Handbook for Sound Engineers

Intercoms 1575

often used for extended periods. Long life is obtainable
because the transmitters are only on when talking.
FM transmission has a characteristic that is called
capture and is normally rated as capture ratio. It occurs
when two or more transmitters are on the same
frequency. The stronger of the two signals at the
receiver captures the receiver, blanking out the other
transmitters. If the transmitters are moving with respect
to the receiver, the stronger transmitter would capture
the receiver, so the communications could be bouncing
between the various transmitters. For this reason all
transmitters must be off when not in use and the new
talker must monitor the channel before transmitting. If
multiple transmitters and receivers are required,
multiple frequencies are required.

43.2.6 Belt Packs

Belt packs are used in remote areas and when the person
has to move around and/or the communications must
not be heard by others such as during a theatrical perfor-
mance, Fig. 43-15, or for football communications
between spotters and coaches, Fig. 43-16. They may be
wired or wireless and incorporate either single or dual
headsets. Belt packs often utilize noiseless, digital elec-
tronic switching on audio circuits. A push-pull amplifier
supplies high levels of audio in the headset and a micro-
phone limiter compensates for user voice variance. The
belt pack may also include a two position gain switch
for normal and high-noise environments. The remote
mic-kill function at the base station enables belt pack
microphones to be shut off from another location to
conveniently eliminate microphone pickup. This is done
by sending a 20–24 kHz ultrasonic signal down the
audio line, turning off the talk gate on each unit on the
line. A visual call signal is provided by high-intensity
LEDs on the belt pack to alert operators who have
removed their headsets.

43.2.7 Telephone Interface

Broadcast intercoms are often connected to a telephone

line. This is accomplished through a micropro-

cessor-based telephone interface that provides commu-

nications between a wet dial-up phone line and an

intercom system. The interface is ideal for the broadcast

industry, and is specifically designed to connect a tele-

phone line to ENG and EFP trucks, production studio

consoles, and TV facilities. The device automatically

answers incoming calls to the intercom system, and

automatic forward-nulling circuitry adjusts internal

hybrids on both sides of the line to achieve a null of up

to 40 dB in less than s. The devices also include

automatic gain control to insure that the incoming tele-

phone audio remains at a constant level. It is also

capable of automatic dial-up IFB, enabling a field crew

to directly access a preset IFB circuit to immediately

communicate with the studio crew.

When the device is set up for automatic answering,

an incoming call is automatically answered, and the

ringing is indicated on the master station and can illumi-

nate a light at all intercom stations. The interface can

automatically hang up or release the line if it detects a

dial tone, resulting from either an intentional hang-up or

a disconnection due to a line problem. If an audio

program becomes too loud or distracting, it can be

momentarily or permanently interrupted by any local

intercom user.

Because telephone-to-intercom interfaces are used

with standard telephone lines, frequency response is

limited to 250 Hz–3.4 kHz ±3 dB. Automatic volume

control (AVC) is about 20 dB and the depth of the null

is greater than 30 dB, 200 Hz–8 kHz.

Many of the digital matrix intercoms offer extensive

direct dial-in access to the system from any touch-tone

telephone in the world. They may include up to 50 two

digit DTMF codes that can be used to select any station,

group, program source, or IFB circuit in the system.

43.2.8 Headsets

Most headsets are designed to work with all major types

of communication systems including party line and

matrix intercoms. They are used in intercom and sports-

caster/announcer applications where audio quality, reli-

ability, comfort, and the ability to hear and talk in noisy

environments are of prime importance.

Light weight and comfort as well as durability are of

major importance. Many headsets are made of flexible

composite materials, which will not be damaged if

dropped, thrown, or stepped on.

Figure 43-15. Football communication system. Courtesy
Telex Communications, Inc.

Defensive

spotter

Offensive

spotter

Defensive

coach

Head

coach

Offensive

coach

Line 1

Line 1 Line 1

Line 2 Line 2

Press box

Line 2

Outlet plates Field

Beltpacks

ac

Control box--offense

or defense can be

isolated from each

other or combined by

the flip of a switch

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