THE WIRELESS MICROPHONE
. . . what it is, what it does . . .
The wireless microphone is actually a system consist-
ing of a microphone, a transmitter, and a receiver. The
microphone is an electroacoustic transducer-a device
which converts sound waves (voice, musical in-
struments, etc.) to electrical inpulses, which are sent to
the transmitter. The transmitter generates a radio-
frequency carrier, modulates this carrier with the
microphone signals, and radiates this modulated RF
signal like a tiny radio station. The receiver is matched
to the transmitter frequency and picks up the transmitter
signal through its antenna. The receiver’s circuitry
detects the modulated RF signal and converts it to an
audio voltage for routing (through a microphone input) to
an audio mixer.
At this point, the wireless system acts like any
microphone input to a sound system. The figure below
shows the complete system, including that which is not
a part of the wireless microphone system.
Frequency Bands.....
Shure wireless transmitters and receivers are single-
channel, crystal-controlled units operating in the VHF-
FM band between 150 and 216 megahertz. A total of 15
frequencies, computer-selected for interference-free
operation, are readily available, and other frequencies
can be ordered on a special basis. This means that up to
12 systems can be operated in a single sound installa-
tion, simultaneously and without intermodulation pro-
blems.
Diversity Reception.....
The most troublesome aspect of wireless microphone
operation can be a phenomenon known as multipath
transmission, or simply, multipath. This appears as a
signal “dropout,”
with annoying static or complete
(although temporary) signal loss. It’s caused when the
direct signal and a reflected signal-bounced off walls,
ceiling, metallic objects, etc.-arrive at the receiver at
different times, literally out-of-phase, or when the signal
path is completely blocked.
You’ve probably experienced this phenomenon with
the FM radio in your car. A dropout location, most likely
caused by buildings, mountains, or even telephone
poles, causes your radio signal to drop to a low, noisy
level -sometimes disappearing completely. This is
multipath.
Multipath is virtually a thing of the past with a unique
antenna/receiver system called DIVERSIPHASE™. The
two antennas supplied with the W25DR receiver are plac-
ed some distance apart, and the signals each receives
are compared at the receiver. The W25DR’s DIVER-
SIPHASE circuit monitors each antenna, and if the direct
and reflected signals appear to be creating an out-of-
phase condition-and the inevitable signal dropout, the
receiver modifies the phase difference between the two
antennas until the two signals add in phase.
TYPICAL WIRELESS MICROPHONE SYSTEM
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