Intercom Page

General information

Intercom is a private telecommunciation system that allows typically two or more locations to communicate with each other like telephone does.

Simple two station wired intercom system can be used as an intercom and doorphone. You can use use an intercom systems from your house to house gate/doot to screen visitors to your house.

Many productions which needs co-operation of more than a few people need special intercoms that cover many users. Intercom systems used in TV and stage productions are usually headset type intercoms connected to one line using party line arrangement. The primary use of this type of system is in live or media productions where (for example) the video director speaks to the camera operators, or where the stage manager speaks to the stage hands and lighting operator, etc.

Intercom systems, by definition, may be comprised of many different types of intercoms and subsystems. The basic building blocks can be categorized, however, into four basic types or elements: Party-line Systems, Matrix Systems, Wireless Systems, and Accessories.

The most basic intercom circuit consist of two intercom stations linked to each other with a push to talk (PTT) switch. This kind of circuits are simple and gneerally consist of only one or two amplifiers and genrally use the speaker as normal speaker and microphone (how it is used depends on talk switch position). Depending on the circuit design there could be one PTT switch on one end, or spearate PTTs on both stations.

When you want go get rid of push to talk switch and want full duplex, things get more complicated to build. In case you have possibility of having four wires between the two nodes and typical headsets on the nodes (separate speaker and microphone, good isolation from sound coming from speaker to the mic element). In this kind of two headset system, you can simply use two amplifiers, one for ound in each direction. Care should be taken to make sure that you don't use too much amplification that can lead to siund feedback.

When you want full dupled through to wires, then the simplest approach to make it is to use build intercom hsing two normal telephones. Other full-duplex teo wire systems need to use the same basic ideas as telephone network used (meaning some form of hybrid circuit).

If you want two wire full-duplex intercom with speakerphone like operation on both ends, things get very complicated in the design. One idea is to use speakerphone telephones for this.

When you want to add more than two intercom stations to one line with full-duplex operation, things also get complicated here. Commercial party line intercoms can do this, but the station circuits that give good performance for this get easy complicated.

Some terms related to intecom systems:

  • Point-to-Point (P-P): Point-to-Point is direct, one-way, simplex communication between two intercom stations or between stations and interfaces. It is the basis of communication in matrix intercom systems and is established by activating a station key. The receiving station does not have to take any action to receive the incoming communication, however, a talk key at the receiving station must be pressed to answer.
  • Party Line (PL): Party-line communication (sometimes called ???conference "line", "net", or "ring") is a group of two or more stations communicating with each other in a two-way, fullduplex mode. Each station must activate the listen key to the desired party line to listen and the talk key to talk. Party lines require two actions to establish even a oneway party-line communication path (e.g., activating both a talk key on the sending station and a listen key on the receiving station). Stations are dynamically added and subtracted from a party line as users activate talk and listen keys. A party line is intended for use as a conference with a significant amount of back and forth communication between users.
  • IFB: IFB stands for Interruptible Foldback and allows a user to hear one audio source, which may then be interrupted by another audio source. A typical application is in broadcasting where talent hears program audio in his ear piece and a producer can interrupt that audio to offer the talent information and instructions.
  • IRF: IRF stands for Interrupted Return Feed. It is same thing as IFB.
  • ISO: The isolation (ISO) function allows a user to speak privately to another user. ISO is generally used to provide private communication between two members of a party line. In broadcasting, ISO is often used by a video operator to speak privately to one cameraman who is one of many cameramen on the camera party line. This was called Camera Isolate as it first was used to remove an individual camera from a conference to allow private communications. Typically this works in the following way: The person who needs to interrupt presses a button or a key, and there is established a private two person conversation with the desired person. Upon releasing the key, the two participants are returned to whatever conversation(s) they were a part of previously.
  • Fixed Group: A Fixed Group is a group of intercom stations and interfaces. A user who has a talk key programmed for a fixed group is able to simultaneously talk to everyone who is part of that fixed group. A fixed group differs from a party line in that the group???s membership is set by the configuration of the intercom system (not changed dynamically). A fixed group is intended to be used for one-to-many type communication.
  • Tally: Tally is a signal sent for the purpose of indicating status for a particular purpose. The sound of your telephone ringing can be described as a tally. In an intercom panel with multiple channels, it can be a visual signal to indicate to which station a calling voice belongs. It can be used to indicate that a particular function is not available due to a conflict - just like the busy signal you get when calling the radio station to try and be the tenth caller and win a year long supply of cat litter.

