Telex 38109-977 User Manual

HANDBOOK OF INTERCOM

S

YSTEMS ENGINEERING

FIRST EDITION

38109-977 Preliminary Rev. 4, 3/2002

The Fine Print

The Handbook of Intercom Systems Engineering, first edition, Copyright© 2000 by Telex Communica­tions, Inc. All rights reserved. Printed in the United States of America. Except as permitted under the United States
Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means,
electronic or mechanical, including photocopying and recording, or by any information storage or retrieval system
without the prior written permission of Telex Communications, Inc., unless such copying is expressly permitted by
federal copyright law.

Address copying inquires to:

Telex Communications, Inc.
Attn: VP, Intercom Products
12000 Portland Ave S
Burnsville, Minnesota 55337 USA

Information contained in this work has been created or obtained by Telex Communications Inc. from sources
believed to be reliable. However, Telex Communications, Inc. does not guarantee the accuracy or completeness of
the information published herein nor shall Telex Communications Inc. be liable for any errors, omissions, or dam­ages arising from the use of this information. Telex Communications, Inc. is not attempting to provide professional
services through the publication of this book, but rather intends only to provide information. If such professional
services are necessary or desired, users of this book should seek such professional assistance.

TABLE OF CONTENTS

Preface- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -3

About the Authors - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -5

Intercoms—An Overview- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Party-Line Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Matrix Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

Wireless Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Before We Begin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

IFB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

ISO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Tally. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

The Rest Of The Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Introduction to Party-Line

Intercom Systems - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -7

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Some Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Party-Line (PL) systems / Conference Line Intercom Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Two-Wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Balanced Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Full Duplex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Decibel (dB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Beltpack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Biscuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Main Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Master Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Sidetone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Crosstalk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

A Short History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Present Day Systems and Manufacturers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

System Components and Their Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Belt Pack Headset User Station Functional Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Speaker User Station Functional Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Master Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Some Technical Notes About The Stations Above . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

How Each System Works. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

System Powering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Headset User Stations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Speaker User Stations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Master Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Cabling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Outstanding Features of Each System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Call Lights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Limitations of Each System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

(Some Definitions) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

(A Short History) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

(Present Day Systems and Manufacturers) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

(System Components and Their Function) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

(How Each System Works) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

(Outstanding Features of Each System) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

i

(Limitations of Each System). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Design of Party-Line

Intercom Systems - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -21

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Defining And Meeting Your Needs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Application 1 Generic Single Channel Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Audiocom Party-Line Intercom Equipment Listing #1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Clear-Com Party-Line Intercom Equipment Listing #1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

RTS TW Party-Line Intercom Equipment Listing #1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Application 2 Two-Channel System: TV, School, Cable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Audiocom Party-Line Equipment Listing #2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Clear-Com Party-Line Equipment Listing #2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

RTS TW Party-Line Equipment Listing #2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Application 3 Theater System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Audiocom Party-Line Equipment Listing #3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Clear-Com Party-Line Equipment Listing #3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

RTS TW Party-Line Equipment Listing #3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Application 4 Training Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Audiocom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Clear-Com . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

RTS™ TW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Application 5 Medium System for Television . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

The IFB System (One Way Communications System) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

How an IFB Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Studio and Some Field Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Field Application, Sports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Field Application, ENG (Electronic News Gathering) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Connecting (Interfacing) to Other Communications Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

A Typical Interfacing Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Interfacing Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Level Problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Signal / Data Conversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Call Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Interfacing Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Interfacing Television Camera Intercom Systems to TW Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . 36

General Camera Configuration Information for Television Cameras (except ENG units) . . . . . . . . . . . . . . . . . . . . . 36

The Problems in Interfacing to Cameras . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Alternatives for Interfacing to Television Cameras . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Some Practical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Headset Cable Lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Headphone Impedances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Wiring Practices/Workmanship Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Unbalanced vs. Balanced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Extended Range On Part Or All Of The System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Cable Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Crosstalk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Crosstalk Through A Common Circuit Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Crosstalk Through A Mutual Capacitance Of Two Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

A Low Crosstalk Approach To Interconnection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Distances/Conductor Sizes/Distributed vs. Central Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

System Current/System Capacitances/Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Temperature Range Consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Cooling Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Moisture / Contamination Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Magnetic Fields: Hum Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

(Defining and Meeting Your Needs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

(The IFB System (One Way Communications System)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

ii

(Connecting (Interfacing to Other Communications Systems)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

(Some Practical Considerations) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Introduction to Matrix

Intercom Systems - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -45

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

History of Matrix Intercoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Modern Day Matrix Intercoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Configurability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Types of Communications Supported . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Ancillary Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Basic Ancillary Functions via GPI/O General Purpose Input / Output . . . . . . . . . . . . . . . . . . . . . . . . . 54

More Complex Ancillary Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Complexity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Design of Matrix

Intercom Systems - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -61

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Back-to-Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

RTS™ Matrix Intercom Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

To Begin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Let’s get started.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Studio A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Control Room . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Other. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Cable Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Audio and Data Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Polling Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Very Large Systems, Split Operation and Trunking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Interfacing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Signal Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Interconnecting Matrix, PL, and Wireless Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Software Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Introduction to Wireless

Intercom Systems - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -85

Introduction to Wireless Intercoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

History of Wireless Intercoms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Modern Day Wireless Intercoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Design of Wireless

Intercom Systems - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -95

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

Back-to-Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

Transmitters and Receivers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

Antenna & Cable Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

iii

Determining Intercom Needs - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 113

Conference Versus Point-to-Point Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

Fixed vs. Mobile Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

A General Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Determining Intercom Needs, two-wire, four-wire, or both?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

Small Studio or ENG Vehicle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

MCE325 Modular Programmable Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

PS15 Power Supply/MCP2 Rack Kit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

TW5W Splitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

IFB325 Talent User Station. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

BP325 Programmable Belt Pack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Headsets and Earsets (not shown) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Medium Sized Studio and Mobile Intercom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Two-wire Case (Medium Intercom). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

803-G1G5 Master Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

862 System Interconnect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

PS31 Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

SAP1626 Source Assign Panel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

BOP220 Connector Translation Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

4010 IFB Central Electronics Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

4025A Splitter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

4030 Talent User Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

MCE325-K Programmable User Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

BP319 Belt Pack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

BP325 Programmable Belt Pack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

Telos Link. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

Headsets and Earsets (not shown) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

Four-wire Case (Medium Intercom) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

Zeus™ DSP2400 Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

KP96-7 Keypanel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

TIF-2000 Intelligent Telco Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

MKP4-K Modular Keypanel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

IFB828 IFB Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

SSA324 System-to-System Adapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

