Studio Technologies 42A 2008 - Manual (Page 20)

Issue 1, February 2008 Model 42A User Guide
Page 20 Studio Technologies, Inc.
an IFB user device, a simple “ohms law”
calculation will tell you the maximum cable
length.
Let’s use the example of a Studio Tech-
nologies Model 210 Announcer’s Con-
sole being connected to a Model 42A IFB
circuit. We’ll select Belden 8412 as the
interconnecting cable. For correct opera-
tion, the Model 210 needs at least 24 volts
DC between pins 1 and 2 of its IFB input
connector. It has a current draw of 105
milliamperes. The Model 42A’s IFB circuit
presents an output voltage of 30 volts
across pins 1 and 2 and can supply a
maximum current of 200 milliamperes. (As
the Model 210’s current draw is well within
the Model 42A’s capability, this is not a
limiting factor.) The difference between
the voltage supplied by the Model 42A
(30 volts) and the voltage required by the
Model 210 (24 volts) allows a 6 volt maxi-
mum drop over the interconnecting cable.
Using the current draw and maximum
voltage drop figures, the maximum cable
resistance can easily be calculated: 6 volts
divided by 0.105 amperes equals approxi-
mately 57 ohms. And finally, with 8412’s
21.8 ohms (total) per 1000 feet of cable,
a maximum of 2615 feet of cable can be
used and still be less than or equal to 57
ohms. Using this example as a guide, en-
tering the appropriate values will allow you
to determine the maximum cable length
for your application.
Cabling Issues – Crosstalk
The Model 42A’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 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 circuit 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 “profes-
sional” 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