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Party Line Intercom
The following article is intended as very general overview for those
who are not familiar with party line intercom systems. There are
many informative articles on websites and in catalogs of other
manufacturers as well.
Party line intercom is typically used "behind the scenes" by the people
who stage a live event such as a theatrical performance, concert,
religious service or sports event. They communicate with each other to
coordinate their actions and those of others with whom they
communicate.
Different size and complexities of systems are created from the
building block components to match the demand of the typical event.
Systems can be permanently or semi-permanently installed in a theatre,
house of worship or sports stadium. Systems are also used in a portable
format. A good example is a concert tour, where the lighting and sound
systems travel from venue to venue. The intercom, which is vital to
running the show, travels with the system and is set up and taken down
at each location. (Intercom for these events requires a fairly basic
system, so the set-up and tear down are one of the simpler actions
compared to the other larger systems used in the production.)
Because of the high quality of the signal and capabilities to use
anything from speaker stations to closed-ear headsets, these types of
systems have found their way into many other applications, including
aerospace, industrial, medical and government. These types of
systems offer much superior speech intelligibility compared to two-way
radios and telephone systems.
Party line intercom is usually "full duplex", which means that anyone
and everyone on a channel can talk and listen at the same time. Of
course this should not actually happen. More likely is one person
at a time speaking to other people on that channel who need to hear
what that person is saying. Party line is like two people
in a house talking on two telephones on the same line to a third person
somewhere else. Either of those first two people can join or
leave the conversation at any time. In addition, as the users
come and go from a properly designed intercom system, the audio quality
or listen levels are not affected. Compare this to two-way radios
where a user can only talk (transmit) or listen (receive) on their
device at any moment and only one user can be transmitting on a
frequency at any moment. (This type of system is referred to as simplex
or half-duplex) A system will often have the capability to
support many more users than are required for a typical show. The
beauty is that by wiring more stations into the channel, the additional
users are easily added. The term party line as used here comes
from the early days of telephone when more than one household would
share a telephone line.
System Basics:
A system consists of at least two users who communicate with each
other. All systems require a power supply or main station which has a
built-in power supply. This component is plugged into a standard
electrical outlet. The power supply provides a low voltage DC
electrical current (usually either 24 or 30 volts) which travels
through one of the wires in a standard mic cable to a beltpack or
remote station. The other wire in the "twisted pair" carries the audio
signal and, when needed, a Call signal. Call signals are used to either
get someone's attention if they are away from their station, have
removed their headset, set their listen level too low or off or to send
a non-verbal cue. ASL products offer both a visual call signal and an
audible call buzzer. Of course for quiet shows, the audible buzzer can
be turned off. The twisted pair wire is wrapped in a metallic shield
which reduces noise from stray electrical, magnetic and radio frequency
energy. This shield also acts as the ground return for the signal and
DC power.
A "channel" is one particular circuit of communication. Most systems
for smaller theatres, churches and tours use either one, two or four
channels. Let's take the example of a small theatre:
The stage manager has a two-channel main station. Channel A is
connected to the lighting director and the people operating the follow
spotlights. The lighting director can direct the actions of the follow
spot operators through the intercom. Channel B is connected to
the audio booth and the properties/props room. The people on channel A
can talk to each other and the stage manager, but not to the people on
channel B and vice versa. Only the stage manager can talk and/or listen
to both channels at once, because he or she has the two-channel station.
As a system demands more complexity, there are two channel beltpacks
and remote stations that go up to six channels and even more.
If someone needs to move within a certain fixed area, requires
communication on one or two channels, and can wear a headset, a
beltpack is ideal. They are economical, small, light and simple to
operate. Facilities are often wired with permanent cabling in the wall.
Access to this wiring is via wall plates. A person with a beltpack
decides where they need to be and connects their beltpack into the
system via that wall plate, using a standard 3-pin XLR mic cable of
whatever length they need. Some wall plates allow a selection of two or
more channels via a switch or simply have separate jacks for each
channel. Both have advantages and disadvantages. Whichever
method is used though, the wire that carries the audio for each channel
must be wired. In less permanent uses, a cable is simply run from a
jack on the back of the main station or power supply to the
beltpack. Users also mount beltpacks to a wall or rest it on the
meter bridge or by the side of a mixing console.
The alternative to a beltpack is remote speaker station. As the name
implies, they contain a loudspeaker, so the channel can be monitored
without wearing a headset. For talking back into a channel, there are
models with either gooseneck or flush mounted panel mics. All remote
speaker stations also accommodate a headset for those times when it is
not practical to have the speaker on. Remote stations offer up to six
channels.