PS15 Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

SAP612 Source Assign Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

MRT327-K Modular User Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

PAP951 Program Assign Panel and UIO256 GPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

Cameras in the Medium Intercom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

Large Studio or Mobile Vehicle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

Determining the Makeup of the Intercom Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

First Step--Determine the Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

Users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

IFB Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

Cameras . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Static Party-Lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Wireless Intercom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Telephones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Studio Announce and Dressing Room Paging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Second Step--Determine the Panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

KP96-7 Keypanel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

KP96-6 Keypanel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Other Considerations in Determining Intercom Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Physical Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

two-wire Conference Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Four-Wire Point-to Point Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

How old is Too Old? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Expandability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Interoperability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

iv

Glossary - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -129

Index - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -151

v

vi

LIST OF FIGURES

Simple Party-Line System - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1
Simple Matrix System - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2
Wireless Intercom Examples - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 3
Example of Interfacing a TW System to a Matrix System - - - - - - - - - - - - - - - - - - - - 4
Complex Matrix Intercom System - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 5
Audiocom® intercom concept. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 19
Clear-Com® intercom concept. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 19
RTS™ TW intercom concept. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 20
RTS™ TW user station block diagram. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 20
Generic single channel Audiocom® system. - - - - - - - - - - - - - - - - - - - - - - - - - - - - 22
Generic single channel Clear-Com® system. - - - - - - - - - - - - - - - - - - - - - - - - - - - 23
Generic single channel RTS™ TW system. - - - - - - - - - - - - - - - - - - - - - - - - - - - - 24
Small TV operation. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 25
Theater application. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 26
Audiocom® based training intercom system. - - - - - - - - - - - - - - - - - - - - - - - - - - - 28
Clear-Com® based training intercom system. - - - - - - - - - - - - - - - - - - - - - - - - - - - 29
RTS™TW based training intercom system. - - - - - - - - - - - - - - - - - - - - - - - - - - - - 30
Medium intercom system for television. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 31
The KP-32 is a good example of an advanced user station (keypanel). - - - - - - - - - - 46
Example of Matrix Ports - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 47
A Comparison 3x3 vs. 9x9 Matrices - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 48
A comparison of the 9400 Intercom System to the 9500 Intercom System (see inset).
The 9500 represented a tremendous reduction in physical size. - - - - - - - - - - - - - - - 50
An example of how multiple signals are “time-sliced” for use in a TDM system. - - - 51
Conventional Matrix vs. TDM Matrix - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 52
Typical Keypanel - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 55
Simplified Low-Cost User Station - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 55
Use of Source Assignment Panels such as this SAP-1626 allow the
rapid reconfiguration of PL systems without changing any cables - - - - - - - - - - - - - 58
Typical ADAM™ Matrix Connections - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 62
A wide variety of keypanel options exist. Here we have a selection of
RTS™ keypanels that fit a range of needs. Small keypanels such as the
(A) KP-12LK and (B) WKP-4 provide an interface for those with limited
keypanel needs. The (G) KP-96-7, a medium sized unit, was the workhorse
of the RTS™ keypanel line until the 1980’s and 1990’s. The (C) KP-32 is
the top of the line keypanel, and can be enhanced through additional options,
such as the (D) EKP-32 expansion panel, and the (F) LCP-32/16 level control
panel. The (E) KP-8T is an example of a specialty keypanel that makes use of
an empty bay in a Tektronix vectorscope. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 66
ADAM™ (including ADAM™ CS and Zeus™) Intercom Cable Connections - - - - - 67

A Comparison of Relative System Sizes - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -70
Separate Studios, Separate Intercom - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -71
Fixed Trunking - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -72
Intelligent Trunking - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -74
Cascaded Trunking - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -76
TW and Matrix Signal Flows - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -78
Wireless Intercom Interfaced to Matrix Intercom - - - - - - - - - - - - - - - - - - - - - - - - -79
GPI/O Implemented PTT (Push-To-Talk) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -80
TW to Matrix Interface 81
ADAM™ and ADAM™ CS Basic Components - - - - - - - - - - - - - - - - - - - - - - - - -82
Matrix Intercom Remote Control - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 83
The first beltpack based wireless intercom system. - - - - - - - - - - - - - - - - - - - - - - - -87
An example of a modern day wireless intercom system. - - - - - - - - - - - - - - - - - - - -88
The RadioCom™ BTR-800 System is an outstanding example of the
next generation of wireless intercom systems. - - - - - - - - - - - - - - - - - - - - - - - - - - - 89
NTSC channel configuration. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -90
The E an H fields exist in two separate planes, 90° apart from each other. - - - - - - - -96
An example of wireless transmission and reception. - - - - - - - - - - - - - - - - - - - - - - -97
An example of electromagnetic waves being radiated. - - - - - - - - - - - - - - - - - - - - - 97
An example of reflected RF waves.- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -98
The orientation of the radiator (antenna) determines the polarization,
and therefore, the orientation of the E and H fields. - - - - - - - - - - - - - - - - - - - - - - -99
Waves that are in phase combine to form a larger wave. - - - - - - - - - - - - - - - - - - - -99
Waves that are out of phase cancel each other. - - - - - - - - - - - - - - - - - - - - - - - - - - -99
An example of combining waves that are not 180° out of phase. - - - - - - - - - - - - - - 100
An example of multipath in its most basic form. - - - - - - - - - - - - - - - - - - - - - - - - - 100
Transmitter block diagram. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 102
Receiver block diagram. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 103
Good linearity is a must for faithful signal reproduction. - - - - - - - - - - - - - - - - - - - 104
A comparison of the radiation patterns for an Isotropic Radiator
(theoretical) vs. a Dipole (practical). - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 105
An example of a Yagi antenna. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 107
Telex®’s ALP-450 is an example of a Log Periodic antenna. - - - - - - - - - - - - - - - - 107
The typical parts of coaxial cable. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 109
Wiring differences between larger conference and point-to-point styles. - - - - - - - - - 114
Figure 3. Block diagram of a medium sized intercom system using two-wire.
The forms of communications depicted here are six conference lines and eight
IFB circuits. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 119
Block diagram of a medium sized intercom system using the Zeus™
four-wire matrix. The forms of communications depicted have
increased to include point-to-point and ISO. - - - - - - - - - - - - - - - - - - - - - - - - - - - 121
Figure 5. Block diagram of a large size intercom system using a twin
ADAM™ configured as a 200x200 matrix. - - - - - - - - - - - - - - - - - - - - - - - - - - - 123

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REFACE

C

Welcome to the Telex Communications, Inc. Handbook of Intercom Systems Engineering. The
idea for this book came, as it does with many books and inventions, over drinks at a bar. A few of
us “intercom types” were discussing our varied histories and experiences. We added up the years
each of us had in the intercom system industry and between the four of us we hit the 75 year mark.
Add the “rest of the gang” at Telex into that estimate and we are well past the century mark,
quickly closing in on the two century mark. It was then we decided that we were getting old and
had spent too much time dealing with intercoms. Someone commented that it was a shame that
“the younger generation” didn’t really know what we seasoned pros did and suggested that we
should pass down our profound body of knowledge for the good of “intercom-kind.”