General intercom inteformation links

Basic intercom circuits

Here are links to some very basic intercom systems that allow two places to communicate with each other. Most of the circuits listed here are simple half duplex intercom circuits.

Full duplex intercom

Full duplex intercom system allows the users of the system to communicate with each other on both directions without any controls (like transmit button pushing etc.).

Implementation of full duplex intercom in generally much more complicated than implementing hanf duplex intercom, especially if you need to communicate with just few wires (a pair of wires) and want spakerphone like operation without handset. For this kind of full-duplex system you will generally need an amplifier at the gate like you have now but you will also need a circuit that isolates the gate circuit's input from its own output, like a speaker phone does.

  • AN 507: AS2507 Two wire Intercom Application Note - This application note describes basic applications for 2-wire intercom solutions with central and/or local supply using the AS2507 circuit. Speech, data transmission and remote DC supply over the same 2-wire bus. simple connection without repeater of up to 60 terminals with local supply, 25 terminals with central supply.    Rate this link

Using normal telephones as intercom

You can use normal telephones as intercom. You can retty easily connect two normal telephones to one wire line, provide them some suitable power and make the conversation with them.

Normal telelphones generally works as intercom when you have two or possibly three telephones. But when you start having more than few phones, you get easily into problems, because the telephones were never designed to use like many phones on one line configuration. Typical problems in many phones in parallel/series configuration is that you have excessive sidetone on all phones and can have problems in powering all of them properly

ClearCom type intercom systems

Intercom systems used in TV and stage productions are usually headset type intercoms connected to one line using party line arrangement. The primary use of this type of system is in live or media productions where (for example) the video director speaks to the camera operators, or where the stage manager speaks to the stage hands and lighting operator, etc. This type of intercom allows communications in two directions between anyone on the line in tha basis that everyone hears everything.

This kind of intercom systems are frequently referred to as 2-wire although this term is misleading since the stations are connected together using 2-conductor shielded microphone cable, and the shield is very much a conductor. This connector is typically used to carry the party line voice in one wire and the station operating power on another wire. The ground is common for both of them. This kind of intercom systems are sometimes called "talkback" systems.

The history for those systems is quite long and related to stage performances. During the 1960???s as rock & roll tours were becoming more sophisticated and technically demanding, the beltpack intercom system evolved so that backstage personnel could talk discretely to each other during rehearsals and performances. This started out as a small box of electronics worn by the user, that was connected to other "beltpacks" by a standard microphone cable. Since then, this concept has been developed by several manufacturers into a range of products, which are now in widespread use.

The advantages of party-line systems include relatively easy setup, simple operation, use of common cabling (standard microphone cables), portability, and clear and reliable communications. Party-line intercoms are standard equipment at performing facilities, allowing communications among the lighting and stage-production staff. Touring sound and lighting companies will carry these systems with them for concerts and events. Locations such as community theaters, school and college auditoriums, sporting and racing venues also use party-line intercom systems. Smaller TV broadcast facilities and mobile production vehicles, especially those that produce new programs, will have 2-channel or 4-channel party-line systems to communicate among camera operators, audio and video technicians, outside broadcast staff, facility technicians and the talent.

There are several interconnect standards for headset intercom systems, none of which are directly compatible with the others. The most common standards are those originally devised by Clear-Com??, Telex?? and RTS?? . The Clear-Com standard is also used by Technical Projects Production Intercom, HM Electronics, and a number of English and European manufacturers.

Clear-Com???s 2-wire Party-Line intercom uses a single audio pair to connect intercom stations and Beltpacks to an intercom master station via daisy-chain wiring. Audio and DC power travel on this single pair and the master station (or alternatively a power supply) provides power to the other stations. The daisy-chained devices form a single channel that is a conference or party line. used for Ethernet are used for wiring Clear- Com matrix intercoms. This leverages the Party-Line systems carry both talk and listen audio as well as DC power on a single pair.

