Studio Technologies 42 - Manual (Page 9)

Issue 1, July 2004 Model 42 User Guide
Page 10 Studio Technologies, Inc.
With these systems maximum Model 42
performance would be provided by reduc-
ing their nominal output levels by 4dB.
With the Artist™ system from Riedel, the
analog ports have a nominal level of
+6dBu. A 2dB reduction in their output
level would be beneficial.
IFB Circuits
As mentioned previously, the Model 42
has four independent IFB output circuits
each supplying DC power and two chan-
nels of unbalanced audio. The IFB circuits
are designed to connect to a variety of
devices that conform to the broadcast IFB
standard. In this implementation pin 1 is
used for a combination of shield, DC
power return, and audio common; pin 2
supplies a combination of DC power and
one channel of unbalanced audio; pin 3
supplies a second channel of unbalanced
audio. The DC power supplied on pin 2 is
30 volts nominal with a maximum current
of nominally 200 milliamperes. The audio
superimposed on the DC power has a
nominal signal level of –10dBu. Its audio
source is the channel one audio input
associated with that specific IFB circuit.
The audio on pin 3 also has a nominal
signal left of –10dBu. Its audio source
is the channel two audio input channel
associated with that specific IFB circuit.
The Model 42’s IFB circuits are interfaced
using four 3-pin male XLR-type connectors
which are located on the unit’s back panel.
The associated interface cables, one for
each IFB circuit, must be terminated with
3-pin female XLR-type connectors. In most
cases the IFB circuits should be wired by
way of an input/output connector panel
rather than directly to the external devices.
It’s also recommended that the panel have
“mults” (multiple connectors) for each
of the IFB circuits. For troubleshooting
purposes it also may be useful to have
the IFB circuits pass through points on
an audio patch bay.
The type of interconnecting cable used
between the Model 42’s IFB circuits and
the user devices will vary by application.
In a fixed installation it would be typical
to use 22AWG, shielded, stranded cable
in either a single- or 2-pair configuration.
With single-pair cable, pin 1 should be
connected to shield. Pins 2 and 3 would
connect to the cable pair.
If 2-pair cable is used, pin 1 should con-
nect to one side of each pair, with pin 2
going to one side of pair one and pin 3
going to one side of pair two. The shields
can either go only to the XLR connector
shells, or to both the connector shells
and pin 1. Shielding unbalanced audio
signals can be a tricky proposition. It
is recommended that the focus be on
using excellent twisted-pair cable, rather
than worrying about whether or not it is
shielded. The typical foil shields used
in much of the contemporary audio cable
generally offers very limited effectiveness.
The best rule to follow is to try to minimize
exposure to large noise sources. (Okay,
so that’s hardly ever practical but at least
it’s a nice dream!)
In the event that very long cable runs
are required, the resistance of the cable
can impact the DC power supplied by the
Model 42. There’s no way to get around
the fact that some DC voltage will be
dropped by the interconnecting cable. A
simple ohms law calculation will tell you
the impact a specific cable run will have.
You’ll need to know the current draw of the