Studio Technologies 5120 2014 - Manual (Page 13)

Model 5120 User Guide Issue 5, October 2014
Studio Technologies, Inc. Page 13
Model 5120
Line/IFB Output Module
Cabling Issues – Crosstalk
The Model 5120’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 are 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 capac-
itance presented and the longer the cable
run, the greater the crosstalk will become. Is
this normally a problem during actual use?
No. But it’s something that should be noted.
Superior Power Delivery and
Audio Quality
One of the Model 5120’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 5120 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
applications that use long cable runs. As
the IFB circuit voltage begins to drop prob-
lems with user device performance can
occur.
Contrast this situation with the perfor-
mance provided by the Model 5120. The
DC voltage supplied by its IFB circuit won’t
“poop out” when loaded over its entire 0 to
120 milliampere range. This will allow IFB
beltpack devices to work correctly in many
more applications. Figure 4 shows the IFB
circuit voltage-current curves for the RTS
4000-series and the Model 5120’s IFB out-
put. The performance differences are quite
interesting. While the RTS Model 4010 is
rated to support a much higher maximum
current (versus the Model 5120) it doesn’t
effectively deliver it. The Model 5120’s
output voltage varies little over its rated
output current range. Not so with the RTS
4010. Well before its maximum rated
current is reached its output voltage falls
off precipitously.
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”
modulator and current limiter. The noise is
an artifact of the design topology and sim-
ply can’t be overcome. How does Studio
Technologies know this? Because our first
“breadboard” designs used this method
and achieved the same poor results! Only
after the problem came to light did work
on an improved circuit begin. The result
was well worth the effort.