1536 Chapter 41
restricted to a 3–4 kHz bandwidth that is adequate for
voice only. Table 41-1 gives the technical specifications
for an eight channel inductive loop system by Auditel.
Inductive loop systems are inexpensive and easy to
install and therefore usually preferred in large portable
and multiple venues where cost is important. Different
systems in adjacent areas can cause crosstalk and secu-
rity can become a problem if a listener can get close to
the loop. Steps can be taken, such as using an
out-of-phase loop around the room about 3 ft beyond
the periphery of the transmission loop, to reduce hori-
zontal coverage, Fig. 41-7D. To reduce vertical interfer-
ence where one system is directly above another, a
double zigzag antenna system can be used, Fig. 41-7E.
This antenna system will reduce the unwanted signal to
50 dB below the wanted signal when the separation
distance is 10 ft (3 m) as opposed to 20 ft (6 m) with a
standard antenna.
The induction loop receivers normally operate on
batteries so the listener can be located or move
anywhere within the induction loop. The receivers use a
ferrite rod antenna, have a battery status indicator, a
volume control, and a channel selector switch. Most
units use alkaline batteries and get upwards of 500 h
between battery changes. High fidelity headsets are not
required because the overall frequency response is so
limited.
41.3 Infrared Systems
An infrared system is a modern version of the wireless
system. InfraRed was designed by Sennheiser in the
seventies as a medium for wireless audio transmission.
It was initially used as a broad band system for home
entertainment and later for assisted hearing systems. It
is applied to multichannel audio systems using narrow
band modulation techniques in the 930 nanometer
wavelength band. Rather than using radio frequencies
for transmitting the signal, it uses infrared frequencies,
which are confined to line-of-sight or reflections off of
objects. These objects can be mirrors, glass ashtrays, or
any other brightwork. Infrared goes through glass
windows and is absorbed by objects and walls that are
dark green or black. It is important that the walls of a
room using infrared be light colored and have a
minimum number of windows.
Two different transmission techniques are used: the
pulse modulated system and the FM multiplexed
system. The pulse modulated system has an advantage
in that the amount of emitted radiation required is inde-
pendent of the number of channels, however, it usually
has poorer audio quality than the FM multiplexed
system. Pulsed systems do not meet IEC audio stan-
dards and can be affected more by high-frequency fluo-
rescent lights.
Most systems used today are based on modulated
carrier techniques using FM. The operating frequencies
for wide-band two-channel infrared systems are 95 kHz
and 250 kHz with peak deviation of ±50 kHz.
Narrow-band systems operate on twelve or more chan-
nels between 55 kHz and 1335 kHz (excluding
455 kHz) with 40 kHz channel spacing and peak devia-
tion of ±7 kHz. These standards are specified in the IEC
76 international standard and insure compatibility
between manufacturers. General specifications for
infrared systems are given in Table 41-2. The system
comprises three sections: the transmitter, the emitter
(sometimes both are combined in one unit), and the
receiver. The transmitter imparts the audio signal onto a
subcarrier that the emitter converts into infrared light.
The receiver decodes the infrared signal to retrieve the
original audio, Fig. 41-8.
The signal enters the listening area via infrared radi-
ators. The IR light emitting diode can cover an area of
70 ft^2 and has a coverage angle of ±25 degrees. TheTable 41-1. Technical Specifications for FM Inductive Loop Systems
Channel No 12345678Channel Frequencies (kHz) 59.7 44. 8 29.9 134.3 119.4 104.5 89.6 74.6
Modulation FM
Normal Deviation ±1.5 kHz
Peak Deviation ±1.7 kHzAudio Frequency Characteristics Transmitter ReceiverFrequency response (3 dB) 125 Hz–4 kHz 90 Hz–4 kHz
Max. distortion (at 1 kHz) <1% <2.5%
Signal/Noise (A weighting) >50 dB >45 dB