This Clear-Com system uses 3 pin XLR connections for station interconnection. The pinout is the following:

  • Pin 1: The pin 1 is common ground.
  • Pin 2: The Pin 2 carries the station power (24-30V DC up to 100 mA per station, typically 50 mA).
  • Pin 3: The audio is carried using pin 3.

The audio connection unbalanced party line which is terminated to 200 ohms load. The nominal signal level for this line is -13dBv (0 dBv max). This is pretty similar to consumer "line level" (usually in 0.5-0.7V range). In some applications DC level in party line is used for signaling (0-4 DVC = receive mode, 10-15 VDC = send mode / call signal).

A well-filtered source of 24-30V DC power is required to run this system. This is often supplied by the fixed base station in building installations, or by a portable unit at the "head end" in transient systems. Power could conceivably be supplied by something as simple as a "wall wart" plug-in power supply, as long as it was sufficiently rated and filtered to meet the requirements of the system.

At ONE point in the system, the audio/call buss must be terminated to maintain proper levels throughout the system. Since the system must also be powered at one point, these functions are traditionally done together. Termination can be as simple as a single 1/4watt resistor between the audio/call bus and ground (typical termination resistance is 200 ohms).

The beltpacks on Clear-Com system use typically a headset that is connected with a 4-pin XLR connector. The dynamic headset connector pin-out is:

  • Pin 1: mic common
  • Pin 2: mic hot
  • Pin 3: headphone common
  • Pin 4: headphone hot
Do NOT connect headphone ground and mic ground together at any point. To insure proper level and performance, the dynamic headset (or handset) should have the following characteristics:
  • Mic Impedance: 150-250 ohms
  • Mic Output Level: -55 dB
  • Ear Element Impedance: 50-2000 ohms

Some system also accept a standard carbon mic headset, and provides a 1/4" phone jack for the input. The carbon headset should contain a 50 ohm carbon-type microphone, a 600-1000 ohm earpiece, and the cord should be terminated in a 1/4", 3-conductor (ring/tip/sleeve) phone plug. The carbon headset connections are:

  • Tip: mic hot
  • Ring: headphone hot
  • Sleeve: ground

There are commercial sources of this type of intercom, the primary one being ClearCom. You can find links to other manufacters at ComClone2 project page.

NOTE: To prevent shock hazard, ground loops, and noises, NEVER connect line connector Pin 1 (common) directly to the station chassis. In addition, never connect Pin 1 to the shell of the mic cable.

Please note that ClearCom is not the only used party intercom system. At least three different party-line standards exist, with variation in levels, pin-outs and call-signal methods.

RTS TW Intercom

RTS intercoms can be found in the vast majority of television stations and production trucks in the United States, in control rooms at NASA, connecting countless broadcasters at Olympic Games, at the Chicago Board of Trade, in nuclear power plants and thousands of other applications around the world.

The TW Intercom System in its simplest form is a two-wire conference- line intercommunications system that allows up to 50 user stations to connect across a common line (also called a channel). The system operates in a full-duplex mode; simultaneous talk and listen, to and from each user station. Individual user stations may be separated from the power supply as much as 1.6 kilometers (1 mile).

The RTS Systems TW Intercom System, in its simplest form is a two-wire system employing just 2 wires for one channel of communications. This means that the number of channels may be increased with the addition of extra wires, i.e., 3 wires = 2 channels, 4 wires = 3 channels, etc. The system utilizes XLR-3 type connectors for interconnection between 2-channel user stations and the power supply. XLR-4 type connectors are used for connection to 3-channel user stations. Since, in the unbalanced version of the TW Intercom System, all channels share a common circuit ground return, crosstalk due to common ground resistance can occur. This crosstalk is proportional to the ratio of the common ground resistance to the system terminating impedance, 200 ohms. The signal level on the line is -5 dBv nominal (+3dBv max).

The units receive electrical power from either (1) a system power supply (26 - 32 volts DC) or (2) a local power supply option (12 - 18 volts DC). Current requirements for each type of user station are listed on the equipment manuals. In case system power supply is ued, the power and communications signals share conductors, it may be necessary to overcome power losses by increasing conductor size over long runs (over 1 kilometer) Normal conductor size is #22 gage. The maximum allowable loop resistance is determined by the power supply voltage, the loop current and the user station minimum operating voltage. Example: a headset station without call light and a dynamic headset (with 25 ohm headphones) plugged in, uses up to 50 milliamperes, the power supply voltage is 26 VDC and the user station minimum operating voltage is 18 VDC. A 500 mA power supply can supply typically 6-12 stations.

