Telex 38109-977 manual Limitations of Each System, Summary, Some Definitions, Short History

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™ system is most useful. If you have severe coupling with power cables, the Audiocom® system will help.

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

Summary

(Some Definitions)

1A 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.

3A balanced line reduces unwanted noise and crosstalk pickup.

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

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

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

7A main station is a multichannel user station.

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

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

10Crosstalk is unwanted interference.

(A Short History)

1Television, theatrical, and concert production crews need Party-Line intercoms. Party-Line intercoms are also used for training and for industrial crews.

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

3In the 1970s, fresh new designs were the beginning of the modern Party-Line intercoms we use today.

(Present Day Systems and Manufacturers)

1Principal “two-wire” Party-Line brand names today are Audiocom, Clear-Com, and RTS. Other brand names are Chaos, David Clark, PortaCom, and Production Intercom.

2With the exception of David Clark, present day Party-Line intercoms are the distributed amplifier type.

16 H a n d b o o k o f I n t e r c o m S y s t e m s E n g i n e e r i n g