Federal Signal AudioMaster 310LD-MV Instrucciones De Instalación Y Servicio página 7

ISOLATED BALANCED/UNBALANCED LINES:
The audio signals for the 310 are transmitted over a wire pair using
balanced line technology. This means each wire carries a signal that is
opposite in polarity of the other. At the input of the intercom is an isolated
balanced line transformer. This transformer subtracts the two signals from
each other providing an output free of noise generated onto the wires by
some other noise source such as a motor or light fixture near the wires.
The isolation provided by the transformer means that the grounds of
the units are not connected by the audio lines. Therefore, ground loops
and other problems caused by non-isolated systems are avoided. The
polarity of the audio lines between the + and - signal terminals need not be
maintained.
A shielded cable can be used in installations where extreme problems
from interference are suspected. The shield should always be connected to
the ground of only one of the intercoms. Connecting the shield to ground
at both ends will cause ground currents to travel through the shield which
could cause hum in the system.
There are several diagrams in the back of the manual to aid in wiring
intercoms together. The wiring diagram section illustrates balanced and
unbalanced line interconnection along with how a shielded audio cable can
be used. (See Figures 5 through 12.) It also illustrates multiple intercom
interconnections.
Figure 3.
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