Studio Technologies 43 - Manual (Page 12)

Model 43 User Guide Issue 1, July 2004
Studio Technologies, Inc. Page 13
Cabling Issues – Crosstalk
The Model 43’s IFB circuit conforms to a
broadcast industry standard for sending
DC power and two channels of audio over
a single pair shielded audio cable. This
implementation allows standard portable
cables, such as are used for microphone
signals, to interconnect IFB user devices.
This method is undoubtedly convenient
and practical, but is not without limitations.
The main audio quality issue is the pos-
sibility of crosstalk between the two audio
channels. This issue arises due to the
capacitance presented by the two wires
that form the twisted pair. The greater the
capacitance presented and the longer the
cable run, the greater the crosstalk will
become. Is this normally a problem dur-
ing actual use? No. But it’s something that
should be noted.
Studio Technologies did some experi-
menting with various cables and the
crosstalk that was created. For example,
a 1000-foot reel of 24-gauge 2-pair un-
shielded telephone cable was used to link
the Model 43’s IFB circuit with an IFB user
device. One pair carried the pin 2 (DC with
channel 1 audio) and pin 3 (channel 2 au-
dio) connections. One wire from the sec-
ond pair carried the pin 1 (DC and audio
common) connection. The inter-channel
crosstalk in the voice audio band was on
the order of –45 dB. Is this a good value
for “professional” audio? Of course not.
But for the intended talent cueing applica-
tions it should be fine. In almost all cases
the audio signals being carried are some-
what or fully phase-coherent. A bit of one
channel getting into the other won’t even
be noticed, especially since monitoring
is generally done using headsets, head-
phones, or ear pieces.
Is it possible to reduce the crosstalk that
is created? Absolutely, as long as a non-
standard cable connection is made. This
becomes a trade-off between an improved
crosstalk figure and ease of installation
and use. Using two full pairs can sig-
nificantly reduce crosstalk. Several con-
nection schemes are possible; the exact
one selected will depend on the specific
installation and personal technical philoso-
phy. Two unshielded twisted pairs can be
effectively used. The first pair would carry
the DC and channel 1 audio signal and
common. The second pair would carry the
channel 2 audio signal, again along with
common. There will still be some capaci-
tance between the conductors carrying
the two audio channels but it should be
significantly less. Two shielded pairs can
also be used, as was discussed in the
Installation section of this user guide.
Superior Power Delivery and
Audio Quality
As previously discussed, one of the Model
43’s strengths is its ability to very effec-
tively deliver energy to the connected IFB
user device(s). This allows more devices
to be supported over longer cable runs.
How does the Model 43 accomplish this?
Simply by having circuitry that is superior
to that used in most of the “industry-stan-
dard” equipment. In most IFB interface
devices, an adjustable voltage regulator
integrated circuit is used as a combina-
tion of audio modulator and current limiter.
While this is a simple and inexpensive
solution, it’s not without significant limita-
tions. The major problem with this method
is the type of voltage-current “knee” that
is created. As the load current increases
past about 50% of the rated maximum the
output voltage begins to decrease. This