0

0

0

The idea for the book sort of hibernated for a bit after that (as did we). Weeks later we found
ourselves planning for a trade show and discussing the appropriate “swag” for giveaways. After
some discussion, we decided a well written, reasonably impartial, complete reference / tutorial on
intercom system design would be a great thing – useful, desirable, business related, and maybe
something inspirational. We hope those that read this book take advantage of the knowledge they
can glean from it and expand the capabilities of their own intercom systems. And, maybe they
will use some intercom equipment from Telex. In the process, we may go down in history as the
“guys who wrote the book on intercoms.”

The book you are starting has a number of goals; it is intended to be a systematic tutorial for the
novice user and an encyclopedic reference for the designer in the midst of a project. It is NOT a
100+ page sales brochure for Telex

®

products. Rather, it is a resource intended to take the reader
through the different types of intercom systems and needs, compare them, point out strengths and
weaknesses, and provide many “real-life” examples of working systems.

This book will be updated regularly to keep pace with changes in technology. On the enclosed CD
you will find a good deal of technical information, systems examples, and some marketing “fluff”
such as Telex
strived to provide real examples with real products. Many of the examples will make use of

®

Telex
RadioCom

®

product sheets, catalogs, operating manuals, etc. Throughout the book, we have

products, as that is what we know best – Telex AudioCom®, RTS™ Matrix, RTS™ TW,

™

Wireless and Telex® Headsets. If we get to a point with an example where the
equipment needed or best suited is not one of our products, we will tell you what that product is
and how to find it.

I have often joked that intercoms are the “stepchild” of the industry – no one (or VERY, VERY
FEW) people decided in high school what they wanted to do with their lives is be Mr. (or Ms.)
Intercom. People tend to get dragged kicking and screaming into dealing with the design,
installation and support of intercom systems because they were in the wrong place at the wrong
time. What they later learn is that they have developed a valuable bit of niche expertise that can be
in great demand.

The one goal above all with this book is to provide a solid body of work, in a useful form to all
those who have been, and will be, dealing with specification and operation, as well as, design,

3

installation and support of intercom systems. In other words, we hope this book helps you get the
absolute most out of your communications systems.

Apart from the story of the bar and the trade show, there is another serious reason why we have
written this book. Intercoms (in our opinion) are a neglected, underrated, taken for granted part of
the technical world – they are not glamorous nor interesting. I have at times made the comment
that intercom systems have a lot in common with toilets (no off color jokes to follow). They are
often the last system designed into an environment, they are often cheaply done, they are
PRESUMED to be always available and always working, and when they are NOT – it QUICKLY
becomes a crisis – and the plumber, all of a sudden is worth ANY AMOUNT OF MONEY to
return the toilet to its normal functioning condition, FAST!

Now, let’s take the same scenario except in the intercom world. Consider a live television show, a
camera fails, or a microphone fails, and the audio operator can’t hear the guest, or a tape jams in a
VTR. No problem, we’ll just TELL the TD to take another camera, and TELL Camera 2 to
change its shot, or the audio operator will ASK the stage manager to get a spare microphone to the
talent, or the director will TELL the talent to ad-lib until the tape can be salvaged…. “WHAT DO
YOU MEAN, NO ONE CAN COMMUNICATE THESE SIMPLE INSTRUCTIONS!?!? Get the
PLUMBER (oops… INTERCOM EXPERT) NOW!!!!”

The intercom system, whether in a television station, on the sidelines of a football game, or in a
factory is critical, and must be seamless, reliable, and work without fault to allow all needed
communications to take place. This book is intended to help make that happen.

We’d love to know if you think we have succeeded, or failed, or fallen short with this effort, so
that, as with all things in life, we can learn, grow and improve. Please send your comments to
[e-mail].

Ralph K. Strader

Vice President & General Manager

Intercom Products

Telex Communications, Inc.

January 2001

4 Handbook of Intercom Systems Engineering

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BOUT THE

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UTHORS

This handbook is the work of a number of past and present Telex employees, as well as, some
outside experts (such as Stan Hubler).

Among the contributors (in alphabetical order) are: Talal Aly-Youssef, Gene Behrend, Larry
Benedict (contributor and editor), Rick Fisher, Stan Hubler, John King, Murray Porteous, Dave
Richardson, Ralph Strader, and Tom Turkington. The credits for each chapter reflect the
contribution of the primary author for that chapter. Through a group effort such as this, the words
may actually be those of a number of individuals in any given chapter.

Many other individuals have directly or indirectly contributed to this book, and not all of them
can be recognized here. Many of the illustrations were prepared by John Yerxa, and many of the
systems examples came from the work of Shawn Anderson, Chuck Roberts, Gene Behrend, and
Geoff Rogers.

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6 Handbook of Intercom Systems Engineering

Introduction

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

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VERVIEW

Party-Line Systems

Wired Party-Line systems are systems in which a number of participants are all involved
in the same conversation. Think of the telephone extensions in your home, if each person
in your family picks up a telephone in your home, you will all be able to hear each other.
You can talk to one another simultaneously and the person “on the other end of the line”
will be a full participant in one “public” conversation.

Depending on where in the world you are from, (presuming English language), you may
also refer to this type of system as “PL” (for “party-line), “TW” or “Two-Wire” from the
telephone systems, where on two wires, a full duplex conversation takes place, or
“conference” denoting the type of activity taking place in the conversation.

Figure 1.1

Note, the physical configuration and implementation of that “PL” or “TW” does not
necessarily need to be on two physical wires, in most cases it is not. The specific
topologies will be addressed in the chapters that follow.

Simple Party-Line System

Chapter 1 - Intercoms—An Overview 1

Matrix Systems

Wired Matrix systems are systems in which a large number of individuals have the ability
to establish private individual conversations from point A to point B. Again, going back to
the telephone system in your neighborhood, you, your next door neighbor, the pizza joint
down the street and the local gas station are all connected to the same central office by
wires from each location back to the telephone company. At any time, you can be talking
to the gas station, while your neighbor is ordering a pizza. The pizza guy does not hear you
ask the mechanic about the repairs on your SUV.

