Studio Technologies 41 2013 - Manual (Page 19)

Model 41 User Guide Issue 3, November 2013
Studio Technologies, Inc. Page 19
in the connecting cable; the lower the re-
sistance per foot (or meter), the longer the
cable can be. (Although cable capacitance
affects high-frequency performance, resis-
tance is the limiting factor in this case.) For
example, a traditional 20 AWG microphone
cable is Belden 8412, which has 10.9 ohms
resistance per conductor per 1000 feet.
Since we’re using two conductors to carry
the signal (pins 1 and 2) you’d get 21.8
ohms per 1000 feet of cable. By know-
ing the cable resistance value, along with
the minimum voltage and maximum load
current required by an IFB user device, a
simple “ohms law” calculation will tell you
the maximum cable length.
Let’s use the example of a Studio Technolo-
gies Model 210 Announcer’s Console
being connected to a Model 41 IFB out-
put. We’ll select Belden 8412 as the inter-
connecting cable. For correct operation,
the Model 210 needs at least 24 volts DC
between pins 1 and 2 of its IFB input con-
nector. It has a current draw of 105 milliam-
peres. The Model 41’s IFB output presents
an output voltage of 30 volts across pins 1
and 2 and can supply a maximum current
of 220 milliamperes. (As the Model 210’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 voltage re-
quired by the Model 210 (24 volts) allows a
6 volt maximum drop over the interconnect-
ing cable. Using the current draw and maxi-
mum voltage drop figures, the maximum
cable resistance can easily be calculated:
6 volts divided by 0.105 amperes equals
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, entering the
appropriate values will allow you to deter-
mine the maximum cable length for your
application.
Cabling Issues – Crosstalk
The Model 41’s IFB outputs 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 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 “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