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Michael Drolet -- 2008


One of the tasks usually relegated to the Sound Dept. (by default) is the setting up of the Headset or Intercom system for the production. The intercom (INTERCOM-munication) system allows members of the production crew to communicate privately during rehearsals and performances, without disturbing the cast or audience.

At John Abbott, the two performance spaces are equipped with Production Intercom  partyline intercom systems. In the Theatre, the Main Stations and much of the cabling are permanently installed.

The intercom is a distributed amplifier system; each Main (Fig. 1) or Beltpack Station (Fig. 2) houses its own microphone preamplifier, headset or speaker power amplifier, and signaling circuitry. Stations bridge the intercom line at a very high impedance (more than 10 kOhms), and place a minimum load on the line. The audio level always remains constant, and does not fluctuate as stations leave and join the system.

Low-impedance mic input lines (200 Ohms) and specially designed circuitry make intercom channels relatively immune to RFI (Radio Frequency Interference) and dimmer noise.
A minimal system consists of one Main Station , two beltpacks and associated cabling.

The Main Station is capable of supplying 2 amps of direct current at 24-30 volts to power Belt Packs and Speaker Stations (Fig. 3). Belt Packs each draw about 30mA (.030 A) and Speaker Boxes about 150mA (0.15 A.). The Main Station can thus power a maximum of 60 Belt Packs. Subtract 5 Belt Packs for each Speaker Box.

{ MS-200 Main

Figure 1 -- MS-200 Main Staion


Figure 2 -- BP-1 Single Channel Beltpack

{LS-3 Speaker Box}

Figure 3 -- LS-3 Single Channel Speaker Station


Intercom stations are interconnected with two-conductor, shielded microphone cable (or individually shielded multi-pair cable as required). Portable stations are connected with 2 conductor cables with 3-pin XLR connectors. One wire, connected to pin 2, carries the DC power (24-30 volts) from a Main Station or Power Supply to all Remote Stations. The other wire, connected to pin 3, carries the 2-way (duplex) audio information. The shield, connected to pin 1, acts as a common ground. One termination per channel is needed throughout the intercom network, and is usually located in the Main Station or Power Supply.
Intercom Line Connection:
The rear panel of the Main Station has three 3-pin male XLR connectors for each intercom line A and B.These connectors are wired in parallel. Any single-channel station or channel of a multi-channel station connected on a line plugged into Channel A of the a Main Station will be "party-lined" with all the other stations on that channel. In a multi-channel system, the goal is to assign specific people to the correct group, i.e. the other people they need to be in contact with the most. This is particularly important when the party line users are on a single-channel beltpack or station; less so if they are on multi-channel stations.

The pinout of the intercom connectors is as follows:

Pin 1 Ground (Shield)
Pin 2 Power (+30 VDC)
Pin 3 Audio
Headset Connector:

The headset connector is located on the front panel of all stations. Production Intercom  headsets are recommended, but others can be used if they meet the following requirements:

Mic Type --- Dynamic; 150 to 250 ohms impedance; -55 dB output level

Headphone --- Dynamic; 50 to 2000 ohms impedance 

The wiring of the headset is as follows:

Pin 1 Mic common
Pin 2 Mic hot
Pin 3 Headphone common
Pin 4 Headphone hot

The mic and headphone wiring in the headset cord must be individually shielded. Do not connect Pins 1 and 3 together. Headset extension cords or headset "Y" cables are not recommended because they will increase crosstalk between channels.
Paging Output:
The Main Station has a 3-pin Combo XLR/ female connector on the rear panel to feed into a studio PA (Public Address) system. Placing the front panel "MIC" switch in the "Page" position sends the signal from the headset microphone to the "Paging" connector. Program audio feed to the Announce Output is selected by setting a jumper on the Main board to the ON position.

The pinout of the "Paging Output"  connector is as follows:

Pin 1 Sleeve
Ground (Shield)
Pin 2 Tip
Hot (+) Signal
Pin 3 Ring
Cold (-) Signal

The audio output is balanced and transformer isolated. It has a 600 ohm impedance and a nominal output level of 0 dBv. A shielded twisted pair cable should be used in the cable wired to this connector.

Because the intercom system is a party-line system, where everyone is connected together, certain etiquette must be observed to assure reliable communication.
If you have to go "off headset":
    • Announce clearly that you are going "off headset",  so the Stage Manager is aware.
    • Turn off your mic., so it won't pickup ambient noise and interfere with communication.
    • Turn your headset volume to minimum, so the headset feed will not distract the actors or audience.
As much as possible keep your beltpack and headset away from sources of (hum-causing) electromagnetic radiation such as:
    • computer monitors
    • electric motors
    • lighting patch panels
    • dimmer packs
Although designed to be rugged, system components can be damaged if tripped over or stepped on in the dark.  Do not leave the headset/belt pack unattended on the floor.  Leave them on a chair or table when not in use.
The intercom system is not a chat-line.  Keep conversations limited to production-related talk.
Background noise picked up by open mics. is cumulative.  Your mic. switch should be turned "OFF" except when you have something production-related to say.