Depending on where in the English speaking world you are, you may refer to these types
of systems as Matrix systems, crosspoint intercoms, point-to-point systems, private lines
(sometimes, confusingly referred to as “PL”), or by some of the brand names used:
McCurdy, ADAM

Figure 1.2

Simple Matrix System

™

, Zeus™, and others.

MATRIX

Yo u

Neighbor

Pizza Joint

Gas Station

X

X

Yo u

Neighbor

X

Pizza
Joint

X

Gas

Station

Like the telephone system, matrix systems have other functions and capabilities.
Conferences, call waiting, busy signals, and other features are common to many matrix
intercoms. They are not limited to simple point-to-point communications. Some systems
even allow inter-matrix routing of signals, similar to long distance telephones calls using
trunks between central offices. Having a matrix system with a number of conferences
configured within it (virtual PLs) is very common.

Wireless Systems

Wireless Intercoms encompass all sorts of systems from the most basic pair of “walkie
talkies” to cell phones to dedicated professional full duplex intercom products. The most
basic feature of wireless intercoms is that they are not tethered by wires. (Didn’t think this
was going to be quite that basic, did you?) Seriously, wireless intercom systems are
employed where the limitation of wireless systems which can include fidelity,
interference, lack of range, lack of security (real or perceived), and battery life limitations
are outweighed by the freedom of being cordless. This freedom can be essential in many
applications—try dragging a wired intercom cable into the containment vessel of a nuclear
reactor.

Wireless intercom systems can be designed, installed, configured and operated in PL or
matrix configurations, and may very likely be connected to a hard-wired PL or matrix

2 Handbook of Intercom Systems Engineering

intercom system at some point. They can range from as simple as a single pair of units

HeadsetControls

talking to one another, to a system in which 24 or more different portable units are
dynamically switched between conversations.

Figure 1.3

Wireless Intercom Examples

Transmit to
Beltpacks

BTR-300



RadioCom

Power

Headset

ExtIntercom AuxAudio

PortableTransmitOn

4

2

1

3

PortableStationConnect

BTR-300

HeadsetControls

Talk

Gain

O/M

PushTwiceto Latch

Volume

Transmit to
Base

TR-300

TR-300 TR-300

TR-300

Mirror Image Pair

Telex TR-500

Base Station with

4 Remotes

Wireless systems will vary tremendously worldwide, due to varying governmental radio
regulations. What is common in America may be illegal in Japan, and may be unsuitable,
for other reasons, in Germany. These units may be referred to by any of the types
mentioned above, but, again, the unifying feature is the freedom from a wire.

Accessories

The fourth and final category is “accessories”. We are giving accessories its own separate
category because of its importance. This book is addressing intercom systems. In all
likelihood, many of the systems you encounter will be an amalgam of the three types
mentioned above. Without “accessories” you cannot have a system, just a bunch of
equipment.

To connect a TW system to a matrix system, a converter is required to change the
combined talk and listen signal from the TW to separate talk and listen signals for the
matrix – a hybrid provides this conversion.

Chapter 1 - Intercoms—An Overview 3

Figure 1.4

Example of Interfacing a TW System to a Matrix System

To connect a matrix intercom system to a Two-way radio system, a contact closure may be
required to activate the radio transmitter. A GPI (General Purpose Interface) between the
matrix and the base station of the radio can solve this problem easily.

To do intelligent trunking between matrix systems, across campus or across the country,
the audio and control signals between the matrices could be transported over fixed pairs of
wires. Realistically, however, installing a set of wires between Omaha and Los Angeles
may be out of your budget – so an interface allowing the use of dial-up telephone lines
may be needed. Other possibilities include muxes and demuxes to allow the audio and data
to be carried over an existing corporate Wide Area Network (WAN), or “piggybacked” as
subcarriers on an existing satellite feed.

4 Handbook of Intercom Systems Engineering

Figure 1.5

Complex Matrix Intercom System

MATRIX

Audio IN,

Audio OUT,

Data

Analog

Audio

Third Party

Terminal

Equipment

LISTEN

FLORTEL2TEL1T1NEWSADTDIFB4PL01RAPRODDIR

Keypanel

PLNUM

AUTO

3

2

1

ISOSUST IFB

PHONE

AUD1ISO2ISO1CHYR

6

5

4

RELAY

E-PANL

COPY

9

8

7

DISPLAY

CLEAR

MULT

CALL

PGM

CLR

0

FUNC

LAN / WAN

Third Party

Terminal

Equipment

Email

System

News

Computers

Third Party

Terminal

Equipment

Audio IN,

Audio OUT,

Data

PLNUM

AUTO

3

2

1

LISTEN

FLORTEL2TEL1T1NEWSADTDIFB4PL01RAPRODDIR

ISOSUST IFB

PHONE

AUD1ISO2ISO1CHYR

6

5

4

RELAY

E-PANL

COPY

9

8

7

DISPLAY

CLEAR

MULT

CALL

PGM

CLR

0

FUNC

Keypanel

In many cases, connection to “the telephone company” is required to allow a reporter to
connect into an intercom from his or her cell phone, or to allow a return program feed to be
fed to a remote location. A telephone interface (TIF) unit provides this connectivity.

The most basic accessory in an Intercom system may be the headset. It may provide
isolation from ambient noise; it may have a noise-canceling microphone to reduce wind
noise, and may have stereo ear pieces to allow program audio and intercom audio to be fed
independently to the right and left ears.

Each of these accessories is vital to creating an intercom system that meets the
communications needs of the users.

Before We Begin

Throughout this book, you will be subjected to the jargon that permeates the intercom
world. In the chapters that follow, you will be presented with definitions specific to the
topic being covered. In many cases, there are common terms that will be applicable to all
these chapters, and so we will present a few definitions to get us started. We have also
provided a comprehensive glossary in the rear of the book.

IFB

Interrupted Fold Back – also referred to as IRF – Interrupted Return Feed. The best way to
explain this is to give an example. A news reporter is on the scene of live accident
coverage. She needs to not only hear what the anchor back at the studio is saying i.e., “So,
Jane, how many chickens were injured when they tried to cross the road during rush
hour?” She also needs to hear instructions from the director back in the studio i.e., “Wrap
it up, 10 seconds.” The IFB function in an intercom system allows a single audio signal to
be sent to Jane, normally containing program audio interrupted by instructions or
information from someone not a part of the program audio.

