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User Manual PS 130 / October 2013 © ASL Intercom BV

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

consist 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, a signal voltage is

developed which can be further amplified and sent to

headphones or loudspeakers.

This principle has three advantages:

1. the use of a single audio line allows several

stations to talk and listen simultaneously

2. 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

3. 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 is sensed by each station and

interpreted as a Call signal.

9.0 CABLING

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 ASL intercom system.

Use high quality cable

Use high quality microphone cable (shielded two

conductor cable, minimum 2x 0.30 mm2).

In case multi-pair cable is used, each pair should

consist of two conductors (minimum 2x 0.15 mm2) with

separate shield and an overall 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 this cable screen to the

metal housing of ASL units or XLR-3 wall boxes.

See section ‘Earthing Concept’.

Connect cable trunks, connection 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

connector 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. When using multi-pair

cable, parallelism is ensured in the best possible way.

Avoid closed loops

Always avoid that 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.

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 outlet, but

avoid using an outlet which also powers dimmer

controlled lighting systems.

In case of more complex installations, don't hesitate to contact us. Please send a block diagram of the planned

network with a list of all user stations and their positions, and we are happy to advise you on cabling lay out.

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