Figure 5 -- Typical Theatre Installation Using Program Input

In the preceding diagram of a  typical Theatre installation, the Stage Manager operates a two-channel Main Station, which also powers the system. Several single-channel beltpacks are connected to Channel A. The beltpacks are used by curtain, spot, patch panel, and light board operators. Communication between these people and the Stage Manager is on Channel A.
An audio mixing console output from the stage pickup microphone(s) is connected to the Auxiliary (Line or Mic) Input of the Main Station. If desired, this audio can be placed at a low (or any) level on Channel A. The level can be adjusted using the  Circ A (or Circ B) Auxiliary (Line or Mic) Level control.  If it is not desirable to have audio from the stage microphones on one of the  intercom channels, the Circ A or Circ B switch should be set to the OFF position.
The Auxiliary (Line or Mic) Level control can be used to adjust the stage microphone audio level heard.  Channel B is connected to Speaker Stations such as the Production Intercom LS-3 which are installed in the green/dressing rooms or Production office. 

The stage microphone audio can then be heard by the actors and actresses in the green/dressing rooms. The stage microphone audio will be heard by the waiting actors and actresses.
Because of the two-channel capability, communication between the Stage Manager and stage equipment operators cannot be heard by the actors and actresses, and vice-versa. This eliminates confusion and miscommunication. If desired, the Stage Manager can turn on the Party Line Link (A+B) switch on the Main Station during rehearsals to combine the communication between everyone on both channels.

{CS-222 Main Staion Block}

Figure 6 -- Clearcom CS-222 Main Station Block Diagram
(similar to Production Intercom MS-200)

Figure 7 -- Cleacom RM-501 Beltpack Block Diagram
(similar to Production Intercom BP-1)

The most common cause of poor system performance or failure is the accidental grounding of the shield in the 2-conductor shielded microphone cable used to interconnect intercom stations. In this application the shield performs 4 functions: as the shield; as the zero volt reference for the +24VDC on pin 2; as the zero volt reference for the +12VDC which appears on pin 3 when signal lamps are activated, and as the return for the audio signal on pin 3. Check carefully that the shield is not connected to the ground lug in XLR connectors, or touching a grounded metal surface inside any junction boxes which are part of the system.
Problem: System does not operate. No power to Main Station or Power Supply. Green 24 VDC  LED not illuminated.
Cause 1: No AC power to the Main Station or Power Supply. Solution 1: Make sure the power switch on the rear panel is turned ON. Check AC connection and cable. Plug into dependable AC source.
Cause 2: Main Station or Power Supply has an internal Power Supply failure. Solution 2: Unit requires servicing.
Cause 3: Short or overload on that channel due to a shorted or miswired cable. Solution 3: Remove cables one at a time from system until the faulty line is located. (The red Short LED will then turn off.) Check for shorts between pins 1 and 2 or improper cable wiring. Once the short is removed, the Main Station or Power Supply will re-set automatically and the power will come back up within several seconds.
Cause 4: Defective Remote Station Solution 4: Check Remote Station and replace if necessary.
Problem: Hum or buzz in system.

Cause 1: Inductive pickup caused by close proximity of this Main Station or connected Remote Stations to power lines or Transformers. Solution 1: Relocate offending unit.
Cause 2: 10 Ohm chassis ground resistor is open. Solution 2: Check the DC resistance for 10 Ohms between the chassis and pin 1 of any inter-com connector.

If this condition (Cause 2) happens, it is because the system ground came into contact with something
that was "HOT" with respect to the Power Supply earth ground. If this occurs, carefully check
the system ground and AC distribution in the area.


Problem: System feedback (Acoustical) .

