1570 Chapter 43
Many intercoms are engineered to receive phantom
power from the 24 Vdc system power supply. Stationary
or permanent user stations usually operate in a dry
mode as they are supplied with power from a local
power line. The term dry refers to an intercom channel
that has audio but not the usual 24 Vdc phantom power
on the channel. Dry operation has several advantages
over wet operation as it is generally quieter, reduces the
need and cost of large system power supplies, and takes
up less master rack space. System configurations can
include a mix of wet and dry channels, depending on the
station equipment assigned to the particular channel.
Generally, most wired belt-pack and loudspeaker
stations require a wet channel and thus need a system
power supply.
43.2.1 Broadcast Point-to-Point Systems
The point-to-point system consists of a centralized rack
(or racks) of amplifiers and signal routing circuitry
controlled from remote stations. The audio signal paths
are analog and simplex or digital.
The system allows a station to route its voice or
other signal to one or more other stations. The origi-
nating talker determines who hears the communication
and the listener normally has no control over who is
received at the individual stations. Normally,
point-to-point systems require direct or home run
cabling from each station to the central control.
Each station in a point-to-point analog system
requires a minimum of one audio transmit pair, one
receive pair, many control or station selection conduc-
tors, and a power pair or local power supply. It is essen-
tially a switch-selected, multiple station, one-way
paging system. A point-to-point digital system has the
same functions plus many more and operates on a single
pair of unshielded wires.
43.2.2 Broadcast Party Line System
In the party line system, each station is equipped with
all of the required electronics for receiving and trans-
mitting audio and for signal routing. Party line systems
require minimal centralized rack equipment, which
usually consists of the system power supplies and
passive assignment switching in multichannel systems.
Party line systems allow groups of stations to
communicate in real-time, full duplex fashion. In fact as
it is a party line, all units on the line hear and are part of
all conversations. Multichannel party line systems allow
users access to several different channels, allowing
them to determine which line they talk and listen to.
Normally there are no private communications such as
point-to-point systems provide.
Two-channel, dual-listen, with monaural output
intercoms with programmable switching let the user
listen to both channels simultaneously and select which
channel to talk on. They include an individual volume
control for each channel, microphone on/off control, a
call signal button and indicators, and sidetone. These
stations are ideal for ENG and EFP mobile production
vans, production studio consoles, and TV facilities.
Straight two-channel units allows simultaneous
listening and talking on two intercom channels. The
headphone output operates in a split-feed stereo mode,
feeding each channel into a separate ear of a
double-muff headset with an individual volume control
for each channel. The operator can talk or listen on
either channel, combine them, or access them separately
without tying both channels together. Often a stage
announce output can be supplied with relay control for
external paging. Microphone or line-level program
inputs may be assigned to either or both channels. The
systems capabilities can include a selectable program
interrupt function, remote mic-kill function,
dual-action, electronic momentary/latching talk buttons,
and a no-fail power supply with automatic reset and
short-circuit protection.
A typical two-channel party-line system by
Clear-Com consists of a main station, which includes a
power supply. Each channel is full duplex allowing
everyone on the channel to choose to both listen and
talk to everyone else. Although the min station provides
two channels, single- and two-channel beltpacks can be
mixed within the same system. This reduces cost and
allows one to control exactly who is assigned to what
channel. All cabling is standard low capacitance
shielded microphone audio wiring, Fig. 43-10. A party
line belt pack is shown in Fig. 43-11.
Some multichannel systems have extensive program-
ming capabilitie, which allows individual stations to be
customized by storing the button setups in nonvolatile
memory. Many programmed setups can be stored, thus
allowing quick and easy switching between setups for
rehearsal and performance or shows and events. Indi-
vidual button assignments can be stored in presets for
instantaneous recall. When programming this equip-
ment, messages prompt the operator through the
programming sequence, simplifying station setup.
A four-channel, two-wire party line system is similar
to the two-channel party line except with two addi-
tional channels. In Fig. 43-12, Channel A is for the floor
crew, Channel B is lighting, Channel C serves audio
with Channel D calling the talent dressing rooms and