Chapter 1 - Intercoms—An Overview 5

ISO

Camera Isolate – This is not reserved strictly for the domain of cameras anymore. This is
truly an isolate function, not unlike the action at a party of grabbing the arm of a fellow
guest, dragging them off to a corner for a private conversation, and then returning them to
their group. There are instances where it is necessary in an intercom system to establish a
momentary private conversation with someone who may be talking and listening to a
number of other people. The person who needs to interrupt presses a button or key, which
establishes a private two person conversation. Upon releasing the key, the two participants
are returned to whatever conversation(s) they were a part of previously. This was called
Camera Isolate as it first was used to remove an individual camera from a conference to
allow private communications.

Tally

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. On an intercom panel with multiple
channels, it can be a visual signal, such as a blinking light, to indicate which station is
calling. It can be used to indicate a particular function is not available due to a conflict –
similar to the busy signal you get when calling the radio station trying to be the tenth caller
and win a year’s supply of cat litter.

The above definitions and many more can be found in the glossary at the back of this
book.

The Rest Of The Book

We have organized this book by the above types of systems – two chapters devoted to PL
Intercoms, two chapters for matrix systems, two chapters for wireless systems, and one
chapter on interfaces, determining systems needs and requirements, technical
requirements for installation, and some real world case studies.

Near the end of the book, we have included references for further information, a glossary,
and a CD full of information on Telex
drawings.

®

Products, technical references, and many system

6 Handbook of Intercom Systems Engineering

Introduction

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2

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ARTY

S

-L

YSTEMS

STAN HUBLER

INE

Leading off this chapter, Some Definitions that may help you understand Party-Line
intercoms terms (and buzz-words). Then, a Short History of Party-Line intercoms will be
presented, leading into a discussion of Present Day Systems and Manufacturers. The
System Components and Their Function will explore the main components of these
systems and what they do. Then, How Each System Works shows how these system
components are put together to make a functioning intercom and some examples of the
different systems. Outstanding Features of Each System describes application areas and
where each system is often marketed. Some important Limitations of Each System are
described and a Summary closes this chapter.

Some Definitions

Party-Line (PL) systems / Conference Line Intercom Systems

A Party-Line system allows a group of people to intercommunicate. For example, one
person can talk, while all the others on the bus or channel can hear. When the system is full
duplex, anyone can talk and the rest can hear or interrupt the speaker at any time. The
Party-Line and distributed matrix systems presently sold today are usually full duplex and
are non-blocking, which means that access to the channel is immediate and there is no
busy signal. Conversations on Party-Line systems are, in general, non-private. It is
important to note that both two wire and four wire type systems support the Party-Line
concept.

Two-Wir e

A communications system where the path is the same for both talk and listen. In electrical
pathways there are, in fact, two wires (one path). Two-wire systems can be two-wire
balanced or two-wire unbalanced.

Chapter 2 - Introduction to Party-Line Intercom Systems 7

Balanced Line

The balanced line concept reduces noise pickup by outside sources. A balanced two
conductor line carries audio that is differentially driven and balanced to ground.

Full Duplex

This is communication that allows simultaneous two-way conversations, that is, one
person can interrupt the other. In data communications, full duplex permits confirmation
of sent data by the receiving terminal echoing, sending back the same data, or confirming
data.

Decibel (dB)

A derived unit of loudness. The human ear perceives a 10 decibel increase as twice as
loud, and a 10 decibel decrease as half as loud.

Beltpack

A portable headset user station. This station is designed to be worn on a user’s belt, but is
also fastened to the underside of consoles, taped to a structure near the user, or mounted on
a piece of equipment. The headset plugs into the user station, as does the connection to the
rest of the intercom.

Biscuit

Marketing buzz word for a portable speaker station.

Main Station

A multichannel user station. There may be one or more of these stations in a system.
Usually the primary station in a system.

Master Station

A user station where a user station and a system power supply are combined into one
package

Sidetone

In the truest sense, sidetone is a small amount of microphone signal fed back to the
earphone of the individual speaking into the microphone. In a two type user station, the
null balance control is sometimes used to adjust the amount of sidetone the user hears.
This control is sometimes (erroneously) called the sidetone control. Other equipment has
both null balance adjustments and a true sidetone adjustment.

Crosstalk

Unwanted interference caused by audio energy from one line coupling (“leaking”) into
adjacent or nearby lines.

A Short History

Party-Line intercoms were needed early on by television production crews to coordinate
their activities. Some of the activities included on-site sport pickups, entertainment on
stage, and videotaping of shows. The crews included camera operators, audio, lighting,

8 Handbook of Intercom Systems Engineering

stage directors, director, assistant director, production assistant, and others. Originally,
these crews shared one intercom channel where the director called the shots. Later, as
intercom developed, additional channels were added so each crew could still listen to the
director, then could switch to their own channels to coordinate activities without conflict
with the director. Party-Line intercom systems were also used by industrial activities to
coordinate manufacturing and testing of large systems such as aircraft.

Early intercom systems (1960-1975) were either homemade or accumulations of telephone
equipment lashed together. Often, the homemade intercoms worked well enough but
lacked the flexibility to expand the system or interface with other systems. The telephone
equipment approach had some flexibility, but performance degraded rapidly as the number
of stations increased above ten user stations.

In the early 1970s, Clear-Com built Party-Line systems for rock-n-roll concerts, and later
for theatrical stage, and eventually for television production. This system was flexible and
expandable, but required one three-conductor microphone cable for each channel. In the
mid 1970s, another company, RTS Systems, designed a system for television production
that had two channels on one three-conductor microphone cable (or one channel on a pair
of wires). This system was even more flexible and expandable with a design that allowed
up to 50 user stations on a single channel. On the East Coast, a company, Chaos, produced
intercoms for the New York and other stages. And, in the Midwest, a company, Telex
Communications, produced a balanced Party-Line system. This system was especially
useful in noisy electrical environments, because it was immune to induced interference.
Other Party-Line systems include systems such as David Clark, which is used for fire
trucks and similar public safety and service crews. And, of course, four wire matrix
systems can emulate Party-Line intercoms.

As Clear-Com
systems of both appeared. They included HME and Production Intercoms for Clear-Com,
and ROH and Anchor Audio’s PortaCom for RTS

®

and RTS™ Systems intercoms became more widely known, compatible

™

. Chaos is similar to Clear-Com,
except it uses a much higher power supply voltage (46 vs. 24 volts). As the markets
expanded, the distinction between theatrical and television production became blurred and
Party-Line systems of all types were used wherever they were needed. So a competitive
atmosphere developed and continues to the present. ROH and HME are no longer in the
wired intercom market.

