Interspace Industries TheatreCue 16 Way V4 System - 16 Outstations - Manual (Page 25)

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Outstation Cables
Cable capacitance reduces the amplitude of the data
signal by storing some of that signal as on electric charge
in the cable.
Cable branches with nothing plugged into them will also
decrease the data signal’s amplitude due to the cable’s
capacitance which is still connected to the circuit.
Cable capacitance is measured in pF (Pico Farad) per foot
or metre and is determined by the cable’s construction and
insulating materials used.
Cable capacitance can be ignored where the total cable
length in an installation is less than about 500m (1640 ft).
Where more than 500m of cable is used, cables with a
core to core capacitance of less than 80pF/m (24pF/ft)
should be selected.
Low capacitance cables have core insulation materials like
polyethylene or polypropylene. They are often specified as
Industrial Twinax or for use in applications such as RS485,
AES/EBU digital audio or DMX. Any of these low
capacitance cables are suitable for the Cue Light system
(but we still need to check that their wire size is adequate
for the cable length proposed).
See table on page 28.
(Continued)
Cable Capacitance
Cables using PVC insulation for the inner conductors have
high capacitance and high losses for digital signals. They
should be avoided for long cable runs or permanent wiring.
PVC outer jackets however are OK.
Most analogue microphone cables use PVC insulation and
can be used where the total cable length in an installation is
less than about 500m (1640 ft).
PVC insulated cables are also OK for use as tails between
permanently installed 3 pin XLR outlets and the Outstations
in large installations. In this case they only form a small
percentage of the total cable in the system and do not
degrade the overall system performance significantly.
Lower cable capacitance = longer possible cable runs due
to less attenuation of the data signal.
25.