Handheld transmitter controls are generally limited
to a power switch, a mute switch, and gain adjustment.
Again, tuneable models include some provision for
frequency selection. Indicators are comparable to
those in bodypack transmitters: power status, battery
condition, frequency. Handheld transmitter antennas
are usually concealed internally, though certain types
(primarily UHF) may use a short external antenna.
"Plug-on" transmit-
ters are a special type
designed to attach
directly to a typical
handheld microphone,
effectively allowing
many standard micro-
phones to become
"wireless." The trans-
mitter is contained in a
small rectangular or
cylindrical housing with
an integral female XLR-type input connector. Controls and
indicators are comparable to those found in bodypack
types and the antenna is usually internal.
Miniaturization of components has also resulted in a
class of transmitters that are integrated directly into
headworn microphones and lapel microphones as well as
units that can plug directly into the output connector of an
electric guitar. The trend toward smaller and more highly
integrated devices is certain to continue.
While transmitters vary considerably in their external
appearance, internally they all must accomplish the same
task: use the input audio signal to modulate a radio
carrier and transmit the resulting radio signal effectively.
Though there are many different ways to engineer wireless
transmitters, certain functional elements are common to
most current designs. It is useful to describe these
elements to gain some insight to the overall performance
and use of wireless microphone systems. (See Figure 2-3.)
TRANSMITTER: AUDIO CIRCUITRY
The first part of the typical transmitter is the input circuitry.
This section makes the proper electrical match between the
input source and the rest of the transmitter. It must handle the
expected range of input levels and present the correct
impedance to the source. Gain controls and impedance
switches allow greater flexibility in some designs. In certain
cases, the input circuitry also provides electrical power to the
source (for condenser microphone elements).
The signal from the input stage passes to the signal
processing section, which optimizes the audio signal in
several ways for the constraints imposed by radio
transmission. The first process is a special equalization
called pre-emphasis, which is designed to minimize the
apparent level of high frequency noise (hiss) that is
unavoidably added during the transmission. The "emphasis"
is a specifically tailored boost of the high frequencies.
When this is coupled with an equal (but opposite)
"de-emphasis" in the receiver, the effect is to reduce high
frequency noise by up to 10 dB. (See Figures 2-4 a & b.)
The second process is called "companding"
(compress/expand),
which is designed to
compensate for the
limited dynamic range of
radio transmission. The
part of the process
performed in the trans-
mitter is "compression,"
in which the dynamic
range of the audio
signal is reduced or
compressed, typically by
a fixed ratio of 2:1. Again, when this is coupled with an equal
but opposite (1:2) "expansion" of the signal in the receiver,
the original dynamic range of the audio signal is restored. A
voltage-controlled-amplifier (VCA) is the circuit element that
provides both dynamic functions: gain is decreased in the
compressor mode and increased in the expander mode.
The gain change is proportional to the signal level change.
Nearly all current wireless microphone systems employ
some form of companding, allowing a potential dynamic
range greater than 100 dB. (See Figure2-5.)
Selection
and Operation
of Wireless Microphone Systems
10
CHAPTER 2
Basic Radio Systems
Figure 2-4b: de-emphasis in transmitter
Figure 2-4a: pre-emphasis in transmitter
Figure 2-3: general transmitter block diagram