Party Line, Technical Concept; Cabling - ASL INTERCOM PS 6379 MK2 User Manual

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13.0 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
•
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 sig-
nal.

14.0 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
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