Present Day Systems and Manufacturers

Note

The three major brands of “two-wire” Party-Line intercoms having the largest worldwide
presence are RTS, Clear-Com, and Telex
Clark, PortaCom, and Production Intercom.

Table 2.1

Brand Name Manufacturer

Audiocom

Chaos Goddard Design Company

Clear-Com Clear-Com Intercom Systems

David Clark David Clark Company, Inc.

PortaCom Anchor Audio, Inc.

Production Intercom Production Intercom, Inc.

RTS

Intercom brand name vs. manufacturer.

®

™

Telex Communications, Inc.

Telex Communications, Inc.

Present day Party-Line intercom systems are mostly distributed amplifier type systems as
opposed to a centralized system where all the headset lines plug into one box (Some David
Clark Systems are of a centralized type). Oh yes, there is a no-amplifier system called a

Chapter 2 - Introduction to Party-Line Intercom Systems 9

Audiocom. Other brands include Chaos, David

sound powered system, but we do not discuss it here. Present day Party-Line intercom
systems may be wired or wireless or both.

System Components and Their Function

The system components for most Party-Line intercoms consist of power supplies (or
master stations), user stations (e.g. belt packs, speaker stations, main stations, etc.),
interconnecting cable, headsets, panel microphones, push-to-talk microphones, and a
system termination.

The power supply (which is normally centralized) generates the DC power for the entire
system (with the exception of self powered user stations). The power supply usually
includes system termination for the audio channel, 200 ohms for RTS and Clear-Com, and
300 ohms for Audiocom. This may be as simple as a capacitor and resistor in a series, or,
an electronic termination, which is integrated into the power supply voltage regulator.

The user station connects to the power supply and intercom line. The human user connects
to the user station via a headset or loudspeaker and microphone or some combination. For
a given channel or channels the user stations are connected to each other in parallel.

The interconnecting cable for most intercoms is standard microphone cable with three pin
XLR type connectors. The female XLR connects towards the power supply and the male
XLR plugs into the user station. This polarity was chosen to prevent putting DC power
onto audio microphones which also use this type cable. There are at least two exceptions to
the use of microphone cable: the RTS
unshielded pairs (12 of the 25 pair in a cable). Another exception is where a twisted pair is
the only connection between two points. The RTS
to a twisted pair, while other user stations need adapters of one kind or another, and power
may have to be supplied at either end.

™

TW master stations connect audio with

™

TW user stations can connect directly

The wired systems are of three wiring configurations: 1) separate power, audio, and return
conductors (example: Clear-Com), 2)an audio pair which includes phantom power and a
common (example: Audiocom), and 3) a conductor that contains one channel and power, a
conductor that contains audio with- or without power, and a return (example: RTS

™

TWTW intercom system).

10 Handbook of Intercom Systems Engineering

Table 2.2

Clear-Com

Pin # Function

1 Common for Audio, Power, &

2 DC power: 30 volts nominal

3 Unbalanced Audio

Audiocom

Pin # Function

1 Common for Audio, Power, &

2Audio + DC Power

3Audio + DC Power

RTS TW

Pin # Function

1 Common for Audio, Power &

2 Channel 1 Audio + DC Power

3 Channel 2 Audio

Intercom connector wiring by various manufacturers.

Shield

Shield

Shield

The wireless systems usually include an interface to the wired systems. Principal
manufacturers include Telex Communications, Vega (now part of Clear-Com), and HME.
We will go into further detail on wireless systems in a later chapter of this manual.

Wired intercoms are mostly of the distributed amplifier kind. The distributed amplifier is
built into a User Station. User stations come in various packages and are of three kinds:
headset, speaker-microphone, or both. The various packages include a belt pack (worn on
the users belt, and of the headset kind), console mount (headset or speaker-microphone),
rack mount (headset or speaker-microphone), desk mount (portable speaker station), wall
mount (headset or speaker-microphone), and console/rack mount Master Station/Main
Station (details later). The distributed amplifier concept allows each user to adjust his/hers
own listening level. The user station also includes a microphone amplifier, a line
amplifier/buffer, volume control(s), talk switch(es). Some user stations also may have a
Call light, status indicators, and a channel selector. The microphone may be in the headset,
fastened to or plugged into a speaker station, in a handset, or in a push-to-talk hand held
unit.

Belt Pack Headset User Station Functional Description

A typical single channel belt pack headset user station has the following connectors:
Intercom Line (XLR-3) and a Headset Connector (XLR-4).
The station has the following controls:
Microphone ON/OFF (sometimes called a TALK switch), and a headset Volume Control.
It may also have a Call Lamp and a Call Lamp Send button. Examples of this station are an

™

BP318 single channel belt pack, or an Audiocom® BP1002, or a Clear-Com® RS-

RTS

501.

A typical two channel headset belt pack user station adds a channel selector switch to the
above. Examples RTS

™

BP351, Clear-Com® RS-502, Audiocom® BP2002

Chapter 2 - Introduction to Party-Line Intercom Systems 11

Alternately, newer units have two talk buttons, two volume controls, and two status
indicators to tell which talk button is engaged. Examples: RTS

®

RS-522-TW, or Audiocom® IC-2B.

Com

™

BP325, BP351, Clear-

Speaker User Station Functional Description

A typical speaker station can function with either a headset or a speaker/microphone. A
power amplifier, a speaker, and a speaker on/off switch are added to the electronics of a
belt pack. In addition, a nulling adjustment is easily accessible. The nulling adjustment
allows for full duplex operation without unwanted feedback. Also added is a connection or
jack for either a panel microphone (rack mount stations) or a push to talk microphone (for
desk mount or portable speaker stations).

Master Stations

The Master Station allows a user to access multiple channels. This allows different crews
to be monitored, cued or updated. If the master station is used for training, again, different
crews may be monitored and guided. These master stations have extra features for special
tasks such as IFB (Interrupted FeedBack) or SA (Stage Announce), relay closures, “hot”
microphones, and microphone kill. Master stations can send and receive call light signals
on any channel. Two examples of the Master station are Clear-Com
channel) and RTS

™

Model 803 (12 channel). Audiocom’s master station is modular and
can be as few as 2 channels or as many as 22 channels. Master stations allow simultaneous
monitoring of any channel, any combination of channels, or all the channels. They can call
or “mic kill” on any given channel. In addition, some master stations can monitor a
program source.

