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User Manual PS 29 M / Issue 2011 © ASL Intercom BV
9.0 PART LINE, TECHNICAL CONCEPT
User stations in an ASL intercom system are
interconnected via one or several ‘party lines’
A party line offers two way (‘full’ duplex’)
communication and consists of standard microphone
(multi-pair) cable. One wire is used for the audio signal
and one wire for the DC power. The screen of the
microphone cable functions as earth/return.
Current drive is used for signal transfer.
Each intercom 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
headphones or loudspeakers.
(the line impedance is situated in the power supply
between XLR pin 3 and pin 1)
This principle has the following advantages:
The use of a single audio line allows several
intercom stations to talk and listen
simultaneously
Due to the high bridging impedance offered by
each intercom station. The number of stations
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.
The intercom lines (the ‘party lines’) 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.
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:
Use high quality cable
Use high quality microphone cable only (shielded two
conductor cable, minimum 2x 0.30 mm2). In case
multi-pair cable is used, there should be an overall
shield and each pair should consist of two conductors
(minimum 2x 0.15 mm2) with separate shield.
Use flexible cable
Use flexible single and multi-pair microphone cable
instead of cable with solid cores, especially when the
cable is subjected to bending during operation or
installation.
Cable screens to XLR pin 1
The screen of each separate microphone cable and/or
the screen of each single pair in a multi-pair cable,
should be connected to pin 1 of each XLR-3
connector. Do not connect these screens to the metal
housing of ASL units or XLR-3 wall boxes).
See Earthing Concept.
Connect cable trunks, connection boxes and
overall multi-pair cable screens to clean earth
Metal cable trunks, metal connection boxes and overall
multi-pair cable screens should be interconnected and,
at one point (the 'central earth point') in the intercom
network only, be connected to a clean earth or a safety
earth (see Earthing Concept).
Keep metal connection boxes and trunks isolated
from other metal parts
Metal trunks or pipes for intercom cables and metal
connection boxes should be mounted in such a way
that they are isolated from any other metal housing or
construction parts.
Keep cables parallel as much as possible
When two (multi channel) units in a network are
connected by more than one cable, make sure that
these cables are parallel to each other over the whole
distance between those units. When using multi-pair
cable, parallelism is ensured in the best possible way.
Avoid closed loops
Always avoid that intercom cables are making a loop.
So-called 'ring intercom' should not physically be
cabled as a ring.
Keep cables away from electromagnetic sources
Keep intercom cables away from high energy cables,
e.g. 115/230/400V mains power or dimmer controlled
feeds for spotlights. Intercom cables should cross high
energy cables at an angle of 90º only. Intercom cables
should never be in the same trunks as energy cables.
Place power supply in a central position
In case of a system powered by a separate power
supply: In order to diminish power losses, place the
power supply as close as possible to where most
power consumption occurs, in other words most user
stations are placed.
Connect ASL powered units to a 'clean' mains
outlet
Master stations or power supplies should be
connected to the mains outlet with a clean earth. Other
audio equipment may be connected to this mains
outlet, but avoid using mains outlets which also power
dimmer controlled lighting systems.
