Studio Technologies 220 2013 - Manual (Page 30)

Issue 5, February 2013 Model 220 User Guide
Page 30 Studio Technologies, Inc.
Technical Notes
Grounding and Shielding
As previously discussed in this user guide,
the pin 1 connections on the main and
talkback outputs’ 3-pin male XLR connec-
tors are “floating,” i.e., not connected to
anything within the Model 220’s enclosure.
Some audio experts might take offense to
this, grousing that this should have been
left to the user or installer to be connected
or disconnected as desired. However,
repeated field testing found that floating
pin 1 on the outputs was the key to main-
taining quiet audio. From Fenway Park
in Boston, to the Orange Bowl in Miami,
and then northwest to Husker Stadium in
Nebraska, lifting pin 1 did the trick.
A simple solution is available if an applica-
tion does require that a ground be avail-
able on the main and talkback outputs’
interconnecting cables. All Model 220 XLR
connectors have a ground connection
that is made to the interfacing connector’s
metal “shell.” And most XLR connectors
have a pin or connection point available
to access its metal shell. By connecting
the cable shield to the mating connector’s
shell terminal, the common connection
typically found on audio interconnections
is created.
IFB Channel Crosstalk
By the very nature of its design, a stan-
dard “wet” IFB circuit is prone to exhibit
crosstalk between its two audio channels.
This occurs because the audio paths are
unbalanced (“single-ended”) and typically
transported on a single shielded twisted-
pair audio cable. The primary cause of
the crosstalk is the capacitance between
the wires in the cable pair. The greater the
capacitance, due to cable type and length,
the more crosstalk there will be. It’s not
surprising to find in sports broadcasting
venues that audio from one channel in an
IFB circuit can be heard “bleeding” into
the other channel. Does this generally cre-
ate a problem? No, as each channel in an
IFB circuit generally carries related audio
content. For example, on-air talent hearing
in their left ear a small amount of program
audio from channel 2 while an interrupt
from channel 1 is active, typically wouldn’t
perceive this as an issue.
There are several ways of reducing IFB
channel crosstalk. Probably the easiest
way is to use cable pairs that are not twist-
ed. Twisted pairs are great for differential
(balanced) signals, but not so great for
unbalanced transmission. This is gener-
ally because the more twists in a pair the
greater the effective cable capacitance.
In a stadium or arena setting, choosing
standard “telco” pairs may actually work
better than “high-performance” audio or
data cable!
Another option is to use two cable pairs
for each IFB circuit. If the pairs are not
shielded the wiring is simple. Common
would be connected to one side of each
pair, and then signal from each channel
would connect to the other side of the
pairs. If the pairs also contain shields the
wiring could be done somewhat differ-
ently. One option is to connect common
to both cable shields, IFB channel 1 (DC
with audio) to one full pair, and IFB chan-
nel 2 to the second full pair. A better op-
tion might be to have common connect to
both shields and one side of the pair that
serves IFB channel 2.
Other options are available if an applica-
tion demands low crosstalk. If resources
in the broadcast or production facility
allow one method would be to run the