®

Model 912 (12

Wiring Notes

Some Technical Notes About The Stations Above

The stations mentioned above generally are designed for the dynamic microphones in the
headsets to have an impedance of about 150 to 500 ohms. The speaker station panel
electret microphones are designed to have an impedance of 1000 to 2000 ohms and require
1 to 5 volts excitation. And, the push-to-talk microphones have around 500 ohms. This
means the actual input impedance of the station microphone preamplifier will range from
470 ohms to 5000 ohms. The low impedance of 470 ohms minimizes the crosstalk in the
headset cord. The headphone impedances expected range from 50 ohms to 1000 ohms.
The 50 ohm headphones along with suitable headphone amplifiers provide enough SPL
(Sound Pressure Level) to overcome the interference from loud concerts and sports events.
The headphones also need to have an acoustic isolation of 20dB or more to protect the
user. These stations generally have a bridging impedance across the intercom line of
10,000 to 15,000 ohms. A bridging impedance of 10,000 ohms assures that up to 50
stations can be plugged into the systems and the level drop will only be 6dB. The level
drop of 6dB corresponds to the level drop when an extension telephone is picked up on an
existing conversation-noticeable but the telephone is still usable.

1 Clear-Com

connect to the intercom line. Clear-Com also offers the Clear-Com
which has two channels on a 3 pin XLR.

2 Clear-Com

headsets and a male 4 or 5 pin XLR connector on their user stations. However, RTS
uses a male 4 or 5 pin XLR connector on their headsets and a female 4 or 5 pin XLR
connector on their user stations.

®

and Audiocom® two channel stations have 6 pin XLR connectors to

®

and Audiocom® systems use a female 4 or 5 pin XLR connector on their

®

RS-522-TW,

3 In any system, pin 1 and the shell of the XLR connector should NOT be connected

together.

12 Handbook of Intercom Systems Engineering

4 The pin out of the headset connectors is as follows:

Four pin XLR

Pin 1 - Microphone common
Pin 2 - Microphone “hot”
Pin 3 - Headphone common
Pin 4 - Headphone “hot”

Five pin XLR

Pin 1 - Microphone common
Pin 2 - Microphone “hot”
Pin 3 - Headphone common
Pin 4 - Left Headphone “hot”
Pin 5 - Right Headphone “hot”

5 Since the power supply has a limited amount of XLR-3 connectors, splitter boxes are

used to expand the system. These boxes have all the connectors wired in parallel.

6 Some user stations have “loop-thru” connectors that allow “daisy chaining” stations

using a single connection to the power supply.

How Each System Works

Note

Drawings at the end of the chapter depict the systems being discussed.

First, please note that although these systems are full duplex and everybody could
theoretically talk at once, this is not at all practical or desirable. The usual operation is the
director or lead person has their microphone enabled all the time, while all other
microphones are switched off. These microphones are switched on only long enough to
supply an answer, make a request, or give data. In some cases, especially in noisy
environments, all microphones are off and only switched on as required. Because the
Party-Line concept has so many signal sources, this operational protocol is the only way
the Party-Line can be effective. And this is the reason for the system “mic kill”
(microphone turn-off) capability, for the situation where a station is unmanned but has its
microphone enabled.

These systems use voltage controlled current sources (or similar electronics) to apply a
signal to the intercom line. All the signals applied are summed and converted to a voltage
at the single termination resistor or electronic impedance. The current sources (or similar
circuits) have output impedances of 10,000 ohms or greater. The loading effect of the
station on the intercom, say in a 200 ohm terminated system is, worst case, 10,000 ohms in
parallel with 200 ohms. This results in a change of the system termination to 196 ohms, a 2
percent change. This, in turn, causes a voltage change of 2 percent or 0.175dB, an
imperceptible change. It takes 20 stations across the line to cause a 3dB change, a
perceptible but not significant change. The volume controls in the user stations easily
adjust for this change. In the “not so” worst-case situation, these systems can work with up
to 75 stations, provided enough DC power is available. The work-around in this case, in
the RTS

™

TW system, is a switch on the power supply which doubles the system

impedance. Then, two power supplies can divide the DC load and are coupled together
with capacitors to end up with the 200 ohm termination and twice the user stations. In the
case of Clear-Com, the system termination is not electronic but a passive resistor. If an
adapter is made, the same trick can be done in a Clear-Com

®

system power supply. In the

case of Audiocom® intercoms, paralleling two power supplies with capacitors would
result in an impedance of 150 ohms which could still be usable in some instances.

Chapter 2 - Introduction to Party-Line Intercom Systems 13

System Powering

Systems can be centrally powered with a power supply or they may be individually
powered with “local power” modules, also known as built-in power supplies. The systems
can also be a mixture of central and local power. In the cases of Audiocom

™

RTS

TW systems, the power and signal share the same wire(s). This means, for those

®

systems and

two systems, the power supplies DC source must be ultra low noise/quiet, circa -70dBu or
better. Most systems can work using main powers of 120 or 240 volts AC. Some
individual stations can be powered with 2 or 3 nine volt batteries in series. Venues such as
the Rose Parade may have to use a pair of batteries from the telephone company just to
cross the street. Since this may involve a mile of copper wire, there is no central DC source
that’s going to make it. Out come the nine volt batteries! The RTS

™

TW power supplies

can tolerate only a 5 volt peak-to-peak signal on the powered line. In this system, each
station can generate a maximum 2 volt peak to peak signal, so two stations talking
simultaneously can add up to 4 volts peak to peak. So, there is just 1 volt of headroom.
Clear-Com specifies a range of signal levels of .5 v p-p to a maximum of 4v p-p, but
doesn’t specify the reference (it is probably dBu or dBv). Audiocom

®

intercoms specify

only a nominal level of 1 volt RMS, which is equivalent to 3 v p-p.

