Specifications

proportional to their relative strength. When both are strong,
the contribution from each signal is equal. If one signal
becomes weaker, its contribution is similarly reduced. Finally,
if a complete dropout occurs for one signal, the receiver uses
only the good signal. Since the combining technique acts
as a continuous balance control rather than as a switch,
it further reduces any audible effects of diversity action.
Historically, diversity receivers have always been used
for critical applications even though their cost was some-
what higher. Today, the cost of wireless systems in general
and diversity systems in particular has decreased to the
point that diversity receivers are used in the majority of
meeting facility applications.
An additional refinement in nearly all recent wireless
systems is some form of noise reduction, or companding,
in order to decrease the inherent noise and increase the
limited dynamic range of radio transmission. The word
companding refers to the two steps of the process: the
signal is modified (compressed) in the transmitter before
it is broadcast and then “un”-modified (expanded) in
the receiver in a complementary fashion. Although the
principle of companding is similar in all wireless systems,
significant differences between manufactured models
make it undesirable to mix transmitters of one brand with
receivers of another brand.
Other aspects of wireless microphone systems that
must be considered in selection and use are operating
frequencies, antennas, and radio interference. All three are
especially important when planning the use of multiple
wireless systems in the same location.
Every wireless microphone system transmits and
receives on a particular radio frequency, called the
operating frequency. These frequencies may be grouped
into four bands: low-band VHF (49-72 MHz), high-band
VHF (169-216 MHz), low-band UHF (450-614 MHz)
and high-band UHF (806-952 MHz). VHF stands for
“Very High Frequency”; UHF stands for “Ultra High
Frequency”; and MHz stands for “MegaHertz” or millions
of cycles-per-second. Use of these bands is regulated by
the Federal Communication Commission (FCC). Certain
frequencies within each band have been designated for
use by wireless microphones as well as by other devices.
It should be noted that while manufacturers must be
licensed by the FCC to sell wireless equipment, it is the
responsibility of the purchaser to observe FCC regulations
regarding their actual use.
Low-band VHF, particularly 49 MHz, is shared not only
by consumer grade wireless microphones but by cordless
telephones, walkie-talkies, and radio controlled toys.
For this reason, it is almost never recommended for serious
applications, even though systems in this range are very
inexpensive.
The high-band VHF range is the most widely used for
professional applications, and quality systems are available
at a variety of prices. Although this range includes U.S.
television channels 7-13, there are ample frequencies for
use in any part of the country.
The two UHF bands, while free of many interfering
signals found in the VHF bands, have traditionally been used
only by very costly systems, due to more complex design
and circuitry. However, as the price of UHF systems declines
and the VHF band becomes more crowded, the UHF band
has become increasingly attractive.
Selection of operating frequency for a single wireless
system only involves choosing a frequency that is unused
by local television stations. Although there are tunable or
frequency agile systems available, most equipment
operates at a fixed frequency controlled by a quartz
crystal which is installed and adjusted by the manufacturer.
Most manufacturers can recommend a frequency
that will generally work in a given geographic area.
The difficulty arises when multiple systems must work
together in the same location.
Due to the nature of radio reception, it is not
possible for a single receiver to pick up multiple
transmitters on the same frequency. Therefore, each
transmitter must be on a separate frequency and have a
corresponding receiver on that frequency. An additional
complication is that simultaneously operating systems,
even though they may be on different frequencies, may
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MEETING FACILITIES
Audio Systems Guide for
Mercantile Stores “University of Merchandising”