Studio Technologies 41 2004 - Manual (Page 17)

Model 41 User Guide Issue 1, October 2004
Studio Technologies, Inc. Page 18
an output voltage of 30 volts across pins 1
and 2 and can supply a maximum current
of 200 milliamperes. (As the Model 200’s
current draw is well within the Model 41’s
capability, this is not a limiting factor.) The
difference between the voltage supplied
by the Model 41 (30 volts) and the volt-
age required by the Model 200 (24 volts)
allows a 6 volt maximum drop over the
interconnecting cable. Using the current
draw and maximum voltage drop figures,
the maximum cable resistance can eas-
ily 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 41’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 41
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 “pro-
fessional” audio? Of course not. But for
the intended talent cueing applications it
should be fine. In almost all cases the au-
dio signals being carried are somewhat or
fully phase-coherent. A bit of one channel
getting into the other won’t even be no-
ticed, especially since monitoring is gener-
ally 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 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 In-
stallation section of this user guide.