Studio Technologies 42 - Manual (Page 13)

Issue 1, July 2004 Model 42 User Guide
Page 14 Studio Technologies, Inc.
the Model 200 (24 volts) allows a 6 volt
maximum drop over the interconnecting
cable. Using the current draw and maxi-
mum voltage drop figures, the maximum
cable resistance can easily be calculated:
6 volts divided by 0.095 amperes equals
63 ohms. And finally, with 8412’s 21.8
ohms (total) per 1000 feet of cable, a
maximum of 2890 feet of cable can be
used and still be less than or equal to
63 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 42’s IFB circuits conform to a
broadcast industry standard for sending
DC power and two channels of audio over
a single pair with shield audio cable. This
implementation allows standard portable
cables, such as are used for microphone
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 capacitance 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.
Studio Technologies did some experi-
menting with various cables and the
crosstalk that was created. For example,
a 1000-foot reel of 24-gauge 2-pair
unshielded telephone cable was used
to link a Model 42 IFB circuit with an IFB
user device. One pair carried the pin 2 (DC
with channel 1 audio) and pin 3 (channel 2
audio) connections. While 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 –45dB. Is this a good
value for “professional” 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
somewhat or fully phase-coherent. A bit of
one channel getting into the other won’t
even be noticed, especially since monitor-
ing is generally done using headsets,
headphones, or ear pieces.
Is it possible to reduce the crosstalk
that is created? Absolutely, as long as
a nonstandard 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 significantly reduce crosstalk. Several
connection schemes are possible; the
exact one selected will depend on the
specific installation and personal technical
philosophy. 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 capacitance 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 user guide.
Superior Power Delivery and
Audio Quality
As previously discussed, one of the Model
42’s strengths is its ability to very effec-
tively deliver energy to the connected IFB