Intercom Basics

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.