Studio Technologies 5132 2014 - Manual (Page 16)

Issue 5, October 2014 Model 5132 User Guide
Page 16 Studio Technologies, Inc.
Model 5132
Party-Line Interface Module
problem has gone away. If the problem is no
longer present, check the user device(s) to
see which is causing the fault. If the prob-
lem has not gone away, review the intercon-
necting cables and find the cause of the
fault condition. Within five seconds of the
problem being “cleared” the Pin 2 Status
LED will stop flashing.
Cable Length
There are no “hard and fast” rules defin-
ing the maximum cable length possible
when connecting user devices to the Model
5132’s party-line intercom output and using
the Model 5132’s internal party-line inter-
com power source. The maximum cable
length is directly related to the amount of re-
sistance in the connecting cable; the lower
the resistance per foot (or meter), the lon-
ger the cable can be. Although cable capac-
itance affects high-frequency performance,
resistance is the limiting factor in most
cases. For example, a contemporary micro-
phone cable is Belden 1172A which has 18
ohms of resistance per conductor per 1000
feet. Since we’re using two conductors to
carry the signal (pins 1 and 2) you’d get 36
ohms per 1000 feet of cable. By knowing
the cable resistance value, along with the
minimum voltage and maximum load cur-
rent required by a party-line intercom user
device, a simple “ohms law” calculation will
tell you the maximum cable length.
Let’s use the example of an RTS BP325
beltpack being connected to the Model
5132’s party-line XLR connector. We’ll
select Belden 1172A as the interconnecting
cable. For correct operation, a BP325 needs
at least 18 volts DC between pins 1 and 2 of
its interface connector. And the BP325 has
a rated maximum current draw of approxi-
mately 60 mA. The Model 5132 presents a
party-line voltage of 28 volts across pins 1
and 2 and can supply a maximum current
of 180 mA. (As the BP325’s current draw
is well within the Model 5132’s capability,
this is not a limiting factor.) The difference
between the voltage supplied by the Model
5132 (28 volts) and the voltage required by
the BP325 (18 volts) allows a 10 volt maxi-
mum drop over the interconnecting cable.
Using the current draw and maximum
voltage drop figures, the maximum cable
resistance can easily be calculated: 10
volts divided by 0.060 amperes equals 167
ohms. And finally, with 1172A’s 36 ohms
(total) per 1000 feet of cable, a maximum
of approximately 4600 feet (1424 meters)
of cable can be used and still be less than
or equal to 167 ohms. Using this example
as a guide, entering the appropriate values
will allow you to determine the maximum
cable length for your application.
Cabling Issues – Crosstalk
The Model 5132’s party-line interface con-
forms to the broadcast-industry standard
for sending DC power and two channels of
audio over a single pair-with-shield audio
cable. This implementation allows stan-
dard portable cables, such as those used
with microphones, to interconnect vari-
ous party-line user devices. This method
is undoubtedly convenient and practical,
but is not without limitations. The main
audio quality issue is the possibility of
crosstalk between the two audio channels.
This issue arises due to the capacitance
presented by the two wires that form the
twisted pair. The greater the capacitance
presented and the longer the cable run,
the greater the crosstalk. Is this normally
a problem during actual use? No. But it’s
something that should be noted.