Headset User Stations

The microphone preamplifier has a maximum gain in the neighborhood of 53 dB. Many
stations have Automatic Gain Control (AGC), which adjusts the gain according to the
incoming microphone signal. Some stations also have a limiter that prevents overloading
the intercom line. An electronic switch is placed between the microphone preamplifier and
the current source (line driver). This substantially reduces noise on the intercom line. A
hybrid connection is necessary to sort out the talk and listen signals (a two wire to four
wire converter would work best). The listen signal goes from the hybrid to the listen
volume control. The listen volume control drives the headphone amplifier that has a gain
in the range of 30 to 40 dB. For a 50 ohm headset, the headphone amplifier produces
maximum peak sound pressure levels of around 105dB. This is the level needed at
concerts and sporting events (along with 20dB acoustic isolation of the headset). In less
strenuous situations, a handset instead of a headset may be used with these stations. These
stations must have a bridging impedance of 10,000 ohms or higher. The current drains
range from 30 to 65 milliamperes. Most systems have signal levels that range from -15dBu
to 0dBu. In the case of Clear-Com
microphone preamplifier tend to keep the level in the -10dB range. This enhances
intelligibility and compensates for differences between voices and headset microphones.
Usually the headset amplifier has enough gain to make up the differences (by readjusting
the volume control).

®

and RTS™ TW systems, the AGC / limiters in the

Speaker User Stations

Most of these stations can operate in both a speaker/microphone mode and a headset
mode. The difference between a headset only station and the speaker station is that a
speaker amplifier, switching electronics, and a null pot are added. Usually the portable
speaker stations use a push-to-talk microphone, whereas the fixed speaker stations use a
panel or gooseneck microphone. The stations that have microphone and speaker on the
same panel have less available speaker level because of feedback. The push-to-talk
microphone has much better isolation. Speaker stations often have “dimming” or
“ducking” which attenuate the speaker output when the microphone is keyed. This allows
more gain and less feedback. Speaker stations use a very substantial amount of current,
about 120 milliamperes. So, fixed speaker stations are ideally operated with local power,
to prevent overloading the central power supply. Some RTS
and do not use central power.

14 Handbook of Intercom Systems Engineering

™

TW are direct AC powered

Master Stations

These are multichannel stations. Some Master Stations are balanced (RTS™ TW Model
802/803) and require an interface (RTS

™

TW Model 862 or 4012) to work with

unbalanced channels. Master Stations can be configured to work with their respective
systems with a minimum of interfacing. Master Stations have many functions which we go
into to detail later.

Cabling

Usually the intercom system’s specifications are based on the use of 22 AWG microphone
cable. Microphone cable of 22 gage measures 3 ohms per 100 feet or about 30 ohms per
1000 feet (round trip resistance). The wire table says 32 ohms per 1000 feet round trip, but
the shield resistance is much lower than the wire resistance. The Audiocom
both wires and the shield to transport DC so the calculations will be different for DC
voltage drop versus distance.

Outstanding Features of Each System

The Audiocom® system is immune to noise and is a lower cost system. It is used in
difficult environments, i.e.: churches, concerts, theaters, and sporting events.

The Clear-Com
available. It is often used for concerts, rock-n-roll tours, and in theaters. It is also used in
remote trucks, uplink trucks, and low budget venues.

The RTS

™

available in most countries world wide. Because the RTS
microphone cable, it is used where many channels are required, such as the Oscar and
Emmy award shows. It is also used for events such as the Superbowl. Most larger TV
trucks carry both a four-wire system and an RTS
interfaced together so the four-wire is used inside the truck and the RTS
outside the truck.

®

system is robust, relatively lower in cost, and rental systems are readily

system is also very robust, reasonable in cost, and rental systems are readily

®

system uses

™

intercom has two channels per

™

Party-Line system. These systems are

™

system is used

In addition to these features, most systems support extra features such as, “microphone
kill” and “call light”. The microphone kill feature allows all microphones in a given
channel to be switched off. In the case of Audiocom and RTS, the signal is an inaudible 24
kilohertz. In the case of Clear-Com, the power is interrupted for a long enough time to
reset the microphones to off.

Call Lights

The Call Light Signal allows user stations to generate and display a visual signal for
attention-getting and cueing purposes. The flashing light of the RTS
systems alerts the crew to put their headsets back on. The steady light of the Clear-Com
system can also be used for this purpose, however, it has another purpose: when the
director holds the call light on, this is a standby signal. When the light goes off, this is the
execute signal (raise/lower the scenery, follow spot on, et cetera). Call signals can also be
used to key 2-way radios, sound alarms, and activate lighting controls. Audiocom

™

systems use an inaudible 20 kilohertz signal for the call signal; Clear-Com

RTS

systems use a DC voltage added to the audio signal. Telex manufactures a call signal
detector / display (Model CIA-1000) which provides both a high visibility light and a relay
closure when a call signal is sent. The CIA-1000 works with RTS
systems. Clear-Com and other manufacturers also provide similar products. The company
VMA supplies a bright strobe lamp that is triggered by the RTS

Chapter 2 - Introduction to Party-Line Intercom Systems 15

™

and Audiocom®

®

and

®

™

TW and Audiocom®

™

system call signal. This

®

strobe is powered from the RTS line but only draws 10 milliamperes. It also supplies a
relay closure and a logic signal.

Limitations of Each System

Cable capacitance, resistance, and crosstalk affect all three systems. The longer cables
(over 2000 feet) limit the number of belt packs at the end. A system with cumulative
cables adding up to 10,000 feet will have a reduction in frequency response due to cable
capacitance. Both resistance and capacitance affect crosstalk.

If all you have is a twisted-pair cable, then the RTS
severe coupling with power cables, the Audiocom

Some of the information in this chapter is repeated in the next chapter, but in a different
context.

Summary

(Some Definitions)

1 A Party-Line system allows a group of people to intercommunicate.

2 “Two-wire” means a communications system where the path is the same for talk and

listen.

™

system is most useful. If you have

®

system will help.

3 A balanced line reduces unwanted noise and crosstalk pickup.

4 A full duplex intercom allows simultaneous two-way conversations.

5 The human ear perceives a 10 decibel increase as twice as loud.

6 A belt pack is a user station designed to be worn on a user’s belt.

7 A main station is a multichannel user station.

8 A master station combines a user station and a power supply.

9 Sidetone is a small amount of microphone signal fed back to the user’s ear.

10 Crosstalk is unwanted interference.

(A Short History)

1 Television, theatrical, and concert production crews need Party-Line intercoms.

Party-Line intercoms are also used for training and for industrial crews.

2 Early intercoms were inflexible and limited to small groups of users and sometimes

short distances.

3 In the 1970s, fresh new designs were the beginning of the modern Party-Line

intercoms we use today.

(Present Day Systems and Manufacturers)

1 Principal “two-wire” Party-Line brand names today are Audiocom, Clear-Com, and

RTS. Other brand names are Chaos, David Clark, PortaCom, and Production
Intercom.

2 With the exception of David Clark, present day Party-Line intercoms are the

distributed amplifier type.

16 Handbook of Intercom Systems Engineering