Phase 1 intercom user station require power on both pins 2 and 3 with respect to pin 1. Newer stations need power only on pin 2. Many new TW power supplies provide power only to pin 2 nowadays (can cause problems with some old Phase 1 devices).

The call signaling on TW system is built using tone system. 20 kHz tone on audio bus indicates that call has bee activated (send frequency 20kHz +-100 Hz level -6dBV, receive frequency range typically 20 kHz +-500 Hz level -30dBv).

The pin-out for TW system on 3-pin XLR connector is the following:

  • Pin 1: Common
  • Pin 2: +VDC and Channel A Audio
  • Pin 3: Channel B Audio

The best type of cable to use for interconnection depends upon the application and electrical environment. In areas of strong electrical fields (radio frequency, hum, digital circuits), shielded cable should be used; if areas of even stronger fields, it may be necessary to employ balanced user stations and shielded, balanced cables. Most 2-channel applications may use either standard microphone cable (for convenience) or 2-twisted-pair cable (considerably less expensive than microphone cable).

The TW Intercom System circuit ground should not be directly connected to "earth" or "chassis" ground (where directly means a connection an ohmeter would show...therefore an ohmic connection). Each user station is bypassed to its own chassis via a 0.1 microfarad capacitor to establish a radio frequency (RF) ground.

Not grounding has two benefits. A single accidental ground can be tolerated by the system until the fault can be cleared and, with luck, before a second ground fault can cause noise or overload or bring the system down. Another benefit of not "earth" grounding the circuit return is to prevent noise introduction through "earth" currents from other equipment. If the RTS circuit ground conducts these currents, it is likely that they will be heard as interfering noise on the communication line.

TW intercom systems need a termianted line. A proper termination impedance is 200 ohms. There are good technical reasons why this termination is needed. Modern TW intercom systems use a simple but effective little ciruit called a "Bilaterial Current Source". This audio circuit is what allows dozens of intercom user stations to be connected to a single partyline without each beltpack significantly loading the audio which would reduce the level to less than usable. This circuit is really a simple oscillator design which makes it very unstable and prone to a lot of funny business. The stability (they are very stable in practicle use) comes from the addition of a resistance on the output of the circuit. This resistane is provided by a termination resistor typically located in the power supply for the the intercom system. Each channel of the system would have it's own terminator.

Matrix intercoms

Smaller TV broadcast facilities and mobile production vehicles, especially those that produce new programs, will have 2-channel or 4-channel party-line systems to communicate among camera operators, audio and video technicians, outside broadcast staff, facility technicians and the talent. As the communications needs of these facilities become greater and more complex, a point-to-point matrix intercom system is considered. Digital matrix intercom systems are the choice of most television broadcast studios, especially those with active news-reporting operations. The larger mobile production vehicles invariably carry matrix systems.

A matrix intercom offers the user great power and flexibility. This is largely because a matrix intercom consists of crosspoints that allow any intercom input to be routed to any intercom output.

Matrix intercom systems consist of central communications frames through which all communications and data are routed: multikey intercom stations that allow many channels of communication and interfaces to other systems such as telephones, two-way radios, a party-line intercom, audio and IFB/cue systems, microwave, and ISDN links and relays to control other devices. These systems are fully programmable so that any type of point-to-point, group or party-line communication can be created, and any or all of these interfaces can be accessed by each user station.

The central communications frame contains a number of circuit-board cards that process and route audio signals and control data coming from the individual stations and interfaces. Each station and interface connects to the frame via a port, such as an RJ-45 connector, with paths for audio signals and control data. A typical configuration might be nine 8-port circuit-board cards for a total of 72. Typically the stations connect to the central communications frame via a single 4-pair cable for analog transmission or single-pair or coaxial cable for digital transmission.

The term crosspoint refers to a one-way audio path from one port???s input to another port???s output. Crosspoints exist between every pair of ports in the system and are connected and disconnected as needed to provide communication paths between system ports.

