User manual

PAGE 7
User Manual PS 430 / October 2013 © ASL Intercom BV
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8.0 PARTY LINE, TECHNICAL CONCEPT
User stations 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 microphone
(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 and place it on the common audio
line where, due to the constant line impedance
(situated in the power supply between XLR pin 3 and
1), 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 simultaneously
• due to the high bridging impedance offered by
each station, the number of stations on the
party line has no influence on the level of the
communications signal
• power and audio to the intercom stations use
the same cable
The Call signal is also sent as a current on the audio
line. It develops a DC potential over the line
impedance which will be sensed by each station and
interpreted as a Call signal.
9.0 CABLING
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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 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 :
Use high quality cable
Use high quality microphone cable (shielded two
conductor cable, minimum 2x 0.30 mm2).
In case multi-pair microphone 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 if 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 unitst or XLR-3 wall boxes.
See section ‘Earthing Concept’.
Connect metal cable trunks, wall boxes and
overall multi-pair cable screens to clean earth
Metal cable trunks, metal wall boxes and overall multi-
pair cable screens should be interconnected and, at
the 'central earth point' in the intercom network only,
be connected to a clean earth or a safety earth.
(see section ‘Earthing Concept’).
Keep metal connection boxes and cable trunks or
pipes 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 part.
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. If using multi-pair cable,
parallelism is ensured in the best possible way.
Avoid closed loops
Always avoid that intercom cables are making a
closed 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.
ASL powered units to a 'clean' mains outlet
Master stations or power supplies should be connected