Handbook for Sound Engineers

Microphones 567

in Japan. This is achieved by careful attention to factors
like antenna position, use of low loss RF cables and RF
gain structure of the antenna distribution system.

There are indeed some applications in which more
RF output power is an appropriate measure; a perfect
example would be a golf tournament, as the wireless
system needs to cover a wide area. There are usually
only a few wireless microphones in use at this type of
function, and those microphones are generally not in
close proximity to each other.

If transmitters with high RF power are close
together, intermodulation usually occurs. At the same
time, the RF noise floor in the performance area is
increased. As a matter of fact, a transmitter in close
proximity to another transmitter will not only transmit
its own signal, but it will also receive the signal and add
this to the RF amplifier stage.

16.11.6.2 Dc-to-Dc Converter

Transmitters should be designed to provide constant RF
output power and frequency deviation throughout the
event being staged. This can he achieved through the
use of a dc-to-dc converter circuit. Such a circuit takes
the decaying battery voltage as its input and regulates it
to have a constant voltage output. Once the voltage of
the batteries drops below a minimum level, the dc-to-dc
converter shuts off, almost instantaneously. The result is
a transmitter that is essentially either off or on. While it
is on, the RF output power, frequency deviation, and
other relevant specifications remain the same. Transmit-
ters without regulation circuits, once the battery voltage
begins to drop, will experience reduced range and audio
quality.

16.11.6.3 Audio Processing

To improve the audio quality, several measures are
necessary because of the inherent noise of the RF link.

16.11.6.3.1 Pre- and De-Emphasis

This method is a static measure that is used in most of
the FM transmissions. By increasing the level of the
higher audio frequencies on the transmitter side, the
signal-to-noise ratio is improved because the desired
signal is above the inherent noise floor of the RF link.

16.11.6.3.2 Companding

The compander is a synonym for compressor on the

transmitter side and for expander on the receiving end.

The compressor raises low audio level above the RF

noise floor. The expander does the mirror opposite and

restores the audio signal. This measure increases the

signal-to-noise ratio to CD quality level.

16.11.6.3.3 Spurious Emissions

Apart from the wanted carrier frequency, transmitters

can also radiate some unwanted frequencies known as

spurious emissions. For large multichannel systems

these spurious frequencies cannot be ignored. They can

be significantly reduced through elaborate filtering and

contained by using a well-constructed, RF tight metal

housing for the transmitter. Also, an RF tight transmitter

is less susceptible to outside interference.

A metal housing is important not only for its

shielding properties, but also its durability. These

devices usually experience much more abuse by actors

and other talent than anyone ever predicts.

16.11.6.4 Transmitter Antenna

Every wireless transmitter is equipped with an antenna,

which is critically important to the performance of the

wireless system. If this transmitter antenna comes in

contact with the human body, the transmitted wireless

energy is reduced and may cause audible noises known

as drop-outs. This effect of detuning the antenna on

contact is called body absorption.

For this reason, talent should not touch the antenna

while using handheld microphones. Unfortunately, there

is no guarantee that they will follow this recommenda-

tion. Taking this into account, optimized antenna setup

at the receiver side and the overall RF gain structure of

the system becomes critical.

This same effect can occur when using bodypack

transmitters, especially if the talent is sweating. A

sweaty shirt can act as a good conductive material to the

skin. If the transmitter antenna touches it, reduced

power and thus poor signal quality may result. In this

case, a possible approach is to wear the bodypack

upside down near or attached to the belt, with the

antenna pointing down. Sometimes this measure does

not work because the talent will sit on the antenna. In

this case, a possible solution is keeping the transmitter

in the normal position and fitting a thick-walled plastic

tube over the antenna, such as the type that is used for

aquarium filters.