A matrix intercom???s power lies in the many functions that can be implemented through computerized control of matrix crosspoints.

In addition to the point-to-point communication described above, matrix intercoms offer Party-Line type conferences, IFB, Fixed Groups, and ISO for private communication, control of external devices including two-way radios through the use of relays and the ability to accept control inputs through the use of GPIs (General Purpose Inputs).

Usually there are interfaces to 2-wire, party-line intercoms; 4-wire circuits; and dial-up telephone lines.

Matrix intercoms offer superior audio quality in comparison to many party-line intercom systems. Matrix systems have separate talk and listen circuits while Party- Line systems carry both talk and listen audio as well as DC power on a single pair. Party- Line systems do not have the power and flexibility that are available with matrix systems.

According to the application, a Party-Line intercom or a Matrix intercom may be most appropriate to meet a users requirements.

Wireless intercom systems

A limitation of these beltpack systems is that the user???s movement is restricted by the cable linking them to the system. The way to get rid of the wires is to use wireless technologies.

Wireless intercom systems consist of full-duplex beltpacks (able to converse two ways) and a base station (the common link among them and a connection nexus to wired intercom systems or other audio systems). In most applications, the wireless intercom is an extension of the wired system, used by those staff members who require mobility for safety or convenience.

To allow users can talk together at the same time, each beltpack must operate on its own unique frequency. Each wireless beltpack requires a transmitter and a receiver working together closely, plus audio circuitry, a mic preamp and a headphone amp. This fact makes them inherently more complex than wired beltpacks, and more difficult to manufacture.

The great advantage of wireless intercom versus other two-way radio technologies is that it is full duplex, so that users can maintain a true conversation, hands-free instead of push-to-talk. Also, they are designed to work in conjunction with wired intercom systems with the proper interfaces and connections to do so.

Standard intercom headsets are typically used with wireless beltpacks. These systems generally operate on VHF or UHF frequencies in the television bands, just like wireless microphones.

Before adding wireless intercom systems to an installation, the installer must determine what frequencies are being used in the vicinity and what is allowed to be used. The next thing to consider is which other wireless devices are used in the installation: microphones, two-way radios, walkie-talkies and so on.

A variant on wireless intercoms is the IFB, or cue system. This type of system is used for listen-only applications such as monitoring an ongoing program, cueing talent, and where the return path for the person's voice is a microphone (e.g., an on-camera television announcer receiving cues from a director).

Mains wire intercoms

Plug in Intercom devices are used to monitor babies and transmit speech over the power-line. The bandwidth occupied by those devices is typically something like 30KHz and the modulating frequency is between 150KHz to 500KHz. To overcome the attenuation of the signal on the mains the output level from these units could be as high as 7Vpp (harmonics can be high).

Intercom systems interfacing

Intercom systems are rarely used in isolation; they are the communications and coordination systems behind a production that often includes other audio and video systems. In larger events, the production communications must include people who are elsewhere within a facility or in a distant location. So devices must be used to match the audio levels, impedance and operating characteristics of these different communications means: telephones, ISDN, two-way radio and walkie-talkie, paging systems, audio consoles, camera intercoms, etc.

Also, not all intercom systems and brands are compatible, yet in some settings they must communicate together. At least three different party-line standards exist, with variation in levels, pin-outs and call-signal methods. All of the digital intercom systems have their own proprietary communications protocol. And often party-line and digital matrix systems are used together, requiring further interfacing.

Aviation intercom systems

Motorbike intercom systems

Misc intercom related circuits and informatrion

  • Headset Wiring Standards    Rate this link
  • Intercom preamp - A very convenient way of making an intercom is to use a loudspeaker as a microphone. This is a preamp with a low input impedance suitable for this purpose.    Rate this link
  • Camp Intercom - Did you ever try to make a telephone call on a campsite by using the tent pegs of your tent? Well, here's how you can do it. This simple electronic circuit is a telephone set that uses only one wire to connect two or more posts. The other wire, the ground return, is not needed. In fact, the ground itself is literally used as the second wire.    Rate this link
  • Hard-Wire Underwater Communications - Hard-wire communications typically means that the divers will be using a system similar to an intercom. The divers will be wearing a helmet or full face mask (ffm) of some kind equipped with an earphones and a microphone.    Rate this link

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