Studio Technologies 41 2007 - Manual (Page 20)

Issue 2, February 2007 Model 41 User Guide
Page 20 Studio Technologies, Inc.
course not. But for the intended talent cue-
ing applications it should be fine. In almost
all cases the audio signals being carried
are somewhat or fully phase-coherent. A bit
of one channel getting into the other won’t
even be noticed, especially since moni-
toring is generally done using headsets,
headphones, or earpieces.
Is it possible to reduce the crosstalk that
is created? Absolutely, as long as a non-
standard cable connection is made. This
becomes a trade-off between an improved
crosstalk figure and ease of installation
and use. Using two full pairs can signifi-
cantly reduce crosstalk. Several connec-
tion schemes are possible; the exact one
selected will depend on the specific instal-
lation and personal technical philosophy.
Two unshielded twisted pairs can be ef-
fectively used. The first pair would carry
the DC and channel 1 audio signal and
common. The second pair would carry the
channel 2 audio signal, again along with
common. There will still be some capaci-
tance between the conductors carrying the
two audio channels but it should be signifi-
cantly less. Two shielded pairs can also be
used as was discussed in the Installation
section of this user guide.
Superior Power Delivery and
Audio Quality
As previously discussed, one of the
Model 41’s strengths is its ability to very
effectively deliver energy to the connected
IFB user devices. This allows more devices
to be supported over longer cable
runs. How does the Model 41 accomplish
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 cur-
rent limiter. While this is a simple and inex-
pensive solution, it’s not without significant
limitations. The major problem with this
method is the type of voltage-current “knee”
that is created. As the load current increas-
es past about 50% 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 41. The DC voltage
supplied by its IFB circuits won’t “poop out”
when loaded over its 0 to 200 milliamperes
range. This will allow IFB belt pack and
announcer’s console devices to work cor-
rectly in many more applications. Figure 10
shows the IFB circuit voltage-current curves
for the RTS 4000-series and the Model 41
Interface. The performance differences are
quite interesting.
It’s 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 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 regula-
tor being used as an “AM” modulator and
current limiter. The noise is an artifact of
the design topology and simply 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 cir-
cuit begin. The results were worth the effort.