Ifb circuit – Studio Technologies 43 User Manual
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Issue 1, July 2004
Model 43 User Guide
Page 6
Studio Technologies, Inc.
of audio to user devices by means of stan-
dard audio cables. These cables, ubiqui-
tous to the audio industry, interface using
3-pin male and female XLR-type connec-
tors. With IFB circuits and standard audio
cables it’s a simple matter to support user
devices such as listen-only belt packs and
announcer’s consoles with no external
power source required. Whether the dis-
tance from the source to the user device is
100 or 1000 feet, reliable operation can be
provided.
In many cases, the Model 43 Interface will
be used in on-air television applications.
No matter if a unit is installed in a fixed
location or as part of a remote facility,
excellent performance can be obtained.
In addition, the Model 43 is applicable for
non-broadcast applications. For example,
audio recording and post-production
facilities can also effectively use the unit.
Combined with stereo or mono listen-
only belt packs, also available from Studio
Technologies, a variety of headphone
cue systems can easily be deployed.
Maintenance facilities and test benches
will appreciate having a Model 43 avail-
able to provide assistance during the
testing and repair of IFB user devices. As
the unit’s audio inputs are compatible with
standard line-level audio signals virtually
any analog source can be connected.
IFB Circuit
The Model 43 supplies one circuit that
incorporates two audio inputs and a
“wet” IFB output. The audio inputs are
transformer coupled, have a nominal
level of +4 dBu, and are compatible with
balanced or unbalanced sources. In on-
air television broadcast applications the
audio sources will often be analog outputs
from matrix intercom systems. If this is the
case, two sources are typically designated
to feed user cue signals to stereo or mon-
aural headsets or headphones. Generally
one source is configured in the matrix
intercom system as “interrupt” while the
other is configured as “program.” An al-
ternate term often used for the “interrupt”
channel is “program-with-interrupt.” This
may be more descriptive as the function
is actually a program source that gets
interrupted with talkback audio. The “pro-
gram” channel is typically a continuous
source of program audio. An alternate
term is “program-only.” For other applica-
tions the Model 43’s audio inputs can
be connected to one or two monaural
sources, or alternately, to a stereo audio
source. This configuration may prove use-
ful in radio broadcasting, audio-with-pic-
ture, or recording studio applications.
Maintaining excellent audio performance
was a major Model 43 design goal—the
hiss, hum, and noise associated with
typical IFB circuits was simply not accept-
able. The Model 43 meets those require-
ments with audio that is “on-air” quality:
low distortion, high signal-to-noise ratio,
and ample headroom. On-air talent and
guests, production personnel, and techni-
cians will all appreciate the clean, quiet
cue signal.
As previously covered briefly, the Model
43’s IFB circuit provides DC power and
two channels of unbalanced audio over
a single 3-conductor output. The DC out-
put is nominally 30 volts with a maximum
rated current of 200 milliamperes. A major
strength of the Model 43 is the IFB circuit’s
ability to effectively deliver DC power over
a variety of conditions. Unlike other inter-
face devices that use a common but less-
than-ideal circuit topology, a unique IFB