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THE AGE OF WIRELESS
- If you're old
enough, you probably remember the precursor of the modern music
video. Rock groups in the late '60's and early '70's would appear on
television variety shows and lip-sync their current hit wearing peace
symbols, bell-bottoms, and carefully coifed hair on a stage decorated
in psychedelic paisley. Ah, the good old days!
One dead giveaway
that they weren't really playing was the common absence of cables
leading from the guitars and even the microphones. Apparently,
producers wanted a clean, uncluttered look on camera. Besides, who
would notice? Everyone would be listening with rapt attention to the
canned music, right?
Wrong. Armed with
a little knowledge of electronics, I remember thinking how strange
this looked. Even stranger, why did they stand so still once they
were unencumbered by cables? I thought rock music was supposed to be
wild and free. Except for Mick Jagger and Pete Townsend, most of
those guys were glued to their spots. Maybe it was force of habit;
after all, when they played for real, they could get hopelessly
tangled up in cables if they moved around too much.
These days, live
performers needn't choose between movement and music. The advent of
wireless technology has freed musicians from the cable's leash.
Today's concerts feature lead singers and guitarists running amok on
stage while belting out their licks. Welcome to the good new days.
TUNING IN
- The concept
behind wireless technology is simple: A transmitter converts a signal
into electromagnetic waves, which radiate from an antenna into the
surrounding environment. The antenna on a nearby receiver detects
these waves and converts them back into the original signal. This is
the basis of all wireless communication, including radio, television,
and satellite transmissions, as well as cordless and cellular phones.
Electromagnetic
waves (also called radiation) occur at different frequencies; the
entire spectrum encompasses an extremely wide range that includes the
radio frequencies (RF), microwaves, infrared (heat), visible light,
ultraviolet, x-rays, and gamma rays. Wireless audio transmitters
convert audio signals into the RF portion of the spectrum before
broadcasting them into the air using a technique called frequency modulation.
Everyone is
familiar with FM radio, which works in exactly the same way as
wireless audio systems. An audio signal is fed into a transmitter,
which varies, or modulates, a carrier frequency above and below its
nominal value in a pattern that corresponds to the audio signal. The
receiver, which must be tuned to the same carrier frequency, extracts
the modulated information and converts it back into an audio signal.
The FCC approves,
assigns, and licenses specific radio frequencies for a variety of
uses. The frequencies available for wireless systems differ from
place to place depending on local television, radio, and other
signals. If you try to operate a wireless audio system on a frequency
already occupied by another signal, interference may render your
system unusable.
Most wireless
audio systems offer a selection of frequencies to choose from; if one
doesn't work due to excessive interference, you can try another one.
In addition, most wireless audio manufacturers offer a frequency
coordination service that identifies locally used frequencies.
STRIKE UP THE BAND
- There are
several bands of frequencies in the RF region assigned to wireless
audio and other applications. In an ironic twist of terms, the VHF
(very high frequency) bands occupy the low end of the region. These
bands include low (49 to 88 MHz), mid (150 to 174 MHz), and high (174
to 216 MHz).
The audio portions
of television channels 2 through 6 use the low VHF band; wireless
auditory assistance for the hearing-impaired uses 72 to 76 MHz.
Cordless phones, radio-controlled toys, walkie-talkies, and other
consumer products use 49 MHz, which doesn't require a user license
from the FCC. Making equipment that works in this band is
inexpensive, but few wireless audio products use it because it's so
crowded. The mid band is also quite crowded with industrial and
government signals. In 1988, however, the FCC allocated eight
frequencies in the mid VHF band that can be licensed by commercial
users of wireless audio systems, including musicians, churches,
theaters, etc.
The best
price/performance ratio for wireless audio systems is in the high VHF
band, but broadcasters use it extensively (it carries television
channels 7 through 13). These frequencies can propagate through walls
and sets, making them well-suited to live performance work. However,
only broadcasters and video production companies can legally license
this band.
The high end of
the RF region is called UHF (ultra high frequency). UHF equipment is
more expensive to produce, but more frequencies are available for
wireless systems. UHF signals can be transmitted with greater power,
which increases the operational range between a transmitter and
receiver. However, this draws more current, reducing battery life or
requiring a larger, heavier battery. In addition, these signals are
reflected more easily, which is a blessing and a curse. Although the
reflections can extend the operational range, they also contribute to
some forms of interference.
The low UHF band
encompasses 450 to 614 MHz, although the FCC forbids its use between
608 and 614 MHz for wireless mics. The UHF mid band, which has been
approved for wireless use, extends from 614 to 806 MHz. Wireless
audio systems don't use the low UHF bands much, but crowding in other
bands will soon lead manufacturers and users to seek licenses for
these frequencies. The newest generation of cordless phones shares
the high UHF band (806 to 960 MHz) with several wireless audio
systems. The 902 to 928 MHz region of this band is license-free.
