Description and Theory of Operation
This project describes an intercom station that is normally used while interconnected to one or more similar intercom stations. The intercom system is a "party-line" style system where everyone can speak and everyone listens simultaneously.
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.
There are several commercial sources of this type of intercom, the primary one being ClearCom. Sources of compatible intercom equipment include:
This is NOT a complete list. If you know of other "ClearCom compatible" manufacturers (not just vendors or resellers) that should be listed here please let me know. email me at
Online auction sites are sometimes also sources for these systems:
Note that I do not sell completed intercom units. My intention is not to compete with these commercial vendors of production intercom equipment. Anyone with the need for this type of equipment should certainly first consider buying commercial equipment. This project is offered for the benefit of those who appreciate studying how systems like this work, school projects, and those with the technical expertise to build their own, but lacking the resources to buy commercial units.
THEORY OF OPERATION
This type of intercom consists of two or more stations interconnected by conventional twisted-pair shielded cable (i.e. mic cable). The stations are identical in the basic functions (send and receive audio, and sending and receive signaling). Other features (speakers, multi-line, etc.) are just variations on the central theme.
Wiring: The three conductors are deployed as:
The audio signals operate at around 0.5 to 0.7 V (similar to consumer "line level") The call signal imposes at least 10-15V DC on the signal bus.
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 is as simple as a single 1/4watt resistor between the audio/call bus and ground. See the schematic diagrams for value.
ePanorama http://www.epanorama.net/links/intercom.html features more information about various intercom systems including this one.
The ClearCom website also contains much valuable information about the system and technical details of their commercial products
Production Intercom also features many documents describing the application of this type of intercom system.
rcrowley.com ComClone Description and Theory of Operation
© 2002 Richard Crowley
Updated: 02 Jun 2003