It is important to note here that each channel in common between
multi-channel stations requires its own wire. While the ground and
power can be shared, a separate, shielded conductor is needed for each
channel. For example, system with four channel stations connected to
each other would require a cable offering at minimum: four shielded
conductors and at least one additional conductor for DC power. A four
pair "snake" cable would be the usual choice with a couple of
alternatives for dealing with the wiring of the DC conductors.
Typical wired party line systems can use up to 1000 feet of 24 AWG
gauge cable before the laws of physics (capacitance, inductance and
resistance) cause noticeable effects on performance. Though still
usable at greater distances, problems such as high frequency roll-off
and voltage drop can make a system sound "muddy" and perform
inconsistently. For fixed installs with long cable runs or more
than 6 stations "daisy chained" together, use of heavier cable is very
important.
One of the best things about party line systems is their wiring
flexibility. In a single channel system, almost any combination
of series and parallel wiring will work. A jack at the back of main
station can be connected to a three-way splitter with each of those
legs connecting a string of "daisy chained" beltpacks. The term
refers to the practice of using the parallel connectors on a beltpack
(or other station) to connect multiple beltpacks to each other.
To add to their utility, intercom systems can also connect to other
common communications devices such as two way radios, telephones,
professional video cameras and paging systems via interface devices.
Most major brands of intercom offer some version of these devices.
Party line intercom works on a standard mic cable with one of the wires
in the shielded pair carrying audio in both directions
simultaneously. The three pins of the XLR are connected as
follows:
Pin 1 Ground
Pin 2 24-30 Volts DC
Pin 3 Audio and optional call signal voltage
For unbalanced systems, the pin 1 ground should NOT be connected to the shell of the XLR plug.
The unique piece of electronics that makes bidirectional communication on a
single wire and ground possible is the sidetone null circuit. All two-way
communication starts and ends as what is called four-wire audio: a pair of
conductors carries an input/receive signal and a second pair carries the
output/send signal. This circuit connects the four-wire audio to the single
wire in such a way as to variably restrict the user's reception of his own
voice on the intercom line, which is often referred to "sidetone". Since
there is often a very high gain between the send and receive circuits of an
intercom, there is a risk of oscillation resulting from acoustic and/or
electronic coupling within a headset or between a speaker and a microphone.
A sidetone nulling control fine tunes the circuitry to best match the
devices to the acoustic conditions near the intercom, as well as to the
electronic conditions on the intercom line. This control sets the level of
the user's voice, relative to other signals, in his own headset. Once set,
the user's own voice level, or sidetone, is then controlled by the listen
level control, along with the incoming audio from others. All wired
stations have a sidetone null control for each channel. They should be set
at the time of system installation and adjusted as is comfortable for the
user. Different headsets with different microphone sensitivities, different
types of voices, and changing personal preferences, as well as major system
configuration changes are all good reasons to readjust the sidetone null
control.
Wireless:
Over the last decade, wireless intercom has become increasingly
popular. It removes the limitation of the cable, so the users can go
anywhere within the system's range. While this has some limitations due
to physical and radio frequency environments, people really like being
untethered. Full frequency response wireless typically costs from three
to eight times as much per user, depending on the system size and
quality. Many systems use a combination of wired and wireless,
using the latter only for those who absolutely require the additional
mobility. Wireless base stations have connectors to hook into one
or sometimes two different channels of the wired system allowing
communication between the users on a common channel, whether wired or
wireless. There are also systems available with frequency
response and system fidelity more comparable to a telephone or
walkie-talkie system. As one would expect, these systems cost
less and often have less options.
Summary:
Wired Party Line intercom is a cost effective solution for many of the
communications challenges encountered in live events. Even
complex systems, once properly installed, can be fairly easy to
operate. Most commercial systems are well built and can withstand
daily, professional use, and even some abuse.
About Us:
ASL-USA is operated by Positive Product Specialties. JX Loeb, founder,
worked for one of the major suppliers of intercom systems for nine
years. He developed products, designed systems and provided
applications support for users all over the planet. Based in San
Leandro, California, ASL-USA offers party line intercom and related
products from ASL Intercom in the Netherlands (factory and
headquarters) and other select manufacturers. ASL Intercom B.V. and
ASL-USA are not associated with any other intercom manufacturer.
ASL-USA is not a subsidiary of ASL Intercom B.V. Any reference to
other brand names is for contextual purposes only. All trademarks
are the respective property of their holders.
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