Cause 1: Listen Level control at this station or a Remote Station is set too high. Solution 1: Adjust.
Cause 2: Sidetone Null control at this station or a Remote Station is not adjusted correctly. Solution 2: Adjust. Refer to the procedure in the Front Panel Controls section of this manual.
Cause 3: Channel unterminated. Solution 3: Set the Main Station or Power Supply termination switch for that channel to the ON position.
Cause 4: A headset extension cord was used.
Solution 4: Headset extension cords are not recommended.
Problem: Excessive crosstalk.
Cause 1: High DC resistance in ground return. Solution 1: Use heavier cable; add additional conductor(s) to ground return.
Cause 2: MULTI-CHANNEL cable pairs are not individually shielded. Solution 2: Replace cable with individually shield pairs.
Cause 3: Headset cables are not wired properly or shielded properly. Solution 3: Correct wiring. Use headsets with properly shielded wiring.
Problem: Program signal sounds distorted.
Cause: Overload of Program Input circuit. Solution: Reduce Program Input level or reduce the gain of the program signal at the source,
such as an audio mixer.
Problem: Call signals do not function.
Cause 1: Excessive DC loading of intercom line. Solution 1: Remove any audio transformers or other equipment which may be connected
across the intercom line. If equipment other than Clear-Com intercom equipment
must be connected to the intercom line, please contact Clear-Com application or
service personnel for advice.
Cause 2: Far too many terminations on the intercom line. Solution 2: Check all Main Stations and Power Supplies to make sure each intercom channel
is terminated at only one point .



Some of the terms used when discussing critical communications for television or theatre may be new to you as they are unique to intercom applications. Although many of the terms are common to other audio applications, to be certain you understand their meanings we offer the following definitions:
All Call: Ability to push one button from the Main Station and talk to all channels at once on a multiplechannel system.

Ambient Noise: Those background sounds which are not part of the specific communication but are picked up by the microphone. Selection of a good "noise-canceling" mic will reduce ambient noise.

Belt Pack: A portable electronics package worn on the belt or mounted on a wall or other convenient location. Interconnects to system with mic cable and is powered by a central Power Supply or Main Station.

Bridging, High Impedance (hi-Z): A method of connecting to an audio line (such as Clear-Com) with-out loading or taking appreciable power from that line. Simply stated, as you add more and more sta-tions to the line, the volume remains constant.

Call Signaling: This feature is included with the majority of Clear-Com products. It is a visual indica-tor on a station (a lamp or LED) used to attract the attention of an operator who has removed the headset.

Channel: A channel is the line that connects parties together within a party line - it is a two-way talk path. For example, if you have six people who need to hear one director, you have a seven-station single-channel need. If the same director needs to speak privately to any one of the six, add a second channel. You now have a seven-station, two-channel system.

Closed-Circuit: Any intercom which is connected via cable (also called hard-wired). The other type would be Wireless. . .we make those too. However, if you want privacy and versatility, you probably want a closed-circuit system or a combination of both.

Cross Talk: Leakage of audio transmissions from one channel to another.

Dry Pair: A telephone term is used to describe a pair of wires (2 conductors) that carry audio but no voltage. Contrast this with a "Wet Pair" that carries both audio and voltage.

Duplex: Duplex refers to bi-directional communications. Normal communication between individuals talking face to face is "full duplex" -- in other words you can talk and listen simultaneously. The alter-native is "half-duplex" such as a push-to-talk situation where one station at a time can talk while others listen. A walkie-talkie is a good example of half-duplex communication.

EFP: Electronic Field Production. An EFP truck contains the necessary audio, video, intercom, and other equipment to create these productions.

ENG: Electronic News Gathering. An ENG truck contains the necessary audio, video, intercom, com-munications, and other equipment to effectively support gathering news and transmitting news reports back to a studio.

IFB: The term means "Interrupt Fold Back." A Fold-Back is a monitor system that allows, for example, talent to hear their voices or musicians to hear their voices and instruments on stage. IFB (program in-terrupt) disconnects the audio source while the talk button on the Main Station is pushed.

ISO: A private conversation path. An ISO channel allows one to simply push a button and transfer themselves and the person they wish to speak with to an isolated channel.

Linking: Linking ties separate channels into one single party line.

Main Station: This is a product that includes both the ability to communicate with multiple channels without connecting them together, and to power all the stations connected to these channels.

Master Station: A Remote Station which requires AC power to operate, and cannot power other stations.

Multi-Channel: More than one channel

Paging Output: Redirects output of the Main Station's microphone to an external destination (such as a PA system).

Party Line (P.L.): Intercom system where all people talking on the system can talk or listen to each other simultaneously. Also called conferencing.

Point to Point: One path to one person.

Program: Audio source that is fed into the intercom channels.

Program Interrupt: Disconnects the audio source while the talk button on the Main Station is pushed. (IFB)

Remote Mic Kill (RMK): The ability for certain Main Stations to shut off all microphones on beltpacks in a system.

Remote Station: Like the belt pack, this would be any of the products connected to the intercom line that allow duplex or half-duplex conversation, but do not contain a Power Supply.

Sidetone: This is your own voice heard in your earphone as you are speaking.

Station: A station is connected to one or more channels. For example, if you have six people who need to hear one director, you have a seven-station single-channel need. If the same director needs to speak privately to any one of the six, add a second channel. You now have a seven-station, two-channel system.

Termination: Passive network that is connected in each channel, usually on the Power Supply or Main Station.