Specifications

Non-diversity receivers work well in many applications
but are subject to a phenomenon known as dropout:
a temporary interruption of the radio signal. The audible
effect may range from a slight “swishing” noise to a
complete loss of sound. Since radio signals become
weaker over greater distances, a dropout can occur when
the transmitter is very far from the receiver antenna.
A dropout can also occur at shorter distances when the
radio signal from the transmitter is blocked by obstacles
such as human bodies, walls, or equipment. Finally,
dropouts may be experienced even at very short distances
by a mechanism called multipath interference. Part of the
signal from the transmitter (which radiates in all directions)
travels directly to the receiver, but some of the signal is
reflected to the receiver by metal objects or other
structures. When the paths of the direct signal and of
the reflected signal(s) are sufficiently different, they will
interfere with each other when they combine at the receiver
antenna. If the interference is great enough, partial or
complete cancellation of the signal occurs, resulting in a
dropout. It is similar to an extremely severe ghost in
television reception, and the cure is the same: move the
receiver antenna relative to the transmitter. This is not
usually practical since it is the receiver antenna that is in a
fixed location, while the wireless microphone location is
constantly changing.
This introduces the concept behind the second
wireless receiver configuration, called a diversity system.
A diversity receiver utilizes two separate antennas and
(usually) two separate radio circuits. When the two
antennas are separated by even a short distance, the
chance of a simultaneous interruption at both antenna
positions is extremely low. The key to the system is
additional intelligent diversity circuitry which continuously
monitors the received signals and takes action according
to the type of diversity circuitry employed.
The simplest effective diversity technique, known as
“phase” diversity, has two antennas but only one radio
circuit. The diversity circuitry adjusts the relative polarity of
the antennas before combining them for optimum
reception. This approach is less expensive, due to the
single radio section, and works well when both antennas
are getting a usable signal. But, it may not give the best
results in the case of severe multipath interference.
Most “true” diversity receivers are of the switching type.
These utilize two antennas and two radio sections.
The diversity circuitry selects the better of the two received
signals by means of an electronic switch. If the switching is
done quickly and quietly enough, the result is nearly
dropout-free performance, with minimal audible side effects.
The third diversity design is known as the combining
type. This method takes advantage of the fact that both of
the received signals are usable much of the time. In this
case, using the signals from both antennas yields better
reception than using only one signal (as in the switching
type). The combining diversity circuitry adds the signals
23
MEETING FACILITIES
Audio Systems Guide for
Multi-path reflections