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SPREAD SPECTRUM
TRANSMISSION
A transmission technique that may have application to
wireless microphone systems is known as "spread
spectrum." The object of this technique is to improve
performance by reducing interference effects and
increasing efficiency of band utilization. Instead of
concentrating the information and energy of the
transmission at a single, continuous frequency, the signal
is spread out over a wide radio frequency range. The two
most common methods are "frequency hopping" and
"direct sequence spreading. "
Frequency hopping systems utilize a transmitter that
changes its operating frequency many times per second
according to a predetermined pseudo-random pattern.
The receiver is synchronized with the transmitter so that it
changes its operating frequency in exactly the same
pattern. At any instant the system is operating on only one
frequency but over time the range of frequencies used
may be several MHz. The information may be carried as
an FM signal or as a digital signal. (See Figure 3-13.)
Direct sequence systems operate around a center
frequency but the effective total modulation (bandwidth) of
the signal is significantly increased. This is accomplished by
modulating the phase of the carrier with a high-speed, pre-
determined pseudo-random digital sequence (pattern).
Again the receiver is synchronized with the transmitter
according to the same pattern. In this system the
information can be carried as an analog FM signal or as a
digital signal, mixed with the phase modulating sequence.
(See Figure 3-14.)
By spreading the transmission power of the desired
signal over a greater portion of the radio spectrum, the
average energy of the desired signal at any one frequency
is reduced. This reduces the potential for interference from
a particular transmitter. In addition, the receiver becomes
less sensitive to undesirable radio sources at any one
frequency because it spreads the energy of the interfering
source over a wider range at much lower average level.
It is not only possible to decrease certain radio interference
effects but also to increase the number of users that may
operate in a given band.
Historically, this technique has been applied to
communication and data transmission, particularly in
military applications. Presently it is found in some
consumer cordless telephone equipment and has been
used in at least one MIDI wireless microphone system.
However, current spread spectrum technology may not
lend itself to the highest fidelity audio transmission in a
wireless microphone package without significant tradeoffs
in size and cost.
RANGE OF WIRELESS
MICROPHONE SYSTEMS
A logical question concerning wireless performance is
the transmission range of various systems. Unfortunately,
the answer is much more complicated than a simple
distance measurement. Ultimately, the receiver must be
able to pick up a "useable" signal from the transmitter.
"Useable" means that the strength of the desired signal is
within the sensitivity range of the receiver and further that it
is sufficiently stronger than (or different from) undesirable
signals and RF noise to produce an acceptable signal-to-
noise ratio at the audio output of the receiver. Elements
that affect useability are the transmitter/antenna, the
transmission path, the receiver/antenna and RFI. Some
characteristics of these elements are controllable,
some are not. (See Figure 3-15.)
Selection
and Operation
of W
ireless Microphone Systems
32
C
H A P T E R
3
Wireless System Operation
Figure 3-13: frequency hopping spectrum
Figure 3-14: direct sequence spectrum
Figure 3-15: loss vs. distance vs. frequency