Handbook for Sound Engineers

Microphones 563

Note: If long runs of antenna cable are used, be sure

they are of the low-loss type.

• Make a walkaround test, i.e., operate the transmitter
  at all positions where it will be used later. Mark all
  points where field strength is weak. Try to improve
  reception from these points by changing the antenna
  position. Repeat this procedure until the optimum
  result is achieved.

Interference is mainly caused by spurious signals
arriving at the receiver input on the working frequency.
These spurious signals may have various causes:

• Two transmitters operating on the same frequency
  (not permissible).


• Intermodulation products of a multichannel system
  whose frequencies have not been selected carefully
  enough.


• Excessive spurious radiation from other radio instal-
  lations—e.g., taxi, police, CB-radio, etc.


• Insufficient interference suppression on electric
  machinery, vehicle ignition noise, etc.


• Spurious radiation from electronic equipment—e.g.,
  light control equipment, digital displays, synthe-
  sizers, digital delays, computers, etc.

16.10.3 Companding

Two of the biggest problems with using wireless micro-
phones are SNR and dynamic range. To overcome these
problems, the signal is compressed at the transmitter
and expanded at the receiver. Figs. 16-144 and 16-149
graphically illustrate how and what this can accomplish
with respect to improving the SNR and reducing the
susceptibility to low-level incidental FM modulation,
such as buzz zones.
As the typical input level changes by a factor of
80 dB, the audio output to the modulator undergoes a
contoured compression, so a change in input audio level
is translated into a pseudologarithmic output. This
increases the average modulation level, which reduces
all forms of interference encountered in the transmis-
sion medium.
By employing standard narrowband techniques at the
receiver, the recovered audio is virtually free of adjacent
channel and spurious response interference. In addition,
up to ten times the number of systems can be operated
simultaneously without cross-channel interference. The
ability of the receiver to reject all forms of interference
is imperative when utilizing expansion and compression
techniques because the receiver must complimentarily
expand the audio component to restore the original
signal integrity.

16.10.4 Waterproof Wireless Microphone Systems

Wireless microphones that are worn are very useful for

coaching all forms of athletics including swimming and

aquatic aerobics. If the instructor always stays on the

pool deck, a weatherproof system might be adequate. If

the instructor is in the water, a completely submersible

and waterproof system will be required.

Hydrophonics assembles a completely waterproof

and submersible wireless microphone system. Assem-

bled with Telex components, the system includes a

headset microphone with a special waterproof

connector and a Telex VB12 waterproof beltpack trans-

mitter. The transmitter can operate on a 9 V alkaline

battery or a 9 V NiMH rechargeable battery. The

rechargeable battery is recommended as it does not

require removing the battery from the transmitter for

recharging and therefore reduces the chance of water

leaking into the transmitter housing. The receiver is a

Telex VR12 for out-of-pool operation, and can be

connected to any sound system the same way as any

other wireless microphone.

An interesting thing about this system is you can

dive into the water while wearing the system and come

up and immediately talk as the water drains out of the

windscreen rapidly.

The DPA Type 8011 hydrophone, Fig. 16-150, is a

48 V phantom powered waterproof microphone

specially designed to handle the high sound pressure

levels and the high static ambient pressure in water and

Figure 16-149. Compression and expansion of the audio

signal. Notice the 80 dB signal is not altered, and the

20 dB signal is altered significantly.

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Dynamic range

without companding

Dynamic range

with companding

Compression

at the transmitter

Transmission

path

Expansion

at the receiver

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System noise

plus modulation

demodulating

noise