Studio Technologies 220 2008 - Manual (Page 30)

Issue 4, October 2008 Model 220 User Guide
Page 30 Studio Technologies, Inc.
connecting the cable shield to the mating
connector’s shell terminal, the common
connection typically found on audio inter-
connections 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 easi-
est way is to use cable pairs that are not
twisted. Twisted pairs are great for
differential (balanced) signals, but not so
great for unbalanced transmission. This
is generally because the more twists in
a pair the greater the effective cable ca-
pacitance. In a stadium or arena setting,
choosing standard “telco” pairs may actu-
ally 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 al-
low one method would be to run the
IFB circuits “dry” (no DC) and differential
(balanced). This would gain the benefits
of differential transmission, including mini-
mizing the crosstalk. The balanced line-
level signals can be directly connected
to the Model 220’s line inputs. However,
if it’s desired to connect cue sources to
the Model 220 using an IFB-type circuit,
the line-level audio signals will need to
be “wetted up” into standard IFB circuits.
This is easily accomplished using one of
several high-performance IFB interface
units from Studio Technologies. For fur-
ther information please refer to the Studio
Technologies website.
IFB Audio Levels
The Model 220 is designed to operate
best with IFB audio levels that are nomi-
nally –10 dBu. This is the nominal level of
most IFB systems, such as the RTS 4000-
series. But actually having the correct level
present on an IFB circuit is often a “hit-or-
miss” proposition. During field testing of
prototype announcer’s consoles, Studio
Technologies’ personnel found that a
wide range of nominal audio levels were
present on “real-world” IFB circuits. Many