc

Ls

Rc

Rs

Ccs

S S

dB 10

P 1

P 2

= log----- -