ASL INTERCOM PS 4379 Manuel d'utilisateur télécharger pdf (Page 16)

PAGE 16

User Manual PS 6379 / May 2011 © ASL intercom BV

13.0 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.

14.0 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 XLR pin 3, DC power is on XLR pin 2.

XLR 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, it

should be of high quality and 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 these screens to the

metal housing of ASL units 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. When 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 to a mains outlet with a clean earth.

Other audio equipment may be connected to this

mains outlet, but avoid using an outlet which also

powers dimmer controlled lighting systems.

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ASL INTERCOM PS 4379 Manuel d'utilisateur Page 16

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