Studio Technologies 5121 - Manual (Page 14)

Issue 3, October 2014 Model 5121 User Guide
Page 14 Studio Technologies, Inc.
Model 5121
Line/IFB Output Module
1 and 2 of its IFB input connector. It has a
current draw of 35 milliamperes. The Model
5121’s IFB circuit presents an output volt-
age of 28 volts across pins 1 and 2 and can
supply a maximum current of 120 milliam-
peres. (As the Model 32A’s current draw is
well within the Model 5121’s capability, this is
not a limiting factor.) The difference between
the voltage supplied by the Model 5121 (28
volts) and the voltage required by the Model
32A (24 volts) allows a 4 volt maximum drop
over the interconnecting cable. Using the
current draw and maximum voltage drop
figures, the maximum cable resistance can
easily be calculated: 4 volts divided by 0.035
amperes equals 114 ohms. And finally, with
1172A’s 36 ohms (total) per 1000 feet of ca-
ble, a maximum of approximately 3100 feet
of cable can be used and still be less than
or equal to 114 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 5121’s IFB output conforms
to a broadcast-industry standard for send-
ing DC power and two channels of audio
over a single pair with shielded audio cable.
This implementation allows standard por-
table cables, such as those used for micro-
phone signals, to interconnect various IFB
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 capaci-
tance presented and the longer the cable
run, the greater the crosstalk. Is this normal-
ly a problem during actual use? No. But it’s
something that should be noted.
Superior Power Delivery and
Audio Quality
One of the Model 5121’s strengths is its
ability to very effectively deliver energy to
the connected IFB user devices. This allows
the devices to be supported over longer
cable runs. How does the Model 5121 ac-
complish this? Simply by having circuitry
that is superior to that used in most of the
“industry-standard” equipment. In most
IFB interface devices, an adjustable volt-
age regulator integrated circuit is used as a
combination of audio modulator and current
limiter. While this is a simple and inexpen-
sive solution, it’s not without significant limi-
tations. The major problem with this method
is the type of voltage-current “knee” that is
created. As the load current increases past
about 50 percent of the rated maximum
the output voltage begins to decrease. This
means that the usable power delivered to
the connected device(s) will start to drop
well before the rated output is reached. This
limitation will become significant in applica-
tions that use long cable runs. As the IFB
circuit voltage begins to drop problems with
user device performance can occur.
Contrast this situation with the performance
provided by the Model 5121. The DC volt-
age supplied by its IFB circuit won’t “poop
out” when loaded over its entire 0 to 120
milliampere range. This will allow IFB belt-
pack devices to work correctly in many
more applications.
It’s also interesting to note the reason why
typical IFB circuit audio quality is less than
pristine. It’s not hard to notice the back-
ground “hiss” that is seemingly always
present on pin 2 (DC with channel 1 audio)
of the interface connector. Technically, it’s
white noise that comes from the adjustable
voltage regulator being used as an “AM”