• Introduction
• Interconnections
• Typical Theatre Installation
• Intercom Etiquette
• Trouble Shooting
• Glossary
• Links
  

INTRODUCTION:
 

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 Casgrain Theatre, much of the cabling is  permanently installed.

A minimal system consists of one Main Station (Fig. 1) , two beltpacks (Fig. 2) and associated cabling.

The Main Station is capable of supplying 2 amps of direct current at 24-30 volts to power Belt Packs. Belt Packs each draw about 30mA (.030 A). The Main Station can thus power a maximum of 60 Belt Packs.
 

INTERCONNECTIONS:
 

Intercom stations are interconnected with two-conductor, shielded microphone cable.  Beltpacks are connected with 2 conductor shielded 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 Beltpacks. 
• 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.

Intercom Line Connector:
 

The rear panel of the Main Station has three, 3-pin male XLR connectors for each intercom line "A" and "B".

Figure 3 -- Intercom Line Connector (3XLM)

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, a 4-pin XLR male, is located on the front panel of all stations.

Figure 4 -- Headset Connector (4XLM)

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.

Typical Theatre Installation:

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.

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 doing so.
• Turn off your mic., so your it won't pickup ambient noise.
  
• 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
  

The intercom system is not a chat-line.  Keep conversations limited to production-related talk.

Your mic. switch should be turned "ON" only when you have something production-related to say.

TROUBLESHOOTING:

Problem: System does not operate. No power to Main Station or Power Supply. Green POWER LED is not illuminated and no SHORT LED's are 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.

Problem: Red SHORT LED illuminated.

Cause 1: Short or overload on that channel due to a shorted or miswired cable.	Solution 1: 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 2: Defective Remote Station	Solution 2: Check Remote Station and replace if necessary.

Problem: Both red SHORT LEDs are illuminated.

Cause 1: System is overloaded.	Solution 1: Remove cables, one at a time from system to help determine where the excess current requirements lie. Re-evaluate system current needs.
Cause 2: Short in multipair cable.	Solution 2: Remove cables, one at a time from system until the faulty line is located. Check for shorts between pins 1 and 2 or improper cable wiring.

Problem: Hum or buzz in system.

Cause 1: Inductive pickup caused by close proximity of this Main Station or connected Re-mote 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.

WARNING:

THIS IS A POTENTIALLY DANGEROUS SITUATION. A SHOCK HAZARD MAY EXIST BETWEEN THE METAL BOOM OF A REMOTE STATION HEADSET AND
GROUND.

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 .

GLOSSARY:

Some of the terms used when discussing critical communications for 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:

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 or Production Intercom) 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 or Production Intercom 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.

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.

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.

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.

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.

• Pro Intercom -- manufacturer of our system
  
• Com Clone -- build your own system.
• Intercom Technology Page -- misc. Information on Intercom Systems