Studio Technologies 5130 2013 - Manual (Page 14)

Issue 5, July 2013 Model 5130 User Guide
Page 14 Studio Technologies, Inc.
Model 5130
Party-Line Interface Module
pin 1 (common) and pin 2 (power with chan-
nel one audio) pins. Troubleshooting should
prove quick and easy. Begin by disconnect-
ing the party-line user devices. Observe the
pin 2 power LED and see if the problem has
gone away. If not, review the interconnecting
cables and find the fault condition. Within
five seconds of the problem being “cleared”
the pin 2 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
5130’s party-line intercom output. The
maximum cable length is directly related to
the amount of resistance in the connect-
ing 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 resistance per
conductor per 1000 feet. Since we’re us-
ing 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 current 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
5130’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
approximately 60 milliamperes. The Model
5130 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 5130’s capability, this is not a limiting
factor.) The difference between the voltage
supplied by the Model 5130 (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 maximum voltage drop figures,
the maximum cable resistance can eas-
ily 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 appro-
priate values will allow you to determine the
maximum cable length for your application.
Cabling Issues – Crosstalk
The Model 5130’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 shielded 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 cross-
talk. Is this normally a problem during actual
use? No. But it’s something that should be
noted.