Studio Technologies 5132 2012 - Manual (Page 15)

Model 5132 User Guide Issue 4, September 2012
Studio Technologies, Inc. Page 15
Model 5132
Party-Line Interface Module
the interconnecting cables and find 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 defining
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
intercom power source. The maximum
cable length is directly related to the
amount of resistance in the connecting
cable; the lower the resistance per foot
(or meter), the longer the cable can be.
Although cable capacitance affects high-
frequency performance, resistance is the
limiting factor in most cases. For example,
a contemporary microphone 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 interconnect-
ing 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 approximately 60 milliamperes. The
Model 5132 presents a party-line voltage
of 28 volts across pins 1 and 2 and can
supply a maximum current of 180
milliamperes. (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
maximum drop over the interconnecting
cable. Using the current draw and maxi-
mum 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 standard
portable cables, such as those used for
microphone signals, 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.