Most VHF and UHF
wireless audio systems multiply the frequency of a crystal oscillator
until they reach the desired carrier frequency. However, a new
technique called frequency synthesis uses electronic oscillators to
directly generate UHF carrier frequencies. Frequency synthesis can
help reduce the problem of interference by providing a greater
selection of frequencies. However, these systems generally require
more power, which, once again, translates to larger, heavier
batteries. Examples of these systems include the Nady 950GS (40
channels in the 800 MHz range) and Samson UHF Synth series (74
channels in the 800 MHz range).
DON'T GIMME NO STATIC
- As we
mentioned earlier, RF signals at nearly identical frequencies can
interfere with each other. This interference, called heterodyning,
results in a windy or whistling noise that sometimes exhibits a
repetitive beat pattern. Manufacturers generally select frequencies
that don't coincide with radio or television frequencies, but other
sources of RF radiation abound, especially in performance venues.
These include digital music and sound gear, computers, fluorescent
and neon lighting, and large motors and generators. Walkie-talkies
used by backstage security personnel can also cause problems.
Broadway represents the worst-case scenario, where almost every
theater up and down the street uses wireless audio systems.
Intermodulation is
another form of interference. In this case, a receiver picks up two
dissimilar frequencies that interact within the electronic components
to produce sum and difference frequencies, which also results in a
windy or whistling noise. With a careful receiver design that
includes high rejection and narrow input filters to attenuate other
signals other than the intended carrier, this effect can be reduced.
Unfortunately, receivers that offer several channels must have
filters wide enough to accept all specified frequencies, which
increases the possibility of unwanted signals entering the system.
In order to
minimize both types of interference, frequency coordination is
essential. Everyone using wireless systems in a venue, including on
stage artists and technical crews, must select their frequencies so
that they don't interfere with each other, or don't coincide with
other local RF sources. This is always difficult, and sometimes
impossible, when many wireless systems are in use at once.
Another common
problem is dropout, in which the signal fades or disappears into a
sea of static noise. This inevitable when the transmitter is too far
away from the receiver, so it's important to minimize the distance
between them as much as possible. The practical range of a wireless
system depends on many factors, including transmitter power, type of
antenna, and environment.
Among the most
common obstacles to RF are walls, sets, scaffolding, lighting grids,
and bodies. Solid metal objects tend to cast a "shadow" in
the presence of short-wavelength UHF signals; if the receiver antenna
is located in this shadow, a dropout is likely. However, nearby
surfaces reflect UHF signals more easily, which might allow the
signal to reach the antenna around the obstacle.
Reflections can
also cause dropout. RF signals radiate from the transmitter's antenna
in all directions. One part of the signal reaches the receiver's
antenna directly, while the other parts are reflected toward the
antenna from surrounding surfaces, particularly those made of metal.
If the difference between the lengths of the direct and reflected
paths is a multiple of 1/4 to 3/4 of a wavelength, the two signals
will partially or completely cancel each other out, resulting in a
dropout. This is called multipath cancellation.
The exact
locations of multipath cancellation zones depend on the relative
positions of the transmitter, receiver, and reflective surfaces. In
an on stage wireless audio system, the receiver and surface are
generally stationary, while the transmitter is moving around with the
performer. This causes the cancellation zones to move around, as
well. When a cancellation zone coincides with the receiver antenna's
position, the signal level drops. It's possible to map out the
transmitter locations that cause cancellation at the receiver ahead
of time, but it's hard to avoid these spots when you're running
around during a performance.
LOUD AND CLEAR
- Wireless
receivers fall into two basic categories: non-diversity and
diversity. Non-diversity receivers use a single antenna to pick up RF
signals. This setup does nothing to reduce multipath cancellation,
but it's inexpensive and works reasonably well with low-power
transmitters over short distances (FM radios are non-diversity
receivers that work over long distances, but the transmitters operate
at very high power level.) The Nady Wireless One and 101, Samson VLP
and Stage 2, Sennheiser VHF 1B, and Shure L3 are non-diversity
systems that operate in the high VHF band.
Diversity
receivers use two antennas to reduce the effect of multipath
cancellation. If the antennas are placed at least 1/4 wavelength
apart, it is highly unlikely that both will fall within a
cancellation zone at the same instant. There are several types of
diversity, each with their own ardent supporters.
Phase diversity
uses two antennas and one receiver. If the signal level at the
receiver drops below a threshold value, the phase of the signal from
one antenna is shifted to compensate for the assumed multipath
cancellation. In some systems, the phase immediately inverted by 180
degrees, while other systems continuously shift the phase according
to the instantaneous signal level at the receiver. For example, the
Telex FMR-100 continuously shifts the phase of one antenna to obtain
the strongest possible signal.
