Party Line, Technical Concept; Cabling - ASL INTERCOM BS 217 User Manual

9.0

PARTY LINE, TECHNICAL CONCEPT

User stations and power supplies in an ASL intercom system are connected via one or
several 'party lines'.
A party line offers two-way ('full duplex') communication and consists of standard micro-
phone (multi-pair) cable. One wire is used as an audio line, one as a power line and the
screen of the cable functions as earth/return.
Current drive is used for signal transfer.
Each station utilizes a current amplifier to amplify the microphone signal. That signal is
put on the common audio line. Due to the constant line impedance, a signal voltage is
developed which can be further amplified and sent to the headphones or loudspeakers.
This principle has three advantages:
the use of a single audio line allows several stations to talk and listen simultane-
•
ously
due to the high bridging impedance offered by each station, the number of sta-
•
tions on the party line has no influence on the level of the communications audio
signal
power and audio to the intercom stations use the same cable
•
Also the Call signal is sent as a current, on the audio wire. It develops a DC potential over
the line impedance which is sensed by each intercom station and interpreted as a Call
signal.

10.0 CABLING

The intercom lines (the 'party lines') in an ASL analog intercom system are of the shielded two-
conductor microphone cable type. The intercom line connectors are of the XLR-3 type. Audio and
Call signals are on pin 3, DC power is on pin 2 and pin 1 is connected to the shield of the cable
which functions as the common return for audio and power.
The audio signal is transferred in an unbalanced way (see 'Party Line, Technical Concept').
To avoid earth loops (hum), the possible effect of electromagnetic fields and to minimize power
loss, certain rules have to be obeyed when installing the cabling of an intercom system:
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