Handbook for Sound Engineers

Assistive Listening Systems 1555

audio signal onto a subcarrier signal which the emitter
converts into infrared light. The receiver decodes the
infrared signal to retrieve the original audio.
To achieve a usable radiated power level, the IR
LEDs are used in multiple arrays. Their light output is
amplitude-modulated by one or more frequency-modu-
lated subcarriers (typically 95 kHz for single-channel
wideband systems; 95 kHz and 250 kHz for two-channel
systems). Each channel’s audio signal frequency-modu-
lates its particular subcarrier, Fig. 42-15.

Two transmission modes are available: wideband,
for one or two channels of high-fidelity audio; or
narrowband, for up to twelve channels with a 70–
7000 Hz response suitable for communications, Fig.
42-16.
Since the transmission medium is a modulated
carrier of harmless invisible light, Fig. 42-17, instead of
radio or audio signals, it is immune to outside interfer-
ence and also causes none itself. No operator licensing
is required for use of infrared systems.
Manufacturers provide detailed instructions for plan-
ning and installing the system. Typical installations are
shown in Fig. 42-18. The rear emitters must cover both
under the balcony plus the balcony. For this reason,
separate emitters may be required to cover both areas.
The advantages of infrared systems are fully realized in
applications where different audio programs are
required in adjacent rooms, such as a multicinema
complex. Each room can be equipped with the same

system without interference among them. No frequency

coordination is required as with radio frequency

systems. The same receiver can be used in any theater.

A useful tool in aiming the emitters is a low-cost,

black-and-white television camera and monitor. Most

monochrome television cameras have useful sensitivity

in the infrared region. With the room lights off, observe

on the television monitor the part of the room illumi-

nated by the infrared beam. The well-illuminated area

will be the area of good reception. A corollary to this

procedure is that the infrared television viewing system

can be used to view a darkened stage—for instance, for

coordination of rigging and prop moves in a fast,

complicated change in the dark.

42.3 Receivers

Receivers are required with all systems, though fewer

are needed with an induction loop because many

patrons will have aids equipped with T-coils. Most

manufacturers of systems for the hearing impaired offer

several types of earphones with their receivers. Typi-

cally these include a single earpiece, a stethoscope-type

dual earpiece, Fig. 42-19, and an induction loop for use

Figure 42-15. Infrared system’s modulation technique.

Time

Time

Time

Time

Infrared light with amplitude modulated subcarrier

AF signal

Infrared light with frequency modulated subcarrier

Amplitude

Amplitude

Amplitude

Amplitude

Unmodulated infrared light (carrier)

Figure 42-16. Infrared systems channel allocation.

Figure 42-17. Light spectrum.

0 100 200 300 400 1 kHz

(^0100) 200 300 400 1 kHz
1 channel wideband (swing ±40 kHz)
2 channel wideband (swing ±40 kHz)
0 40
(^2060)
80 200 300 1 kHz
100
Multichannel narrowband (swing ±7 kHz)
Amplitude
Amplitude
Amplitude
Frequency–kHz
Frequency–kHz
Frequency–kHz
Visible
light
Infrared
light
Ultraviolet
light