Studio Technologies 200 - Manual (Page 16)

Model 200 User Guide Issue 3, August 2004
Studio Technologies, Inc. Page 17
have a pin or connection point available to
access its metal shell. By adding a jumper
wire from the mating connector’s shell
terminal to its pin 1 and cable shield, the
common connection typically found on
audio interconnections is created.
IFB Channel Crosstalk
By the very nature of its design, standard
“wet” IFB circuits are prone to exhibit
crosstalk between the two audio channels.
This occurs because the audio path is
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. Is this generally an is-
sue? 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 be
a problem.
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 differ-
ential (balanced) signals, but not so
great for unbalanced transmission. This
is generally because the more twists in
a pair the greater the 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
channel 2 to the second full pair. A better
option 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. If optional line input
cards have been installed in the Model
200, the balanced line-level signals can be
directly connected. However, if it’s desired
to connect cue sources to the Model 200
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 further informa-
tion please refer to the Studio Technolo-
gies website.
IFB Audio Levels
The Model 200 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 cor-
rect level present on an IFB circuit is often