Studio Technologies 42A 2013 - Manual (Page 20)

Issue 2, December 2013 Model 42A User Guide
Page 20 Studio Technologies, Inc.
signals, to interconnect various IFB user
devices. This method is undoubtedly con-
venient 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 great-
er the capacitance presented and the lon-
ger 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.
Studio Technologies did some experiment-
ing with various cables and the crosstalk
that was created. For example, a 1000-foot
reel of 24-gauge 2-pair unshielded tele-
phone cable was used to link a Model 42A
IFB output with an IFB user device. One
pair carried the pin 2 (DC with channel 1
audio) and pin 3 (channel 2 audio) con-
nections. One wire from the second pair
carried the pin 1 (DC and audio common)
connection. The inter-channel crosstalk
in the voice audio band was on the order
of –45 dB. Is this a good value for “pro-
fessional” audio? Of course not. But for
the intended talent cueing applications it
should be fine. In almost all cases the
audio signals being carried are some-
what or fully phase-coherent. A bit of one
channel getting into the other won’t even
be noticed, especially since monitoring
is generally done using headsets, head-
phones, 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 sig-
nificantly reduce crosstalk. Several con-
nection schemes are possible; the exact
one selected will depend on the specific
installation and personal technical philoso-
phy. Two unshielded twisted pairs can be
effectively 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
significantly less. Two shielded pairs can
also be used as was discussed in the
Installation section of this guide.
Superior Power Delivery and
Audio Quality
As previously discussed, one of the Model
42A’s strengths is its ability to very effec-
tively deliver energy to the connected IFB
user devices. This allows more devices
to be supported over longer cable runs.
How does the Model 42A accomplish this?
Simply by having circuitry that is superior
to that used in most of the “industry-
standard” equipment. In most IFB inter-
face devices, an adjustable voltage
regulator integrated circuit is used as
a combination of audio modulator and
current limiter. While this is a simple and
inexpensive 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 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 output volt-
age begins to drop problems with user
device performance can occur. Contrast