Theatre Design & Technology - Summer 1983 - 25

induction loop for use with the patron's
T-coil. Two types of induction loops are
available-one hangs around the neck
and may be used to support the receiver,
while the other is a small coil that hooks
over the ear close by the hearing-aid.
The FM transmitter costs as little as
$700. The receivers cost from $50 up depending on brand.
AM Broadcast systems are largely
unregulated by the FCC. The basic rules
are: I) the system must not cause any interference to an existing licensed service,
2) the radiation must be restricted to the
user's building, 3) the operating frequency must be in the AM broadcast
band or below, and 4) if an open-wire
antenna is used, it cannot exceed 10 feet
in length.
The quality of an AM broadcast signal is better than a magnetic induction
loop but not as good as FM. Although
there is the possibility of more noise
from electric line interference than with
FM, the AM system is usually not as
noisy as an induction loop. If a pocket
AM radio works in the space, then the
low-power broadcasting system will
probably work as well. Systems operating in the commercial broadcasting band
may be picked up on any AM receiver.
Systems operating below the standard
broadcast band are available with fixedfrequency, non-tunable receivers.
It may seem attractive economically
to select a system operating in the regular broadcast band because patrons
could be expected to furnish their own
receivers. But much can be said in favor
of the special non-tunable receiver. Picture a person in the third row trying to
find your stage broadcast with his or her
earphone unplugged and the receiver
blasting as she or he tunes across every
rock and roll station in town.
The coverage area to be expected
from an AM system depends upon the
type of antenna employed. The three
type are open-wire, "lossy" co-axial
cable, and carrier current. The quality
of the broadcast sound is not \'ery dependent upon the antenna type; only the
coverage area is affected by the antenna.
Open-wire antennas are limited by the
FCC to 10 feet in length. Their coverage
ranges from 50 to 150 feet depending
upon the amount of steel in the building. The fixed-frequency, below-thebroadcast-band systems usually employ
an open-wire antenna. These systems
are used by many churches.
2-1

Lossy co-axial cable systems employ
a special type of co-ax cable that has a
very loose or open shield, thus allowing
a little radiation to occur all along the
cable. Usable reception range is 50 to 75
feet from the cable. The cable may be
quite long if necessary. Common uses of
the lossy co-ax technique are drive-in
cinemas, stadiums and arenas, and for
broadcasting flight arrival and departure
information along the highway approaches to airports.
The most common type of AM system is carrier current. In this system the
output of the transmitter is connected
into the main power distribution wiring
of the building. The radio signal travels
throughout the building on the electrical
wiring. This system is widely used on
college campuses for limited-coverage,
student-operated radio stations. Carrier
current would be an inexpensive way to
provide lobby and backstage monitoring.
The costs of the open-wire transmitter and fixed-frequency receivers are
about the same as FM systems. The
lossy co-ax and carrier current systems
may cost from $1,000 to $4,000, depending upon power required and length

of co-ax. Any of these systems can be
installed easily by following the manufacturer's instructions. Both the lossy
co-ax and carrier current systems should
be planned through consultation with
the manufacturer of the transmitter.
Infrared Light can be used to broadcast a very high quality signal. Presentlyavailable systems can broadcast up to
nine different programs on the same
emitter, thus making infrared very useful for large-scale language translation
systems. Unlike the other techniques
discussed, it can be used to transmit in
stereo. Also unlike other systems, infrared broadcasts are completely contained
within the room. Infrared behaves like
visible light-it cannot go through a
wall; even a heavy cloth is an opaque
barrier. This control of the broadcast
range is a signi ficant factor where confidentiality is important-as in corporate meeting rooms. For this reason and
because of its outstanding sound quality, in frared is the system chosen by almost every professional theatre in ew
York and Los Angeles.
There is a unique limitation to the
use of infrared-it cannot be used in

Figure 1: Infrared Emirrer
Coverage Parrerns

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Theatre Design & Technology USITT SUlllmer 1983

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