The most common
type of diversity is called switching, or true diversity. This method
uses two antennas connected to their own separate receivers.
The system
monitors the signal level from each antenna and uses the audio output
of the receiver with the strongest signal at any given instant. This
requires sophisticated circuitry to switch with no noise.
Nevertheless, most high-end wireless audio systems use switching
diversity to minimize multipath cancellation.
Combining
diversity is a variation of switching diversity in which the audio
signals from the two receivers are combined in the optimum proportion
according to their relative levels. If both are strong, both are
used; if only one is strong, it is used on its own. This scheme
eliminates any noise that might be cause by switching from one
receiver to the other. The Nady 201 and Shure EC4 and L4 receivers
use combining diversity and operate in the high VHF band.
Another factor to
consider is signal-to-noise (S/N) ratio. RF transmissions can achieve
an S/N of no more than about 65 or 70 dB. As a result, virtually all
wireless systems now use companding to improve the S/N ratio; this
technology is probably responsible for the current viability of
wireless audio systems. The transmitter compresses the signal,
typically with a compression ratio of 2:1. The receiver then expands
the signal accordingly, which yields an S/N in the 90 to 113 dB
range. Companding also extends the practical range of the system by
reducing the required RF level at the receiver.
Of course,
companding has its own problems. The S/N ratio from one system to
another can vary by as much as 20 dB, depending on the specific
design of the companding circuit. Systems with lower S/N ratios may
also exhibit audible "breathing." Other anomalies include
high-end and low-end roll-off, as well as increased feedback
sensitivity. These problems can be reduced or exaggerated depending
on factory parameters such as threshold.
APPLICATIONS
- Vocal
microphones are among the most common applications of wireless
technology. Many manufacturers build the transmitter directly into
the body of a hand-held mic. For example, the Shure Beta 58 and 87
are available in wireless versions. Another option is a headset mic,
such as the Samson system with the AKG 410 used by Garth Brooks. A
headset mic is connected to a body-pack transmitter attached to the
performer's belt. Tiny lavalier mics are also used with body-pack
transmitters for lecturers and stage actors. The receiver is
connected to the house audio system with standard cables.
Guitarists are
using wireless systems in greater numbers, as well. Like headset and
lavalier mics, the guitar output is connected to a body-pack
transmitter attached to the performer's belt or guitar strap. The
receiver is connected to the guitar amp in the normal manner.
Examples of wireless guitar systems include the Nady Wireless One GT,
Telex R-10G and R-20G, Samson Concert II, Sennheiser VHF 2G and UHF
2G, and Shure Guitarist.
Wireless audio is
now in common use for electronic news-gathering (ENG). These systems
include a wireless handheld or lavalier mic and a small receiver that
usually mounts directly on a video camera. This receiver can be of
the non-diversity type because the distance between the camera and
reporter is generally very short. Several companies make ENG systems,
including the Sennheiser UHF 2B, Lectrosonics Pro-Mini, Telex ENG-1,
and Samson MR-1.
Other applications
include wireless intercom systems for the technical crews in a venue.
These systems can take the form of walkie-talkies, or headset
mic/earphones with belt pack transmitter/receivers. For examples, the
HM Electronics Systems 800 UHF and Telex Radiocom VHF intercom
systems both accommodate up to four full-duplex (simultaneous send
and receive) headset/belt-pack units with each base station.
Large performances
often require several wireless mics and/or guitar systems operating
simultaneously. In such applications, it's important to remember that
each receiver can pick up only one signal at a time; if two or more
signals are on the same carrier frequency, interference rears its
ugly head.
Practically
speaking, ten or twelve channels can be used together in each VHF
band, while UHF can accommodate up to 40 channels simultaneously.
Some wireless systems offer many more channels in a given band, but
it may not be possible to use them all.
If the number of
antennas on stage is unwieldy, an active antenna splitter can
streamline the system. One set of antennas picks up signals from
several transmitters and sends them to their respective receivers.
For example, the Nady AD-4, Shure WA-404, Samson DA-4, and Telex
AD-200 can each accommodate up to four diversity or eight
non-diversity receivers; multiple units can be connected to serve
more receivers. The Sennheiser SAS100, 200, 300, and 400 can
accommodate up to six, twelve, eighteen, or 24 receivers, respectively.
Several companies
offer multiple receivers in a single unit with one set of antennas.
The Sennheiser EM 1046 offers up to eight switching-diversity UHF
receiver modules in a single card cage and a computer monitor system.
The Nady 750 offers two switching diversity, high-band VHF receivers
in a single unit; up to five units can be connected for a total of
ten simultaneous channels. The Samson UR5D includes two separate UHF
synthesized receivers with 74 channels each. |
