Specifications

CHAPTER SIX
WIRELESS MICROPHONE SYSTEMS
A wireless microphone is actually a system consisting of
1) a microphone 2) a radio transmitter and 3) a radio
receiver. The function of the microphone is unchanged and
the function of the transmitter and receiver combination is to
replace the microphone cable with a radio link. Although
this objective is simple, its accomplishment is not. However,
with some knowledge of the components and characteristics
of wireless microphone systems, and a clear idea of the
intended application, the selection and use of wireless
microphones can be made relatively straightforward.
1) The Microphone: How does sound
enter the wireless system?
The selection process for the microphone part of
a wireless system is exactly the same as for wired
microphones. The microphone must be matched to the
desired sound source and to the sound system. In this
case, the sound system consists not only of the devices
that make up the rest of the audio chain but the input to the
radio transmitter as well. Acoustically, wireless and wired
microphones behave identically. Proper microphone
choice and placement is still necessary to get the best
sound and to avoid problems such as feedback.
Available microphone choices for wireless include
dynamic or condenser types, with flat or shaped frequency
response, omnidirectional or unidirectional polar patterns,
and a variety of physical designs: lavalier, handheld,
headworn, etc. Almost any type of microphone may be
used as part of a wireless system, the notable exception
being phantom-power-only condensers. The choice
depends on the specific application.
2) The Transmitter: How does the microphone
signal become a radio signal?
The transmitter uses the audio signal from the
microphone to vary the frequency of a radio signal which is
broadcast to the receiver. The principle is called Frequency
Modulation (FM) and is identical to that used by commercial
FM radio stations. Electrically, the transmitter input must
be compatible with the microphone output both in level
and impedance. The transmitter input may also supply
bias power for some condenser microphone elements.
(Note that bias power is not the same as phantom power.)
The transmitter itself is always battery-powered.
Physically, the transmitter takes one of two forms. The
first is a small box, called a body-pack or belt-pack, that
can be clipped to a belt, placed in a pocket, or otherwise
attached to the user. The microphone connects to it by
means of a small cable. Some models have a detachable
cable which allows the transmitter to be used with a variety
of inputs. This form is most often used with lavalier
microphones but can also be connected to head-worn
microphones or hand-held types with appropriate cables.
All transmitters have a power on-off switch and many have
a mute switch to silence the microphone without turning off
the radio signal itself.
The second form is a transmitter that is built into
the cylindrical body of the microphone itself. This is used
almost exclusively for handheld speech microphones and
results in a package only slightly larger than a conventional
wired microphone.
3) The Receiver: How is the radio signal turned back into
an audio signal?
The receiver picks up the radio signal broadcast
by the transmitter and extracts or demodulates the audio
signal from it. Again the principle is the same as that of an
ordinary FM radio. The output of the receiver is electrically
identical to a microphone output and can be connected to
any typical microphone input in a sound system. Some
receivers have additional amplified outputs for headphones
or auxiliary connections to sound systems. Although most
receivers operate on ordinary 120V or 230V AC power,
battery-powered types are available for portable use.
Wireless receivers are also designed in two different
configurations. The first is called non-diversity and consists
of a single antenna and a single radio circuit. An ordinary
FM radio is an example of a simple, non-diversity receiver.
22
MEETING FACILITIES
Audio Systems Guide for
Wireless microphone system components